i think hot water heat heat is the cleanest most comfortable heat but is also expensive because you need another system for ac.

are you sure that your cast iron radiation cant be converted
to hot water and just re pipe them? depending on how they are built some of they can be used on water. Later Paulbm

You have a steam boiler, not a furnace. Are you happy with the steam system? If so, stick with it.

A steam system, properly regulated, is a perfectly viable way to heat a home. I grew up in a large house heated with steam, and loved it. One concern is making sure that, if you DO convert to hot water and use the existing rads, that they are large enough to put enough heat into your home. Steam operates at roughly 215 deg. F., while hot water rarely goes above 180 deg. F.

Built when your house was, it's a toss up whether you have a one pipe or two pipe system. A two pipe system is easier to convert to hot water using the existing pipes and radiators, but the problem is still one of getting the radiators flushed and cleaned properly so that minerals and scale that has built up in the steam rads over the years doesn't kill your new system by plugging the pipes or the circulators.

Another possible option is to run a separate hot water exchange coil out of your current steam boiler and to a heat exchange blower that would deliver the heat to the rooms in your addition.

Another solution would be not to tie to the old system at all and go with a heat pump type system like a Mr. Slim. That would depend on where you are in the country. Probably not a good idea if you're in Massachusetts.

Some other things to consider. Steam radiators, which might run about 1 1/2# of pressure, would be asked to run at 12-20# of pressure. They may leak.

It's true that hot water is more efficient because it requires lower water temperatures, but junking a working steam system doesn't make sense. kframe has a good idea about using hot water to make warm air and you should be able to hook an A/C into that system.

Your plumber wants to install an indirect water heater, such as an Ergomax. These are nice if you have large demands for hot water. They also retain their heat loss to about 1/2 degree an hour in standby vs. 10 +- degrees per hour with a boiler. If you have average use of hot water, there is nothing wrong at all with the coil that can be found in a boiler. Your current boiler may even be able to be fitted with a coil.

Steam is most efficient in tall structures and less in large footprint low buildings.

If it were me, I know I'd go with the current steam system and come up with something else in the addition.

I know electricity is expensive, but have you considered an electric boiler? They hang on the wall and will heat your addition with hot water.

Thanks for all the advice. A couple of followups/answers:

1. I live in the Northeast, no heat pumps are not an option and electricity is expensive.
2. There is not enough capacity in the existing steam boiler to heat the addition, either through providing hot water to coils, or otherwise. That's why if I keep the existing steam system, I need to add a third gas devive to the boiler room, which seems inefficient.
3. I do have large demands for hot water (2 adults and 4 children) so I am also concerned that the existing 75 gallon tank is insufficient.

Does this change the recomendations? Assuming my radiators can handle hot water, am I better off converting the whole systems to hot water, or just using forced air for the new addition?

Thanks.

Stick with the steam that is the most confort as far as its not dry heat like a forced hot air. Why anyone would go to forced out air is just taking the cheap way out. A hot coil in an air handler will work better. Also have e you thought of radiant floor system at all or off the return of your boiler you can have a circulaing pump and add baseboard to the new room. if 75 gallon water heater isn't cutting it you may want to try a tankless water heater. Don't go with forced hot air you wont be happy.
Here would be my options that you can run by a good plumber.
1 tankless water heater to do radiant floor or coil on a air handler, with a heat exchanger you can also do domestic water and never worry about hot water. the neg side about this is tankles has to be on a maintnace program for scalling and general PM.
2 put a B&G pump on the return of your steamn boiler and use that hot water to do a basboard or new radiaters in your new addition.
for your a/c I would go witgh a ductless systeem if its only one big room.
here are the three manufactures I would go with
1. tankless I would go with takagi ( make sure its sized correctly)
2. air handler I would go with first company.
3. fujutsi if you where using a ductless.
If it was my house I would use high baseboard and heat it only to 140 degrees stick that on a tankless water heater and get my domestic hot water thrue that also. or radiant floor instead of baseboard. Use the fujitsu unit for the a/c of the house.

I have just removed my boiler due to asbestos and have been told that steam heat is obsolete and I should convert to hot air. Which would be more efficient and would the conversion mean all piping changed.

A modern hot air system would be more efficient in terms of annual operating costs. The down side is all the radiators and piping would be removed and you would have to add duct work. It may not make financial sense.

This thread is over 6 years old. If you have more questions start a new posting. I grew up in a house with steam heat so I find the topic interesting.