Sizing boiler off installed emitters?
Miilking a tenant's 1970's Bryant 3 section hydronic boiler, but probably coming up to its end of service. Is it okay to just add up the length of baseboard at 180 degree SWT and make that the boiler size? Any slop I should add in?
Note, apt. is comfortable with 180 SWT and i don't notice short cycling. Thinking a Peerless MI-03 as of now.
Comments
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The boiler can't eject any more than the emitters can emit, so there is no reason to size any larger than that. However, a heat loss calc is still a good idea to possibly be able to downsize the boiler and gain some efficiency.
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if you are supplying 180, use the 170 degree column for the fin tube output, that will be the average temperature in the loop, assuming a 20 degree delta design.
The number will be a little more accurate this way.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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Make any sense to measure the present return temp? I can tweak that with pump selection I guess. Go with 4 gpm? 1 zone, 2 loops. I can guesstimate the longer loop length. All 3/4.
Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.0 -
most designers use the 1 gpm row.
4 gpm options gives you a bit more output, it may require a larger pump to get that, depending on the pipingBob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
It's probably closer to 1gpm given the pump that's on there. Little Sunstrand.
Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.0 -
This is correct, but be sure to read the fine print. I've seen output charts where the column headings say "aquastat setting." So the column for 180F aquastat setting is already giving you 170F average sent water temperature.
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The proper way to size a hot water boiler is to do a heat loss calculation for the space. The emitters likely are oversized and the system will heat more evenly and run longer cycles with a boiler that matches the heat loss. The only reason that steam boilers are sized to the installed edr is because you have to more or less fill the whole system with steam to heat all of the emitters (with careful balancing and with 2 pipe systems it can work with less but this is the reasoning to size to the edr).
You should double check that the emitters can output your calculated amount of heat but you don't need to make it bigger than the heat loss.
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Yes, I get that. Seeing how I only have to deal with one property with hydronic, I'm just trying for good enough. I do have wiggle room with the SWT.
Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.0 -
Size it for the baseboard and don't add the 1.15 pu factor if you don't want to. The boiler can't output more than the radiation needs. Even if that makes the boiler oversized how much will that be in the grand Scheme of things.? Not very much.
Size the boiler for 170 average water temp. You can always raise the water temp.
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it is wise to read the footnotes if you know the brand and model of the board. There could be a pick up factor listed in the notes
This graph or formula will remove it to get a more accurate output number.
You adjust for output with different air temperatures entering the fin, if you want to take it that far😗
More info here
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
i think if they wanted to take it that far they'd do a heat loss calculation…
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Awesome, Thanks Bob and everyone.
Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.0 -
There will be guesstimations all along the process. A perfect heat load number for an existing building is not so easy to nail down.
Insulation type and installation quality, air leakage, how you use the home, internal gains from people and equipment. Outdoor temperature, wind speed and direction, solar gains, shading. These are all factors that vary from building to building. Don't make yourself crazy trying to get down to the exact BTU load number.
Ideally you want to run the heat emitter at them lowest possible temperature at any given time. Match the boiler input to the ever changing load. Outdoor reset and modulating boilers get you as close as you can get.
A fixed output boiler will be oversized 80% or more of the heating season.
You can develop a graph like this from your local weather station, if this type of information interests you.
Assume design day condition of 0° for this location, notice how many hours a year you need the full boiler output.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
~40,000 btu/hr based on emitter length @ 170. Smallest Peerless is 52k net. Still oversized but a lot better than what's in there now: 90k in/72k out 😨
Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.0
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