Extending two pipe steam

Bill Marsh

I have lived for 5 years in a 200 yr old house in New Hampshire with a 40 year old addition.

The house has 2 pipe steam in the old house. When we moved in, the steam didn't work at all (someone had removed the main vent) but now, even on the coldest day of the year, it produces too much heat, and on our coldest days the boiler runs perhaps 30% of the time. We have managed this by turning off radiators and opening windows; for this year I have installed thermostatic radiator valves.

The pipes are really well insulated. It looks like someone spent lots of money in an attempt to make the steam work.

The prior owners lived in the addition using its hot air heat. I hate hot air heat as it is dusty and makes me sneeze. We still run this furnace as the steam doesn't extend into the addition, but I figure I can extend the steam by adding 6 radiators, which would increase the capacity of the steam system by 33%. Then I would only have to run one furnace.

The problem is that if I add up the capacity of the radiators, then size the boiler per Dan Holohan's The Lost Art of Steam Heating, my current boiler is sized correctly for my current system.

Probably the technically correct thing is to replace the boiler with a bigger one. But this is obviously a considerable expense. And I am not convinced I need the extra capacity as my boiler, even on really cold days, only runs about 1/3 of the time.

I am considering instead adding these radiators as a zone with ball valves in the supply and return lines. (Maybe I only need them on the supply -- any comments?) Then, if on cold winter days the boiler can't keep up, I could just shut these valves and fire up the hot air. I really doubt if this would happen as the system acts like it has excess capacity. And if it happened too much, I could replace the boiler later.

I was wondering if anyone had any thoughts for me.