how does the pressurestat work together with the thermostat

jackmeyer1

Let's say the boiler was off, room temp is 63degs, and I raise the T-stat to 68 degs.
Pressurestat is set for 0.5 cut in and 1lb cutout, so 1. 5 lbs pressure. The pressuretrol reaches cut out and shuts off the boiler at 1.5lbs, rests, and cuts back in at 0.5lbs. Will it keep repeating this on and off cycle until the T- stat is satisfied ?

Jamie Hall

That's exactly what it will do. The presssuretrol's purpose, when it is acting as a control device (not a safety device) is to ensure that the boiler is producing exactly as much steam as the radiators can condense. Any more and you are wasting fuel. Any less, and not all the radiators will be as hot as they can be. It does this by turning off the boiler from time to time to allow the radiators to catch up.

mattmia2

Yes. If the boiler is sized so it only produces as much steam as the system can condense it will never build pressure. The pressuretrol/vaporstat is a safety control to tame an oversized boiler. Ideally it would only shut down if there were a problem with the system, with a perfectly sized boiler.

Jamie Hall

Big if, there, @mattmia2 . But we've been around this barn before. And you are quite right, of course -- if the boiler is perfectly matched to the radiation it will never build more pressure (a few ounces) than is needed to move the steam.

Problem. That is a practical impossibility. Why do I say that? Simply because the condensing capacity of the radiation varies with the temperature in which it is located. Now I will quite happily agree that the installer should endeavour to ensure that the boiler is as closely matched to the radiation as is reasonable -- I would be (and am) very happy with Cedric, for instance, which takes almost an hour to reach that point if the house is at 60 (more like 45 minutes if the house is at 65). It is not always possible to get that close, however, even with extensive fiddling which most people don't want to do (or may not be able to -- TRVs on the system throw the whole project into the dumpster).

Solution. Modulate the boiler. Now if you can dial down the fire in response to very small pressure changes, so much the better. That's what some of the old coal fired boilers did. They didn't do it very efficiently (the efficiency on low fire was really abysmal). Most residential size steam boilers can't do that. So the next best way is to pulse modulate the boiler -- that is turn it on and off so that the average output is exactly what the radiation needs.

And that is exactly what the pressuretrol or vapourstat does, when it is used as a control device. It's elegant and simple.

I keep hammering on one additional point. The control device -- vapourstat or pressuretrol -- is not, repeat not, a safety device. If you want an overpressure safety cutoff -- and I think you should have one, make no mistake -- that should be a separate pressuretrol, on its very own separate pigtail, set at a pressure which will protect the system.