Furnance BTU Question

[gmhill33]

Greetings,

I have a old 1983 carrier furnace that is 150,000 btu. The thing is probably 80% efficient at best. What btu furnace would be equivalent to that with todays high efficiency furnaces?


Thanks,
Gary

 

[aandpdan]

To size a furnace or boiler you need to do a heat loss calculation. Don't rely on your current furnace as it is likely GROSSLY oversized.

Cool dog!

 

[mygarageone]

To size a furnace or boiler you need to do a heat loss calculation. Don't rely on your current furnace as it is likely GROSSLY oversized.

Cool dog!

Exactly !

 

[gmhill33]

To size a furnace or boiler you need to do a heat loss calculation. Don't rely on your current furnace as it is likely GROSSLY oversized.

Cool dog!

Exactly !

Thanks for the reply. We have a old house that has no installation and old wood windows. The wife says when the furnace was working it didn't do that great of a job. I thought that 150,000 was a bit oversize myself for our small 1400 sq ft house, but that is what the original owner had put in all those years ago.

I had heard someone say that a 120,000 96% or 98% efficiency furnace would be equivalent to the one I have now since it is so inefficient. So I thought I'd come on here and ask the experts.

Thanks again for the replies.

Gary

 

[dfiler2]

If your current furnace is 80% and you went to a 96% efficient furnace, the new one would be 16% more efficient. Using that you could go with a furnace about 24,000 BTU's smaller. However, many times older furnaces were not sized correctly in the first place and you may have added insulation, replaced windows or done other upgrades since the original furnace was installed. I would do two things find an online heat loss calculator, preferably one from a university, and follow it closely and see what number you come up with. The second thing I would do is to call a local reputable heating contractor and get their opinion on what size they would generally recommend for a home like yours. Most of the time the input BTU's and the output BTU's are listed on your existing furnace, the difference between the 2 numbers will give you the approximate efficiency.

 

[yeldogt]

An oversized furnace will cycle on and off ... best to have your house properly sized for any new heater. The proper size heater will run longer and keep you house more comfortable.

Many states have energy programs -- with free audits and rebates .. with no income restrictions.

 

[DC73]

Agree with others that it is best to right size a furnace.

But, to answer your question about equivalency, the math is pretty simple. An 80% furnace loses 20% up the vent pipe so only 80% makes to the heated space.

150,000 x 80% = 120,000 BTU to the heated space.

For an equivalent 96% furnace, divide the first heated space calculation by 96%.

So, 120,000 / 96% = 125,000 BTU which is your equivalent 96% furnace size to get the same BTUs to the heated space.

DC

 

[DC73]

We have a old house that has no installation and old wood windows. The wife says when the furnace was working it didn't do that great of a job.

Installing insulation in a house that currently doesn't have any can be a no brainer for payback. Especially if you can do it yourself. Walls might be out of the question but the ceiling might be pretty easy to insulate. Air sealing all penetrations through the walls and ceiling with caulk or cans of spray foam is also a low cost solution with near immediate returns. Storm windows would also have a reasonable payback. I would do as much as you can to insulate and stop air leaks before sizing the next furnace. You'll end up needing a smaller furnace and will use much less energy in the process.

As mentioned, your state or local power company might have an energy audit program to help you with advice at least but they also might be able to help with rebates on new equipment.

DC

 

[gmhill33]

An oversized furnace will cycle on and off ... best to have your house properly sized for any new heater. The proper size heater will run longer and keep you house more comfortable.

Many states have energy programs -- with free audits and rebates .. with no income restrictions.

Agree with others that it is best to right size a furnace.

But, to answer your question about equivalency, the math is pretty simple. An 80% furnace loses 20% up the vent pipe so only 80% makes to the heated space.

150,000 x 80% = 120,000 BTU to the heated space.

For an equivalent 96% furnace, divide the first heated space calculation by 96%.

So, 120,000 / 96% = 125,000 BTU which is your equivalent 96% furnace size to get the same BTUs to the heated space.

DC

Installing insulation in a house that currently doesn't have any can be a no brainer for payback. Especially if you can do it yourself. Walls might be out of the question but the ceiling might be pretty easy to insulate. Air sealing all penetrations through the walls and ceiling with caulk or cans of spray foam is also a low cost solution with near immediate returns. Storm windows would also have a reasonable payback. I would do as much as you can to insulate and stop air leaks before sizing the next furnace. You'll end up needing a smaller furnace and will use much less energy in the process.

As mentioned, your state or local power company might have an energy audit program to help you with advice at least but they also might be able to help with rebates on new equipment.

DC

If your current furnace is 80% and you went to a 96% efficient furnace, the new one would be 16% more efficient. Using that you could go with a furnace about 24,000 BTU's smaller. However, many times older furnaces were not sized correctly in the first place and you may have added insulation, replaced windows or done other upgrades since the original furnace was installed. I would do two things find an online heat loss calculator, preferably one from a university, and follow it closely and see what number you come up with. The second thing I would do is to call a local reputable heating contractor and get their opinion on what size they would generally recommend for a home like yours. Most of the time the input BTU's and the output BTU's are listed on your existing furnace, the difference between the 2 numbers will give you the approximate efficiency.

Thanks everyone for the replies and advice, I have some research to do now.

Information is our best friend and with all the advice I can now make a better decision.

Thanks again,
Gary

 

[CNGsaves]

Thanks for the reply. We have an old house that has no installation and old wood windows. The wife says when the furnace was working it didn't do that great of a job. I thought that 150,000 Btu was a bit oversize myself for our small 1400 sq ft house, but that is what the original owner had put in all those years ago.

I had heard someone say that a 120,000 96% or 98% efficiency furnace would be equivalent to the one I have now since it is so inefficient. So I thought I'd come on here and ask the experts.

Thanks again for the replies. Gary

Wowsa, that furnace is MEGA sized !!!

I've got 60K Btu furnace (90% efficient) for slightly smaller 1955 house but all-brick and new windows. Think mine is slightly undersized as prior owner converted the garage to big sunken living room. They did build oversize detached 4 car garage instead.

I'd pursue the state program for improving house (ie new windows and blown-in cellulose insulation). That is very purpose of program to fix houses with massive heat losses. Good luck.

 

[aandpdan]

Thanks everyone for the replies and advice, I have some research to do now.

Information is our best friend and with all the advice I can now make a better decision.

Thanks again,
Gary

Any HVAC contractor that merely replaces the furnace/boiler with the same size unit, and does not perform a heat loss calc, is taking shortcuts.

The good ones will do their homework and research too.

 

[woodzy]

As stated - if possible please insulate as the price of fuels go up insulation seems to be the best bang for your buck. I would guess that your furnace is way oversized but that is only my opinion. I just had a 2230 square foot ranch built with just 4" walls, and they only installed a 60,000 BTU 95% efficient furnace. No issue with sub zero Michigan temps and keeping it 72°.

 

[Showkey]

Similar story......replaced two 125,000 btu 80% with two 70,000 btu 96%.
Worked perfect and NG fuel cost dropped by 30%.

There are many heat loss calculators online that will give you a very good estimate.

One other question that will come up is two stage or single stage units.