Steam convectors with Shell & Tube

Double D

In sizing a replacement boiler, how much should I add to the total square footage of the steam convectors for this B&G shell & tube? Model # 5-260-06-060-002. Total btu's of hot water baseboard and fan forced convectors is 280,000. The baseboard and fan forced convectors are being used for perimeter heating at all of the entrances. The Webster convectors are being used to heat another section of the building.

Steamhead

Why not propose two boilers- one steam, one HW? That's what I'd do.............

Double D

That sounds like a better idea. Definitely easier on servicing. Total sqft of steam radiation is only 354. Existing boiler is grossly oversized. 800,000btu input.

EBEBRATT-Ed

@Double D

The shell and tube rating varies with steam pressure and a lot of other variables, but If your hot water load is 280,000btu/hour that's what I would add as long as your HW load is accurate.

280,000/240 (btu's in a sq ft of steam)=1167 square feet of steam=280 lbs of steam

Double D

@EBEBRATT-Ed I'm confident the hot water load is accurate. Add the BTU HW load to the boiler input?

EBEBRATT-Ed

If your steam load is 354 sq ft and the hot water is 1167=1521 total so you need a boiler that will do 1521 sq feet.

1521 sq feet x 1.33 piping and pick up=2022 sq feet. Assuming the boiler is 80% efficient you need 2022 x 1.25= 2528 sq feet of input. 2528 sq feet x 240 btu/sq foot =606,600 input.

But you can just pick a boiler based o how many sq feet it will heat you need 1521 sq feet

mikeg2015

Better off with 2 boilers. Can run the HW at 86-95% efficient Most of hte heating season with outdoor reset.

Steam boilers run different duty cycles than low mass radiant. Do you plan on having zone valves to isolate the steam radiators? Or do those steam radiators have TRV’s?

If two boilers, I’d round down to 125k input if its well insulated. 150k if not. 125k if there are TRV’s.

Then go back and check if it’s 280k is the heat loss of the building or the total of the emitters and determine design temp and consider diversity. A 300k input commercial condensing boiler if you hav some diversity of loads (which you likely do) or a 400k if you need the pickup and 160+ water temp on a design day.

Also,, fan convector underperform when dirty.... which they are much of the time. So consider that. Hate to have you put n a 400k boiler and it never fires over 50% except on startup or morning warmup.

Double D

Thanks for all of your input. All three of you were very helpful. There are zone valves on the piping now for the shell and tube and the radiators. All but one of the steam radiators have TRVs. 280 it the total of the emitters on the water side.

mikeg2015

A building load calc might save some money in the boiler. You can also take gas bills and work backwards. I’d bet actually peak demand is probably 200k. But some commercial buildings like schools use weekend and evening setbacks and need extra pickup.