Heat Loss Calculations - Surprising Results! - Need Advice...


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Old 10-06-10, 05:00 AM
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Heat Loss Calculations - Surprising Results! - Need Advice...

Hey guys, we are getting all of our ducks in a row for a new forced hot water baseboard system (upgrading from steam). As an engineer and overall geek I like to educate myself on projects that will cost alot of money, especially when I have to hire someone else to do it. I downloaded Slant/Fin's "Hydronic Explorer" Heat Loss Program to make my life a bit easier. I was shocked at the results!

We have a 110 year old two-story colonial that measured in at 1525 sq. ft. It is fairly well insulated. About 20-30 years ago (a guess), the previous owners had blown-in insulation installed in the walls and attic. Assuming it hasn't settled too much the walls should be R13 and the attic should be R19. I have installed new double-pane low-e windows throughout and insulated steel entry doors as well. The house is fairly tight and does not have many drafts at all.

That said, I meticulously entered all the data room-by-room for both floors into Slant/fin's program using a 70 degree desired indoor temp, 0 degree outdoor temp. I was shocked that I only had a heat loss of 41,500 btu's!. For those of you who are familiar with heat loss calculations....does that sound about right for a 1525sqft, two story house that is fairly well insulated?

The reason I was shocked is because the boiler for our existing steam system has an output of 90k btu. Of course, the current steam system only provides heat on the first floor.

I did have a question on the heat load calc that wasn't too clear to me:
When calculating the window/door square footage, do you have to include the interior doors? i.e. the entrance door for a bedroom?

Thanks for the advice guys!
 
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Old 10-06-10, 05:31 AM
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Heat loss

That sounds about right. I have a home about the same size but poorly insulated and all one level with a heatloss of about 60K. Your steam boiler should be sized to the connected load which was probably sized for the heat loss at the time it was installed.

The only doors you figure in are exterior doors since the r-factor for them is going to be less than the exterior walls.

BTW, I use a rule of thumb to give me a guesstimate of heat loss that comes fairly close, at least close enough to know if I'm in the right ball park. For a well insulated home such as yours I multiply the square footage by 25, average construction by 35 and a barn by 45. 25 X 1525 is just over 38K.
 
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Old 10-06-10, 06:55 AM
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I found the same thing. Converting from steam to hot water, I found my heat loss to be around 60-70k BTUH. My existing steam boiler was a 145k model!

Oversized boilers aren't unusual at all. I chose to go with a modcon boiler that can do 88k, but can throttle down to 25% of that. With outdoor reset, I should see much lower energy use with better comfort.
 
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Old 10-06-10, 06:30 PM
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Any chance you can keep the steam rads and use with hot water? That would be a terrific high-mass system and a great candidate for a modcon boiler.
 
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Old 10-06-10, 06:54 PM
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A few years back I calculated HL on my place. It's a converted Cape, added full second floor in '95. SlantFin told me around 70K or so. Then, for the next two years I 'clocked' my oil usage versus degree days. The ACTUAL heat loss averaged around 45-50K for those two winters. This place is about 1800 sq ft.

We've (myself and others on this board) have found that in general, SlantFin's numbers are about 25% high, and this is probably the norm for any Manual J calculation.
 
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Old 10-07-10, 04:21 AM
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Originally Posted by xiphias View Post
Any chance you can keep the steam rads and use with hot water? That would be a terrific high-mass system and a great candidate for a modcon boiler.
Well...probably not. We don't have heat on the 2nd floor, the radiators are in the worst possible places (aesthetically of course), and are starting to show thier age. (one has a hairline crack).

Plus, we are trying to keep it low budget. We are probably going to get a Weil Mclain CGI or CGA, Burnham 203H or PVG3, or Lochinvar CBN boiler. All can be had for less than $1,750.
 
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Old 10-07-10, 07:41 AM
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There are good federal and MA tax credits for high-efficiency equipment. Plus MA gas utility rebates. Makes them very cost-competitive if not cheaper than conventional. And certainly large potential upside for fuel savings over the life of the system. Especially in a house with low heat loss.

Check out dsireusa.org for list of credits, incentives, rebates.
 
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Old 10-07-10, 08:38 AM
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Originally Posted by xiphias View Post
There are good federal and MA tax credits for high-efficiency equipment. Plus MA gas utility rebates. Makes them very cost-competitive if not cheaper than conventional. And certainly large potential upside for fuel savings over the life of the system. Especially in a house with low heat loss.

Check out dsireusa.org for list of credits, incentives, rebates.
Uggggggh. It seems the timing in my life is always off. It looks like it will be spring when we install this system, of course. I don't think I can justify jacking up the credit cards just to take advantage of the credits. Hopefully they extend the $1,500 credit for another year or two.
 
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Old 10-07-10, 01:58 PM
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Calculating the heat loss of a house is not that difficult. If anyone is interested, here is how.

H = U*A*Dt where

H = Heat loss (BTU/hr)
U = Overall heat coefficient of heat transmission of the material (Btu/hr/Deg.F/sq.ft)
A = Area of heat transmission surface (sq.ft)
Dt = Temperature difference (interior temp exterior temp) F

The only unknown is the U which can be googled for your particular wall / roof / glass type

First you calculate the heat loss through all exterior glass surface.
Then the heat loss through the exterior walls (on North walls or prevailing winds we add approx. 10%)
Then the heat loss through the roof
Finally the infiltration heat loss (approx. two air changes per hour).

Then we add all together and we have the total heat loss of the house

Example for windows: (same principal for the walls and roof)
Lets say that we have a house with 10 double glass windows totalling 150 sq.ft glass area
The U for double glass is 0.45 (Btu/hr/Deg.F/sq.ft)
Dt = 70 0 = 70 F
Thus H = U*A*Dt = 0.45*150*70 = 4,725 BTU/hr)

Example for infiltration:
House volume = 30 long x 40 wide x 25 high = 30,000 cubic feet
With 2 air changes per hour, the volume of infiltration air that must be heated is
2*30000 = 60,000 cubic feet per hour (CFH)
The heat that must be supplied to raise the temperature of this air is
H = (CFH*Dt) / 55 = (60,000*70) / 55 = 76,363 BTU/hr
 
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Old 10-07-10, 02:50 PM
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Thanks for taking the time to type all that in... it is kinda informative to know the actual formula's for heat loss. Might give some a better understanding of what is actually occuring within the computer programs that do all the number crunching for you.

This is the way that it had to be done 'back in the day', and still the way that it's taught in the schools (at least the ones that I'm familiar with). Nowadays it's just too easy to load up a program and punch a few numbers in. Problem is that you don't really gain an understanding of the 'process' when you do that.
 
 

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