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Should I Care About an Oversized Furnace?

A Right-sized High Efficiency Furnace Installed.

The bottom line is that, yes, you SHOULD care about an oversized furnace. Furnace oversizing has been a common practice among heating contractors for as long as heating systems have been installed in homes, and is still rampant today. However, in the last 30 years, research has shown overwhelmingly that oversized furnaces contribute to a number of common problems that affect our homes, our pocketbooks, and the environment.

In order to understand the implications of improper furnace sizing, one should first understand the basic operation of a furnace. Furnaces use a gas burner to create heat from an energy source, and this heat is then distributed to the inside of the home to replace heat that is lost when it is colder outside than inside. Heating energy is commonly measured in British thermal units (Btu). A 1,000 square foot Bay Area home built in the 1940s would likely lose around 60,000 Btu per hour on a cold winter night, and as heat was lost, the home’s thermostat would trigger the gas furnace to replace this lost heat. However, a typical existing furnace in that home would be capable of producing 125,000 Btu –more than twice as much heat as the home would ever actually need.

Over 15 minutes, the house would lose 15,000 Btu on this cold winter night, reducing the indoor temperature by 2-3 degrees and triggering the 125,000 Btu furnace to cycle on and replace the lost heat. It would take the furnace about seven minutes to replace the lost heat, and then the furnace would cycle back off. 15 more minutes, 15,000 more Btu, and another seven minute cycle. Rinse and repeat. Over that hour, the furnace would run four seven-minute cycles to maintain the desired indoor temperature, switching on and off, on and off, on and off.

In contrast, a 60,000 Btu furnace – sized to match the home’s heating load – would run more or less continuously to maintain temperature. One continuous cycle. So, what are the implications of running six short cycles versus one long cycle to produce the same amount of heat?

Excessive Energy Consumption If you’ve ever ridden a bicycle, you know that it’s way harder to get up to speed every block after a stop sign than it is to maintain the same speed as you zip through a string of green lights. In the same fashion, a furnace consumes extra energy every time it starts a new cycle. It takes the furnace about two minutes to produce enough heat to trigger the furnace fan to turn on and deliver heat to the home. Then, once the thermostat is satisfied and signals the furnace to turn off the burner, the furnace fan runs for about another minute to remove excess heat from the heat exchanger. So, this seven minute cycle actually takes 10 minutes: nine minutes of burning gas, and eight minutes of a fan running – about 30% of the furnace’s run time is dedicated to startup and cool down, rather than actually heating the home! In addition, combustion efficiency is reduced during startup. Just as most cars get better gas mileage on the highway than around town, your furnace runs more efficiently when it can cruise at steady state instead of stop and go.
Higher Heating System Installation Costs Hank Rutkowski, the mechanical engineer responsible for developing the Manual J heating equipment sizing methodology, estimates that homeowners would save $500-$1,500 by installing properly sized heating equipment instead of oversized equipment. From additional burners, to bigger distribution fans, to additional sheet metal in the heat exchanger and exterior cabinet, to added freight costs for bulkier equipment, furnace oversizing adds up to substantially higher equipment cost.
Poorer Reliability and Shorter Equipment Life Expectancy Not only is excessive cycling bad for efficiency, it is also harder on the furnace components. The rapid, repetitive expansion and contraction of components as the furnace heats up and cools down over and over again puts stress on the heat exchanger and other components. (By the way, a cracked furnace heat exchanger leaks carbon monoxide into the home!) The furnace flue (exhaust system) does not work properly until the furnace warms up the flue pipe, so repeated cold starts result in poor exhaust of combustion gases, leading to the accumulation of chemical-laden moisture in the flue system and internal components, which causes corrosion both internally and externally. Ironically, most duct systems are UNDERsized relative to the furnaces they are paired with, which hinders the system’s ability to move heat from the furnace heat exchanger into the home. When temperatures inside the heat exchanger get too high, a limit switch is triggered, causing the furnace to shut off. Usually, the furnace will reset and start up again, but on some models, tripping a limit switch can knock the whole system offline and result in a service call. Finally, excessive cycling causes many parts, such as the igniter and gas control valve, to operate much more frequently than they were designed to, resulting in excessive strain and premature failure.
Noisier Operation The amount of air a furnace moves corresponds directly to its size – the larger the furnace, the more air it is designed to move. As we discussed above, duct systems are commonly undersized relative to furnaces, which leads to higher velocity, more turbulence, and ultimately more noise at the duct registers where supply air enters the home and return air is pulled from the home.
Reduced Comfort Have you ever noticed that, especially when it’s not that cold outside, if you set your thermostat to 68°F, it often feels much warmer when the heat is on, as well as right after it shuts off? Most furnaces are not capable of modulating the amount of heat they put out, so when the system turns on, you are getting full blast whether you want it or not. This can be an issue even with properly sized furnaces – which is why a market has emerged for dual stage and modulating furnaces, which adapt their heat output to the heat load – but the problem is especially pronounced when equipment is oversized.

In response to this knowledge, a small but growing number of heating contractors and home performance contractors have begun to implement furnace sizing guidelines to ensure that properly sized equipment is installed on their projects. This methodology, published by the Air Conditioning Contractors of America (ACCA), is called Manual J. Manual J heat load calculation takes a number of factors into consideration, including climate, orientation, home size, insulation values, and airtightness to determine how much heat a home will need to stay warm in the coldest weather, and consequently how large the heating equipment needs to be.

It is up to your building analyst, or other heating/energy professional, to determine whether your furnace is oversized, and whether that oversizing is enough of a problem to warrant replacing the entire furnace. Oversizing is a matter of scale, and should be taken into account as one of many factors that will influence your decision of whether to replace your furnace. If your furnace is two years old, high efficiency, and 20% oversized, it is probably not worth the investment to replace. However, if your furnace is eight years old and 100% oversized (twice as powerful as it needs to be), you would have a strong case for replacement, even if it is a high efficiency model.

Finally, even if your furnace is properly sized for your home’s current heating load, implementing energy efficiency measures such as air sealing, insulation, window replacement, and duct improvements will could reduce your heating load by half or more. If you are considering a furnace replacement, and have performed or plan to perform energy efficiency work on your home, be sure to discuss proper furnace sizing with your contractor to ensure your new heating system is as wise an investment as it can be. If your contractor is not aware of ACCA Manual J sizing, or balks at the idea of running a load calculation to determine which furnace to install, look for a different contractor!

Sources:

Green Building Advisor http://www.greenbuildingadvisor.com/blogs/dept/musings/saving-energy-manual-j-and-manual-d

Green Building Talk http://www.greenbuildingtalk.com/buildcentral/sip/article_hvac_sizing.aspx

Wikipedia http://en.wikipedia.org/wiki/British_thermal_unit Btu definition http://en.wikipedia.org/wiki/Building_performance home performance contractor def

Carsondunlop.com http://www.carsondunlop.com/2011/12/heating-efficiency/ steady state def

Howstuffworks.com – http://home.howstuffworks.com/home-improvement/heating-and-cooling/how-to-maintain-a-furnace4.htm Limit switch def

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