Questions about Dan's "How to set a Pressuretrol" post

PEvans

Link:

https://heatinghelp.com/systems-help-center/how-to-set-a-pressuretrol/

I get how he found the Pressuretrol cut-in pressure in his example, at double the 4 oz system pressure drop or 8 oz (0.5 psi), and why the differential should be 1 psi.

However, for a Vaporstat he advises lower values for the cut-in pressure (4 oz) and the effective differential (6 oz) if I'm reading it right.

I don't see why the desired values would not be the same for the Pressuretrol and the Vaporstat.

ethicalpaul

Don't overthink it too much unless you are working on a large building. There is no system pressure drop in a residential system, so don't worry about that…you are simply telling the boiler when to shut off on high pressure, then when to turn back on after shutting off. Here is a link to the article for those interested.

Pressuretrols are typically used on single pipe systems because those systems can handle higher pressure, and Pressuretrols can set a cutout pressure of 1.5 psi minimum. And the minimum is where you want to set it.

Vaporstats are used on two pipe systems usually because those systems usually need to run at lower pressures than 1.5 psi.

In both cases, ideally, you would have your system sized so that the pressure control rarely or ever comes into play.

HVACNUT

Because if you tried to set 4 oz. on a PA404A, the linkage would fall part. You can just dial in better with the Vaporstat. More sensitive to low pressure than a PressureTrol. And of course the system must be piped properly to accurately read those pressures.

Jamie Hall

On some two pipe systems — Most "vapour" systems — there is or at least was once a gadget which kept the differential pressure between the steam main at the boiler and the dry return at the boiler from rising above a certain value — typically 8 ounces differential.

Several reasons for that, all of them good. If there are orifices or calibrated inlet valves, that's what they are set for. It prevents excess differential pressure on the traps and thus avoids damage. And you don't need any more than that.

However, that also means that any boiler pressure over 8 ounces gauge doesn't do anything additional to provide heat out in the radiation — all the extra fuel is doing is compressing the steam in the entire system. A complete waste of time.

So… if you have a vapour steam system, the vapourstat must cutout at or below 8 ounces gauge, There is no point, however, in setting the cutin too low — might just as well fire up again as soon as you can — so a differential of 4 ounces or so works just fine.

Now before someone starts commenting that the pressure control is a safety, not a control, I beg to differ — with an additional comment. The vapourstat set as above is very definitely a control, not a safety, and is used to modulate the effective firing rate of the boiler, since most steam boilers do not modulate otherwise (some coal fired boilers could, and did — but that's another topic). You also need a safety for overpressue, however, and that should be a pressuretrol — preferably a manual reset one, so someone will have to find out why the vapourstat didn't work — set at some reasonable pressure, such as around 2 to 3 psig.

PEvans

He sets the pressuretrol cut-in to twice the system pressure drop as a "safety factor." Are you saying with Vaporstat you don't need to be so conservative, you could set the cut-in to closer to the actual system pressure drop?

He sets the Pressuretrol differential to 1 psi to make sure the radiators see 215F steam temperature. I don't see why that would be different for a Vaporstat.

For what it is worth, my system is residential, 2-pipe, and I have installed a 4 psi Vaporstat and a 3 psi low-pressure gauge in place of an old Pressuretrol. So I can dial in the settings and read the actual boiler pressure when the device operates. I have also done the system calcs per Dan's method, so I have some idea of what the system pressure drop could be. I'm thinking my present settings might be too low, and I do have radiators that I believe used to heat up and now don't. I would rather understand this before jumping to conclusions.

ethicalpaul

I guess you are ignoring my assertion that there is no "system pressure drop" which is fine, but put another low pressure gauge at the far end of your main if you want to verify it.

PEvans

@Jamie Hall I'm not aware that my system is a "vapour" system. I found it with higher pressure settings on the old Pressuretrol than I am using now.

@ethicalpaul I'm not ignoring you assertion. I am more wondering why the differential shouldn't always be 1 psi based on Dan's post. I could certainly set the "main" or cut-out to 1 psi or 1.1 psi or whatever.

ethicalpaul

on a pressuretol, the diff should always be 1 because that’s the lowest setting.

If you raise it to 2, then cutout won’t happen until 2.5

On a vaporstat it can be lower.

On mine, it’s a custom control that cuts out for 10 minutes to let the radiators put their heat into the house for awhile

Jamie Hall

As @ethicalpaul has said, you can ignore any pressure drop in the system mains. So long as you have any pressure greater than atmospheric at the end of the main, you're fine.

That you found it with higher pressure settings than are recommended just means that someone along the line didn't understand what pressures were necessary, and just cranked them up. A very common error.

@ethicalpaul 's comment on the differential is also correct — you can't set a pressuretrol's differential to less than 1, nor the cutin to less than 0.5. It's the way they're built. You can set a vapourstat's cutout to whatever you need, but the cutin — main minus differential — should always be greater than about 2 ounces gauge which limits the cutin to perhaps 4 ounces or so.

If you have some radiators which are not heating up properly at … well, let's say 1 psig — there's something else amiss. On a two pipe system the most likely canddates are failed traps (either open or closed — a failed closed trap will affect its own radiator, a failed open will affect others on the same return) or a sag in a pipe trapping condensate and keeping steam from moving. That is, always assuming that you have ensured that the inlet valve is open.

Another possibility — a little rarer, and one which will affect all the radiators on a given return — is that the main vent on that return is failed closed, or is much too small.

PEvans

https://forum.heatinghelp.com/discussion/comment/1814124#Comment_1814124

OK, great point. I hope people with Vaporstats reading Dan's post will find this thread. When Dan says set the cut-out at 10 oz and the effective differential at 6 oz on a Vaporstat it's because the Vaporstat lets you do that.

When I calculate my system pressure drop (sorry!) I get 3.2 oz. Right now on my Vaporstat the diff is set to 7.5 oz which is, I think, how it came. My notes say it cuts out at 0.7 psi on the gauge or 11.2 oz, which would imply a cut-in of 3.7 oz.

I was thinking this cut-out and cut-in are a little low, but it sounds like maybe not.

Jamie, I don't want to hijack this thread with why some of my radiators may not be heating. I have had bad radiator traps, bad F&T traps, sticking zone valves, and failed thermostats since I started to bring this system back, so it is sketchy to draw any conclusions from my past experience with it.