Oversized steam boiler options in classic home

motordiscord628

We’re moving into a new house and the boiler is kind of beastly. The house is 100 years old with essentially zero insulation, 3100sqft of living space (basement and attic not included), and two pipe steam heat. The boiler was replaced in 2018 with a Peerless 63-05 236,500 BTU/196,000 BTU output unit. We’re in zone 6b/7a so cold but not frigid. The plan is to get the floor in the attic insulated with cellulose and the big picture windows are getting storms.

At napkin math of a generous 40 btu/sqft I’m at like 125,000btu boiler capacity. Since the system is two pipe (sneaking suspicion it was vacuum steam 50 years ago…) we can easily modulate the radiators and the plan is to eventually automate that with motorized TRVs.

My concern is that I’m going to be short cycling the beast, especially with the radiators trimmed in for unused rooms and the brand new insulation. Valid concern? Should I consider a modulating scheme? With a boiler this new, replacement isn’t really on the table but a multistage pressure triggered valve is. Thanks!

pecmsg

Steam

Is measured by EDR not BTU/h.
you need to measure the radiators and determine the total EDR to see what size boiler

Jamie Hall

Unfortunately you say nothing about how much radiation that boiler is powering — and that is what you need to know before worrying about how oversize it is — or isn't.

Steam boilers are sized to match the radiation they are powering, and are not concerned with the heat loss of the structure. It is quite possible in your situation that the boiler is actually not that far off. If we assume — and it is only an assumption, mind, and quite possibly wrong — that the radiation was sized correctly for the house and that you have, therefore, somewhere around 520 square feet of effective radiation (referred to as "EDR") then the ideal boiler would be rated for that square footage (may be shown on the rating plate as EDR or square feet of steam). Now boilers are commonly rated with a margin, called the "pickup factor". In use that factor has been found to be too large, in general, but if the piping isn't insulated it isn't. So I would expect a reasonably sized boiler to have an EDR, as I said, of about 520 square feet — and a net BTUh rating of somewhere around 170,000 BTUh. output.

So yours may be too big, but not overwhelmingly so — and there is no way of telling until we know how much radiation it is powering.

Will it run up to pressure and shut itself off before the thermostat is satisfied? Probably. If the above assumptions are all correct, I'd expect it to run a cycle of around 8 minutes on and 2 minutes off, starting after all the radiation is full of steam. This is no big deal, however. There is some loss in efficiency, but not that much.

The new insulation won't make any difference to the timing within a cycle; all it will do is make the length of a single thermostat call shorter, and the time between thermostat calls longer.

Cutting off some radiators, however, will make a difference to the cycling, since it reduces the amount of radiation the boiler is powering, and you will get shorter "on" times once the remaining radiation is filled.

Now be careful of pressure. No residential steam system should be allowed to go over 2 psig for the cutout, and around 1.5 psig is better. If this was originally a vapour system, then the cutout should be around 8 ounces (0.5 psi) and the cutin around 4 ounces. Extra pressure over that will get you exactly nothing in terms of more heat — all it will do is spend your money for you.

Hope this helps…

mattmia2

If it is a vapor system and still has the metering valves on the emitter, you could in theory set the stops on the metering valves to limit the steam to say half the capacity of the emitter and use a smaller boiler but this would have to be done with extreme caution and a lot of tweaking would likely be necessary to get it balanced and all heating.

EBEBRATT-Ed

Try it and see I wouldn't scrap a 6-year-old boiler without trying it. As others have mentioned the boiler has to be able to supply the installed radiation regardless of the house heat loss.

If your load is really 125.000 and you add the 1.33 pick up factor you will see your boiler may be not that oversized.

The Steam Whisperer

https://forum.heatinghelp.com/discussion/comment/1819972#Comment_1819972

We have found this is not difficult at all. Many old systems were intentionally set up with orifices so a small fire in the coal boiler would still heat all the radiators evenly. We have a number of small and large homes (60 radiators) and up to 30 living unit buildings ( around 160 radiators) which are now running with orifices with boilers much smaller than the radiation load and they heat evenly. You do sometimes have to be careful at cold starts, so you have to have enough capacity to prime some systems. We have 1 building that we bring the modulating burner to about 600,000 btu/hr to prime the system and then once the long main is heated to the end, drop back to the outdoor reset modulation level of the burner. This 41,000 sq ft building has 100 radiators and about 5000 Sq ft or radiation…1,209,000 btu/hr….. (original was probably about 5400) and the boiler runs on direct outdoor reset from about 350,000 btu/hr output in warm weather to about 1,000,000 btu/hr output @ -10F outdoor. It also used to be equipped with a vacuum pump, which we removed from service.

109A_5

Hello motordiscord628,

Figure out your actual EDR. Then go from there.

motordiscord628

I have to be honest, I was wondering about that steam, sqft number. It didn't make sense why a 3 dimensional substance would be measured in 2 dimensions. I'm used to dealing with mass when doing mass energy calculations but it seems like residential steam has a slightly different parlance. Going to have to crack open a Holohan book… thanks all, I'll figure out that EDR and get back to you.

ethicalpaul

Yeah "Net Sq Ft of Steam" or "Steam, sq ft" isn't great language.

The correct way to think of it is "Net sq ft of steam radiation" which as you can figure out, is exactly what EDR is (Equivalent Direct Radiation). But once you know, you know.

delcrossv

Since radiation is emitted from the surface, a 2 dimensional (sq. ft.)unit makes sense. Although I'll admit the usage (sq. ft. of steam) is a misnomer. As @ethicalpaul says, it's really square feet of radiating surface.