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Thermostatic radiator valves vs. motorized zone valves

I’ve been away from steam (and you nice folks) for about six years, but now I’ve found myself in an apartment building with a 2-pipe steam system. Eager to exercise a little more control, I started looking at options.

Having used thermostatic radiator valves very successfully on a 1-pipe system, my first thought was to go that way here. I even thought I could add electric operators like Danfoss TWA or ABRA, but the specs for those recommend against steam use beacuse high temperatures can keep the wax molten, which keeps the valve from closing.

My next thought was to go with a more traditional motorized zone valve like the familiar Zone Sentry or PopTop. But I remembered from my younger, steamier days that Dan told everyone not to make that mistake (and again here). So I wonder: what gives?

Why are TRVs OK but motorized zone valves not OK?

For that matter, what makes a motorized zone valve any more of a problem than fully closing/opening a radiator’s supply valve?

Or are all of the problems with motorized zone valves strictly about 2-pipe gravity return systems? There’s a condensate pump on the returns in my building. Do I get a pass?

Hello again and thanks!


  • Jamie Hall
    Jamie Hall Member Posts: 17,060
    If you read those two articles thoroughly, you will see that @DanHolohan is referring to the bigger motorized zone valves which might be used to control whole sections of a system. These can, indeed, be problematic although there are ways to cope with the problems. A motorized TRV for one radiator, however, should not give trouble. The dry return pressure -- which is what needs to be maintained will be held by the other radiators feeding the return.

    If some rooms are too warm, keep in mind that with two pipe steam it is perfectly alright to partly close the feed valve on the radiators which are overenthusiastic. That might be a cheaper solution...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • superMARKet
    superMARKet Member Posts: 87
    Thanks, Jamie, that’s good to hear. I see now that Dan was talking about zone valves in the boiler room and zoning off entire wings of buildings.

    A quick follow-up: why isn’t a vacuum breaker needed in this configuration? I’m thinking that when the supply valve (whether it’s a TRV, motorized zone valve, or just a hand valve) closes fully against a steam-filled radiator, some of that steam is going to condense and put the radiator into a vacuum (relative to the system, at least). As soon as the trap opens, the radiator will see some higher pressure via the return. I bet you’re going to say “a vacuum breaker wouldn’t fully help anyway” and “it’s not a problem,” but I’m wondering WHY it isn’t a problem.

    (Maybe it’s only a problem if another radiator sharing my return has a faulty steam trap. But in that case, would having a vacuum breaker help protect my own trap?)
  • Jamie Hall
    Jamie Hall Member Posts: 17,060
    On a two pipe system, keep in mind that the dry return should be vented. The radiator trap isn't designed to hold a vacuum, so all that will happen is that some air may be pulled into the system and relieve that local vacuum (unless your main vents are Hoffman 76s, in which case the vacuum will pull into the whole system -- not a problem).

    I don't think that a failed open trap elsewhere on the return would damage your trap -- but it might give you some steam ghosting into your radiator on occasion, if the radiator vacuum goes deep enough to pull the trap open. On the other hand, as I'm sure you know, that failed open trap may raise the return pressure enough so that the radiators on that return heat poorly, if at all, anyway!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 15,028
    The Caleffi Z-one zone valve is rated for hot, chilled and low pressure stem, if you need a small spring return valve up to 1-1/4" and 7.5 Cv.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream