Pressurtrol mystery

Rightrudder

I swapped out a seemingly malfunctioning 404A Honeywell pressuretrol for a 408J vaporstat, seeking greater accuracy. I set the cut in at .5 PSI, and cut out at 1.5. The result: the boiler cuts out at 1.5, and cuts in at .75 about 20% of the time. The other 80% of the time, it cuts out at 1.5, the pressure drops all the way to zero, wait a minute or two,  the main vent breaks the vacuum, wait a minute, and the boiler kicks on again and fills the system with steam again. Thought maybe I had the wiring wrong, changed that, same result.
Thinking that maybe the vaporstat was a bad unit, I went and got a brand new 404A pressuretrol to see what would happen. Set the cut in at .5 psi, and the differential wheel set at 1. The result: the pressure kicks off at 2 PSI, drops all the way to zero, wait a minute, main vent breaks the vacuum, wait a minute, boiler kicks in. Here we go again. Reversed wiring, same result.
All pressures verified by two 3#  gauges, each on its own clean pigtail.
I even brought in a “pro” to show what was going on, and he didn’t seem to have a problem with it. I paid him his fee and escorted him out of my basement.
I really need to try to get one of these devices to keep the steam in the system, ideally with a .5 cut in and 1.5 cut out, continuously until the thermostat is satisfied. Just like my system used to act in the good old days.

Any ideas out there?

KC_Jones

It’s unclear to me what you are trying to accomplish.

” I really need to try to get one of these devices to keep the steam in the system”

I don’t quite understand this statement?  Are you saying you think the pressure is helping?

Rightrudder

The problem is that the cut in doesn’t seem to be occurring properly in either of these devices. In the vaporstat it was working perhaps 20% of the time, but the rest of the time the system would simply depressurize to zero and then eventually the boiler would kick on again. Cut in with the new pressurtrol is not happening at at .5. (bottom of the scale), but rather long after the pressure in the system has reached zero and the main vent has opened. If I try setting it higher than .5, all it does is drive the cut out higher. I really don’t want it any higher than 2 pounds, which is where it is currently occurring (even though I have the differential set at one, which should be yielding a 1.5 cut out)

EBEBRATT-Ed

@Rightrudder

You don't have to have any measurable pressure on the system for it to heat properly. Most residential systems will heat without any pressure on any gauge if the venting is working and everything else in the system is ok.

I would put the vaporstat and the L404a back on the boiler and wire them in series so either one will shut the boiler down.

Set the L404A to cut out at 3 psi with a 1 psi differential. It will never need to function as it is a safety back up.

Set the vapor stat to cut out a 1.5 psi and set the differential so the burner cuts back on just above 0.

Stop reversing the wires it makes no difference on a two wire circuit

Make sure all the pigtails are clean as well as the piping going into the boiler.

What do you have for main vents? From a cold start time how long for the supply piping to leave the boiler to get hot. Then time how long for the steam to get to the ends of the mains.

The we will have more information.

Take some pictures of the boile so we can see the piping

Dan_NJ

You could assemble a little test rig similar to Gordo's here to confirm the operation at expected pressures:
https://youtu.be/mE1-QuUx4_c

Aside from the pressuretrol micro switch possibly not closing as expected, do you have a Cycleguard LWCO that could possibly be shutting the boiler off intermittently? Or is there a vent damper not opening, or something else delaying the firing?

EBEBRATT-Ed

@Dan_NJ
Makes a good point. The vent damper (if you have one) closes when the pressure control opens. When the pressure control re starts it take a little while for the vent damper to open to allow the boiler to fire

Rightrudder

I have two three-quarter inch Gorton # 1’s at the end of each main. 

Steam gets rolling pretty quickly from a cold start. 

Wiring in series sounds like a great idea, but I can’t get the damn things to cut in at a proper setting at all. Both of them simply cut in long after the pressure has dropped to zero.

KC_Jones

The vaporstat and pressurtrol are notoriously inaccurate.  If you want that level of control you will have to crewte a test rig, as suggested, and tune them, then maybe you can achieve expected results.  I have 2 vaporstats and a pressurtrol in mine and even after tuning and maxing adjustments, they still don’t work.  I just set them with the pressure gauge and ignored the scale as it appeared to be useless.

Honestly, for me, it isn’t going to make much of any difference on system performance.  I’m assuming your boiler is oversized or inadequate venting that you are even hitting that pressure, which means those safeties are now functioning as controls, not really how you want the system to be, but many are stuck with.

Rightrudder

Interesting question about the vent damper. Actually, I had one go bad a few years back-of  course on the coldest night of the year. I then replaced it, but subsequently deactivated it in the open position so as not to have another moving part fail in the future. The idea came from a conversation I had with a plumber who felt that the energy savings the damper affords wasn’t worth a cold night with the boiler shut down.

Dan_NJ

Is that a Cycle Guard at the bottom of the picture? In the burner off state? If I'm not mistaken the green light indicates it would have the burner off for a water level check. Does the burner come on when that green light goes off?

Rightrudder

LWCO is McDonnell.Green light always on during normal operation, red comes on in a low condition.

Jamie Hall

Is the entire pigtail really and truly completely clean? All the way into the boiler? Not just I think I can blow through it clean, but really clean? Water flows freely? One unappreciated possible problem is that gunk in there, while it can allow pressure in, may easily block pressure coming out again -- holding the vapourstat (which is very sensitive) off.

Second, is there a post purge/pre purge delay sequence going on at the same time? If it's an oil burner particularly, that can give the illusion that the problem is in the vapourstat, when it isn't. The vapoustat resets just fine -- but the post purge is still happening. Then there's a pause... and the pre purge occurs, by which time the pressure is zero, and then the burner refires.

Since you are fiddling, find out whether you are operating the vapourstat on 24 volts or 120, and rig a test light of appropriate voltage across the terminals. That will tell you, with no further effort, what the state of the vapourstat switch is, assuming the rest of the circuit remains powered. Light on? Vapourstat terminals open. Light off? Vapourstat terminals closed.

Rightrudder

One thing Imay have failed to adequately mention is that both Pressuretrol and vaporstat seem to be cutting OUT at reasonably close to the pressure expected. But as I watch the pressure decline towards zero, I can hear the microswitch click at approximately the pressure that I was expecting for cut IN.  But then nothing happens. The pressure drops to zero. Both gauges are in synch.  No evidence of residual pressure. The main vent breaks the vacuum after a few minutes, and then the boiler cuts back in a minute or so later. Any idea why both devices might not respond, even when I know the microswitch is activating at about the proper set point?
(LWCO is fine, vent damper deactivated and left an open position. I have checked and cleaned both pigtails and the tee right back to the boiler, and all are pipe-cleaner clean.

Jamie Hall

If you hear the microswitch click where you expect it to (oh for the old mercury switches where you could see what was happening!) -- it's not the vapourstat (although there'd be no harm to measuring the voltage across the terminals with a multimeter, if you have one)). Something else in your system is causing the delay.

So... you now have the interesting project of finding the something else. Vent damper? They take time to close and open? A built in refire delay on the burner control? Many have that, even without post-purge/ pre-purge.

You're focussed on the vapourstats. Start looking at the whole system.

Rightrudder

Thanks for your intuitive responses. It’s a gas boiler, and the vent damper is disabled in the open position (intentionally). And the odd thing, the vaporstat fires at about the right time only about 20% of the time. Hard to understand why it occasionally does what it’s supposed to.
Pressuretrol fires way too late all the time.

Jamie Hall

I'll try again. If the pressuretrol and vapourtrol microswitches are clicking -- and you have verified that contact is being made -- it's neither the pressuretrol nor the vapourstat.

Next time the puppy doesn't fire, get out your multi-meter and verify that the voltage on both terminals of the vapourstat and both terminals of the pressuretrol is the same -- or without changing anything else at all, turn off the power and verify that the resistance between the terminals of each is zero or certainly very close to it. Microswitches have been known to fail, but it's not common.

Chris_L

And why do even want the system to come back on while there is still pressure in it?

If you have pressure, it means all your radiators have been fully heated. Assuming they are cast iron, they have lots of heat to give up.

A number of us do just the opposite under these circumstances--keep the boiler off for a period of time to smooth out the heating cycles, and reduce the constant cycling of the boiler.

Rightrudder

Thanks, Jamie,I will break out the multimeter and check it out. I may also try to reconnect the vent damper, and see if by having taken that out of the loop I may have caused some kind of problem. But I thought by staying disabled in the open position it would not be a problem.
I do realize that with both Vaporstat and Pressurtrol  doing the same thing, there is a deeper problem that I must attempt to figure out. 

Chris, far as keeping pressure in the piping, with the pressure constantly rising and then dropping totally to zero, the main vent keeps breaking the vacuum caused by the water coming down the risers every couple of minutes. Then the boiler kicks back in, the rush of steam from still-hot-as-hades water pushes the air out of all the radiator vents very aggressively. A lot of hissing going on here. 

By having a nominal amount of pressure still in the pipe (half a pound or less?) when the boiler kicks back in, the system isn’t constantly replacing air with steam on  every couple of minutes.
At least it seems to be what I am experiencing right now, and also conveys my level of understanding of how this darned stuff works anyway. 

Always open to learning, though. 

Thanks to you all for your input. 

Chris_L

@Rightrudder, yes, when the closed main vent (or closed radiator vents) break open, the system will fill with air. But when the boiler restarts, most of that air should come out of the main vents. If you are hearing a lot of air hissing out of the radiator vents, I have to wonder if your main venting is sufficient.

Keeping some steam in the system will keep the vents closed. But for most people, having fully hot radiators when the thermostat is satisfied will lead to the set point being well exceeded.

That is why some of us put a delay in after the boiler shuts off (e.g., 15 minutes). Yes, the air has to be expelled again (unless you have a vacuum system), but usually it is only once or twice. And with good main venting, you won't notice it.

EBEBRATT-Ed

Also make sure your controls are wired right Maybe you can post a diagram. Almost seems like when the controls open you taking power off something that takes time to re-initialize when the power comes back

Jamie Hall

Once we figure out what it is that is causing the erratic and delayed restart, then we can begin to figure out what is causing the cycling on pressure -- and what, if anything, is to be done about it.

But let's tackle one problem at a time here...

Rightrudder

Looks like I’ve some more work to do, guys. I will report back. 

Thanks again for all your input. 

This site is such a wonderful resource thanks to people like you!

Fred

Reading this thread seems a bit crazy. These gauges, Vaporstats and Pressuretrols are not precise enough to try to sync any two of them up to be exact so that you really know the actual pressure down to the ounce, let alone trying to sync two gauges, a Vaporstat and a Pressuretrol together. The fact that a gauge drops to zero doesn't mean that the system is at zero. It just means that, within the variables of that particular instrument, it looks like zero. The same is true for all the other components of the system, vent temp ranges included.
If you are getting some hissing, when the system is running, you most likely have a clogged pigtail/pipe at the Vaporstat or Pressuretrol. Just because youican blow into it doesn't mean the pipe is open into the boiler. With the pipe configuration I see under the pressuretrol, you could be blowing air in any of two or three directions without actually blowing it back into the boiler. I don't know what the pipe configuration is for the vaporstat since I don't see it in the picture.
Also, when you add a Vaporstat to a boiler, the Pressuretrol typically is used as a back-up, in the event of a failure of the Vaporstat. It is typically set at a higher cut out, Maybe 3 to 5 PSI, so as to not interfere with the cut-in and cut-out operation of the Vaporstat.