How to Get Furnace Sizing Right

Canada has developed a furnace sizing protocol that uses actual gas meter readings to size replacement furnaces.

September 02, 2007
September/October 2007
A version of this article appears in the September/October 2007 issue of Home Energy Magazine.
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Residential furnaces work best when they are properly sized. A furnace that is too large will cycle on and off quickly, with some inefficiencies of operation. This can cause the rooms farthest from the furnace to remain cool and can cause furnace chimneys to deteriorate, due to excessive condensation. It can also cause some furnace controls or parts to break down sooner than expected. Furnaces that are undersized may not keep the house at a comfortable temperature in the coldest parts of the winter. It will take much longer for an undersized furnace to bring a house back up to temperature after the thermostat has been turned down for a period (such as night setback or vacation).


State-of-the-Art Sizing

There is a Canadian Standards Association (CSA) standard on how to size furnaces properly. CAN/CSA F280-M90 (R1998), “Determining the Required Capacity of Residential Space Heating and Cooling Appliances,” provides a procedure to calculate house heat losses for the “design temperature” at a given location. The standard lists the design temperature for most Canadian communities. The design temperature is equivalent to the coldest temperature a community is expected to experience in any given winter. (Canadian design temperatures are at the 2.5% level, and the difference between this number and the coldest day of the year is marginal.) The standard specifies that the heating system shall have an output no more than 40% larger than the steady-state heat required for the house at that design temperature.

The 40% allowance provides some margin for error in the calculation and also permits some oversizing to bring a house back up to temperature. Basically, this sizing calculation will recommend a furnace that runs at least 35 to 40 minutes every hour on the coldest day of the year.

There are several ways that contractors can estimate house heat loss. The most accurate way is to time the furnace run time on the coldest day of the year and, using furnace size and efficiency, calculate house heat loss. However, few contractors have the foresight or interest to do this. A second way is to perform a calculation based on CAN/CSA F280. This is possible, but it is expensive (a good estimate will take several hours), and the contractor must have the appropriate skills. In practice, then, most heating contractors do not properly estimate house heating loads.

For replacement furnaces, contractors simply specify a furnace about the size of the existing furnace output, or perhaps marginally smaller, if energy-saving measures have been undertaken. Homeowners rarely recognize that a furnace is oversized, so they do not complain. They will complain, however, if a furnace is undersized. As a result, most gas or oil furnaces in Canada are grossly oversized—up to 250% bigger than they need to be.

Finding a Better Way

In 2003, the Canada Mortgage and Housing Corporation (CMHC) hired the Saskatchewan Research Council (SRC) to develop a furnace sizing protocol that uses the actual meter readings from gas billing to size replacement gas furnaces. (Oil furnaces present a problem, because they do not have meters. It may be possible to adapt this protocol to oil billing in the future, but for the present the protocol applies to gas furnaces only.) The second part of the project was to quantify the energy savings attributable to proper furnace sizing when replacing heating systems.

SRC located 26 houses with cooperative owners and good utility records. The goal was to find a relationship between the gas consumed and the heating degree-days (HDD). A heating degree-day is the number of degrees of heating required over the course of 24 hours as compared to a base temperature of 18°C (approximately 65ºF). For example, if the average daily outside temperature is 10°C, the number of heating degree-days for that day is 18°C - 10°C = 8 HDD. The HDD at each location throughout Canada are available from Environment Canada Weather Office (www.weatheroffice.gc.ca/canada_e.html).

In the SRC study, once the relationship between HDD and gas consumption was established, gas consumption was calculated for a Saskatoon design temperature of -35°C (which works out to 53 HDD on that cold day). The natural gas usage of other gas-fired appliances in the houses was estimated from SaskEnergy data, and this figure was subtracted from the total for the period in question to isolate the gas requirement for heating.

Three sets of data were analyzed.

January calculation procedure. Ten householders had specific gas meter records for the start and end of January.

Historical procedure. All 26 householders had long-term utility billing data from which various consumption periods could be extracted.

Utility procedure. All 26 households were also tested using a single season, midwinter utility bill. The local gas utility records meter readings on a three-month schedule. The difference between the March reading and the December reading was analyzed on all 26 houses.

For calculations of savings attributable to proper sizing, six houses were chosen at random and modeled using the HOT2000 building energy simulation program software (Version 8.72).

Here is an example calculation, using the three-month meter (utility) reading for an example house where a conventional (72% efficient) furnace is being replaced by a midefficiency (80% efficient) furnace:

  1. Total gas consumption from actual meter readings December to March is 46,600 ft3 (1,320 cubic meters (m3)).

  2. Estimated consumption for other appliances (from SaskEnergy) is 10,800 ft3 (306 m3).

  3. Therefore, gas consumption during the period for heating is 46,600 – 10,800 = 35,800 ft3 (1,014 m3).

  4. Heating degree-days for that period (from Environment Canada data) is 2,840 HDD.

  5. Heating consumption versus degree days is 35,800/2,840 HDD = 12.6 ft3/HDD (0.3570 m3/HDD).

  6. Heating consumption at 53 HDD/day is (53 HDD/day) x (12.6 ft3 /HDD) = 668 ft3/day (18.9 m3/day).

  7. Where gas has an energy content of 1.06 megajoules per cubic foot (MJ/ft3) (37.5 megajoules per cubic meter), and the furnace has a steady-state efficiency of 72%, then the heat loss at 53 HDD/day is (668 ft3/day) x (1.06 MJ/ft3) x (0.72) = 510 MJ/day or 21.3 MJ/h.

  8. Since 3.6 MJ/h  is 1 kW, then 21.3 MJ/h is 5.9 kW.

  9. This heat loss would require a furnace that produces an output of 5.9 kW, or 20,000 Btu per hour.

  10. If we allow the CAN/CSA F280 allowable oversizing of 40%, then the proper furnace output sizing would be (1.4)(5.9 kW) = 8.3 kW (or 28,000 Btu/h).

The new furnace being installed in this house has a rated input of 35.2 kW (120,000 Btu/h). At the rated efficiency of 80%, this results in a heat output to the house of 28.2 kW (96,000 Btu/h). Therefore, the furnace size provides 342% of the heat required, or 242% more than is required.


Findings

The three furnace sizing methods provided consistent results. The historical procedure provided the most extensive, and probably the most accurate, results. The calculated heat losses using the more variable January and utility records were within ±10% of the historical records, with only a couple of explainable exceptions. Replacement furnace sizes (for 80% efficient furnaces) were calculated by each of the three procedures and were compared to the existing furnaces in the houses (most of which were conventional efficiency furnaces). Results showed that the majority of the existing furnaces were grossly oversized (see Figure 1).

The other part of the study—calculating the energy savings attributable to proper replacement furnace sizing—proved less successful. (In Canada, furnaces and duct systems are usually installed within conditioned space, so duct leakage and heat loss from furnaces in garages does not factor into heating efficiency.) There are data showing that proper sizing of conventional (natural-draft) furnaces reduces gas consumption. However, in Canada, energy efficiency regulations forbid the sale of new conventional furnaces. The only gas furnaces now sold in Canada are either midefficiency (80% steady-state efficient) or high-efficiency (90%–95% efficient) models. There are no good data showing whether properly sizing these mid- and high-efficiency furnaces saves energy. The limited current data do suggest that there is a marginal increase in efficiency with short cycling of high-efficiency furnaces. The HOT2000 simulations showed no appreciable savings.

A properly sized furnace will run longer and cycle less frequently than an oversized one, which may lead to longer component life. Bigger furnaces tend to have bigger circulation fans, which use more electricity, so a properly sized furnace may save energy by reducing electrical usage. However, we cannot know the real advantages of proper sizing until we can test the cyclic efficiencies of mid- and high-efficiency furnaces.  

Right-Sizing Benefits

The three procedures for calculating house heat loss were all remarkably successful. This means that most households or contractors now have alternative ways to calculate house heat loss in existing houses and to properly size heating equipment. However, we currently lack proof that proper sizing of higher-efficiency furnaces leads to energy savings. The lower cost of a smaller furnace will save the homeowner some money. And there are other benefits to using a right-sized furnace, such as lower noise levels, and lower air velocities. All in all, getting furnace sizing right makes good sense.

Don Fugler is a senior researcher at Canada Mortgage and Housing Corporation in Ottawa, Ontario.


For more information:

CMHC publishes research results such as that described above as Research Highlights. To find more Research Highlights and other information about Canada’s housing research, visit the CMHC Web site at www.cmhc.ca, or contact:

Canada Mortgage and Housing
Corporation
700 Montreal Rd.
Ottawa, ON
Canada K1A 0P7
Tel: (613)748-2367
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