boiler sizing???

singlepipefitter357

When replacing an old boiler(30 years) should you just put in one of the same size? This monster seems to be rather large for the house. 114,000 btu's and there is only a total of 104 feet of fin that is spit up into 2 zones. Plus, the 30 gallon indirect. If someone could explain to me how I should figure the proper size boiler for this house I would really appreciate it. Thanks in advance!

kpc_14

there are no...

shortcuts. You need to do the math by way of a heatloss analysis. upgradesd to wall insulation and windows over the years will change what was the heatloss then to now. kpc

Steamhead (in transit)

And the easiest way to do that

is with a laptop that you bring to the site. The software is free- click Resources above, then Heat Loss Calcs which will take you to Slant/Fin's site. You can download their free calc program if you have broadband- I'd recommend ordering the CD if you still have dial-up.

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Brad White_185

Kevin and Steamhead have your back

Always start with a calculated heat loss if a hot water system (as this likely is with fin-tube). If steam, you size based on connected radiation.

Here are a couple of postulates and comparisons you may want to consider:

1. There is no point in installing more boiler than you have radiation in a HW system. No point in generating in a boiler what cannot be distributed to the heated spaces.

Your 104 feet of fin, if it puts out 500 or 600 BTUH per LF at 180 degree water, can only emit 52,000 to 62,400 BTU's per hour to the space. Your boiler is twice that size, roughly. What a waste.

2. Radiation is not heat loss. Sure, you may now have twice the boiler compared to radiation, but I am not saying that the 52,000 to 62,400 BTU's I cited is an accurate heat loss. It is just the potential of what can be emitted to the space at a certain temperature of water supplied to it.

3. Your actual heat loss may well be that 52,000 to 62,400 BTU's. Fine. Size the boiler to meet that. My guess is that, given available increments, your boiler might have an 80 MBH input.

4. If your calculated heat loss comes to, say, 40,000 BTU's, that is OK too! You can run your system at a lower water temperature given a radiation surplus (to be determined).

For the indirect, so long as the boiler puts 100% into the production, I would not up-size the boiler to compensate. Chances are you will have enough margin within the incremental size to cover you. Even if not, the time to make domestic water passes quickly and you are back to heating before you know it, assuming you do not have a hot tub whirlpool or car-wash type shower.

A.J.

Well put

That he said!

singlepipefitter357

Thanks Brad!

well put Brad White! Thanks to all that have replied. I am now checking out the slant/fin program so I can learn to do these things myself. I'm beginning to see why people just buy a boiler that's way too big. I won't follow suit, though. Not with prices the way they are.

Bob Bona_4

man

you are GOOD Brad!