Heat Loss Questions

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Old 02-22-14, 02:16 PM
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Heat Loss Questions

I just did a heat loss calc with the Slant Fin software. This was for my parents place which is about 1800 sqft, oil boiler with baseboard. I'll post a couple images below. The heat loss came out to about 50k for the whole house, a lot lower than I thought, so I'm not sure if I did something wrong because given these results, the boiler seems way too big (as I now know is common).

Also, there's 62' of baseboard on the 1st floor, and 70' on the 2nd floor. The first floor is in 2 loops, not sure why, I thought you only did this if you exceeded 67' for 3/4" pipe.

The boiler says input BTU is 140k, and the IBR is 103k. Which one of these ratings is what should get compared to the house heat loss?

I should also add that there is a tankless coil in this boiler, and I'm not sure how many BTUs get added for this.

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Old 02-22-14, 04:49 PM
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Use the I=B=R net rating to compare to the calculated heat loss. That would suggest that your boiler is about 2x oversized. (Now, for more modern boilers, you would use the DOE rating.) The sq ft of your house is relatively small. How recently was it built and what about the insulation?

During very cold periods, time the duty cycle of the boiler (% time the burners are on). That will give you another estimate of the boiler's over-sizing.

Your Slant/Fin print-out didn't include the input data, e.g., design ambient temp, etc.
 
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Old 02-22-14, 05:44 PM
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Your boiler has three sizes depending on nozzle selection. Verify then nozzle size. When you do a heat loss you always use the DOE not the NET. The difference between the DOE and NET is jacket and piping losses. These are normally within the structure and the heat loss is buffered by about 20%. If you have 3 or more zones the boiler can also be reduced by another 10%. So using the DOE makes all the sense. I have been using the DOE for over 30 years with no problem. Do not add anything for the DHW coil as it is prioritized by the aquastat. In fact, do yourself a favor and get the new boiler less coil. This is the most expensive way to make hot water.
I would guess your calculation is correct. My home us 30 years old 2800 sq ft and a heat loss around 40k.
 
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Old 02-22-14, 06:58 PM
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What is the DOE rating, is that the 119k rating in my case? It seems close to the input rating multiplied by the efficiency. I used 0 deg and 70 deg for temps. When the heat is on, the boiler doesn't cycle off... Which is weird for being so oversized. House was built in '75... Only 2 zones.
 
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Old 02-22-14, 07:22 PM
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When the heat is on, the boiler doesn't cycle off... Which is weird for being so oversized. House was built in '75... Only 2 zones.
What kind of baseboard? Copper finned, cast iron?

When you say does not cycle off do you mean the t stats satisfy before the boiler kicks off on hi limit? (180F)
 
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Old 02-22-14, 07:57 PM
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Yeah it's fin-tube. Yeah the thermostats satisfy then the boiler shuts off.
 
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Old 02-22-14, 09:33 PM
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What the temp and pressure gauge read on a call for heat?

So the boiler never reaches 180F?

Your way over ratiated is probably the reason. You have 75k btu of baseboard for a 45k btu heat loss...

If and when a new boiler goes in pipe it P/S piping.Boiler will heat up better.

Trooper has that issue I believe with his boiler.
 
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Old 02-23-14, 06:20 AM
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Trooper has that issue I believe with his boiler.
Yeah, I think the highest water temp all winter has been a couple times when it was close to zero overnight that the water temp hit 160F. Outdoor reset on this system has no effect because the target temps are always well above the temp requirement of the water needed to heat, and that requirement to heat is low enough that I don't dare risk 'tweaking' the curve on the ODR because the temps will be low enough then to cause too much condensation in the boiler and chimney.

Not really an 'issue' though, as there doesn't appear to be a major (there is some) condensation issue inside the boiler. The MPO is tolerant of 'lower' return water temps due to it's design.

The condensation in the chimney is another matter though... even with a 4" INSULATED stainless liner INSIDE a 6" INSULATED manufactured chimney, there is still considerable condensation due to the lower flue gas temps from a higher efficiency boiler.

But we digress from the original thread!
 
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Old 02-23-14, 07:45 AM
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Actually the boiler fluctuates between about 175 and 205 I believe, even though the aquastat settings are 160/180... maybe the temp gauge is bad, I'll have to install another one to really verify it.

1. I'm assuming installing 'more' baseboard than the SlantFin software suggests would heat a room faster, and that's why an installer would do this? Obviously when doing this, your heat loss doesn't matter at this point and you select the boiler based on how much baseboard you've installed?

2. Is P/S piping when you have the closely-spaced tee and the boiler return has its own circulator to keep the amount going into the boiler the same at all times?
 
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Old 02-23-14, 07:51 AM
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1. I'm assuming installing 'more' baseboard than the SlantFin software suggests would heat a room faster, and that's why an installer would do this? Obviously when doing this, your heat loss doesn't matter at this point and you select the boiler based on how much baseboard you've installed?

Years ago BB was sized to the boiler from what I know.

Yes it would heat a room faster at 180F. The benefit of being over radiated is you can run lower water temps to produce the same heat needed.

Example. I am over radiated and heated my home successfully at 150F boiler water. Even when it was 0F out this year... Size the boiler on heat loss not radiation.


2. Is P/S piping when you have the closely-spaced tee and the boiler return has its own circulator to keep the amount going into the boiler the same at all times?

Basically yes....


The problem you face, And trooper can offer aditional advice, is an oil boiler is only so small.. I believe the smallest is around 70K btu...

No gas in the area?
 
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Old 02-23-14, 08:26 AM
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No gas, and the gas company says they have no plans of bringing it there either. I'll definitely take these things into account when this boiler needs replacing.
 
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Old 03-02-14, 05:26 PM
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When you do a heat loss, do you include unheated areas (basement, garage, etc.)?
 
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Old 03-02-14, 05:47 PM
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Are you using the slant fin heat loss calculator found in a sticky at the top of this forum?
 
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Old 03-02-14, 06:55 PM
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I am...............................
 
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Old 03-04-14, 09:10 AM
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Is that a yes? Or did I say something stupid?
 
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Old 03-04-14, 09:59 AM
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Yes include unheater areas but for the basement you would input the correct data for the floors above.

Do it both ways and see the difference you get...

Its not like its an outside wall with no insulation....
 
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