# How many BTU do I need



## coppermouse (Oct 1, 2007)

I want to add a wood furnace to my existing furnace system. I live in Ohio. I have a 2500 square foot 2 story house with a full basement. It was built 10 years ago. Well insulated, double pane glass. What BTU rating do I need?


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## TMonter (Oct 1, 2007)

Most high efficiency furnaces for a house of that size depending on how well insulated you are would be between 60,000 and 120,000 Btu/hr. A good rule of thumb for the whole house of that size would be 80,000 Btu/hr.


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## webbie (Oct 1, 2007)

How much oil or gas did you use in a cold month? - we can work backwards from there....

Also, you should not buy a furnace by BTU rating alone - firebox size and efficiency are important. For instance, maybe a furnace can put out a lot of heat, but only for 4 hours on a full load! It might be better to have one which produces less heat for a longer period.

When a real heat loss study is done on a house like yours, you might be surprised at how little heat it needs on an hour-by-hour basis. For instance, a 120K BTU furnaces might have a 70% efficiency (older oil or gas), so it only puts out 85K. Then you might find it only runs 1/3 of the time when it is 25 degrees outside, which means the house only needs 30K in that weather!

So do the math based on your existing fuel use - or we will if you let us know the numbers.


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## coppermouse (Oct 1, 2007)

Ok thanks, just wondered because the recommendations from the wood furnace manufacturers vary quite a bit


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## jjbaer (Oct 1, 2007)

My home is a 1650 sq ft ranch built in 1950. It has 2x4 walls with blown-in fiberglass, is fairly air-tight, has double plane windows (no gas between them though) and used only about 147 CCF of NG last Dec and 100 CCF in Jan and in Feb. Since about 15 CCF is for hot water each month, Dec used about 132 CCF which means about 15,000 BTU/hr is required as heat input to the home after taking into account inefficiencies (furnace is 96% eff and 15% duct losses).

132 CCF/month x 100,000 BTU/CCF x 0.96 eff x 0.85 eff x month/30 days x day/24 hrs = 14,960 BTU/hr

In Jan and Feb it only used about 10,000 BTU/hr. So, if you run a wood stove or attached wood unit, the hourly input of heat into the home is small. Scaling up to your home, I'm guessing about 22,000 BTU/hr to heat the two main floors and more if you're heating the basement. 

Remember though, the figures I quoted are for my home after taking into account MY inefficiencies. As Craig said, your existing unit may be less inefficient and the real heat input your house requires is lower than you think. That's why he suggested you look at prior bills and take into account furnace and duct losses. I think wood stoves are rated as heat OUTPUT (which means you only have to take into account duct losses and any air-to-air or air-to-water exchanger losses) so for you to get say, 30,000 BTU input to the home, your wood stove needs to output about 47,000 BTU/hr. 

30,000 x (1/0.85 duct loss) x (1/0.9 exchange loss) = about 40,000 BTU/hr 

I don't think max ratings run this low so it would seem that most any typical sized unit would work, but as Web said, start with your previous bill for a cold month and work it backwards taking into account the inefficiencies. If you want, tell us the month, the energy used (tell us if it includes energy for hot water) and tell us the type furnace and efficiency and we can get a ball park idea. I'm assuming this is a wood unit that heats water and circulates it through a heat exchanger installed in your heating ducts to then extract the heat. Is this correct?


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## coppermouse (Oct 1, 2007)

I cannot really say as far as the previous gas usage as I have always used a wood stove to heat with. 
No it does not heat water, it just has a heat exchanger built into the unit.


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## jjbaer (Oct 1, 2007)

coppermouse said:
			
		

> I cannot really say as far as the previous gas usage as I have always used a wood stove to heat with.
> No it does not heat water, it just has a heat exchanger built into the unit.



So you have an existing furnace (which you have not been using) and instead, you've heated with one wood stove.  Is that correct?  If so, tell us this:

1) the stove type/model, 2) efficiency 3) amount of wood you go through in a year or during a cold month like December and 4) how many stoves and where are they located.


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## jjbaer (Oct 1, 2007)

coppermouse said:
			
		

> I cannot really say as far as the previous gas usage as I have always used a wood stove to heat with.
> No it does not heat water, it just has a heat exchanger built into the unit.



New thought:  if you just moved into the home, you can call the gas co and they'll tell you the gas consumed over the last x-number of months........same with electric......


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## coppermouse (Oct 1, 2007)

That is correct I normally don't use the gas furnace except when traveling, then I turn it low (like 45)
I have lived in the house since I built it 10 years ago. I will have to check on the stove, it has been a long time, I think it is an Englander, and it has 4 (1 1/2") round pipes coming out the top front of it that a blower is attached to. There is a catalytic converter in it. Also I cut my own wood and it is difficult to quantify, maybe 4 cords or so, but that is just a quess.


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## jjbaer (Oct 1, 2007)

coppermouse said:
			
		

> That is correct I normally don't use the gas furnace except when traveling, then I turn it low (like 45)
> I have lived in the house since I built it 10 years ago. I will have to check on the stove, it has been a long time, I think it is an Englander, and it has 4 (1 1/2") round pipes coming out the top front of it that a blower is attached to. There is a catalytic converter in it. Also I cut my own wood and it is difficult to quantify, maybe 4 cords or so, but that is just a quess.



Assume 4.5 cords at 20 million BTU/cord = 90,000,000 BTU.  Assume most is used between 1 Oct - 30 April and that your current stove has 75% efficiency (that many pipes with fans might get you 75%) and you get about 14,000 BTU/hr as an average BTU/hr over those 7 months.  My 15,000 BTU/hr was for a smaller home but during Dec.  When I ratio my Dec usage to what I use in those same 7 months, Dec usage is about 1.8 times the average usage so you could expect your Dec heat input to also be about 1.8 times your avg of 14,000 which gives you 25,000 BTU/HR.  Say you want your home a little warmer than it's been, and 30,000 BTU/hr is in the ballpark. 

Assuming your new (proposed) wood unit either heats water or ducts air into your existing ducts, then you need to figure 15% or so duct losses and about 10% loss for the exchange system so to deliver 30,000 BTU you'd need about 40,000 BTU output but would need more to cover things we've under-estimated (duct loses may be higher and/or there may be no exchange losses or thay may already be figured in elsewhere).  So.......50,000 is looking good.  Now, since you probably wouldn't want to be running it full tilt all the time, 60,000 is looking good.  Bottom line:  you need to go through this calculation for yourself and insert your own losses and estimates on current wood consumption to arrive at the correct answer but thinking like this helps you arrive at ballpark figures.


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## rmcfall (Oct 2, 2007)

How do you calculate BTUs/hr needed based on a house with a heat pump?  I've got a 16 seer Carrier Infinity Heat Pump in a 2300-2400 sq ft ranch house with block walls (walls are not insulated...at least not yet) and full basement.  Based on my electric bills, I imagine I am losing a lot of heat through my uninsulated walls and my basement (not insulated yet either).  Last January I used 5493 KWH (includes 40gal electric hot water heater), and in February I used 6439.  




			
				castiron said:
			
		

> My home is a 1650 sq ft ranch built in 1950. It has 2x4 walls with blown-in fiberglass, is fairly air-tight, has double plane windows (no gas between them though) and used only about 147 CCF of NG last Dec and 100 CCF in Jan and in Feb. Since about 15 CCF is for hot water each month, Dec used about 132 CCF which means about 15,000 BTU/hr is required as heat input to the home after taking into account inefficiencies (furnace is 96% eff and 15% duct losses).
> 
> 132 CCF/month x 100,000 BTU/CCF x 0.96 eff x 0.85 eff x month/30 days x day/24 hrs = 14,960 BTU/hr
> 
> ...


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## coppermouse (Oct 2, 2007)

The manufacturer I am looking at has 2 models. They say their 120K BTU furnace is for a 1600 sq ft house and the 150K BTU unit is for 2500 sq ft. So the smaller one should be ok for my application?


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## mayhem (Oct 2, 2007)

I would lean toward the manufacturer's recommendation for your size home.  There's a reason why they recommend the larger unit for the bigger home...it may be strictly money, but three's probably some good data that suggests the smaller unit is probably going to be overworked in your larger home.

I'm surprised at the btu numbers you guys are throwing out there.  I have a 1750 sq ft log home and run an oil burner rated at 86% and 180,000btu.  Its very adequate, but by no menas am I over-boilered.  The one caveat I have is the heat load of my front room is quite large...its a chalet-style house (see avatar) with a 26' cathedral ceiling in the main living area and about 80% of the front wall of the house is glass.  I don't know how to draw up the numbers to run it myself, but I have to belive about 70-80% of the whole house heat load is in that front room with the huge air mass, large glass surface area and low quantity of baseboard area...we installed two fan forced kickspace heaters to even up the odds and on those few nights a year when its well below zero with high winds I keep it all running foll bore most of the night just to maintain high 50's in the room.

To the original poster, most heating systems in the last 10-20 years should have a label right on the main unit somewhere that lsits its heat output.  I would suggest you check what your current btu output capacity is and try to get in the neighborhood with the wood furnace.

Don't forget to check with the installer or salesman about an extended chimney too, all the wood furnaces in my area have low stacks and the smoke oozes out and tends to create a haze in the yard...you want to try and get that smoke moving and get it out of the breathing zone.


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## webbie (Oct 2, 2007)

Mayhem,

here is a fast way to look at it - if I had a 140,000 BTU oil furnace and my house needed that capacity full time (an unlikely scenario, but just for purposes of this thread), than I would use one gallon per hour, 24 per day and 740 or so per month. In the real world, a person with that furnace is more likely to use 250 gal per month, which means they are using it at about 1/3 capacity. Of course, design must be for PEAK load - like getting your house up to at least 65 on that day that is 10 degrees below zero (or more).....


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## elkimmeg (Oct 2, 2007)

What ever normal is 8' ceilings decent insulation one usually needs 1000 BTU's for every 40 sq ft of floor space. Zone also play a part of this northern colder zones will require more BTU's


 Glass area also play a part of the formula  what is really needed it a heat loss take off.. But if moderate glass area  add 20% to that figure

Part of heating furnaces and boilers is sizing them to do the job  Oversizing them leads to short inefficient short cycling Undersizing them will over work them to keep up
 that too is not the most efficient way to opperate them

 A wood furnace posed more issues there is no on off cycles Heat ranges will be much hotter with a constant heat supply and not short cycles of you existing furnace

Most duct systems are not designed for constant flow or the higher possible heat range If your system has flexible duct work it is probably limited to 140 heat range Above 140 the plastic iner core will melt out They do make flexible ducts with higher tollarance heat cores ,but usually not found in existing systems Another common problem is now return bays in floors are
 comprised with combustible  materials and they too can not sub stain higher heat ranges nor should the be non combustible./ Really what is required. is a system completely hard piped ducts

 There is a bit more thought and planning involved than just adding a wood boiler It might be it cannot be added to you current duct system, but have to exist on a separate system designed only to meet the wood fired heat range..  I can't say for sure if this applies to your situation , but I want to point out more planning may need to be done, for you to successfully make a wood boiler work  Again I would need to read your wood boiler specs and examine your current system to give accurate code compliant advice.

Sizing the boiler may be the easiest part of your decision ,retrofitting it to your existing ductwork may pose the greatest challenge


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## karl (Oct 2, 2007)

Elk, please clarify.  1000btus for every 40 square feet.  You are refering to a furnace not a wood stove right?


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## elkimmeg (Oct 2, 2007)

The basic formula or rule of thumb is for every  40  sq ft floor space  to maintain 70 degrees at zero outside 1000 btus needs to be developed by a heat source furnace boiler wood stove ect

 then one has to factor window area  or called glazing assumimg double pane glass usually add 20% to the prior calculations


 As an inspector I have to figure is the design load of heat is adequate for each room in the house. Your basic common Baseboard heat radiator puts out 450 BTU per ft


 a common 12 /12 room only needs 140 cfms from  an air duct in this case one 6" feed there are many way to figure out heat loads,
 but doing an inspection I want to use a fast rule of thumb  to determine if the area is covered. 


 Loading up a f room on the first half of a zone is also not advised too much heat is dissipated before it completes a zone run I have to check out the zone runs as well.

There is nothing worse than to have that far bedroom that is never comfortable because the heat has been dissipated long before it reaches it

 Fortunately with HVAC the flow can be adjusted and balanced to force more heat there and less in the front side of the run.

Another way Forced hot water radiator systems is to add more heat or higher capacity radiators at the zone end. Even though the heat has dissipated,  there is still valuable heat to extract but more radiator exposure is needed or higher  capacity radiators are needed.  This is why it is really important to insulate those pipes exposed in the basement, to prevent heat loss in transmission


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## karl (Oct 2, 2007)

I'm a bit confued Elk.  Using your calculations, I need 60,000 btus to heat my house.  The output on my furnace is 180,000.  Yes it's an old furnace and it does heat the really well.  I just can't see where you use the same numbers for a stove and a furnace.  One cycles and the other doesn't.  I can buy needing 60,000 continuous to heat this house to 70 on a 0 degree day, but I would hate to not have any excess left.


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## titan (Oct 2, 2007)

I always liked using "30 btu's per sq.ft" as a rule of thumb.Not very scientific but when you do a proper heat -loss calculation on a home, it's eerily close to that #.


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## elkimmeg (Oct 3, 2007)

Karl  I should have noted this is new construction using the existing energy codes of today. It does not work in older homes little or no insulation single glazed windows no weatherstripped 
exterior doors

 furnaces /boilers should be only oversized by no more than 25% or they will short cycle my home for instance is 3 zones 2800 sq ft +-  heated with a boiler 105k  BTUS

 does the job just fine.


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## jjbaer (Oct 8, 2007)

karl said:
			
		

> I'm a bit confued Elk.  Using your calculations, I need 60,000 btus to heat my house.  The output on my furnace is 180,000.  Yes it's an old furnace and it does heat the really well.  I just can't see where you use the same numbers for a stove and a furnace.  One cycles and the other doesn't.  I can buy needing 60,000 continuous to heat this house to 70 on a 0 degree day, but I would hate to not have any excess left.



What Elk is saying is this:  if your home is 2400 sq ft and meets the insulation requirements that his "1,000 BTU's per 40 sq ft" was based on, then your home requires about 60,000 BTU/hr continuous to keep the temp at 70F when the outside temp is 0 deg F.  Very few places (and certainly NOT where you are located) have average outside temps of zero degrees so your requirement is far lower than this figure.  

Your current furnace of 180,000 BTU output may deliver (after duct losses) about 135,000 BTU/hr to the home.  To get 60,000 BTU/hr delivered to your home, your furnace therefore only runs about 40% of the time and remember, that's when it's 0 deg F outside.  Since your average temperature is much higher, most of the time your furnace will only run about 20-25% of the time.  Now, to get those same 60,000 BTU/hr output from a wood stove, you'd have to get a stove that delivers about 75,000 BTU because you can't constantly fire a stove to it's theoretical limits and expect it to last very long so you'd need one capable of more than 60,000 BTU/hr.  But again, that's if it's zero outside.  And even if you had such a monster stove, most of the time you can't evenly distribute the heat to other parts of the home so you end up over heating the room the stove is located in while other (remote) rooms are far cooler.  So, realistically, unless you have two stoves located in different parts of the house, or unless you have a home design that allows heat to reach all areas evenly, you can get by instead with a stove smaller than 60,000 BTU/hr and supplememt when needed with your furnace to heat the farther rooms.


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## karl (Oct 8, 2007)

I understand.  What is troubling me is that a furnace cycles more than a stove, so I would think a stove wouldn't need to be as big.  Anyway, my Summit is oversized for the house according to Elk's numbers, so I won't have to worry about being cold.  The big difference between me and most people on here is that I let the house cool off alot.  I am out of town 3 or 4 days a week and set the thermostat on 50 when I'm gone.  After a few days, not only is the air cold.  The block is cold, the furniture is cold, etc.  That's a huge amount of mass to bring up to temperature.  With the gas furnace it cycles on and off alot to heat the house.  It gets the air warm and turns off and then the furniture and walls suck the heat out of the air and it starts all over again.  I'm hoping to use the Summit to bring the house up to temperature when I get home.  That will save me tons on my gas bill.


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## jjbaer (Oct 8, 2007)

karl said:
			
		

> I understand.  What is troubling me is that a furnace cycles more than a stove, so I would think a stove wouldn't need to be as big.  Anyway, my Summit is oversized for the house according to Elk's numbers, so I won't have to worry about being cold.  The big difference between me and most people on here is that I let the house cool off alot.  I am out of town 3 or 4 days a week and set the thermostat on 50 when I'm gone.  After a few days, not only is the air cold.  The block is cold, the furniture is cold, etc.  That's a huge amount of mass to bring up to temperature.  With the gas furnace it cycles on and off alot to heat the house.  It gets the air warm and turns off and then the furniture and walls suck the heat out of the air and it starts all over again.  I'm hoping to use the Summit to bring the house up to temperature when I get home.  That will save me tons on my gas bill.



Yes, but your furnace cycles on and off just to get you 60,000 or so BTU's/hr on average.  If a stove is to provide this same level of heating, it needs to continuously put out this same heat so that means a stove capable of 60,000 BTU/hr all the time (if you can distribute it all).  Your best bet is to slowly start to insulate your interior walls because even 8" of brick have virtually no insulation.  Elks estimate was based on newer homes WITH insulation and you have virtually NO insulation which leads me to believe that your furnace must run a LOT more than even 25-50% of the time......


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## karl (Oct 8, 2007)

I didn't say I had virtually no insulation.  I have 2x6 walls that are insulated and about 12" in the attic. I have good windows.  My only week place is the basement and crawl space.


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## jjbaer (Oct 9, 2007)

karl said:
			
		

> I didn't say I had virtually no insulation.  I have 2x6 walls that are insulated and about 12" in the attic. I have good windows.  My only week place is the basement and crawl space.



sorry...got your walls confused with rmcfall's walls...LOL


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## Vermontster (Oct 9, 2007)

I am not a stove builder, salesman or affiliated with the stove business other than as a consumer. So, I am no expert, but I grew up in Northern Vermont in a house about 2600+ square feet. We had a combination oil/wood furnace in the basement. It rocked, I mean kicked a$$. We had to open windows often to cool down, My father fitted it with a hot water heater and we came home one day to find the hot water had melted the washers in the faucets. Crazy. If I could put one in my house, this what I would have. http://www.yukon-eagle.com/FURNACES/EAGLEIHUSKY/tabid/55/Default.aspx


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## Nofossil (Oct 9, 2007)

At this point, I don't know if you feel like your question has been answered or not. I do think that you're right to consider the thermal mass that you will want to warm up quickly. When I built our house, I neglected to take that into account when sizing the oil hot-water baseboards. I painstakingly modeled every heat loss in the house and determined that we needed 52' of baseboard for our 3500 sq ft house, which works out to about 34000 btu/hr. That turns out to be almost exactly right (I can maintain 72 degrees inside at -30 outside) but it takes nearly forever to change the temperature of the house even one degree.

When I added the wood boiler, I had more problems as it doesn't always provide hot water at 180 degrees. At lower temps, the baseboards don't put out as much heat.

As with many things in life, a little overkill is a good thing.


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