Pressuretrol and cold rooms (2-pipe pumped condensate return zoned system)

PEvans

This relates to my steam heating system in this thread:

https://forum.heatinghelp.com/discussion/184767/bringing-a-2-pipe-pumped-return-steam-heating-system-back-in-service

There are details on the system, zone valves, and installation of the low pressure gauge there.

Here are the Pressuretrol and gauges. The low P gauge and Pressuretrol are together on a pigtail.





Edit: There was a lot of debate in the prior thread about the venting of this system. I'll just say that up until the situation I describe here, every steam main and every radiator was working fine.


So all of a sudden two radiators in one room are not warming up. These both happen to have newly installed Barnes & Jones cage units in the radiator traps and I know for sure the distribution lines are getting steam. The system seems to shut off at the Pressuretrol cut-out before these two radiators really warm up. It will then sit there as long as the zone valve is open/calling, cycling on and off with the pressure.

One change that I can think of is the steam distribution pipes for these two radiators run through a part of the basement that is more exposed and now quite a bit colder now that it is really winter. Another change is the header with the low P gauge and Pressuretrol has been flooded, but this was weeks ago.

I don't really trust the low-P gauge anymore. It used to agree with the main 30 psi gauge on the boiler and now it doesn't, but here goes.

With both zone valves open the system seems to operate at .3 to .4 psi on the gauge. If it stays there long enough the two suspect radiators will eventually heat up. At least I have seen this once. Eventually the pressure rises to about .9 psi and the system shuts off ,so that must be the cut-out. With the other zone valve closed, the system seems to operate at .8 to .9 psi on the gauge. This is close to the cut-out, and it will run there for a bit and then shut off.

I tried raising the cut-out pressure on the Pressuretrol. This seemed to have the actual effect of raising the cut-in pressure to close to the cut-out pressure. I raised the cut-out a couple more times, and also increased the differential. This finally got to where the system would stay on at just under 1 psi on the gauge to allow the two named radiators to heat up.

The pictures above were taken as I left it. You can see that the settings on the Pressuretrol, for what they are worth, are high. The boiler main gauge is also reading high. Also, under these settings I'm getting a lot more steam in the condensate. Plus I just hate to raise the pressure when it had been fine; cranking the pressure up is the most basic steam sin, right?

So I feel like this is not really working out correctly or I am looking at it wrong. Suggestions?

EBEBRATT-Ed

If you are trying to change the difference between cut in and cut out to get a longer run time and you raise the cut out the differential will stay the same it will follow the cut out up the scale. If you want more run time raise the cut out and increase the differential.

If you are cutting out at .9 you can go up a bit

Jamie Hall

Check the return(s) from those two radiators to ensure that there is no possibility of a water trap in them -- even a very shallow one.

PEvans

EBEBRATT-Ed said:

If you are trying to change the difference between cut in and cut out to get a longer run time and you raise the cut out the differential will stay the same it will follow the cut out up the scale. If you want more run time raise the cut out and increase the differential.

If you are cutting out at .9 you can go up a bit

it might be just me, but it seemed raising the cut-out pressure actually had the effect of raising the cut-in pressure (and reducing the differential). The Pressuretrol could be wacky but you guys have said these are pretty bulletproof. More likely my error.

Increasing the differential did seem to get the system to run longer and not cut out as low.

PEvans

Jamie Hall said:

Check the return(s) from those two radiators to ensure that there is no possibility of a water trap in them -- even a very shallow one.

They actually share a return. I will check it with level.

When our weather gets a little better I think I will get some insulation for the pipes for these radiators.

PEvans

Jamie Hall said:

Check the return(s) from those two radiators to ensure that there is no possibility of a water trap in them -- even a very shallow one.

I would say very little possibility. All of the returns have a visible (with a level) slope back to the boiler. Some segments are pretty close to level but nothing is fully level or reverse sloping.

It is not totally out of the question that there was some freezing in these lines. However, I have sealed up the space and it is much warmer now than it was last week, so I think that is less likely now.

PEvans

Here is a crazy thought:

If you look at the Pressuretrol in the photo above it says the "cut-in point equals the cut-out point plus the differential". This has never made sense to me. Also, you can't see here, but this Pressuretrol is a Honeywell model P404B1. Someone here may know the Honeywell model designations better than I do. But if I look on Supplyhouse the PA404B has the following switching logic: "breaks at setpoint on pressure fall. Spst makes at set point plus differential on pressure rise." The main scale is also marked “cut out.” Normally such a device would be used to turn on some other device like a fan when steam is present and turn it off as steam pressure decreases.

So is it possible that this Pressuretrol is just the wrong device for a high pressure cut-out switch? Perhaps it has been re-wired to function as an additive differential switch. The switch is definitely closed, not open, when pressure is below the setpoint shown on the bx. If this is in fact the case I would raise the cut-out point by increasing the differential.

Too crazy?

Jamie Hall

Is that your only pressuretrol? I hadn't looked closely at it. It looks as though it belongs in a museum. It is possible that, internally, it has an SPDT, not SPST switch -- a lot of them do -- and is wired to be on at low pressure and break on rise. With an SPDT, you can wire it either way.

In any case, may I humbly suggest that changing it for a more modern unit might not be such a bad idea?

PEvans

Well, my crazy idea is either not the case or only partially. Adjusting either the differential or the setpoint has the same effect, raising the cut-in pressure.

I think Jamie's humble suggestion is right (thanks :-) ). I need to replace the pressuretrol and, I think, the fairly new low-pressure gauge, to have any confidence in my understanding of what is going on.

That would possibly help illuminate but not answer the question of why I have radiators that had been working fine, with new cage units, now fail to fill with steam.

EBEBRATT-Ed

If it is a PA404B its the wrong control. A PA404A opens the circuit on a pressure rise B closes the circuit on a pressure rise

PEvans

EBEBRATT-Ed said:

If it is a PA404B its the wrong control. A PA404A opens the circuit on a pressure rise B closes the circuit on a pressure rise

Thank you. That is what I came to suspect. It is actually a P404B (no "A") but I'm willing to assume the "B" means the same thing given the annotations on the cover.

This pressuretrol has been on this system for years, of course. So I think maybe the switch was reversed so it can function as some sort of high pressure limit. It is also old, and maybe now not working properly at all. Per Jamie's suggestion I have a vaporstat ordered.

I'm still wondering why a system that had been delivering steam to all the radiators fine now isn't doing it at what seems to be the same pressure.

Jamie Hall

I suspect you're right about somebody fiddling that old pressuretrol And I'm glad you're getting a new one.

We did get somewhat distracted from the original problem -- sorry about that. I'm not going to read the whole thread, but did I ever ask you if you would see if you can trace just how far the steam does get when it's trying to heat those radiators? That may help to figure out what changed (because something did).

PEvans

The steam can get as far as the radiator inlets if it does not shut off on pressure. And as I said, all of the radiators used to heat fine.

This seems to be expanding to more radiators in the same zone, but I'm willing to put that aside until I put in the new pressure control.

Jamie Hall

Somewhere on the return side in that zone I think you may find some sort of blockage. Not complete, and probably not all that apparent for water -- but enough to keep air from passing. Assume nothing.

PEvans

I have the new VaporStat installed; it is the 4 psi variety. I started with the high setpoint at 1.5 psi and the diff as set in the factory, about 7.5 oz.

Honestly, the behavior of the system is identical. Steam doesn't reach these suspect radiators. It might get there eventually after enough cycles (the distribution lines are hot).

On other odd thing. I tried raising the setpoint on the VaporStat. This seemed to have no effect on when the burner cuts out, same as with the pressuretrol (though it does raise the cut-in pressure). It is almost as if something else is shutting the burner down.

There is some steam coming from the condensate receiver; this is not new. I'm sure this does not help, but it doesn't explain what has changed.

Jamie Hall

Ah... steam coming from the condensate receiver? There is a failed trap -- or traps -- somewhere, or your system has some drips which were supposed to be water sealed and aren't and now need traps.

With this new information, I'm going to say you're not going to get much of anywhere with those radiators until you find out where this problem is coming from -- and fixing it will likely make the whole system work better.

On the cutin and cutout. I think you now need to do two things: first, find out for sure what is shutting the boiler off. It may not be the pressuretrol. And second, get a low pressure gauge installed so you can actually see what the boiler is doing.

PEvans

Thanks. I have a low-pressure gauge but I don't trust it, so I'm going to replace it.

It is easy for me to check to see if the VaporTrol switch is still "made" when the boiler shuts off.

I am going to try lagging the pipes for these couple of radiators.

All the drips have traps. I had planned to service or replace all of the F&T traps, but I had hoped to make that a warm weather job.

EBEBRATT-Ed

@PEvans

It is common when a trap fails (and they usually but not always fail open) that you will get steam out of the overflow/vent on the condensate tank.

This should be fixed because you lose a lb of steam =a pound of water lost and a pound of new make up water added which isn't good for the boiler or piping.

Since most of the time the traps fail open that radiator will likely be hot and the ones near it may be cold as the bad trap lets steam through back pressuring the other rads and keeping them from working.

PEvans

Solved.

The actuator linkage on one of my zone valves had slipped, and the valve was only about 1/3 open. So the steam for this zone was 2/3 blocked. I wrote about these linkages in my prior thread linked above.

So with the valve fully opened the radiators heated up right away. I was able to turn the cutout pressure setpoint on the VaporStat down to a little under 1 psi. When I replace my low-pressure gauge I can set it more precisely using Jamie's method.

I could hear the Vaporstat clicking when it shuts the burner off at high pressure, so that is good evidence that there is not something affecting it.

At lower pressure there is less or no steam in the condensate. I have found plenty of radiator traps that had failed closed and the radiator did not heat. I have not found radiator traps failed open, but think I may have F&T traps failed open. I have a pretty good idea which one, but perhaps I can let it go until the weather is warmer; then if I have to replace the trap it won't be as big a deal.

Jamie Hall

Wonderful! You have just discovered one of the real merits of low pressure -- the load on traps is much less. And I'm glad you found that bad valve -- and not surprised it was causing trouble partly open. Saturated steam really doesn't like partial port valves...