Thread: Load calculation form for properly sizing a furnace

Does anyone have an official form that determines what size (BTU) furnace is needed for the size and style of the house being built and the type of material that is being installed to determine what size furnace is needed?

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It's not so much a question of having the right form but using the right standard. Depending on what type of inspection you are doing there are many odds and ends resources you can find on the web. Type in 'CFM, duct sizing, furnace sizes' etc. and all kinds of stuff pops up you can get by with. A good heating guy can guesstimate very accurately by experience.
If however this is a lawsuit/court/arbitration defect inspection the default standard to use is the ACCA Manual S- equipment sizing, Manual D- duct sizing and Manual J- Ac sizing. If you do a load calc using these manuals you've got nothing to worry about.
Note- I've done a few for cases ... maybe it's because I'm merely a half-wit but I still have a hard time doing them easily or quickly.
Good luck
Markus

[Markus Keller]
If however this is a lawsuit/court/arbitration defect inspection the default standard to use is the ACCA Manual S- equipment sizing, Manual D- duct sizing and Manual J- Ac sizing. If you do a load calc using these manuals you've got nothing to worry about.

The heating company I use to work for in Wisconsin many many years ago had to do load calculations for low income people eligible for a free furnace and/or water heater. So I knew there was something out there.

When it comes to a lawsuit/court/arbitration, this indirectly is the reason why I'm asking. I have come across heating companies that vary greatly when sizing a furnace for the house. I just wanted something (for personal knowledge) that can get me closer to what size furnace is really needed.

Thanks for the info.

Originally Posted by Kevin Luce

I have come across heating companies that vary greatly when sizing a furnace for the house.

That would be an energy load calculation.

It could vary greatly, depending on the orientation of the house, where and how large the windows (and sliding glass doors) are, how many windows and doors there are, whether or not the windows are double glazed or not, the amount and type of insulation in the walls and amount and type of insulation in the ceiling, amount of roof overhang, etc.

Originally Posted by Jerry Peck

That would be an energy load calculation.

It could vary greatly, depending on the orientation of the house, where and how large the windows (and sliding glass doors) are, how many windows and doors there are, whether or not the windows are double glazed or not, the amount and type of insulation in the walls and amount and type of insulation in the ceiling, amount of roof overhang, etc.

This would be referring to the same house.

Some heating companies around here figure the square footage times 21 times 2 to determine the size of furnace needed. Other heating companies around here figure the BTU's by the number of registers in the house. I even had one heating company that only went by the size of the existing ductwork only. As a result, these estimates ranged from a 50,000 BTU 80 plus furnace up to a 100,000 BTU 80 plus gas furnace. The air conditioner ranged from a 2 ton to a 3 ton air conditioner. So from the information I have seen, the heating companies are not on the same page.

Again, these companies are giving estimates on the same house.

Throw all that out, at last the curb size furnace sizer. You dont even have to get out of your truck. Cut out the shapes, hold the template at the curb and see which one fits your house.

Originally Posted by Kevin Luce

these estimates ranged from a 50,000 BTU 80 plus furnace up to a 100,000 BTU 80 plus gas furnace.

That's not that far off when you look at it differently.

Let's just say a 'rule of thumb' puts the heat at 75,000 Btu, plus or minus 25,000 Btu. Not too far off from each other.

Let's say a 'rule of thumb' puts the a/c at 2-1/2 ton, plus or minus 1/2 ton. That's not too far off either.

Originally Posted by Kevin VanderWarf

Throw all that out, at last the curb size furnace sizer. You dont even have to get out of your truck. Cut out the shapes, hold the template at the curb and see which one fits your house.

lol. I always thought people like to simplify things to much and that is a great example.

Originally Posted by Kevin Luce

This would be referring to the same house.

Some heating companies around here figure the square footage times 21 times 2 to determine the size of furnace needed. Other heating companies around here figure the BTU's by the number of registers in the house. I even had one heating company that only went by the size of the existing ductwork only. As a result, these estimates ranged from a 50,000 BTU 80 plus furnace up to a 100,000 BTU 80 plus gas furnace. The air conditioner ranged from a 2 ton to a 3 ton air conditioner. So from the information I have seen, the heating companies are not on the same page.

Again, these companies are giving estimates on the same house.

Kevin

In Canada most companys use a heatloss calculator. HRAI - The Heating, Refrigeration and Air Conditioning Institute of Canada
The size of your duct work only tells you the maximum amount of air flow that it can handle. Counting registers, well... that will tell you if theres a heat supply in every room or NOT.

Sounds like its time to find a new heating contractor that wants to size the equipment correctly and sell you what you need, not what he has sittting in his shop.

Tom

Originally Posted by TOM DEEHAN

Kevin

The size of your duct work only tells you the maximum amount of air flow that it can handle. Counting registers, well... that will tell you if theres a heat supply in every room or NOT.

Tom

Agree

A proper heat or cooling load calculation starts with the design temperature (that is the temperature than which it is warmer or colder 99% if the time). This is very location-specific. There are tables for your location.

The technician then determines the U-value of every square foot of the house. The U-value is the number of BTUs that pass through a square foot of a given surface per degree difference in temperature between the inside and the outside. Then he or she adds 'em up. Factor into this the amount of air leakage in the building (most accurately determined using a blower door), the orientation, and shading or wind shielding conditions such as adjoining buildings, heavy wooded areas, and so on. The equipment should be sized to deliver enough heat or cooling at design temperature if it runs 100% of the time. Since equipment is only manufactured in certain sizes, some rounding off is inevitable.

An oversized furnace will not run long enough to reach steady state efficiency or to provide even heat throughout the building. It's like pumping your gas pedal when you should be using cruise control. An oversized A/C blows cold air, but it does not run long enough to dehumidify the environment and remove the latent heat that is entrained by the water vapor in the air.

Every purchaser of heating or cooling equipment should insist on a written heat load calculation and a thorough explanation of why efficient equipment should not blow really hot or really cold air for short periods of time. If the HVAC guy can't deliver these services, the client should find another contractor who can.

People need to be educated to understand how these systems work with regard to heat transfer and moisture management. If they want to squander energy and create conditions that can result in mold growth, well, it's still a free country. But we should advise and educate our clients in a way that saves at least some of the remaining available resources for our children. I think that's a great reason for home inspectors to educate themselves in this subject.

Kevin, load calc's for wisco and chicago are going to be essentially the same, unless your in the UP area. A decent reference point for inspections is such: Unless the house is a shack, 90,000 BTU 80+or90+ minimum on the heat side; 2-1/2 to 3 ton on AC side minimum. Anything less is unlikely to provide sufficient heat or cool in our region.
45000 - 75000 btu apt/condo size 1-1/2 to 2-1/2 ton AC
120000 90+ can do upto 2500 to 2800 sq ft if system is installed properly and house is good construction, but that's pretty much max.
Anything over that is going to require a second furnace.
Be mindful of ductwork on some of the older houses, if there was an old gravity feed system. Lots of times guys won't change any ductwork just the unit. Air distribution becomes very poor at that point due to minimal static pressure. I always take a temp reading when I enter and when leaving.
As you can see from all the posts a proper load calc is not a simple thing. I've greatly simplified it above and will probably get flack for it but for HI purposes it's close enough for now.
Good luck, Markus

Hi Markus.

Here's your flack.

Best regards.