Oversized steam boiler?

wam525

So, I've been wondering what is the downside to an oversize steam boiler (other than the initial extra cost). I have an old Victorian house with a boiler sized for all the 19 radiators (≈ 900 sq ft) in the house. However, with the kids mostly gone, we don't use many rooms, and I've just turned many radiators off. The system seems to work just fine for the remaining radiators. So, while I can see how undersizing a boiler might make it work all the time, I'm wondering what the problem might be with oversizing a boiler.

ethicalpaul

It's just cost really, as long as it's not INSANELY big (like 4x). There's the extra cost on installation, but that's just spent once.

But a bigger boiler takes more fuel to begin steaming. That is waste because no heating can occur until steam production begins. Then when the call for heat ends, the boiler loses its heat to an unconditioned space where lots is lost (some makes it to the living space).

An oversized boiler is going to be more likely to have shorter cycles where it stops firing due to high pressure. But this isn't that wasteful...the water stays at 212 degrees while the pressure drops, then the fire comes back. Not a huge loss there.

But to prevent wasting fuel building unnecessary pressure, you have to have your pressure control set very low, and that increases the cycling. Again, I don't feel cycling itself is that big a deal...the only moving parts affected might be the gas valve (which is rated to hundreds of thousands of cycles), and maybe the automatic flue damper. Many people do find the short cycles annoy them, and that's valid enough--it's a constant reminder that you spent too much on your boiler.

In your case, I think it makes sense to disable radiators in unused rooms. Any waste from the oversized boiler is more than offset by not having to provide steam to those rooms. But maybe I'm wrong! It's very hard to measure this--so many variables.

dabrakeman

The biggest thing is you are heating more water each cycle to make steam than is necessary...$$$ I have a a very similar situation to yours and have grudgingly learned to live with it. Balancing the system will be more difficult and managing your temperature swings and cycle times will be more critical because you can quickly fill the working radiators and start then building pressure. If you don't do large setbacks and recoveries you will have less issues although there are ways to manage those by limiting cycle on times as well.

EdTheHeaterMan

Short cycling is the most common problem.

• Any burner takes time to reach the most efficient burn, called steady state efficiency.
  
• On some burners that can take up to 5 minutes or longer.
  
• If you need 25,000 BTUh for your home, and you use a 100,000 BTU burner to get that, then on the coldest day of the year you will only operate the burner for 15 minutes per hour.
  
• The problem is that the burn is not 15 minutes on and 45 minutes off. In most cases the burner only operates for 4 minutes than is off for 12 minutes, then on for 4 and off for 12 then on 4, off 12, on 4, off 12. In this situation, the burner never operates for 5 minutes so it never gets to steady state efficiency.
  
• You are always operating as some number less that the burners most efficient ability.
  
• These short cycles cause premature part failure. If a part will last for 1,000,000 cycles on average, then on most systems it will last pretty much forever. But if you are constantly turning it off and on over and over, then those cycles will add up much faster and perhaps fail in 10 years or less, not 50 years or more.
  

Other than that, there is no real problem

Jamie Hall

Aside from all that... there is one other consideration. Make sure that the rooms you have shut off stay above the dewpoint of your air at all times. Any sign of condensation, other than on the outer panel of a storm window, is or will become a real problem. Heated humid air from the part of the house you are using will get into those rooms and will, if given a chance, condense. On walls, furniture, books, wood trim... equals at best mildew and mold, at worst real damage.

dabrakeman

@Jamie's point is valid but perhaps like me you have turned off a couple of the kids bedroom radiators. Doors ajar and being upstairs they usually stay warm enough. Just makes managing the oversized boiler that much more critical.

wam525

Thanks for all the comments. it's sort of what i thought. The fact is that when about half of the radiators are shut off, the burner doesn't short cycle. There are some long runs to the most distant radiators, so maybe that helps. There's not much we can do about it now, but if we ever sell, at least the capacity to run all the radiators is there. i haven't timed the cycles, but when the whole family is home and all rooms used, i don't notice any obvious difference. The boiler is relatively new so it should have good efficiency.

dabrakeman

Probably OK as long as you avoid large setback/recoveries.

wam525

What do you mean by setback?

jesmed1

wam525 said:

What do you mean by setback?

Setback is when you program your thermostat for a lower temperature overnight to save fuel. Then when the "morning" program kicks in back to the higher "normal" temperature, the boiler has to run longer to "recover" from the lower overnight temperature in the house.

wam525

Oh, yes, I see. We drop the setting to 50 overnight, so it's mostly always off at night. But it doesn't seem to be a problem in the morning, with either all or half the radiators open.

ethicalpaul

That is a massive setback but if it works, it works.

The issues you might notice are that the boiler cycles on pressure a lot during the recovery. I don't think that's particularly harmful for a steam boiler, but others get freaked out.

dabrakeman

If it is quiet and working well then I guess no problem even if short cycling numerous times on recovery. Still more wear and tear on gas valve, auto damper (if you have one) and radiator vents.

Just trying to understand how oversized you are? You say your connected EDR is 900sqft? What is the sqft rating of the boiler? How much EDR in radiators are you shutting off?

ethicalpaul

Radiator vents don't see more wear from cycling on pressure

dabrakeman

They do if they are seeing wet steam.

ethicalpaul

if if if