Furnace sizing errors: what happens if the heating furnace is too big for the home's heating requirements.

This article explains why it's a mistake to install a heating furnace whose heating capacity is too large for the space or building it serves. We list operating, safety, and heating fuel effects of oversized furnaces and include research and standards on furnace sizing and furnace size or capacity measurements.

Page top photo: a heating furnace installed in the closet of a single-wide mobile home.

This article series describes how to diagnose & fix just about any problem with forced air heating & cooling systems.

Oversized Heating Furnace Problems & Sizing Standards

Most research on furnace sizing comment that most heating furnaces are oversized. ( Brand 2012, Siegel 1998,Phillips 1998, Regester 1981, Newmann 1977, Beach 1961 et als).

This article explains why it is a mistake to install a heating furnace that is too big for the home or building it is serving.

If the furnace is too big it wastes money, is uncomfortable for the building occupants, and ultimately it's unsafe.

A heating furnace that is too small (not detailed here) is a different problem: "too small" means "cant' keep up" - in the coldest weather occupants will not be warm enough.

Question: if our used gas furnace came from a home 3 times bigger, will it work in our home?

2019/11/02 Anonymous asked:

We put a natural gas furnace from a triple wide trailer in a single wide noble home. Will this work.

We had a heating and air guy check everything and he was the one that said about the size to big but would work. So I was checking.

The map shown here gives approximate number of BTUs needed per square foot of living space for five heating climate zones in the U.S.

[Click to enlarge any image]

Reply: Why is an oversized heating furnace particularly bad in a smaller mobile home or other small space?

The heater may "work" in that it will provide heat but if it was properly sized for a much larger home it is almost certainly over-sized for yours. That means that it will not operate efficiently and it may be safe.

Watch out: if the installation was not by a qualified, trained heating service technician it could be unsafe; risks include carbon monoxide poisoning (fatal) or fire or other injury.

How Big Should a Home Furnace Be in BTUs?

First, just how much too big is your furnace? I mean, by how much is your furnace oversized.

If it's just slightly too big you may not care.

If it's much too big there can be annoying or even dangerous problems with the heater.

Take a look at the input BTUh rating of your furnace (tell us the number).

Then take a look at the furnace sizing table given below, choosing the data for the heating zone where you live.

OPINION: if your furnace is between 90% and 110% of the recommended size you may decide you can live with that error and you may be able to compensate by making other adjustments such as to the home's air sealing (if the heater is too small) or to the fan speed if the heater is too large.

Watch out: If your furnace is really 3 times too big, as suggested by your question, that's huge and has some serious disadvantages that we will explain below.

How to Select the Proper Furnace Size (in BTUs/hr) for a Single-Wide Mobile Home

Using the U.S. DOE map data above, typically for a singlewide mobile home that's 600-1300 square feet in floor space we can make a rough calculation of the proper furnace size.

Notes to the table above

The BTU rating of the heating furnace that you can easily find is the Input BTUh from the heater's data tag. The actual output BTUh of the heater into the occupied space will be less and will vary depending on many factors such as system cleaning and adjustment and also due to heat losses in the duct or heat distribution system.

1. Calculation: 600 sq.ft. x the lower number in the BTU-range for this heating climate zone, e.g. 30 BTUh x 600 sq.ft. = 18,000 BTUh

2. Calculation: 1300 sq.ft. x the higher number in the BTU-range for this heating climate zone,e.g. 35 BTUh x 1300 sq.ft. = 45,500 BTUh

These same calculations can be used for larger homes.

Furnace Size in BTUh = (Total home sq.ft. of heated area) x (BTUs/Sq.Ft. for your Heating Zone)

Example: Furnace for a 1800 sq.ft. home in heating zone 3:

40 BTUs/sq.ft. x 1800 = 72,000 BTUh

or

45 BTUs/sq.ft. x 1800 = 81,000 BTUH

Your furnace will probably need to be in the range of 72,000 BTUH to 81,000 BTUh.

Watch out: discuss your furnace sizing with your heating installer. The actual heating capacity you need for your specific home will vary considerably from these very general guidelines depending on your home's heat loss rate. The rate at which the home loses heat to the exterior depends on a number of variables such as

• The difference between indoor temperature (set on the thermostat) and the outdoor temperature. More difference = faster heat-loss
• The home's air leakage rat
• The home's insulation

Current Climate Zone Map for the U.S.

Below is a more-recent climate zone map fdor the U.S. provided by the U.S. Office of Energy & Renewable Energy

You can see that in this newer map the DOE has extended the cold zone south and has defined several "very cold" climate zones.

Four Problems Caused by an Oversized Heating Furnace

Oversized Furnace Problem #1 - furnace short cycling may be unsafe

When you install a forced warm air heating furnace that is over-sized for the space being heated, it is likely that the "heat-on" cycles will be quite short - the air volume and speed and possibly even temperature will be so great that especially in a small space such as a singlewide mobile home, the furnace will shut down quickly after the thermostat has called for heat.

Occupants may not complain - after all, the space is warm-enough.

But at least some heating service companies and manufacturers will point out that the short-on heater cycle can create such thermal stresses on the furnace heat exchanger that it will fail by cracking, leading to both an unsafe condition (risking flue gas and dangerous carbon monoxide leaks into the occupied space) and a shorter life for the heater.

Watch out: your example of a furnace that is 300% oversized is very significant, meaning that you should take seriously the warnings we give here.

Oversized Furnace Problem #2 - furnace is inefficient in burning fuel:

When any fossil fuel heater such as furnace (air heat) or boiler (hot water heat) first turns on, because the combustion chamber is cool combustion of the fuel is incomplete, thus inefficient.

More of the gas or oil the occupants have paid-for is going not into the form of heat in the home but rather up the chimney as exhaust.

A longer furnace "on" cycle means that the heater spends more of its time running in a more-efficient mode as the combustion chamber is hot and fuel is better consumed. A shorter "on-cycle" means that the furnace spends more of its time in that inefficient mode.

Bottom line: heating fuel costs will be higher if the furnace is over-sized.

The fuel efficiency rating of a specific furnace or "AFUE" is derived from that heater after it has run long enough to reach full normal operating temperature. So running your furnace more in a "cooler" state means your furnace will never reach the AFUE rating it was given. You're not getting the efficiency (thus heating cost savings) that the furnace maker says you should.

Oversized Furnace Problem #3 - increased wasted heat lost through the duct system

The U.S. DOE notes that the short hot "on" cycles caused by over-sizing a heating furnace means that the ductwork gets heated up quickly but spends a greater portion of its time with the blower fan "off" (as the occupied space got warm and the thermostat turned off the blower).

That may mean that a greater portion of heat produced by the furnace is actually going to be transferred through the ductwork and its heat losses out of the building rather than into the heated space. (Brand 2012)

Oversized Furnace Problem #4 - comfort issues

An oversized furnace may mean less comfortable heated space because of frequent temperature swings and also wide temperature swings away from the desired room temperature set on the thermostat (York 2002).

Depending on the room thermostat's location, an over-sized furnace also means that heat throughout the home may be uneven, leaving some areas too warm while other areas are too cold, because the short "on" cycle of the furnace means that heat has not traveled to the cooler areas before the thermostat is satisfied.

While the rate at which a heated building cools off varies considerably by details such as the building's air leakiness, shape, insulation, and of course local climate, you may find that those short "on-and-off" heating cycles are annoying or even uncomfortable for building occupants.

What to Do About an Over-Sized Heating Furnace

Bottom line: your oversized furnace was free and you want to live with it, at the very least it is critically important to

1. Install safety alarms: Be sure your home has working smoke detectors and carbon monoxide detectors properly located and installed and tested monthly. (Carbon monoxide poisoning can kill building occupants)
2. Inspect the furnace: Have the heater serviced and inspected for safety every year.
   
   See HEAT EXCHANGER LEAK TEST
   
   Also if your furnace is a high-efficiency condensing unit there are additional safety inspection points suggested (Brand 2012).
3. Adjust the night time thermostat set-back as low as you can tolerate - this will reduce overall heating costs by running the heater less-often. (Koening 1978)
4. Adjust the fan speed? Discuss with your heating service technician whether it's possible to run the furnace fan at a slower speed (without overheating the furnace itself) as that may reduce the duct-heat-loss and short-cycling inefficiencies of an oversized system.
   
   At FURNACE FAN CYCLES DURING HEAT we discuss other problems that occur if the fan speed is set too high for an individual heating furnace system
5. Reduce burner capacity? a few service techs have suggested capping off one or more burners in an oversized gas furnace - possibly an improper or even dangerous suggestion.
   
   Watch out: Do not try capping off or blocking some of the burners of your furnace without approval from the unit's manufacture because doing so can create uneven temperatures inside the unit that damage it, making it unsafe.

 

Furnace Sizing Research, Standards, Data

• ASHRAE / NFPA 90, Standard for Installation of Warm Air Heating and Air Systems
• Beach, William A., Robert L. Burke, and David R. Hogin. "Domestic heating devices." U.S. Patent 3,003,546, issued October 10, 1961.
  
  Excerpt: In most cases, oil-fired heating units or devices are oversized,
• Brand L., & W. Rose, Measure Guideline: HIGH-EFFICIENCY-NATURAL-GAS-FURNACES [PDF] (2012) U.S. Department of Energy, retrieved 2019/11/03, original source: https://www.nrel.gov/docs/fy13osti/55493.pdf
  
  This study was specifically focused on optimum furnace size when replacing older heaters with a new high efficiency (natural gas) furnace. Actually high-efficiency furnaces are LESS sensitive to over-sizing than conventional heaters - according to this study, but the recommendation to avoid over sizing remains. In fact the study tested furnaces at 70%, 100% and 120% of proper size.
  
  Excerpts: PARR conducted laboratory-based research on the AFUE of high efficiency furnaces that were significantly oversized or with high external static pressure (undersized ducts, high pressure drop coils, dirty or high efficiency filters) to determine if those factors would influence installed performance. The research results are summarized below and in Appendix D.
  
  Implications for furnace replacements are that furnaces should be sized as small as possible to match the duct size or the ducts should be replaced if a higher furnace capacity is required. Some practitioners recommend replacing the ducts if the furnace capacity is significantly reduced to increase air velocity and improve comfort in the space (Chitwood and Harriman, 2010).
• Franco, Victor. "Furnace blower electricity: National and regional savings potential." (2008).
• Ingersoll, John, and Joe Huang. "Heating energy use management in residential buildings by temperature control." Energy and Buildings 8, no. 1 (1985): 27-35.
  
  The authors point out that in some climate conditions, A furnace sized to meet the larger peak demands will be oversized for other periods, which will result in periodic on and off switching ...
• Koenig, Kenneth. "Gas furnace size requirements for residential heating using thermostat night setback." ASHRAE J.;(United States) 84, no. 2 (1978).
  
  Abstract:
  The effects of night setback of thermostats and of furnace sizing on both energy savings and temperature recovery times were studied using a digital computer program to simulate in detail a system consisting of a residential structure, its environment, gas-fired hot air heating equipment, and thermostat. The results show that the optimum furnace size and level of setback depends on the relative importance placed on energy savings and short recovery.
  
  It is recommended that the use of night setback be encouraged as a means to reduce energy consumption.
  
  In applications where the furnace is fixed and greatly oversized, setback should be as high as possible until the morning recovery becomes objectionable. Alternatively, setback should be no more than 5/sup 0/C in cases where the homeowner may take undesirable action to eliminate prolonged recovery on very cold days.
• Kratz, Alonzo P., Seichi Konzo, and John A. Henry. Rating equations for hand-fired warm-air furnaces. [PDF] Engineering Experiment Station. University of Illinois at Urbana-Champaign, 1945.
• Kratz, Alonzo Plumsted, Warren Skinner Harris, and Maurice Kendall Fahnestock. Performance of a hot-water heating system in the I= B= R research home at the University of Illinois: a report. [PDF] University of Illinois at Urbana Champaign, College of Engineering. Engineering Experiment Station., 1944.
• Newman, Jerry Okey, Joseph Winslow Simons, and Benarthur Castle Haynes. Effect of Furnace Output and Operation on Temperature Uniformity in a Prototype Research House. No. 1560. Department of Agriculture, Agricultural Research Service, 1977.
• Phillips, Bert G. "Impact of blower performance on residential forced-air heating system performance." ASHRAE Transactions 104 (1998): 1817.
• Pigg, Scott. THE ELECTRIC SIDE of GAS FURNACES [PDF] Home Energy Magazine 1187 Thorn Run Road Extension Suite 340 Pittsburgh, PA 15108 Email: contact@homeenergy.org Home Energy 20, no. 6 (2003): 24-29. Retrieved 2019/11/04 original source: https://homeenergy.org/graphics/HomeEnergy_20-6_feature.pdf
  
  Excerpt: In fact, most furnaces are considerably oversized for their heating load, a fact that is incorporated in the GAMA ratings.
• Regester, R. H., and R. H. Reinauer. Sizing of residential heating equipment. Philadelphia Electric Co., PA, 1981.
  
  Abstract: According to this evaluation, the ASHRAE guide's method of sizing residential heating equipment is sound but somewhat conservative, particularly if air infiltration is overestimated. Furthermore, designers and contractors take these conservative data and round them upward, leading to oversized appliances that are less efficient on a seasonal basis and more expensive.
  
  Gross oversizing frequently occurs when customers convert from oil- to gas-fired boilers and furnaces. Many new gas-boiler customers have 140,000-Btu units installed when a 60,000-Btu model would be adequate.
  
  The difference in equipment cost alone is $600, and installation costs are higher as well due to the larger chimney openings and bigger piping involved. PECO and PSE and G have studied these practices nationwide; they propose simplified methods of calculating gross heat loss that will encourage proper equipment sizing.
• Siegel, Jeffrey, Bob Davis, P. Francisco, and L. Palmiter. MEASURED HEATING SYSTEM EFFICIENCY RETROFITS IN EIGHT MANUFACTURED (HUD-CODE) HOMES [PDF] In Proc. ACEEE Summer Study 1998, pp. 2-189. 1998.
  
  Excerpt: Furnaces are generally oversized by at least a factor of two in manufactured homes; even homes with relatively high heat loss often have oversized furnaces ...
• YORK G8C / FG8 UPFLOW / HORIZONTAL AUTOMATIC IGNITION FURNACE INSTALLATION MANUAL [PDF] (2002) Unitary Products Group, 5005 York Drive, Norman OK 73069 USA retrieved 2019/11/03
  Excerpts:
  
  Consideration should be given to the heating capacity required and also to the air quantity (CFM) required if A/C is to be installed along with the furnace or at some future time.
  
  These factors can be determined by calculating the heat loss and heat gain of the home or structure.
  
  If these calculations are not performed and the furnace is oversized, the following may result:
  
  1. Short cycling of the furnace.
  
  2. Wide temperature fluctuations from the thermostat setting.
  
  3. Reduced overall operating efficiency of the furnace.
• Tietsma, G. J., and Bradley A. Peavy. The thermal performance of a two-bedroom mobile home. Vol. 102. US Department of Commerce, National Bureau of Standards, 1974.
  Abstract:
  
  Tests were conducted on a mobile home located in an Environmental Climatic Laboratory for the purpose of evaluating its thermal performance. The Heating demand greatly affected the part-load efficiency of a gas-fired, forced-air, sealed-combustion furnace system. The practice of installing oversized heating platns was shown to result in low seasonal operating efficiencies.
  
  Air leakage measurements were performed using a pressurization technique to quantify the amount of air leakage through the various parts of the mobile home. Separate air infiltration tests using the SF6 tracer-gas technique showed that somewhat higher air infiltration rates were induced by operation of the mobile home heating plant.
  
  [Watch out: in our OPINION this might point to inadequate return air or inadequate combustion air depending on how and where the heater is installed.
  
  This can be an unsafe condition and risks CO poisoning. - Inspectapedia editor] A thermotraphic survey of interior surfaces showed that the technique used to install the wall insulation may allow wrinkles formed in the surface of the insulation to form air paths running the height of the wall cavity. Convective air flow through these paths may create heat leaks on the building surface which can have an impact on the overall heat-loss rate.
  
  Separate tests were also conducted to identify places in the mobile home envelope having high condensation potential.
  
• Yousa, M. J., and J. N. Hicklin. "The performance of gas furnaces in five well-insulated houses: a field study of gas furnace performance in relation to furnace sizing and space heating load. A research report." (1982).
  

...

Continue reading at MOBILE HOME HEATING SYSTEMS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see FURNACE FAN CYCLES DURING HEAT FAQs - questions and answers posted originally on this page.

Or see these

Recommended Articles

• AIR FLOW IMPROVEMENT, HVAC
• AIR HANDLER / BLOWER UNITS - home
• BTU CHART for AIR CONDITIONERS / HEAT PUMPS
• BTU CHART for HEATING FURNACES
• FAN, AIR HANDLER BLOWER UNIT
• FURNACE FAN CYCLES AFTER HEAT
• FURNACE FAN CYCLES DURING HEAT
• HEAT EXCHANGER LEAK TEST
• HEAT EXCHANGER LIFE & WARRANTY
• FURNACE OPERATING TEMPERATURES
• MOBILE HOME HEATING SYSTEMS
• UNDERSIZED RETURN DUCTS - not enough return air

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