Photograph of furnace rust damage.Furnace heat exchanger leak standards
Allowable amounts of gas leakage from a residential heating furnace heat exchanger

  • HEAT EXCHANGER LEAK ALLOWED - CONTENTS: definition, standards & explanation of the allowed rate of leakage from residential heating furnace heat exchangers
  • POST a QUESTION or READ FAQs about the allowable leakage from a furnace heat exchanger, how the leak rate is calculated, how carbon monoxide hazard dangers are calculated inside a building
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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".

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Minimum Hole Size Allowable in a Furnace Heat Exchanger

LARGER VIEW of heating furnace parts and air flow, showing the 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].

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:

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

size 12 L sub C = 24 cubic feet per hour of flue gases allowed to leak.


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

Flue gas leak problem photo

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 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 computed Leak Opening for a Furnace Heat Exchanger

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

Square root of (4 x Area divided by Pi), as .254 in. and the diameter of an orfice of .025 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

Colorimetric gas tube detector method for testing for carbon monoxide leaks at a furnace heat exchanger (C) Daniel Friedman

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 sub A , size 12 N,V, > < >and

size 12 R> < >it would be possible to calculate

size 12 CO sub R> < >after a time

size 12 T > < >< >representing equilibrium. The

size 12 CO sub R > < >value would be the CO concentration in the house at equilibrium.

Editor's note: the equation computes COR, 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

Building air exchange rate ACH per hour (C) InspectAPedia DeWerth

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

[Click to enlarge any image]

size 12 N> , room air changes per hour.

size 12 CO sub R = {CO sub A left ( {1 - 1 over e sup NT} right )} over NV >

size 12 CO sub R> = CO in room, ppm

size 12 CO sub 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

[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 8>

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

size 12 CO sub R = size 12 14.9> < >ppm.

Using the size 12 CO sub 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

TIF 5000 refrigerant gas leak detector for halogens

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

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

The combustible gas detector can be purchased from

Photograph of a Drager hand pump used to measure carbon dioxide levels in the environment.



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