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Cost Comparison: Gas Fireplaces vs. Furnaces

Q: I have a new gas fireplace rated at 20,000 - 45,000 btu/hr, and 80% efficiency. I want to compare the amount of gas used by the fireplace with the gas used by our forced air furnace. What is the gas per hour used by this fireplace? I am unsure of the type of measurement used in gas use. How much, if any money, we would save by using the fireplace to heat this winter?
-Mrs Anderson

Sweepy All gas appliances are given a BTU consumption rating by the manufacturer. This rating tells you how much gas that particular appliance burns per hour. If the flame height is adjustable, the rating will show two numbers: the amount of fuel consumption at the lowest setting, and the amount of fuel consumption at the highest setting. The rating for your fireplace tells us it consumes 45,000 btu/hr at its highest setting.

Now, for comparison sake, let's say your furnace is also rated at 45,000 btu/hr. Since there are 100,000 BTU's of heat value in one therm of natural gas, both appliances will consume one therm of natural gas every 2.2 hours when the burners are lit and the fireplace is adjusted to its highest flame setting.

It would seem at this point that these two heaters would consume exactly the same amount of gas to heat the same area, but this isn't necessarily the case. We must also consider how many hours the burners in each of the two heaters will need to be lit each day to keep the living space at the desired temperature. One factor that determines this is extraction efficiency, which is a measurement of how effectively a given burner system extracts heat from the gas. The other factor is delivery efficiency, which is a measurement of how effectively the extracted heat is delivered into the living space.

Furnace manufacturers list the extraction efficiency only. They really can't list the delivery efficiency, as no two delivery systems (the ductwork or radiators that deliver the heat to the house) are alike. The extraction efficiency rating is simply a measure of how well a particular furnace's burner and plenum design turns burned gas into heat: for example, a 15-year-old gas furnace might have an extraction efficiency rating of, say, 75%, while a new Pulse furnace might have an extraction rating of 94% or more. Delivered efficiency is the unknown quantity with regards to furnace systems. Given all the variable factors in a furnace installation, such as heat loss per foot of air  ducting, it is very possible for a furnace with a 94% extraction rating to have an actual delivered efficiency of only 60% or so, depending upon the heat loss ratio of the ductwork or radiator system used to get the heat from the furnace into the living space.

Since the heat from a gas stove or fireplace is delivered directly into the living space without the need for air ducting or water piping, the testing laboratories are able to obtain a delivered efficiency rating. When you see an efficiency rating on a gas stove or fireplace, you're not just being told how well the burners extract heat from gas: you're being told how much of the heat produced is being delivered into your house.

You mention that your new fireplace has an adjustable BTU consumption rating of 20,000 - 45,000 BTU/hr, and a delivered efficiency rating of 80%. This means that, when the fireplace is adjusted to its highest flame setting, it is burning 45,000 BTU of gas and delivering 36,000 BTU's of heat into your living space every hour. A 45,000 BTU forced-air furnace with a burner efficiency of 94% might only be delivering 22,000 - 27,000 BTU/hr, depending upon how much heat is lost in the delivery system. To illustrate the difference, here's something to try: next time your furnace is running, hold your hand directly over one of your heating registers and feel how cool the incoming air is after it has traveled through the duct system. Then, light up your fireplace and hold your hand the same distance from the viewing window. You'll get an instant demonstration of the difference between ducted heat delivery and direct delivery.

As you can see, whether your furnace will be cheaper or more expensive to operate will depend upon its BTU consumption rating, extraction efficiency rating, and the heat loss ratio of its delivery system. Since the most important of these factors, the heat loss ratio of the delivery system, is unique to your installation and impossible for you to measure accurately, the only way you're ever really going to be sure which heater is more economical will be to pick two equally cold months and burn the fireplace exclusively for one month and the furnace for the other, then compare gas bills.

If we had to bet on which of the two systems will be less expensive to operate, we'd gamble on the gas fireplace. Today's gas stoves and fireplaces have delivered efficiency ratings hovering around 80%, which would rival even a 98% efficient gas furnace with the best imaginable forced air delivery system. Also, much of the heat produced by a stove or fireplace is radiant heat, which is absorbed directly by the body and warms it much faster than the convected (heated air) heat produced by a forced air furnace. Another advantage is the ability to turn the stove or fireplace's burners down, and "sip" gas to keep the temperature constant. This can be more economical than letting the living space cool until the thermostat lights up the furnace burners at full throttle to bring the house back up to temperature, in much the same way as freeway driving gives you better gas mileage than stop-and-go city driving. A final advantage to today's gas stoves, fireplaces and inserts is the flame display. Our customers frequently report that the cheery flames just make them feel warmer, even when the burner is turned down to its lowest (and most economical) setting.

Sweepy Stay with us, gentle reader, the matter has been put to the test:

January 21, 2004

Man, you guys know your stuff. I bought my new gas insert because I was impressed with all the info on your site, but must confess that I didn't really 100% buy the article about comparing a gas fireplace to a gas furnace (that the fireplace would be cheaper to run). As it turns out, we finished the install the evening of November 30, so I figured what the heck, let's give it a try. The next morning, Dec. 1, I turned off the gas furnace and fired up the insert, and we used it exclusively to heat our home for the entire month (didn't turn the furnace on at all). So we get our bill, and guess what? Even though December was actually a little colder than November, WE BURNED 35% LESS GAS! And we were warm as toast the whole time. Great info, and great website. Keep up the good work.

PS: You can use this on your website if you want, and feel free to include my E-mail address. I scanned the gas bills into the computer and will be happy to E-mail them to anybody who wants to see them.

Roger Wilhelm


March 4, 2005

Your comments about gas furnaces costing more to run than other gas heaters wouldn't have rung true with me, until I traded in our old gas insert for a new one. I hadn't realized this, but evidently there's 2 types of gas inserts: some are "decorative" rated and some are "heater" rated. The lady at the local fireplace shoppe informed us that our old insert was the decorative kind, and sold us a heater-type.

What a difference! Whereas before we had to keep the furnace on even when we lit up the insert, now the new insert heats our whole house! And, just like you said, our gas bill has gone down tremendously. Now that I know the difference, I can't imagine why the previous owner installed a decorative insert in the first place, or why those things even exist!

I really liked your web site, but have a suggestion: you should tell your site visitors the difference between decorative-rated and heater-rated gas burners, and specify which of the models you sell are which.

Marty G in Salt Lake City

Sweepy Hi Marty,

Thanks for the letter! Decorative gas stoves, fireplaces and inserts are popular in warm-climate areas like Florida, Southern California, Hawaii, the Gobi Desert, etc., where the ambiance of a cheery fire might be desirable, but the heat isn't.  They're also used quite often by builders of "spec" houses, as they're less expensive than heater rated fireplaces.

All of our gas stoves, fireplaces and inserts are heater rated.


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