Results of Heat-Loss Test & Comparison to Calculated

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  #1  
Old 12-10-09, 07:46 AM
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Results of Heat-Loss Test & Comparison to Calculated

During last night, I used the cold snap and high winds to measure the houses's actual heat loss, based on burner run time. It came out to 88,700 Btu/hr.

This compares to a calculated heat loss, using the Slant/Fin program, of 106,000 Btu/hr.

So, the program's results are 20% high.

The burner fired 68% of the overnight hours and the thermostat was satisfied with 150 deg water temp. Since the radiation is obviously oversized, there would be nothing to be gained by adding outdoor reset. (I wouldn't want to run below 150 deg to provide margin above condensation in the flue.)
 
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Old 12-10-09, 08:01 AM
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Is it fair to compare those two numbers. I doubt your temp was down to the design temp the slant/fin uses. If/can you adjust your design temp for the slant/fin to match your actual last night.

And what size is your boiler? Just curious.

Bud
 
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Old 12-10-09, 08:57 AM
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Originally Posted by Bud9051 View Post
Is it fair to compare those two numbers. I doubt your temp was down to the design temp the slant/fin uses.

And what size is your boiler?
ASHRAE design heating temp for this area is -3.6 deg F (@ 99.6%) and +2.3 deg (@ 99%). I had used -3 deg for the Slant/Fin program. (That's a number plugged in by the user.)

Usually, when it is the coldest here, skies are clear and wind is relatively calm. But last night, the low temp was -2.3 deg and the west wind was fierce - reportedly gusting to 40 mph. (The longest dimension of the house is aligned N-S.) So, yes, I think the two numbers are comparable, and if anything, I suspect the "real" heat loss was impacted more by the wind than the wind infiltration assumptions built into the Slant/Fin program.

Others on this forum have reported that Manual J heat-loss programs over-estimate by, as I recall, as much as 25%.

Boiler output is 124,600 Btu/hr.
 
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Old 12-10-09, 09:20 AM
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If Slant/Fin is using the typical 15% pickup factor that will account for most of the overage (leaves 5% over). I did the same thing last Winter when it went to design temperature along with a Slant/Fin heat loss.

IIRC (I should find the thread) I found that Slant/Fin was about 19% higher then the measured loss.

Al.
 
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Old 12-10-09, 03:31 PM
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Could I figure out the real world heat loss by reading the gas meter? Assuming that no other appliance uses gas.
 
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Old 12-10-09, 03:47 PM
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Originally Posted by drooplug View Post
Could I figure out the real world heat loss by reading the gas meter? Assuming that no other appliance uses gas.
Yes, that's the preferred way to do it.

First, clock the gas meter with the boiler burner running (and only the boiler burner). That will give you the Btu/hr input of the burner. Convert to boiler output knowing (or assuming) the boiler efficiency.

Install a 24-V elapsed-time meter in parallel with the fuel valve.

Wait unitl a cold snap, and record elapsed time on the fuel valve overnight. That will allow you to compute measured heat loss.

Without the elapsed-time meter, read the gas meter when you go to bed and again when you get up in the morning. Adjust for boiler effieciency. Make sure not other gas appliances are using fuel.

There are a couple of advantages of an elapsed-time meter connected across the gas valve. First, you don't have to worry about disabling other gas-fired appliances. Second, if your gas meter is outside, you don't have to trapse around in the ice, snow, and cold.
 
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Old 12-10-09, 03:58 PM
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No, but you could come up with a reasonable figure as to how much heat you add during that particular day. This figure would include all the losses from the combustion equipment and delivery losses but not account for any gains such as solar or from cooking or just people radiating their own body heat.

I did a similar thing last night only I did it by timing the cycles on my forced air furnace. I determined that I was running on the first stage about 30 minutes of every hour, actually 15 minutes on and 15 minutes off. The furnace has an input on the second stage of 70,000BTUs and the first stage is 60% of the second stage along with it being an 80% AFUE furnace. The numbers are still approximate but I took 60% of 70,000 and got 42,000. Then 80% of 42,000 gives me the heat output to my duct system of about 33,600. Dividing by two (on only 30 minutes per hour) gives me a heat input to the house of about 16,800 BTUs per hour. (It's really less because my ductwork is not the best and it runs through a vented crawlspace.) Dividing that by the square footage of my house (1,500) gives me a heat input of 11.2 BTUs/square foot/hour at an outside temperature of about 28 degrees. (It has been unusually cold the last several days here.) Which, I guess, is not a bad figure. I wonder what it would be if I increased my insulation and sealed up my air leaks?

This also shows that my furnace is horribly oversized for the size of my house but I got the smallest two-stage, variable speed furnace Lennox made. I'm sure glad I made them take out the 90,000BTU furnace they first tried to install.
 
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Old 12-10-09, 04:21 PM
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Originally Posted by Mike Speed 30 View Post
Yes, that's the preferred way to do it.

First, clock the gas meter with the boiler burner running (and only the boiler burner). That will give you the Btu/hr input of the burner. Convert to boiler output knowing (or assuming) the boiler efficiency.
Would this be combustion efficiency and not AFUE?
 
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Old 12-10-09, 04:24 PM
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Originally Posted by drooplug View Post
Would this be combustion efficiency and not AFUE?
Combustion efficency, not AFUE.
 
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Old 12-10-09, 08:18 PM
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Good experiment. Keep going for it.

It takes longer than one overnight for a typical residential structure to equilibrate to design conditions. Couple of cloudy days and overnights at design is better.

Infiltration is commonly about 30% of heat loss. Wind loading goes essentially as the cube of the velocity. IIRC, Manual J is based on 20-25 mph winds. 40 is a lot more oomph.

But regardless, Manual J estimates are often over by 15-30%.

IMHO, an overabundance of radiation is a very compelling reason to use outdoor reset, even in partial reset applications. The simple ODRs are not expensive, and automate things toward higher overall efficiencies. Ample payback over time in many situations.

Also remember that max system dT is at/near design. At temps above design, dT is less. Point being that you can probably nudge down from 150 if your design condition dT is ~20F or less.
 
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Old 12-11-09, 08:34 AM
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Originally Posted by xiphias View Post
IMHO, an overabundance of radiation is a very compelling reason to use outdoor reset, even in partial reset applications. The simple ODRs are not expensive, and automate things toward higher overall efficiencies. Ample payback over time in many situations.
Thanks for all the info.

Now, on this ODR business, you're suggesting adjusting the setpoint to take advantage of the lower delta-T (and, therefore, higher return temps) when outside temps are warm, correct? But, let's say the house's inside temp is maintained at 70 deg, no matter what the outside temp is. So, during a heat call, the baseboards should emit the same heat, Btu/hr, whether it's frigid or balmy outside, right?

There is one thing that I think would apply: in cold weather, the thermostat demand cycles are longer - and in warmer weather, the cycles are shorter. During longer heat calls, the boiler water temps (supply and also return) get dragged down, even with the burner going. So Yes, in that situation, it might be good to boost the setpoint a little. And with shorter heat-call cycles, the setpoint could be trimmed a bit.

Are we saying the same thing here?
 
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Old 12-11-09, 08:58 AM
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Originally Posted by Mike Speed 30 View Post
But, let's say the house's inside temp is maintained at 70 deg, no matter what the outside temp is. So, during a heat call, the baseboards should emit the same heat, Btu/hr, whether it's frigid or balmy outside, right?
No. The colder it gets outside, the more BTU's you lose per hour. So the baseboards need to emit more BTU's. You do that by raising the water temperature. When it is warmer out, the baseboards need to emit less so you lower the water temperature.
 
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Old 12-11-09, 10:48 AM
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Originally Posted by drooplug View Post
No. The colder it gets outside, the more BTU's you lose per hour. So the baseboards need to emit more BTU's. You do that by raising the water temperature. When it is warmer out, the baseboards need to emit less so you lower the water temperature.
That's the theory behind outdoor reset. But I think we're talking about what happens without having ODR.

If the water temp is, say, 150 deg and the indoor temp is 70 deg, the heat emitted by the baseboards will be the same - doesn't depend upon outdoor temp.
 
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Old 12-11-09, 10:56 AM
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Originally Posted by Mike Speed 30 View Post
That's the theory behind outdoor reset. But I think we're talking about what happens without having ODR.

If the water temp is, say, 150 deg and the indoor temp is 70 deg, the heat emitted by the baseboards will be the same - doesn't depend upon outdoor temp.
Ohhhhhhhhhhhhhhh. Then I agree.
 
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Old 12-11-09, 12:35 PM
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Increased cycling hurts efficiency, and causes more wear on equipment. A good ODR control will do some optimization of differential, firing times, etc. to reduce cycling and increase overall efficiency.
 
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Old 12-11-09, 01:15 PM
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Is there a way to determine the best differential for a given system? Right now mine is set to 15 degrees. However, by the time the boiler actually fires, it's usually an additional 5 degrees. Does that matter?
 
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