Furnace heat exchanger leak standards, hole sizes, permitted leakage allowances: this article describes the allowable leakage for furnace heat exchangers, explaining that zero combustion gas leakage may not be achievable.

We review an industry expert's calculations used to determine just how much carbon monoxide leakage is allowable in a building and we translate the allowable gas leakage into an equivalent heat exchanger hole size of abouit 1/8".

Minimum Hole Size Allowable in a Furnace Heat Exchanger

[Click to enlarge any image]

How much combustion gas or CO leakage from a furnace leak exchanger is allowed? Is there a difference in safety between leakage in a new heat exchanger and leaks that occur in use due to damage, rust, or a crack?

It is almost impossible to construct a heat exchanger that is entirely air tight. Therefore any test method developed to detect flue gas leakage needs to have quantitative aspects.

It would not be desirable to identify as unacceptable any heat exchanger leakage that meets the requirements for heat exchanger joints according to the American National Standards.

Our photograph (left) shows a brand new gas furnace heat exchanger we installed in a New York home. Even this new unit may have trace-levels of flue gas leakage.

What is the Allowable Amount of Gas Fired Heat Exchanger Leakage?

Some gas industry experts (Douglas DeWerth & others ) have published studies indicating that in a new gas fired furnace heat exchanger and system, the allowable flue gas leakage rate can be equal to about the leakage expected from a 1/8" diameter hole. This effort at making a reasonable standard recognized that there could be safe imperfections in heating system manufacture. [1] through [11].

• Studies such as DeWerth (1991) (HEAT EXCHANGER LEAK ALLOWED)[1] and standards such as published by AGA (american gas assoc) may define the "allowable leakage" from a heat exchanger, but DeWerth's "1/8 equivalent leak hole size" standard quoted by Friedman in the ASHI Technical Journal article was intended to address the condition of NEWLY manufactured units and assumed particular equipment size, room size, air movement etc. that may not pertain to an in-use older or damaged heating system.
• The actual leakage from a crack or hole in a heat exchanger depends on variables during operation as well as its location - for e.g. when a blower is running air may  go into the combustion chamber through the crack rather than the other way around.
• We expect that heating service technicians who examine a furnace for unsafe heat exchanger leaks as well as the furnace manufacturers will advise that if a crack has occurred the unit needs repair or replacement. (Sometimes repair by welding may be acceptable but it's doubtful).
• Ultimately ANY crack in a heat exchanger that was caused by wear and tear or some event like improper operation is potentially dangerous because we don't know that it won't suddenly increase in size or change how it leaks flue gases into the building air as operating conditions change. So even a small leak, depending on its cause (acceptable manufacturing defect versus a crack or rust hole) might be unsafe.
  
  For that last reason we'd be surprised if any HVAC contractor would risk the lives of the occupants (nor a lawsuit) by saying leaving the unit in place is OK.

Analysis of Furnace Heat Exchanger Gas Leak Standards

The standards for Gravity and Fan Type Direct Vent Wall Furnaces (Z21.44.1981), and for Direct Vent Central Furnaces (Z21.64-1978), requirement for tight joints in heat exchangers is met if the combustion chamber-vent section does not leak more than 2% of the flue gases.

This test is conducted with the internal pressure of the heat exchanger raised to 0.1 w.c. static pressure.

The leakage from the minimum allowable hole is calculated from:

size 12 L sub C = .02^V^I> where:

* re-writing from original text formatted in Ventura Publisher for the ASHI Journal article by DF as size 12 L_C =~ .02 V

size 12 L_C =~ maximum allowable flue gas leakage rate, cubic feet per hour,

size 12 V =~ 15 cubic feet of flue products per 1000 BTU, assuming 50% excess air,

size 12 I =~ Input rate, 1000 BTU/hour

For an 80,000 BTU/hour furnace:

size 12 L_C =~ ( .02 ) ( 15 ) (80 )

size 12 L_C =~ 24 cubic feet per hour of flue gases allowed to leak.

See HEAT EXCHANGER LEAK TEST STANDARDS

Figuring the diameter of a heat exchanger leak opening from the area concentration of leaked gas that will occur as a result

It is possible to calculate the hole size needed to leak 24 cubic feet of gases using an orfice flow equation assumed equal to a particular or specified opening diameter.

Passing 24 cubic feet of flue gases per hour would require a leaking orfice of 0.051 sq.in.

For a pressure drop across the orfice of .4 inch w.c. the orfice would be 0.025 square inches.

Watch out: when inspecting and testing furnace heat exchangers for leaks, don't forget to look for other flue gas or carbon monoxide leak sources such as shown in our photo at left. Failure to observe a gross safety hazard such as this one risks focusing on the wrong hazard.

0.51 sq.in. computed Leak Opening for a Furnace Heat Exchanger

The diameter of an orfice for .051 sq.in. was found to be the computed size of an acceptable equivalent total flue gas leakage opening as follows:

DeWerth [8] used a desired rate of flow in cubic feet per hour, a constant of 1658.5 to convert units, a .9 orfice factor, the area of the orfice in sq. in., a pressure drop across the orfice of 0.1 in. w.c. - which is conservatively high for all but power burners, and the specific gravity of flue gases assumed equal to 1.

Passing 24 cubic feet of flue gases per hour would require a leaking orfice of 0.051 sq.in. For a pressure drop across the orfice of .4 inch w.c. the orfice would be 0.025 square inches.

The diameter of an orfice for .051 sq.in. is computed as follows:

Square root of (4 x Area divided by Pi), as .254 in. and the diameter of an orfice of .025 sq.in. is .180 in. [GRI 84/0162 p. 10]

1/4" diameter Hole in a Heat Exchanger - Unacceptable at 80,000 BTUH example

This calculation implies that it would take a 1/4 inch diameter hole in the heat exchanger to leak an unacceptable amount of flue gases for our example of an 80,000 BTUH furnace with an internal pressure of 0.1 inch w.c.

In an effort to select a conservative condition in the development of the test method, a 1/8 inch hole was selected as the minimum size hole that should be detected. A hole with one half the diameter results in an area one fourth that of the 1/4 inch diameter hole, which would reduce the flow by a factor of four according to the orfice equation. [2].

Maximum Leakage Allowed in a Furnace Heat Exchanger

After conservatively determining that the minimum hole size representing an unacceptable condition was a 1/8 inch diameter hole, it was necessary to select the maximum CO concentration to be allowed in the flue gases leaking through the 1/8 inch hole.

A 100% safety factor was used on the maximum 400 ppm air-free CO allowed by the American National Standards for Furnaces Z21.47-1983. Thus, 200 ppm air-free CO was used as reference for detection.

The appropriateness of the selection of 200 ppm CO can be verified using an analysis based on the maximum allowable room concentration of CO. An equation was developed

Carbon Monoxide Hazards From House Heaters Burning Natural Gas, G.W. Jones, et al., Technical Paper 337, Department of the Interior, Washington, D.C. 1923. which relates the room concentration to emission levels over time. Figure 1 [next page] shows this equation together with all the involved factors.

From this equation it can be seen that if assumptions can be developed to define

size 12 CO_A, size 12 N,V, and

size 12 R it would be possible to calculate

size 12 CO_R after a time

size 12 T representing equilibrium.

The size 12 CO_Rvalue would be the CO concentration in the house [or in the "room"] expressed in ppm, at equilibrium.

Editor's note: the equation computes CO_R, the CO in the room, in ppm, based on the CO in air-free products of combustion ppm, the number of air changes per hour in the room, the volume of the room, the volume of dry air-free combustion products per 1000 BTU of fuel burned (8.52 cu. ft. for natural gas), the burner input rate in BTU/hr., a Naperian logarithmic base of 2.7133, and T, the time needed to reach a given concentration (of CO) in hours.

House Air Changes Per Hour in a Building Related To Furnace Heat Exchanger Leak Standards

A review of literature on tightness of house construction was used to determine a value of

[Click to enlarge any image]

size 12 N , number of room air changes per hour.

size 12 CO_R = CO_A ( {1 - 1 over e^NT} ) / NV

size 12 CO_R = CO in room, ppm

size 12 CO_A = CO in air-free products of combustion (in FAF duct), ppm

size 12 N = Number of room air changes per hour

size 12 V = Volume of room, cubic feet

size 12 P = Volume of dry air-free combustion products, cu. ft. per 1000 BTU of fuel burned (8.52 cu. ft. for nat. gas)

size 12 R = Input rate, thousands of BTU/hr.

size 14 e = Naperian logarithmic base (2.7133)

size 12 T = Time to achieve a given concentration, hours.

A great deal of what is written on house infiltration cites an old rule of thumb that leakage rates vary from 0.5 to 1.5 air changes per hour (ACPH) and average about 1.0 ACPH.

This overall average is shown in the ASHRAE Handbook of Fundamentals and is also used by the EPA in its proposed text for inclusion in DOE's rule making for the Residential Conservation Service (RCS) program.

The current generation of houses being built, however, appear to have an average infiltration rate between 0.6 and 0.86 ACPH. Assuming that the average ACPH for houses built two decades ago had an average of 1.0 ACPH, the average has dropped 15 to 20 percent over that time.

Current air infiltration studies generally support the 0.6 to 0.86 ACPH figure.

Gas furnace heat exchanger leak research

• Natural Ventilation of Modern Tightly Constructed Homes, AGA/LIGT Conference on Natural Gas Res. & Tech., Chicago, IL, July 1982.
• Air Leakage Characteristics and Weatherization Techniques for Low Income Housing, DOE/ASHRAE Conference on Thermal Envelopes, Orlando, FL December 1979.
• Residential Air Infiltration , ASHRAE Technical Paper, Philadelphia, PA 1979
• Building Energy Data Compilation Analysis and Demonstration , DOE Contract W-7405-ENG-48, Lawrence Berkeley Laboratory, 1980.

[Discussion of contents of the references by the author is in his original report.]

House Air Change Rate

Based on the above data and references, a realistic, conservative house room air change rate would be 0.5

size 12 (N = 0.5 size 12 )

House Volume: A small house with 9000 cubic feet (1125 sq.ft. x 8 ft. ceiling) was assumed.

size 12 (V = size 12 9000)

Furnace BTU Rate: The house was equipped with an 80,000 BTU/hour furnace with a 50 percent load factor.

size 12 R = size 12 .50(80,000)

Flue Gas Spillage Destination:

100 percent of the flue gases generated were assumed to spill into the house (which would never occur under realistic conditions)

Time to Equilibrium: From exercising the equation it takes about eight hours for equilibrium conditions to be reached.

size 12 T = size 12 x 8

CO Level at Equilibrium: size 12 CO_R = {( 200 ) ( 8.52 ) ( 40 ) ({ 1 - 1 over {e sup (.5)(8)}} )} over {( 0.5 ) ( 9,000 )}

size 12 CO_R = size 12 14.9 ppm.

Using the size 12 CO_R equation above, this would mean that after a period of eight hours the house CO concentration would be no more than 15 ppm.

Allowable CO Exposure in buildings

The CPSC, in its report on Health Effects of Carbon Monoxide evaluated the scientific basis for suggesting long term exposure limit for CO and concluded that the value should be no more than 15 ppm as a time weighted average. OSHA Concentration Limits for Gases as shown in the Federal Register (Volume 39, Number 125, 6/27/74) specifies a maximum eight hour weighted average of 50 ppm. Much higher than the 15 ppm allowed here.

The American Conference of Governmental Industries Hygienists also discusses their recommended Threshold Limit Value of 50 ppm in Documentation of the Threshold Limit Values (3rd Ed., 1971). This source also reports that the CO limit in the USSR is 18 ppm and in Czechoslovakia 30 ppm.

Therefore by developing a leak detection method that would allow no more than 200 ppm CO to leak from a crack or hole in a furnace heat exchanger the home environment should remain safe if 100 percent of the emissions from a properly adjusted furnace were released into the indoor air.

Specification of the Test Gas for Detecting Heat Exchanger Leaks

A mixture of 14.3 percent methane in nitrogen was used as the tracer gas as this mix cannot be diluted in air to obtain a combustible mixture.

[We've omitted an interesting but lengthy section which explains the reason for selection of the particular test gas, including an explanation of why certain concentrations of combustible gas, including high concentrations, will not support combustion (insufficient oxygen).-Ed.]

[Note that some heat exchanger test procedures make use of refrigerant gases, products readily available to the tradesman and easily detected (though not quantified) using a test instrument such as the TIF 5000 shown at left and discussed in detail

at TIF 5000 GAS DETECTOR.

Watch out: refrigerant gas detection by release into a heat exchanger as a tracer gas if used to check for heat exchanger leaks - is something no longer recommended nor permitted where discharge of refrigerants to the environment is a possibility.

Description of the Test Procedure for Heat Exchanger Leaks

The developed method traces the migration of 14.3 percent methane in nitrogen from the combustion side to the air side of the heat exchanger.

The presence of the gas mixture in the circulating air side is detected with a combustible gas detector which is calibrated to respond to about 200 ppm CO, the maximum leakage concentration chosen.

Figure 2 shows the set-up for the Three-Step Method. [DeWerth]

Step One - Visual Inspection

Step One is to conduct a thorough visual examination of the heat exchanger. Clean any loose particles on the visible surfaces of the heat exchanger, use a mirror and a strong flashlight. Inspect the internal sections for signs of split seams, open cracks, severe deterioration.

Examine joints between flue gas passages of the heat exchanger and other parts of the furnace. If construction is such that a portion of the heat exchanger or radiator is in the cold air return compartment, special care should be given when examining these parts.

Access for visual inspection of the heat exchanger is frequently limited by evaporator coils, etc. therefore a removable inspection plate, access panel, or heat register on the plenum would be helpful to visually examine the exchanger from the air side. Any visible crack or hole in the heat exchanger is reason for requiring repair.

Step Two - Flame Observation

The furnace is then turned on and Step Two: an observation of the flames before and after the circulating air blower comes on is made. After the unit is hot, the gas and electrical power to the furnace are shut off (the blower is not operational for the rest of the test).

Observe the flame pattern for floating flames and flame rollout or any flame distortion. These observations indicate a possible split seam, open crack, severe deterioration of the heat exchanger or mechanical separation of the heat exchanger from the jacket. Disturbance of the flame by the blower is a reason for requiring repair of the heat exchanger.

Step Three - Tracer Gas

Step three is then performed: the tracer gas is injected into the combustion chamber and the calibrated gas detector is used to check for the presence of methane on the air side of the heat exchanger.

Prepare an access hole in the plenum over the heat exchanger, as close to the heat exchanger as possible. If you cannot get within 3 inches, any opening as close as possible will be acceptable but you'll have to allow more time for reaction.

Allow the furnace to operate at least 5 minutes, then quickly conduct the rest of the procedure while the heat exchanger is warm.

Check the vent connector for any blockage

Turn off the main burner and pilot and power supply to the unit

Insert the gas detector probe into the selected area in the plenum and null out any background disturbance

Place the injector probe for the tracer gas in the bottom of a heat exchanger section. Adjust the flow rate of the tracer gas to seven cubic feet per hour. Maintain this flow rate through the balance of the test. For multiple section heat exchangers do one section at a time.

As the heat exchanger is flooded move the gas detector probe to cover the top of the heat exchanger section for at least two minutes.

If an unacceptable leak is present the calibrated indicator

If the tic rate increases during the probing period but the light does not go on, there is no unacceptable leak. But it may be desirable to further investigate if the tic rate increase is substantial.

If the light goes on, the leakage rate is unacceptable and the source of the leak should be investigated by further probing. This is a reason for requiring replacement of the heat exchanger.

Repeat the procedure for the remaining heat exchanger sections

Any access openings made in the furnace plenum to conduct the test must be closed or sealed.

If no reason for corrective action is indicated, re-light the pilot and turn the furnace back to its ready condition in accord with the manufacturer's rating plate or instructions. Seal the hole in the plenum with a small piece of sheet metal.

Combustible Gas Detector Specifications & Where to Buy a Combustible Gas Detector

• Alkaline battery, low battery indicator light, hand held, portable, maximum weight two pounds.
• Warm up time maximum 30 seconds.
• Maximum operating temperature for probe and instrument: 150 °F.
• Must detect CH4 and CO.
• Calibration: internal is desirable with a tick indicator at a low level (less than 20 ppm CO); an indicator light for a low level (200 ppm) of combustible gas; an alarm light and audible signal at 50 percent of the LEL for gas leak detection.
• Calibration should be plus or minus 5%.

The combustible gas detector can be purchased from

• Drager GAS DETECTORS - Draeger Instruments, shown at left
• GAS DETECTION INSTRUMENTS - separate article
• TIF 5000 GAS DETECTOR - separate article
• J&N Enterprise, PO Box 188, Wheeler IN 46398 219-759-1142
• Pragmatics Inc., PO Box 737, Manchester MO 63011 314-225-6786
• Sierra Monitor Corporation, 1050K Duane Ave., Sunnyvale, CA 94086 408-746-0188
• The calibration gas (200 ppm CO/N2) and working tracer gas (14.3% CH4/N2) from Matheson, Dayton OH 45424 513-236-3021
• AIRCO, Chicago, IL 60628 312-468-4200.

Heat Exchanger Leak Standards, Measurements, Calculations

• HEAT EXCHANGER LEAK TEST STANDARDS - includes a more-complete set of researh citations
• American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209
• Gas Appliance Manufacturer's Association, 1901 North Fort Myer Drive, Arlington, VA 22209
• Gas Research Institute, 8600 West Bryn Mawr Avenue, Chicago, IL 60631
• Douglas W. DeWerth, P.E. is an ASHRAE Member and is Group Manager, Residential Appliances/GATC, Research and Development, for the American Gas Association in Cleveland, OH. Readers should also note two relevant and interesting articles provided to the Journal by R. Matzen and A. Carson, respectively:
• John N. Kirkpatrick, MD, Occult Carbon Monoxide Poisoning,The Western Journal of Medicine , (January 1987 146:52-56) available from the author, The Mason Clinic, PO Box 900, Seattle, WA 98111-0900
• F. Steel, Airtight Houses and Carbon Monoxide Poisoning,Canadian Building Digest , Division of Building Research, National Research Council Canada, Ottawa, Canada K1A OR6 ISSN 0008-3097 UDC 614.8:728 pages 222-1 - 222-4.

• [0] Keith R Prach, Residential Furnace Heat Exchangers : History of Metal Failure and Behavior Inspection Techniques and Procedure [at Amazon.com], KRP Enterprises, 1993, ISBN-10: 0963742426 ISBN-13: 978-0963742421
  
  You may also be able to purchase this book from its author: Keith Prach, K&P Enterprises, Inc., 1800 E. Lincoln Suite C-3, Fort Collins CO 80524
• [0a] Ellis Prach,Heat Exchanger Experts Residential Furnace Inspection Manual [at Amazon.com, may be out of print], Heat Exchanger Experts Residential Furnace Inspection Manual [at Amazon.com],
  
  Quoting: This full-color residential furnace inspection manual will broaden your understanding of how defects occur in furnace heat exchangers and will enable you to accurately assess the integrity of any furnace.
  
  You will be led through the steps to identify defects, according to the history of metal failure in a variety of commonly used residential furnaces, and taught the techniques required to conduct furnace heat exchanger evaluations. The Heat Exchanger Experts Manual will prove to be invaluable for seasoned technicians, novice technicians and everyone in between
• [2] "Heat Exchanger Testing, Who's Right?" Dan Friedman, The ASHI Technical Journal, Vol. 2 No. 1, July1991. DJ Friedman (editor of InspectAPedia.com) served as a professional home inspector and forensic investigator of building failures from, 1986 to 2010. He previously operated a heating and cooling service and repair company and a residential construction and renovation firm. Mr. Friedman served as on and chair of the ASHI Exam committee, ASHI Ethics Committee, ASHI Standsards Committee, and ASHI Technical Committee (not simultaneously).
• [3] "Case History: LP Gas Leak - Using the TIF 8800," Dan Friedman, The ASHI Technical Journal, Vol. 2 No. 1, July1991
• [4] "Choosing and Using a Carbon Monoxide CO Monitor," Dan Friedman, The ASHI Technical Journal, Vol. 2 No. 1, July1991
• [5] American Gas Association, Fundamentals of Gas Combustion, 7th printing, September 1985. Prepared by American Gas Association Laboratories for American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209 and Gas Appliance Manufacturers Association, Inc., 1901 North Fort Myer Drive, Arlington, VA 22209.
• [6] National Fuel Gas Code, NFPA 54 1988 and ANSI Z223.1 1988, available from AGA at the address above, or from the National Fire Protection Association, Batterymarch Park, Quincy MA 02269
• [7]NFPA and AGA, National Fuel Gas Code Handbook, Z223.2, same source as above
• [8] "Residential Gas Furnace Heat Exchanger Testing", Douglas DeWerth, P.E. American Gas Association (AGA) Laboratories under SAM number 630-92 9/86. , Refrigeration Service Engineer’s Society’s Service Application Manual (SAM)"
  
  The manual outlines and explains the many test methods that are used and have been used to inspect and test heat exchangers. This manual also goes into detail about the test equipment used in the 3 step method that the AGA developed. RSES members can view the SAM minus the photographs and diagrams at http://www.rses.org. If you are not a member or want to purchase a complete copy call RSES (800)297-5660 for availability and cost.


• [9] Furnace Heat Exchanger Leak Test, American Gas Association .
  • Step 1 Visual inspection.
  • Step 2 Burner flame deviation test.
  • Step 3 injecting a nitrogen/methane gas mixture into the burner chamber. The heat exchanger outlet of the heat exchanger is plugged and a combustible gas detector is used to check for gas leakage on the exterior of the heat exchanger. A detailed fact sheet on the AGA test procedure is of the heat exchanger.
  • Source: http://www.aga.org/NR/rdonlyres/A156C36A-9324-4978-91B4-E78DB63DAD7D/0/8612FURNHEATEXCHNGLEAKTEST.pdf
• [10] Jennifer Moore, Sales Administrator, Nextteq, LLC, Tampa FL, Nexteq 813-249-5888. Nextteq is the master Distributor for Gastec in the United States. According to the company's website, Gastec Gas Sampling Pumps are the industry’s first and only pumps to provide on-the-spot measurement of ambient temperature. [Private email, JM to DF 5/23/08]


• [11] AGA mixed gas test method for heat exchangers: see http://www.aga.org/pdf/publicinfo/co...facts8612b.pdf also see the gas test kit available from http://www.testproductsintl.com/gas.html

Reader Q&A About Heat Exchanger Leak Test Allowances

Question: review of the heat exchanger leak rate allowance calculation

2019/08/15 WM wrote by private email:

I think there is a glitch in the printing -

The question I have is on page two of the article (bottom of page) where it starts:

“The leakage from the minimum allowable hole is calculated from:

size 12 L sub C = .02^V^l> where:

size 12 L sub C = maximum …..

What is the size 12 L sub C ?

Also, Are you aware of the issues with the Lennox 13GEP packaged gas fired units?

Reply:

It looks as if there was a formatting error [that I have now corrected]: the original article text gave

size 12 L sub C = left ( {.02} right ) left ( {15} right ) left ( {80} right )

The "C" is properly written as a subscript

size 12 LC =~ ( .02 ) ( 15 ) (80 )

size 12 LC =~ maximum allowable flue gas leakage rate, cubic feet per hour

For an 80,000 BTU/hour furnace:

size 12 LC =~ ( .02 ) ( 15 ) (80 )

size 12 LC =~ 24 cubic feet per hour of flue gases allowed to leak.

This test is conducted with the internal pressure of the heat exchanger raised to 0.1 w.c. static pressure.

Now reading

Size 12 L_C =~ .02 V

In the heat exchanger leak calculation and allowance article found on this page, using the assumed 0.1" W.C. static pressure in the heat exchanger (which I think is rather low), I wanted to calculate just how big a hole would be represented by the total allowable leakage that one could detect (e.g. using a tracer gas) to get 24 cu ft/hr. leakage.

Watch out - and do comment: I have long argued that the whole approach to heat exchanger testing is fraught with pitfalls. I worry that a false negative (no leak found when there is in fact a leak ) is a very serious error. Just how big is that risk?

It depends on what dope is doing the testing. We have people who test with the blower on, for example, that completely changes the pressures involved. When the blower turns on, depending on just where a crack or hole is physically located, we may be blowing building air INTO the hole and into the heat exchanger rather than drawing combustion gases (and CO) out of the exchanger.

In my view, particularly dangerous is relying on a combustible gas detection instrument alone, placed who the hell knows where near the furnace (or elsewhere).

Yes I'm aware of the Lennox issue - we have published that information at https://inspectapedia.com/heat/Lennox_Pulse_Furnace_Notice.php Lennox PULSE FURNACE SAFETY WARNING

Reader follow-up:

Have you ever seen any testing on the actual pressure inside the heat exchanger?

I am wondering about the following:

Lennox claims the induced draft fan is at -0.06” to 0.09” wg; the supply fan is at 1.0” wg; thus there is no way combustion gases can enter to supply air stream.

In looking at the holes and cracks is they are on the leaving side of the exchangers curved surface, I feel you could induce the combustion gases into the supply air stream.

Also, you are taking in ambient air into the combustion burners and heating it to 350 degrees F (±). This changes the volume of the air from 12.5 CF/Lb. to 20 CF/Lb.

There has to be an increase in pressure inside the heat exchanger due to the heating of the air even though the exchanger is open to the atmosphere.

Like any air flow system the static pressure will vary from the inlet to the discharge. This means the negative pressure is higher at the fan inlet and lower at the burners.

If there is a slight increase in pressure due to the heating of the air, then you could actually have a positive pressure in the heat exchanger due to the volumetric change.

I cannot find anything on this aspect of furnace heat exchangers.

Moderator reply:

There are of course other heat exchanger inspection articles online but as far as I know no one else did the detailed engineering as did DeWerth.

If you see math or other errors in our material I'd be grateful for specifics so that we can address those.

I am doubtful about Lennox's assertions. It's too easy to sneak by a no-worries claim by hiding the shortcomings and omissions in the fine print of assumptions.

I agree that depending on a leak's location negative pressure in that area could literally suck combustion gases out of the heat exchanger.

Your point on the effects on volume of heat is interesting and significant and isn't one I've seen addressed.

IMO no one has done realistic testing. SUBJECTIVELY in the field we've seen a big hole in a heat exchanger that was so pressurized when the fan ran that no leakage was detected.

However before the fan ran the flue gases poured into the building supply plenum and of course ductwork. So we only poisoned people a little bit - and just for a little while as we waited for the system to heat up and the limit switch to flip on the blower. IMO leak testing is most reliable done twice: before the blower runs and after it's on.

Problem with "after it's on" is that even though we may be sucking gases out of the h. exchanger, we're also moving so much air that the result is so dilute that the instrument may not find a thing. Does that mean it's safe? ... it depends.

I think we should make NO assumptions about where the leaks occur on heat exchangers, except that they're damned hard to see without a total disassembly - if then.
However I SUSPECT that there are three classes that appear in different locations:

- manufacturing defects, probably rare, small, occurring probably at seams anywhere, probably varying also by temperature of the steel
- overheat cracks, probably most-common right over the flame, probably worse on hotter oil fired equipment
- rust holes and damage, probably most-common where overhead A/C coils or humidifiers drip right onto the top of the heat exchanger - and potentially the biggest holes.

Besides these there are of course leaks in the flue vent connector, leaks at the draft hood, and unsafe chimneys themselves. I nearly killed everyone in a home in Port Jervis NY simply by pointing out that their chimney was blocked and their gas boiler unsafe. They called an idiot who "solved" the problem by replacing the draft hood with straight flue pipe. My advice to fix the system led to bringing in someone who made it worse.

I am disappointed to have to say I'm a bit overwhelmed by the complexity of this problem and I'm left doubtful that once an engineer has made all the necessary simplifying assumptions that we can have much confidence in a theoretical claim about whether or not there area leaks.

My bottom line is consumer safety: what should we tell people to do or not do about the possibility of CO poisoning and flue gas leaks at furnaces?

I want to see better inspection and testing procedures, always used together (testing alone is not reliable and testing will sometimes catch a problem you could never have seen), to catch as many unsafe conditions as possible, supplemented by steps to assure that there's working CO detectors properly located and installed.

Question: Strong winds may overcome exposed direct vent chimneys or flues for heating appliances?

Concerning sidewall power vent to one of my residential gas furnaces. It is a proper code compliant side vent sloped properly with condensate drain etc. However, it is on a wall exposed to a wide open area - thus winds can be strong against the house. Do I need to install something like an open vent collar to reduce the back pressure variations caused when winds are heavy? - Sack from VA 12/2/12

Reply:

Sack

Interesting question, I don't know but if you can tell us equipment brand and model we will research the question - or you can all the the manufacturer who can tell us.

I've never seen a power vent with wind protection installed, and we have presumed that the blower fan that provides positive draft for the direct-vented heating appliance is designed & tested by the manufacturer to provide more than adequate draft provided that you have followed the manufacturer's installation instructions. Those instructions typically state that

The combustion air intake shall be installed upwind of the vent outlet when exposed to prevailing winds.

Avoid locating the vent terminal on a wall facing prevailing winds and wide-open areas. When impractical, choose a location that protects the vent from strong winds, such as behind a fence or hedge. [15]

Do you have a copy of the installation instructions and can you give the brand and model of your heating appliance?

Question: what is the required clearance between adjacent houses & a neighboring direct vent fireplace?

Is there a minimum distance between brick houses in Toronto for venting termination of a fireplace using direct vent? - A.W., 12/30/2012

Reply:

I have not found building code citations that refer to nearby or adjacent buildings when specifying clearances for direct vent fireplaces, but it seems likely that your local building code inspector would agree that the clearance requirements for the building in which the fireplace is installed would set the minimum acceptable clearances that would then apply also to a nearby or adjacent building.

In other words, if the fireplace vent distances and position relative to your own building windows or doors were LESS THAN those to the windows & doors on the building where the fireplace is installed, and presuming for a moment that the fireplace was installed properly, meeting local building permit and code requirements, then a violation probably exists.

If the fireplace vent distances and position relative to your own building windows and doors is GREATER THAN those to the windows and doors where the fireplace is installed, and presuming for a moment that the fireplace was installed properly, meeting local building permit and code requirements, then a violation probably does not exist.

Take some some sharp photos of the installation that concerns you, or make a sketch and send those along and we may be able to comment further.

Thanks to Alan Carson, Carson Dunlop Associates, Toronto, for assistance with this topic.

Question: neighbor's power vent 18" from power line

(Mar 8, 2014) James S said:

My neighbor just installed a power vent for his furnace on the outside of his house. The house is 18" from the property line (this is a grandfathered non-conforming setback), but the new very loud power vent extends a further 10" into the space, so stops 8" from the property line. Is it ok to extend into the non-conforming set back like this when the code says they can't build 3' from the property line?

Reply:

James,

Good question, for which I don't have a sure answer; this is a question for your building department.

Most communities indeed have restrictions on just what can be close to the property line, with variations depending on whether you're considering a front, side, or rear property boundary. Keep me posted.

Question: clearance distance from direct vent furnace exhaust to clothes dryer exhaust vent

(Apr 8, 2014) Paul T said:

what is the clearance for a direct vent furnace to a dryer exhaust vent?

Reply:

Paul I'm not aware of a clearance specification between the furnace and a dryer exhaust vent, and I'm not sure if your question is distance to the vent pipe or distance from the direct vent furnace combustion air intake or distance from its exhaust, or distance from the heating appliance itself.

But I'd say that ANY dust emitting source close to any fuel burning appliance is a concern if the dryer lint can enter or clog combustion air inlets (very dangerous where carbon monoxide may be produced) or other air vents such as cooling vents on equipment.

I can suggest two approaches to getting past speculative arm-waving:

1. if you see dryer lint in or on the appliance that's a potential safety concern that needs to be addressed

2. Check with the appliance manufacturer directly, for the particular brand and model of heater, starting with a review of its installation instructions and if needed a call to the manufacturer.

Watch out: if your clothes dryer were a model whose air intake were at risk of drawin in combustion products that would be a dangerous situation.

Question: repairs for Sears Homart® direct vent gas fired wall-mount furnace

(May 18, 2014) Anonymous said:

I have an old Sears Homart Direct Vent Gas Fired wall furnace model #867.72542
It has worked well for 29 years with only the fan being replaced 3 times. As of late, it has been getting so hot behind the front panel that the wires going to the 2 limit switches are melting causing smoke. I'm not sure how to fix this or what the problem is. Any suggestions would be greatly appreciated. I can't seem to find anybody that knows anything about these old Homart furnaces. Anteup711@aol.com
Lawrence Markowski

Reply:

Lawrence

The overheating you describe is dangerous and suggests backpressure in the combustion chamber. I'd expect that to happen if the system has not been properly serviced or adjusted and/or in particular if the exhaust venting is blocked or not working properly.

Shut off the system and ask your heating service technician to diagnose and repair the cause for overheating. Any overheated switches and wiring should be replaced as well, as if they're damaged the system is unsafe and risks a fire.

The problem is not one likely to be unique to the Homart Furnace.

Finally, if the total proposed cost of repairs is a significant portion of the cost of a new furnace, I'd give that option consideration.

Question: direct vent Weil McLain gas boiler may be too close to neighbor's windows

(July 17, 2014) Concerned said:

A neighboring house has a Weil-McLain gas-fired boiler with a sidewall direct vent that is directly across from the window of my 3' x 4' powder room. There is less than 9 feet between our houses- 7 feet on his side and 2 feet on my side of the property line. If I open my window, you can see the condensation come in through the window. You can smell and taste it

. I put a carbon monoxide meter on the windowsill and got a reading of 15. I called the company and they originally told me that there is no specification as far as a required distance from the vent to a window and that exhaust is being emitted and that the city should follow code.

I called the inspector and he told me that it should be 10 feet and that it had to be corrected, but it was a "sensitive issue" because they passed the inspection. After about a year and various phone calls he told me that according to the 2009 International Residential Code the manufacturer's instructions needed to be followed which requires 6 feet from an adjacent wall- no mention of a window- which brings this around to the beginning again.

I read a copy of the instructions and on the very first page it has a warning saying "Perform steps in the order given. Failure to comply could result in severe personal injury, death or substantial property damage." There is a section which comes before any clearances are given which says,

"Consider the surroundings when terminating the vent: Position the vent where vapors will not... be objectionable. Avoid possibility of accidental exposure of flue products with people or pets."

These are just 2 of the 7 considerations that should be given before proceeding to the clearance distances. Again I called the company and they told me that they would not guarantee the safety of this exposure and that I should keep my window closed. I met with a lawyer and he told me that it was a trespass on my property and that I would have to take my neighbor to court and not the installer. I talked to my neighbor about it, but no response. What to do?

Reply:

Concerned,

If the neighbor is not interested in addressing this concern, and before launching a costly lawsuit that will upset everyone for some time, I'd try speaking with your local building department. If your local code officials agree that the installation you describe is improper and violates local building ordinances that alone may be enough to encourage the neighbor to change the installation as needed.

Question: clearance distance between direct vent propane fireplace exhaust and oil storage tank

(Aug 26, 2014) Elsbeth said:

Is there a required clearance for a direct vent from a propane fireplace and an outside oil tank? Thanks

Reply:

Elsbeth

I'd respect the same distance for oil storage tanks as that required for clearance from the oil burner, since like an oil burner, a propane fireplace will involve flame - that's ten feet from the burner - which is not the same as 10 feet from the vent.

Propane tanks also have clearance requirements, typically ten feet

Reader follow-up:

Thanks for your response- Are there any requirements preventing an oil tank being in close proximity to a propane tank? We're moving and want to install a propane fireplace and the oil tank is outside near the living room wall which is where we want the fireplace.

Question:

9/8/2014 Pam said:

We had a natural gas boiler installed in our house to replace a diesel burning unit. We asked that the new boiler be vented through the old chimney but the installer said that the new gas water heater was vented through there and that only one appliance could be vented through the chimney. We live in Alberta and from everything i have read my understanding is that 2 gas appliances through one flue is no problem so long as they don't exceed the max BTU output. .please confirm or correct.

Also, I have been reading here on testing new systems for leaks. please let me know how that test is performed as I don't think it was done here. Yesterday I noticed an exhaust leak where the pvc didn't meet the outside wall vent cover and liquid was leaking down the siding of the house. That and other issues with the installation has made me lose confidence in the installer and now want to make sure all was done correctly.

Reply:

two appliance through one flue .... "is no problem " may or may not really be the case. In addition to total BTUs vs. chimney venting capacity there are guidelines for how appliances are connected and how their draft hoods are arranged. If done wrong, for example, a higher BTU appliance can back-vent out through the smaller appliance flue vent connector, particularly when the smaller one is OFF,

Also venting a small appliance into a large chimney can cause it to fail to vent properly when it's running alone - the chimney may be too big and too cold to develop proper draft.

In sum, you want a qualified chimney or gas appliance technician to be sure that the installatin is correct and safe If not the CO risk can be fatal.

...

Continue reading at HEAT EXCHANGER LIFE & WARRANTY or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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Or see these

Furnace Heat Exchanger Articles

• BACKDRAFTING HEATING EQUIPMENT
• BLUE vs YELLOW COMBUSTION FLAMES
• CARBON MONOXIDE WARNINGS: HOME HEATERS
• CO DETECTION OPTIONS
• COMBUSTION APPLIANCE CONTAMINANTS
• COMBUSTION GASES & PARTICLE HAZARDS
• COMBUSTION PRODUCTS & IAQ
• CONDENSING GAS FURNACE INSPECTION & TESTS
• HEAT EXCHANGER CORROSION
• HEAT EXCHANGER LEAK TEST
  • HEAT EXCHANGER CLEANING
  • HEAT EXCHANGER LEAK 3-STEPS
  • HEAT EXCHANGER LEAK ALLOWED
  • HEAT EXCHANGER LEAK TEST STANDARDS
  • HEAT EXCHANGER LIFE & WARRANTY
• GAS DETECTION INSTRUMENTS
• ODORS FROM HEATING SYSTEMS
• SAFETY RECALLS CHIMNEYS VENTS HEATERS
• SMOKE DETECTOR TEST MSDS [PDF]
• SMOKE PENCIL / SMOKE GUN SOURCES

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