Troubleshooting/Understanding Pressuretrol

PGSmick

Hello all,

I am trying to understand how my Pressuretrol unit works in my system especially as it relates to the thermostat.

The system: a late 90's Burnham Low Pressure steam boiler (Model PV73SBP) fired by oil. It has a Honeywell Pressuretrol unit, a probe-type low water cut out, a Honeywell R8184G burner control and a simple Nest two-wire thermostat.

The issue began when the Nest thermostat recently complained about not having power from the furnace. Troubleshooting confirmed that the burner control box was not getting power. I tracked that down to the Pressuretrol unit. The Pressuretrol is a simple 2-wire affair wired in series with the burner control, and if its internal microswitch does not close, then the burner control does not get 120v and hence the thermostat won't get power. That was indeed my situation even though the furnace was stone cold. I took the microswitch out of the Pressuretrol and confirmed with a multimeter that the switch itself was working fine. I put the switch back in back in, and everything now seems to be working...

...Except that I now don't understand fully the function and purpose of the Pressuretrol unit. The unit Cut-In pressure is set for 1 psi and the differential is set at 5. According to the label inside the Pressuretrol I think this means that the furnace will operate at any pressure between 1 and 6 psi. (If my furnace pressure gauge is working right, it's telling me that the boiler is generating at least 10 psi. So that's another puzzle.) I assume that as the furnace operates it will generate internal pressure and when the pressure reaches 6 psi, the Pressuretrol will shut the furnace off.

But that means that the thermostat will report an error anytime that happens because the burner control won't be receiving power. And that doesn't seem right. Indeed, in all the many years that this setup has worked fine, I have never seen the thermostat complain about not having power, and this tells me that the Pressuretrol unit has never cut out.

And that implies that the Pressuretrol is more a safety device and is not meant to govern normal furnace operation unless and until a pressure problem arises.

Can anyone help explain this to me?

Many thanks
Peter

Jamie Hall

OK.

Well, the pressuretrol is a safety device, which is why it cuts power to the boiler when its set pressure is reached. It is also, in your case, a control device, to ensure that the boiler operates within a reasonable pressure range.

Step 1 here. Your pressuretrol is set way too high. Your cutin should be 0.75 psi or thereabouts, and the differential should be 1 psi, for a cutout of about 1.75 psi. You don't need any more pressure than that. In fact, you don't want any more pressure than that (as we have often noted, the Empire State Building runs on a bit less than 2 psi...).

Which leads to step 2. The odds are very good indeed that the little pigtail -- the pipe connecting the pressuretrol to the boiler -- is blocked. Therefore, the pressuretrol has no idea what pressure the boiler is actually operating at. So that needs to be taken apart and either replaced (if it's not been off for a decade or so, I'll bet on replacement) and passage into the boiler checked to be sure it's open. The blocked pigtail will prevent the pressuretrol from cutting out when it needs to -- but it will also prevent it from cutting back in.

That the thermostat has never complained before leads me to the same conclusion -- the pressuretrol has not been working, which means your boiler has no overpressure safety, which is very very dangerous.

Get this fixed as soon as possible, preferably before you run or try to run the boiler again.

Now to go back to Step 1. Call it step 1A. If your boiler has actually been running at anything much over 3 to 5 psi, particularly if it has been doing it consistently, you have some bad news: all your traps and vents are almost certainly toast. I'd at least assume they are. The working pressure for such gadgets is normally 3 psi, and the withstand pressure (one shot overpressure) is usually not more than 15 psi.

Now for Step 3. Nests do need power from the boiler or from somewhere. They use a lot of it. You have gotten by until now simply because you are running much too high a pressure, and the boiler wasn't shutting off. There are nice little wall wart devices, made especially for the Nest and similar power gobblers, which make them independent from boiler power. You need one.

Not a step, but a little bit of good news. If you actually get the boiler pressure dialed down to where it should be, you'll probably save enough on fuel to pay for the wall wart...

PGSmick

Thank you so much. I'm shutting the boiler down tonight and when the system is cold again, I'll dive into those inspections and repairs.

Just for my own peace of mind, does the actual pressure relief valve provide protection from a malfunctioning Pressuretrol?

Thanks again for the quick and understanding reply!

Peter

Jamie Hall

The pressure relief valve will open when the boiler reaches its set pressure -- for a residential steam boiler, 15 psi. It tends to be a rather spectacular event... and will keep the pressure from rising farther. However, it will not shut the burner off, and with that valve open you are losing steam -- and therefore water -- at a pretty good clip. Hopefully the low water cutoff will shut off the burner when the water gets too low.

But notice that pressure, and think about what I said about any vents or traps...

PGSmick

I have now replaced the PTrol and the pigtail. I also installed a couple of new pressure gauges, one on the boiler with its own pigtail and the other teed into where the PTrol is tapped. With the differential set to 1, I found that the control would not cut in until I had increased the cut-in to about 1.75 lbs. But I'll let it heat a cycle and see what happens and then monitor the performance of the radiator vents and see how many are toast. I am relieved and reasonably secure in the belief that my boiler won't end up on the roof of the house! Time will tell.

Jamie Hall

Just what you've done is a vast improvement... though what is the cutout with the cutin at 1.75? The real values, that is, as measured by a decent pressure gauge...

PGSmick

I'll let you know about actual pressures I see. One thing that occurred to me about the Nest thermostat: When the LWCO shuts the furnace off-as it does every few minutes to take an accurate reading-the thermostat does not complain about the furnace power even though it is, in fact, getting no power. So it must have an internal circuit that delays issuing its warning for some period of time to allow for these periodic shutdowns. If the PTrol cycles similarly for pressure reasons, the thermostat might just wait that out, and this could explain why I've never seen the Nest complain. And it was only after an extended period of the PTrol being off that I ever saw the warning. That's my theory.

ethicalpaul

I think that's a good theory

Jamie Hall

Not only is it a good theory, it's correct -- the Nest and some similar widgets have a small internal power supply -- some of them it's a battery, some it's a supercapacitor -- which is good for a few minutes to, say, half an hour of power. Which works just fine with forced air, which runs short on and off cycles. Doesn't work for beans when the run time (during which the battery isn't charging) is long.

EBEBRATT-Ed

@PGSmick if killing power to the Nest is or becomes a problem you can always add a transformer and a relay to solve that issue. See attached.

I have used the attached on some oil boiler when the transformer in the control is not large enough to power the Nest

PGSmick

I see. No, the question for me isn't about powering the Nest per se. In fact, providing constant power to the Nest thermostat defeats the thermostat's ability to report a problem, doesn't it?

Yesterday I ran through several successful heating cycles. Neither of the new pressure gauges I installed budged from 0, so that's a minor mystery. I think my earlier claim that I was generating at least 10 psi was erroneous and based on a very whacked pressure gauge. All the radiators heated normally, and all of the vents appear to be working.

At one point in the day, the thermostat did go offline again, and I had to tweak the PTrol up another couple of turns on the cut-in adjustment (which is now at 2 psi) to get the furnace running again. But I can see no harm in that as long as the differential limit of 2 + 1 is respected. All it means is that if the pressure ever does exceed 3 psi and the furnace cuts out, it won't cut back in again until the pressure falls below 2 psi--assuming the thermostat is still calling for heat. Cranking the Ptrol up to 2 does not require the furnace to operate the system at or above 2 psi. And the question is really academic if my system pressure never actually gets the gauge up off the 0 pin.

Anyway, I think my problem is solved for now.
Thanks for everyone's help.

ethicalpaul

The gauge that ships with a boiler fails approximately 2 days after installation as you have found. Keep an eye on your new gauge during a long call for heat (such as a recovery from a setback or a very cold day) and you should see a little pressure build eventually. Congrats on your system running well.