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#### ruby_jane

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The thread on heaters made me think--how do you determine how many BTU's your heater should be according to the square footage you are heating? :shrug:

#### haypoint

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There is a lot more to it than just square footage, even cubic feet isn't enough information. It depends on the area where you live and the insulation in your house. Lots of windows is another factor. I'm sure a HVAC specialist could figure it out for you. You could see what others have in simular homes in your area and go from there.

#### The Paw

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I don't know of a site with an on-line calculator, but there must be one somewhere.

the number of btu's needed will vary depending on:

-square footage
- outside temp
- insulation/R factor
- leakage (chimneys, door jambs, etc.)
- size and number of windows

#### mdharris68

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Research the "Manual J". It is all about load calculations for figuring heat loss on a structure. Trying to figure btu's by the square foot is like guessing how many marbles it takes to fill the ocean. Actually both methods are guesses, but one of them is calculated.

You may be able to find a heat loss calculator online that you can download for free and use it once or twice before it asks you to buy it.

Here is the calculator that I used.

http://www.slantfin.com/heat-loss-software-get.html

It is put on the net by a boiler manufacturer, but it asks the same questions about your structure to determine your heat loss.

#### Ross

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Yeah it's a fairly involved process to determine the heat loss of any building. There are 2200 sq foot homes here that rate 84,000 btu and similar sized ones needing over 120,000!! You'll need to know the room sizes, wall composition (what is in them, like insulation cladding and windows etc.) same for the roof and basement, the house's orientation, and location (there are tables that have a typical winter's heatloss for every area in N America (probably ones for every part of the world really) all of which factor into the whole building's heatloss.

· Voice of Reason
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Of course, how a home is constructed & insulated, as well as climate, all make a difference in how large a furnace you will need, but a standard 2x4 constructed home with R-13 insulation normally requires 50 to 70 btu/hour/sq.ft. of floor area.

#### Dragonflea

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The formula is simple, and 1500W heaters all put out the same heat.

there is a 10-1 ratio for watts to sqft. 1500w=150sqft heated.

Watts divided by 10 = SqFt.

to figure out how many heaters you need for a given area:

SqFt divided by 150 = # of 1500watt heaters needed.

example: 3000 SqFt will require 2 1500 watt heaters.

It doesn't matter the type of electric heater you buy, 1500 watts can only put out around 5K BTU/hr.

Conversion from Watts to Btu/hr is to multiply Watts by 3.4144.

· Voice of Reason
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The formula is simple, and 1500W heaters all put out the same heat.

there is a 10-1 ratio for watts to sqft. 1500w=150sqft heated.

Watts divided by 10 = SqFt.

to figure out how many heaters you need for a given area:

SqFt divided by 150 = # of 1500watt heaters needed.

example: 3000 SqFt will require 2 1500 watt heaters.

It doesn't matter the type of electric heater you buy, 1500 watts can only put out around 5K BTU/hr.

Conversion from Watts to Btu/hr is to multiply Watts by 3.4144.
By your formula, you would provide about 5,100 btu/hour for every 150 sq.ft. of floor area, or about 34 btu/hour/sq.ft.

I suspect you could get away with that in a lot of areas, but that estimate seems a little low for colder climates. I would be more comfortable sizing a furnace to more like 50 btu/hour/sq.ft. of floor area.

#### Dragonflea

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well, it is just a base formula.
lol

I agree, at a certain level of cold you will need even more heat.
and more even, when your insulation factor sucks.

#### ruby_jane

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Well, our current "structure" is a 2 bedroom single wide 12' by something...very small, though. It's an eat in kitchen, living room, bathroom and the 2 small bedrooms. We have an oil fired heater but we haven't put oil in it yet, so we have electric heat to use. We've tossed around the idea of one of those electric heaters that looks like a wood stove, and surprisingly our bill is very low for this area--last month it was \$172. That's heat, hot water, the washer and dryer in the garage along with the lights for the garage, too (our cooking is propane).

Looking to move eventually north or west as soon as we get our budget in gear (2 year plan), and I am SO lookin' forward to a woodstove!

#### SolarGary

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Hi,
You can use my calculator to get an estimate:
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

For example, a 1000 sqft house, insulated to R10 walls, R20 ceiling, a few windows, typical infiltration, and an outside coldest design temp of 0F would gives you a design heat loss of 23,000 BTU/hr.
You would need a heater at least this large to heat the place on a 0F night.

Don't pick an extreme design temperature, as oversizing some heaters makes them cycle more and run less efficiently. There is a help file there with design temps for various places.

The calculator also gives a rough estimate of heating season heat use, fuel cost, and CO2 emissions. For the example above in a 6000 deg-day climate on propane at \$2.10 per gallon, it comes out \$1300. It all depends on R values and infiltration, so if you double the R values and cut the infiltration in half, you cut the fuel use in half.

All of these kinds of heat loss calculators give approximate answers, so don't depend on them to be right on the money.

Gary

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A calculator like that is probably a lot more accurate for multi-story homes than my 50 btu/hour/sq.ft. rule of thumb. My cabin is a 2-story home, so it wouldn't make sense to add-up all the floor area both upstairs and downstairs, since it's really just one tall room.

I have just over 500 sq.ft. of floor area for both floors, and I installed a 20,000 btu/hour furnace. That gives me about 40 btu/hour/sq.ft., but the furnace isn't on very much of the time. If I had 500 sq.ft. all on one floor I probably would have installed more heating capacity, since there would have been a lot more square footage of heat-loss surface in a single-story home.

As I said before this isn't an exact science, and it doesn't have to be. Furnaces are manufactured in relatively coarse increments, so you're just trying to get into the ballpark to select an appropriate furnace. The question is not whether you need 22K btu/hour or 23K btu/hour furnace, the question is if you need a 20K btu/hour or a 30K btu/hour furnace, since small furnaces are manufactured in 10K btu/hour increments.

Also, you can get away with being off on your btu estimate quite a bit. Last winter my propane regulator went out one evening (I was using an ancient regulator I took off of an old travel trailer) so I used a 10,000 btu/hour kerosene heater just for the night until I could go to town and get a new regulator. It got down to below zero that night, but the 10,000 btu/hour heater still kept us comfortable.

#### ruby_jane

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· Voice of Reason
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One thing I didn't mention is that the furnace I have isn't strictly an on-off controlled thermostat. The one I have can throttle the burner from 10,000 btu/hour up to 20,000 btu/hour, depending on how far from the setpoint the temperature in the room is. Mine is like this one, and has that variable flame feature.

http://cgi.ebay.com/COMFORT-GLOW-20K-PROPANE-WALL-HEATER-VENT-FREE-TSTAT_W0QQitemZ330281231785

The reason for mentioning this is that you can use a furnace like that in an application where a smaller furnace might be more appropriate, since the smaller 10K btu/hour flame will avoid the problems of a furnace too large for the application. It will only burn with a flame as large as it needs. Therefore, the furnace size calculation is not nearly so critical.

#### cfabe

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The best practice industry standard way to come up with a number is to use the Manual J calculation or use a piece of software to do it for you. If the software doesn't ask about orientation, building materials, sqft of doors and windows, etc it's just guessing for you. When I had to install a new furnace I paid for a homeowner version of HVAC-Calc software from www.hvaccomputer.com. It's \$50 but I think it's worth it to get the right size heating system.

Any of the rules of thumb mentioned in this thread are just that, rules of thumb that might get in you in the right ballpark for a given set of conditions. Where I live, trying to heat a 3000 sqft home with two 1500w heaters would be rediculous. And using Nevada's rule of thumb would oversize my furnace by a considerable margin.

As a point of reference, using the software I mentioned, my 1650 sqft house with 2x6 walls, insulating sheathing, about 12" of attic insulation, on an unheated crawl with average ammount of double pane windows and doors has a heat loss of about 70k btu at the design temperature. Before I did the calculation, some online calculators got me close to this number, some were off enough that I would have bought the wrong size furnace.

Installing an undersized furnace will mean you may be cold during extreme weather. Installing an oversized furnace will mean you will get short cycles and probably won't see anything near the advertised efficency. It pays to get it right on this one.

#### SolarGary

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The best practice industry standard way to come up with a number is to use the Manual J calculation or use a piece of software to do it for you. If the software doesn't ask about orientation, building materials, sqft of doors and windows, etc it's just guessing for you. When I had to install a new furnace I paid for a homeowner version of HVAC-Calc software from www.hvaccomputer.com. It's \$50 but I think it's worth it to get the right size heating system.

Any of the rules of thumb mentioned in this thread are just that, rules of thumb that might get in you in the right ballpark for a given set of conditions. Where I live, trying to heat a 3000 sqft home with two 1500w heaters would be rediculous. And using Nevada's rule of thumb would oversize my furnace by a considerable margin.

As a point of reference, using the software I mentioned, my 1650 sqft house with 2x6 walls, insulating sheathing, about 12" of attic insulation, on an unheated crawl with average ammount of double pane windows and doors has a heat loss of about 70k btu at the design temperature. Before I did the calculation, some online calculators got me close to this number, some were off enough that I would have bought the wrong size furnace.

Installing an undersized furnace will mean you may be cold during extreme weather. Installing an oversized furnace will mean you will get short cycles and probably won't see anything near the advertised efficency. It pays to get it right on this one.
Hi,
The calculator that I mentioned here:
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

uses the calculation methods listed in the ASHRAE Fundamentals Volume -- essential the same as the manual J calcs. It does ceilings, walls, windows, ... all with the R values you plug in. IMO there is really no need to pay for a calculation unless you have a very complex situation.

The calcs are not even close to rocket science -- three simple equations covers it all:

The first is for heat loss through anything that can be characterized with an R value (walls, ceilings, most floors, windows) -- this is
Loss = (Area)(Temperature difference)/(Rvalue)

The 2nd is air infiltration:
Loss = (Temperature Difference)(Volume of house)(Air Changes/hour)(Density of air)(Specific heat of air)

There is another simple equation for losses out of concrete slabs.

That's really all there is to it. The calculator is a JavaScript, so you see the source code by just clicking on "show page source" in the View menu of your browser -- you can see that all of the actual calcs are done in half a dozen lines of code -- the remaining 99% just do things like checking inputs for sensibility.

The calculator takes your inputs for areas and R values and gives you a heat loss. If you put in a value for the number of heating degree days for your area, it will also give you a rough estimate of total yearly heat heat loss, what your fuel bill will be based on the fuel you use, and green house gas emissions.

I've had the calculator up for more than a year, and have asked people to report anything that looks like an error. I've made a couple small changes to reflect comments, but nothing significant -- it gets a couple hundred uses a day, so I think I would have heard about any serious errors.

All of these methods, including the Manual J and ASHRAE calcs are just rough engineering approximations of real world heat transfer, and can easily be wrong by a fair margin for reasons listed with the calculator, but luckily you don't have to be right on.

I do certainly agree that a calculator that does not know how much insulation you have, or how good your windows are could be subject to huge errors.

There is a program called HEED that does an actual hour by hour simulation for the whole year using hour by hour weather from a file for your area. I think this will give a better picture of what goes on, and will probably be more accurate. It includes some additional things, like solar gain, but is still subject some errors. Its a free download, and is easy to use (it was written for use by CA utility customers).
Its listed here with some other helpful software:
http://www.builditsolar.com/References/energysimsrs.htm

The calculator and the HEED program are are great ways to see which kinds of insulation, window, or sealing programs it might pay you to do, and how much.

Gary

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And using Nevada's rule of thumb would oversize my furnace by a considerable margin.

As a point of reference, using the software I mentioned, my 1650 sqft house with 2x6 walls, insulating sheathing, about 12" of attic insulation, on an unheated crawl with average ammount of double pane windows and doors has a heat loss of about 70k btu at the design temperature.
Not too far off. At 50 btu/hour/sq.ft. a 1650 sq.ft. home would need an estimated 82.5K btu/hour, yet you calculate needing 70K btu/hour to meet your calculated heat leak estimate. That's certainly in the ballpark. I wouldn't have a problem installing an 82.5K btu/hour furnace in a home with an anticipated 70K btu/hour heat leak.

#### Charleen

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I thought BTUs stood for Bunched Together Until Spring?