Combination steam and air heating and Smart TRV

pabel2001

We have a 1930s house with cast iron radiators and a steam boiler that heats much of the hose. We are adding an AC capability with natural gas fired furnace for warm air as back up plus for the areas of the house that are not served by the steam radiators.

Question is whether it makes sense to out in an integrated control system, potentially using sensors and/or smart TVRs on the steam radiators.

Any thoughts or experiences appreciated

Jamie Hall

Will your new system also heat the areas of the house which are now served by the steam system? If so, you may want to arrange the thermostats for the two systems so that the steam does not get called unless the new system can't keep up.

TRVs on steam systems are a little problematic, for a whole variety of odd reasons.

jumper

Jamie Hall said:

TRVs on steam systems are a little problematic, for a whole variety of odd reasons.

Presumably you speak for one one pipe? Hear good things for two pipe.

Jamie Hall

Actually they can be made to work and work well on both. Just not always quite the way people think they might.

On either one, one has to remember that they can reduce the heat -- but not increase it, unless there is some fancy way to tell the boiler to turn on when they open (on hot water, for instance, you can do that with a smart pump -- if a TRV opens, the pump senses that and comes up to speed and the boiler senses the temperature and starts. Doesn't work that way on steam).

One also has to remember that the "V" in TRV refers to a valve for two pipe and a vent for one pipe.

On one pipe there is an interesting problem They can reduce the amount of heat the radiator can produce -- but they can't shut it off until the pressure in the system drops far enough for them to open and let air back in. Then they can prevent the radiator from heating any more until the room temperature drops, since they would be closed and prevent the air from getting out and the steam from getting in.

Thus, on one pipe, it is essential -- if TRVs are to be used to really control the heat -- that the boiler shuts down from time to time and lets the pressure drop to zero. Seems weird, but that's the way it works.

If one is aware of the quirks and limitations, they're OK.

motoguy128

You have to be careful not to add too many TRV’s. TRV’s are essentially non-calling zone valves. The master zone where the thermostat is needs to be selected carefully, and ideally needs to be one of the coldest rooms in the house and on the north side of the house without much solar gain.

If you have too many TRV’s the system will short cycle off pressure limits. If the boiler is already sized with EDR+ pickup, you will likely have issues. IF you use TRV’s IMO boiler should somewhat closely match boiler output with minimal pickup and venting needs to be reduced as slow as possible, main vents maximized.

I have TRV’s on approx. 40% of my EDR. But my boiler only has about 10% pickup with current installed EDR. I moved 2 radiators to hot water last spring.

jumper

>> short cycle off pressure limits<<
perhaps a bigger range between cut in & cut out?