Two boilers instead of one?

wam525

Our old oil-fired steam boiler died, and one contractor suggested that, since the large boiler we have may not be available for many many months, we might consider putting two available smaller boilers (each with half the capacity) in parallel. Does this sound reasonable, even if the cost were not that much more?

Jamie Hall

Has anyone checked what size boiler you really need? While there are times when the old boiler is the right size, it's also unfortunately common that it is bigger than is needed. Replacing like for like is not always the best choice.

Fortunately, with steam, its really easy to do: add up all the connected radiation in the structure, and compare that to the EDR rating of the boiler. The size you need is that, or slightly less. Don't oversize.

Getting two boilers to play nice together is certainly possible -- but it isn't quite as simple as just putting in two boilers and hooking up some pipes.

ethicalpaul

Definitely have your current sq ft of steam (EDR) of your radiators measured before you even think of ordering a boiler.

I wouldn't even think of trying to run two boilers in a home, you're just asking for trouble and a lot more expense. That advice would have me looking for a new steam contractor.

EBEBRATT-Ed

With steam two boilers are more expensive and the piping can be tricky unless the installer is very good you could have problems controlling the water line. You need a lot more controls to twin two steam boilers together successfully which is expensive.

I would avoid it unless the building is quite large

The Steam Whisperer

Depending on the system, using two boilers can be much more efficient if the system is properly set up to take advantage of a variable input steam heating plant. I just installed a twin heating plant at my church 2 years ago. Its great to see the heating plant running during the week when much of the building is in setback with only a single 345,000 input boiler heating 15,000 sq ft right down to -5F. However, as others have said, the designer really needs to know what they are doing. These two 345,00 input steam boiler along with a 150,000 input hot water boiler replaced a 2,100,000 input steamer. .

wam525

Thanks, guys. All good comments. To me, two in parallel sound pretty tricky to set up especially if not absolutely needed. The total radiator sq footage is around 900, so on the higher end of residential.

Steamhead

Look at the Weil-McLain SGO-9 or Slant/Fin TR-60.

mattmia2

If it were hot water i'd say it gives you redundancy and better match to load on milder days but like others said, steam is a lot harder to do properly.

SteamingatMohawk

To better inform the HHers, what are the ratings on the boiler nameplate, including the model number? Compare that to your EDR and go from there.

Use the KISS method, don't make it unnecessarily complicated. Some day you will sell the building.

The Steam Whisperer

Could maybe even due a single 380.. when the castings become available.

mattmia2

A discussion about how well the EDR matches the building and what happens if we make the boiler a little undersized, can we still make it heat evenly might be warranted here too.

ChrisJ

mattmia2 said:

A discussion about how well the EDR matches the building and what happens if we make the boiler a little undersized, can we still make it heat evenly might be warranted here too.

The Steam Whisperer

With two pipe steam, that is certainly something to look at since it is so easy to "virtually downsize" the existing radiators with orifice plates. For buildings built after around 1905, it seems that the radiation is usually about 60% oversized for the original building load. If the building has had thermal upgrades that number can grow significantly.