Sizing a steam boiler
There is only one way to size a replacement steam boiler.
Wrong ways:
Match the existing boiler(s) size
Go 1 size bigger than the existing, just to be safe.
See what fits in the boiler room.
Use a BTU per square foot rule of thumb.
Estimate based on the height of the building.
Ask people if they’re too hot or too cold in the building.
The Right Way:
The only way to properly size a replacement steam boiler is to perform a complete survey of the radiation in the buildings.
Other types of heating systems are sized based on the calculated heat loss of the building.
Steam boilers are sized based on the EDR - Equivalent Direct Radiation. This is essentially the total surface area of all radiators in the building.
Steam boilers should never be replaced with the same size boiler without doing a proper radiation survey.
Most steam boilers are oversized. Why?
In pre-war buildings, the original boilers were sized to heat the building on the coldest day of the year with the windows wide open. Fun fact - after the Spanish Flu, it was deemed healthy to keep the windows open, even in the dead of winter. Of course, it is healthy to have fresh air, but few people want to leave their windows open.
Each time your boiler was replaced over the last 50, 100 years, the contractor may have decided to go up a size on the new boiler. Better safe than sorry! (In his eyes).
Radiators are removed - apartments are renovated and the “unsightly, inconveniently located” radiators get removed. Now there’s less EDR connected to the boiler.
Why does it matter?
An oversized boiler:
Costs more to operate - wasted fuel.
Costs more to purchase - wasted CapX money.
Can cause banging and other issues in the system.