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# Heat Loss - 1952 house

#1
10-18-14, 03:37 PM
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Join Date: Oct 2008
Location: USA
Posts: 2,588
Heat Loss - 1952 house

I know we all like actual data points for comparison purposes. Here is one, based on the Slant/Fin heat-loss program:

Calculated heat loss = 106,000 Btu/hr (includes basement)
Living area, 2-stories (excluding basement) = 3,600 sq ft
Heat loss per sq ft = 29.4 Btu/hr sq ft

Input Data:
Outdoor design temp = -10 deg F
Indoor design temp = 70 deg F
Windows: combination storms

Basement heat loss (based on just exposed foundation) = 20,000 Btu/hr

Based on clocking the gas meter, I measured about 83,000 Btu/hr heat loss, which is 27% higher than calculated. (It is known that the Manual J calculations tend to over-estimate heat loss - so no need to add a safety factor.)

#2
10-18-14, 03:45 PM
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Join Date: Oct 2006
Location: USA
Posts: 19,710
I measured about 83,000 Btu/hr heat loss, which is 27% higher than calculated.
I know you meant 27% LOWER ...

And that fits right into my 'sanity check rule of thumb'.

My opinion is that Manual J adds that extra factor for a number of reasons.

The first being that one of the things considered is a constant wind which can happen I suppose. I've personally never had the opportunity to determine my ACTUAL heat loss with that constant wind.

The other would probably be to assist in 'recovery time' from steep thermostat setback periods when some extra horsepower is needed to bring the building back up to temperature.

I don't really believe that one would see much in the way of fuel saving by cutting out that 25%... the savings if they existed at all would probably be down in the 'noise' of the data.

Me prefers a little more horsepower than me needs.

#3
10-18-14, 04:11 PM
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Join Date: Dec 2010
Location: Galivants Ferry SC USA
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I know we all like actual data points for comparison purposes. Here is one, based on the Slant/Fin heat-loss program:
I did this a while back. Slant fin calculated me at 27k btu.

1200 sq ft x's 25 btu = 30k btu.

I believe I clocked in at 22k btu on the nearest coldest day of the year..

Was around 20% diffence..( typical fudge factor)

The fudge factor is what is input in the slantfin calc too. Probably why you got 27%

Ill have to find that thread...Trrop and I worked on it some

#4
10-18-14, 04:14 PM
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Location: Galivants Ferry SC USA
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I don't really believe that one would see much in the way of fuel saving by cutting out that 25%... the savings if they existed at all would probably be down in the 'noise' of the data.

Dont forget the IBR vs DOE sizing factor too....

#5
10-18-14, 04:17 PM
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Join Date: Oct 2008
Location: USA
Posts: 2,588
Yeah, I should have said "lower."

Around here, the most bitterly cold weather is usually accompanied by very calm winds. Maybe the worst case is around +10 deg and a strong west wind?

I too, wouldn't want to eliminate that 25% cushion in Manual J calculations. I have cast-iron baseboards, and in the morning when I first turn the heat up, the boiler temperature drops like a rock, at least temporarily, until it can catch up. Fin-tube units, maybe not so much?

That initial drop in morning boiler temp is reduced somewhat since my boiler is set up for warm start, and has a relatively high volume of water.

Last edited by gilmorrie; 10-18-14 at 04:56 PM.