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Hot water zone from steam boiler question

Norm Kerr
Norm Kerr Member Posts: 2
I have a question regarding supplying a hot water zone from below the water line of existing steam boiler.

The question is to confirm if I do have enough boiler to supply this new zone or not. I read Dan's newsletter article from Aril 3, 2000 on this topic but still want to get a confirmation from you guys about my specific situation before I proceed.

The house already has:
Boiler BTUh DOE Gross = 142,000
Steam Radiator area = 346.5 sq ft
Calculated steam radiator load = 124,750 (using a 1.5x pick up load factor) and ~83,000 BTUh without the pick up load.

The shop that I want to heat is 800sq ft, PEX tube in slab radiant with SIP construction (very well insulated) and the calculated input calls for a minimum boiler of 27,000 BTUh output. The shop will be heated all winter, usually only to ~60F but it would be nice to be able to get 70F for special occasions.

If I use my existing pick-up load to supply this new space I think it will be ok because my pick-up load is over 40,000 BTUs, right? (124,000 - 83,000)

If you all agree with this so far, then the next concern for me will be the 50~60 feet between my steam boiler and my shop manifold, so I will need to decide the correct diameter feed line and best insulation to minize heat loss and pumping losses. But this part is more straight forward once the first question is confirmed OK.

Thanks to anyone who answers,
Norm Kerr

P.S. to Dan,
I miss Freemont, he used to be able to answer anything like this for me directly here in Ann Arbor (I had the pleasure of having him check over my system before he passed away).

P.P.S to anyone,
does anyone know a good way to compare the operation costs of this versus a dedicated hot water source out in the shop?
Obviously the installation cost is lower to use existing equipment, but wonder how to compare the gas consuption of my existing 80% eff boiler Pick-up load vs. a new 95% eff. direct water heater like a Rinnai.


  • clammy
    clammy Member Posts: 3,094
    steam to radaint

    Personally i would not do it unless i used a heat exchanger like a small tube and sheel then a good brozen non cartridge circ. pump i would guess that this done properly would cost a little bit but it's a vast improvement from pumping all that bottom sludge out into your new radiant tubing not a good thing .Have you given any though to a small condensing boiler like a munchkin which is a modulating unit which you to also use with a indirect tank making it do more then one job .Oh yeah after the heat exchanger remenber all those other goodies ya got to have extrl water feed backflow high temp limit aquasta circ relay and some purge and isolation valved personally i would opt for a small condensing boiler peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
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