Heat Loss calculations


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Old 03-19-06, 02:32 PM
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Heat Loss calculations

I am plannign a major heating system rebuild in my house this summer (see the converting from steam to hot water thread), and am trying to do the heat loss calculations as shown in the "Modern Hydronics" book. The problem is, I have no idea what's in my walls.

I know the inside is plaster over wood lath, with paneling in some rooms, and I know that the outside is vinyl siding, but I have no clue what, if any, insulation is in there. The house was built in 1928, but has had several major refits, most recently in the mid-90s (this is all by previous owners), so it could be anything in there. What can I do? Is there any way to measure this without ripping open walls? In a few places (attic, basement) I have access to look inside, but in most of the house, I don't.

Thanks,
Juliean.
 
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Old 03-19-06, 04:21 PM
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Pick a minimal value - how well its sealed is probably going to have a bigger impact in the heatloss calcs.
 
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Old 03-19-06, 04:45 PM
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Insulation

When in question, you can remove an electric box & replace it with an "old work" box. Another option is to enlarge the hole under the cover plate a little & install an oversized cover plate.
 
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Old 04-04-06, 12:58 PM
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So the next question is where do I get the 97.5% design dry bulb temperature for my area (Nassau County, NY)?
 
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Old 04-04-06, 01:18 PM
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Try this:

http://www.engineeringtoolbox.com/us...ity-d_296.html

And I found this at www.crownboiler.com back when I was messing with heat loss calcs myself. I think these are purported to be ASHRAE outdoor design temps.

NEW YORK
Albany 0
Auburn 2
Binghamton 1
Buffalo 6
Glens Falls -5
Kingston 2
Massena -8
New York City 15
Oneonta -4
Oswego 7
Plattsburg -8
Rochester 5
Rome -5
Schenectady 1
Suffolk County 10
Syracuse 2
Utica -6
Watertown -6

You might also ask locally, e.g., your supply house.

If you really want to go for it, have a blower door test done, and some IR thermography too. Buddy of mine had both done by a heating consultant for his ~1830 house. Really showed where he needed to tighten the envelope. Also showed where the blow-in insulators missed about 30% of the stud bays (they're coming back...).
 
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Old 04-04-06, 08:50 PM
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Well, the 1st link says 0F, Crown (which may be ASHRAE) says 15F, I ran an analysis on the straight weather data, and I'm getting 21F, and the Dept. Of Agriculture says that average minimum teperatures are -5F to 5F.

So what am I supposed to use as my design temperature? That's a significant range - I did the heat loss calculation for just my 1st floor, and the floor requires 45000 BTU if the design temp is 0F, but only 35000 if it's 15F...
 
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Old 04-04-06, 09:33 PM
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weather underground is a good source

look at the lowest temps year by year
 
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Old 04-05-06, 08:12 AM
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OK, use this from the Hydronics Institute, which is definitive.

http://www.gamanet.org/GAMA/inforeso...oss%20Form.pdf

0F is too low for your area. I'm more or less in between Boston and Providence and a common ODT here is 5-10F. I haven't read up on how they figure these temps, but it's probably based on a fairly long time series at each location with averaging over coldest months, etc. Looking quickly at some cities I know, it would also appear that cities at the same latitude but different distances away from the coast are affected by the ocean (Atlantic City 13, Vineland 11). Also some urban heat island effects (Newark NJ Airport 14, New Brunswick 10; or NYC environs 15, Suffolk County 10). That they use a 97.5% confidence interval makes these tables pretty conservative to start with. Being below your ODT 2.5% of the time over a long time series average makes it pretty unlikely you'll ever see much in the way of real ODT for any length of time.

And now an opinion about engineering.... I'm not an engineer, but know enough about it to say with reasonable intelligence that "safety factor" is all over this stuff. Manual J is a conservative estimate of heat loss. Pump curves are a conservative estimate of pump performance. Boiler output is probably conservative, too. I've been conversing with a real hydronics pro lately, and he gave me a good example. See if I can translate properly. Consider a modulating boiler. On a design day, it should modulate its output (and thus the radiation output of the system) such that it matches the heat loss. So it should run 100% of the time on a design day because the heat input exactly matches the heat loss. Yet he's never seen a system like that do any more than about 50% runtime, so even at a theoretical perfect match between input and loss on a design day, it's still oversized. If these systems and their specs did not have this kind of safety factor, then during the occasional "winter cold spell," _everybody_ would be complaining they don't have enough heat. Some people do, but I bet if you did a heat loss calc and looked at their radiation and boiler, you'd see that they have plenty of boiler output, but insufficient radiation to match the heat loss. Give 'em a can of Great Stuff, tube of caulk, some insulation and tell them to improve the envelope. Or add more radiation. Better investment in the envelope, though.

If you're still unsure, call someone at Brookhaven National Lab and see what they use. Lots of energy testing stuff done there. Maybe the authors of that paper?

FWIW, my 2100 sf house at 10F ODT has a heat loss of 44k BTU/hr, or about 21 BTU/hr per square foot. My limited understanding of "tight" construction is loss at around 15-17 BTU/hr per sf. "Loose" is around 30 BTU/hr per sf. Using an approximation, if your house is 3000 sf, then at 35k BTU/hr for your ~1500 sf first floor you're at 23 BTU/hr per sf.
 

Last edited by xiphias; 04-05-06 at 08:31 AM.
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Old 04-05-06, 12:14 PM
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I am an engineer and everything that xiphias is true.

By installing a bit more radiation than called for you are able to run a lower temperature on your loop and therefore save a bit on your energy input.

Everything in a heating system, especially a hydronic system, interacts with everything else. I like to design to a system temperature of 170 degrees and then if it gets really cold i can jack up the temperature of the boiler to compensate.

All heating systems are usually designed with certain "fudge factors" including an efficiency figure, fouling factor, heatloss of building and outside "design" temperature. Most engineers throw in a fudge factor when doing heat loss calculations over and above what is really necessary.

Generally speaking the "published" design temperature factors are (in my opinion) excessively low. I would look at the lowest temperatures experienced in your local area over the last five, ten and twenty years to come up with a more reasonable figure. Published "design" temperature for the Seattle area (where I live) is 0 degrees F. but in reality the lowest temperature I have experienced in the last fifty years was about 5 degrees. The real truth is that temperatures below 20 degrees are quite uncommon and it may be several years between reaching these low temperatures.

If one designs to the published "design" figures rather than a more normal, or expected, low temperature you will have an oversized system with its inherent inefficiencies running most of the time. This can be somewhat alleviated by the use of modulating burners but generally speaking a boiler will have maximum efficiency when operating at about 80% or so of its ultimate firing rate.
 
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Old 04-09-06, 08:15 PM
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Originally Posted by furd
I like to design to a system temperature of 170 degrees and then if it gets really cold i can jack up the temperature of the boiler to compensate.
I would recommend designing to a lower system water temperature. When using a condensing boiler, you should aim for the 140* area on a design day. This way you will run at a much greater efficiency. When using ODR, on a normal winter day, you could be running as low as 100*, which would give you 98% efficiency.

Even a standard boiler could be run closer to 150-160 and see higher efficiencies.

With energy prices that are just going to keep jumping up, you want to acheive the highest efficiency possible. Of course sealing/insulating the house is the biggest step and should be done first.



The design temp for my area listed in the GAMA brochure seems to be right on. I am near Milwaukee which is -4. This winter our all time low at my house was -17 (only happend for a couple of hours on 2 separate nights). There were a couple of nights where it got down to -5. The rest of the winter was at or above 0.

The best thing to look at is the wunderground site and see what your area actually did. Just remember that in many areas of the country it was slightly warmer than average this year.

Also, since you're not likely to find a boiler that will exactly match your heat loss, you'll probably be slightly oversized no matter what. The only case would be if your heat loss came out to be just a little bit higher than a partucular size boiler. In this case I would use the smaller one since there are certain "fudge factors" built into the calculations.
Michael
 
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Old 04-10-06, 01:20 PM
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Originally Posted by aemeeich
I would recommend designing to a lower system water temperature. When using a condensing boiler, you should aim for the 140* area on a design day. This way you will run at a much greater efficiency. When using ODR, on a normal winter day, you could be running as low as 100*, which would give you 98% efficiency.

Even a standard boiler could be run closer to 150-160 and see higher efficiencies.

With energy prices that are just going to keep jumping up, you want to acheive the highest efficiency possible. Of course sealing/insulating the house is the biggest step and should be done first.



The design temp for my area listed in the GAMA brochure seems to be right on. I am near Milwaukee which is -4. This winter our all time low at my house was -17 (only happend for a couple of hours on 2 separate nights). There were a couple of nights where it got down to -5. The rest of the winter was at or above 0.

The best thing to look at is the wunderground site and see what your area actually did. Just remember that in many areas of the country it was slightly warmer than average this year.

Also, since you're not likely to find a boiler that will exactly match your heat loss, you'll probably be slightly oversized no matter what. The only case would be if your heat loss came out to be just a little bit higher than a partucular size boiler. In this case I would use the smaller one since there are certain "fudge factors" built into the calculations.
Michael

My design temperature of 170 is assuming baseboard convectors, the efficacy of baseboard convectors falls off pretty rapidly below 140 degrees F.

Of course if you are using radiant panels (tubes in the floor, walls or ceiling) you may very well get by with temperatures much lower than with baseboard convectors. Even fan-forced cabinet convectors and unit heaters operate quite well with 120 degree water.
 
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Old 04-11-06, 12:12 AM
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I have baseboard only in my house. It was built in the late 1800's but was remodeled in the late 60's. Not too leaky, but not as tight as new construction. I have R-13 walls, R-19 ceilings, and pretty good Anderson wood framed windows. My upper floor is 1000 sq ft, and has a heat loss at -4 of 21,000 BTU. My downstairs is 1600 sq ft and has a heat loss of 31,800 BTU So total of 2600sq ft and 52,800 BTU.


The published output for Slant Fin Fine/Line 30 is as follows:
220* - 890 BTU
210* - 810 BTU
200* - 750 BTU
190* - 680 BTU
180* - 610 BTU
170* - 540 BTU
160* - 480 BTU
150* - 400 BTU
140* - 340 BTU

Plotting these as a curve on a graph and you can estimate outputs for lower temps:
130* - 280 BTU
120* - 220 BTU
110* - 170 BTU
100* - 120 BTU
90* - 80 BTU
80* - 40 BTU
70* - 10 BTU
65* - 0 BTU

In each of my rooms, the outside wall is lined with baseboard (rooms are each on a zone valve so they don't have to be sized in relation to other rooms, but they actually are pretty close to being balanced). Total element for the upstairs is 61 ft, and for downstairs is 90 ft. On a design night of -4 degrees, my water temp is 145 degrees and my zone valves still occasionally cycle off! On a typical 30 degree day, I'm running at 115 degrees.

So baseboard is very capable of running at low water temperatures.
 
 

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