Why do I have dual pressuretrols?

PeterD2

Hi. I'm glad to have found this helpful web site. Here's my question:

I have a Burnham 405B residential steam boiler on a single pipe system. The system is about 30 years old as it predates my purchase of the house. The radiators are connected to the main steam pipe and there is a separate return pipe that seems to trap the condensation from the radiators and return it to the boiler. There are two Honeywell pressuretrols and I’d like to understand the purpose and function of these dual controls. (I think I understand the basic concept of the pressuretrol.)

I have seen it said that on residential systems the pressuretrols should not be used for operational control of the boiler cycles, but my thermostat does not have the ability to cycle the boiler and in any case doesn’t have any way to know the boiler pressure, so it seems valuable to configure the pressuretrols to manage the boiler pressure and cycles.

Here is a photo of the system model and rating card:



We have had some cold weather recently and I noticed that the system was running with excessive pressure. I determined this because 1) it seemed that the high pressure relief valve had released some steam. There was a small puddle near the valve which had no other obvious source and did not return after evaporating. In addition, 2) the water level in the glass viewing tube got below the bottom of the tube at one point. I keep the tank filled to exactly half way up the viewing tube so this indicated that the system was running with very high pressure. (The water level returned to normal once the system cooled off.)

Because of the pressure issue I am trying to reduce the pressure and to establish an effective cycle time.

I checked the settings on the pressuretrols and reduced the cut-in setting for one of them. This is when I noticed that there are two of them. The pressuretrols are wired in series. One is connected to a pigtail on the water drain/low water cut-off and the other is connected to a pigtail that appears to be connected directly to the boiler. There is also an internal syphon pressure gauge which seems to be functioning although I don’t know whether it is accurate. With the current pressuretrol settings, it ranges from zero to about 5 PSI before the pressuretrols shut the boiler off.

Below is a picture of the system showing the two pressuretrols, electric wiring and the water level tube.



I realize that 5 PSI is high for a residential system and want to make sure I am not really running at that level. But when I adjust the pressuretrols to cut out at a lower pressure, they don’t turn on reliably. At 5 PSI the water level gauge drops a little more than an inch, so it doesn’t look like I’m building up as much pressure as I was before!

Since these pressuretrols seem to be inherently inaccurate, it is not possible to exactly calibrate them together. The effect seems to be for them to operate together as separate low pressure and high pressure cut-offs. The one that has the lower high pressure cut-off setting operates as high pressure control. The one with the lower low pressure cut-in setting operates a low pressure control. At this time, it seems that one of them (the one on the low water cut-off) is doing all the work and the cut-in and cut-off points on the other are both higher than the first pressuretrol.

My question is, is this normal? Why was this system configured with two pressuretrols and what is the correct way to set them?

Thanks for any suggestions!

nicholas bonham-carter

Generally the second pressuretrol, set to a higher cutout pressure would be one with a reset button which when activated must be reset. This will alert the owner that something is amiss with the primary pressuretrol with no reset button.
Somehow yours has deviated from this usual setup, and you have just the two regular ones.
The pigtails should be kept clean on both, and they should be set to cut in at no more than eight ounces, and cut out at two pounds. An accurate low pressure gauge (0-3 psi, rated in ounces of pressure), is not only helpful, but should be considered a necessity, as main and radiator vents can be damaged by pressure over three psi.—NBC

Jamie Hall

I prefer to set them a little differently than @nicholas bonham-carter does -- but the same basic idea. I would use one -- doesn't matter which one -- as the "operating" control, and set it to cut in at about 0.7 psi -- not as low as it can go, but close -- with a differential of 1. The other one I would set for a cutin of about 2 psi with a differential of 1. That one will act as a backup safety.

Pressuretrols are not precision instruments -- but they shouldn't be too far off with those ranges.

SeymourCates

We also set them very close to the levels recommended by NBC. We set .5 lb. as the cut-in pressure and verify it with a 3 lb. gauge. We also set 2.5 lb. as the cut-out pressure as it is always preferable to run the boiler a bit longer rather than suffer with the miserable short cycling efficiency that occurs when it cycles on pressure.

nicholas bonham-carter

If the second one had a reset button, then I would also set its pressure higher, then you could be aware of a problem when you had to reset it.
It’s too bad there are no status lights on all the safeties, so you could see how they are behaving.—NBC

Jamie Hall

nicholas bonham-carter said:

If the second one had a reset button, then I would also set its pressure higher, then you could be aware of a problem when you had to reset it.
It’s too bad there are no status lights on all the safeties, so you could see how they are behaving.—NBC

Indeed. One advantage of the old mercury ones (there are many) is that you can see exactly what they are doing. I prefer to have the safety backup one a manual reset, too. Much better. And status lights would be a great idea. Not hard to implement...

PeterD2

Thank you all for your helpful comments. The differential on both pressuretrols is 2 so I will reduce that to 1 and monitor to get the actual cut-in and cut-out points as low as possible. I don’t think I can rely on the markings on the pressuretrols themselves to achieve the settings since they don’t currently seem to reflect the actual behavior, but I’ll go over to my plumbing supply store and see whether I can purchase a low pressure gauge and setup so that I can measure the pressure more accurately.

nicholas bonham-carter

Here is my setup, and you can see the reset button on the right side pressuretrol.—NBC

ch4man

a boiler with that low of input does not require two pressuretrols, but redundant safeties are fine..

the fact the safety valve is lifting is concerning.

is it faulty, or doing its job??

time to start with a thorough cleaning of the pigtails and control tree, a new 0-3 PSI gauge also my be in order. also main venting may need attention

Fred

I agree with @ch4man . We seem to be focused on the settings of the Pressuretrols when, in fact the pressure Relief valve seems to be spilling some water/steam. Either the pressure is running up to 15PSI or that valve is faulty. Find out which is the case. If the boiler is running up to 15 PSI, neither of those Pressuretrols is doing their jobs and both pigtails may be clogged. If the PRV is faulty, replace it. A basement full of steam is not what you want/need.

Chris_L

I have the same boiler though a bit smaller with a millivolt gas valve.

I did not see in your post whether you said this a gas millivolt system or not. If it is, you have two pressuretrols because it is code--at least here in MA if not in other places--and you do not want to set them the same. They have two different functions.

Your thermostat doesn't need to know the pressure, only the temperature. One of the pressuretrols cycles the boiler on pressure. (In my case it is the one on the LWCO).

The other is an auxiliary, which acts as a safety in case the first one fails. You set it higher because if it trips, the boiler shuts down completely and you have to restart it manually (again assuming you have a millivolt system). The auxiliary pressuretrol doesn't need to have a reset itself because you have to relight the pilot for the gas valve to work once it trips.

Also, check the manual for your boiler. On mine, the normal water level is 26" above the floor, which puts it about 2/3 to 3/4 of the way up the sight glass.

EBEBRATT-Ed

All boilers require an 1. operating control and 2 a safety control.

In most residential work the thermostat is the operating control and the 1 pressure control is your safety. That is ok if the stat runs the boiler and the pressure control is not called into play.

The minute that you use the one pressure control as an operating control then you are supposed to add a second pressure control.

The second control should be manual reset but older jobs always used two regular controls before MR was required. In MA anything over 200,000 requires manual reset on steam and hot water boilers.

I think this is a lesser btu requirement than most other states

mikeg2015

Properly sized steam boiler shouldn't build much pressure unless it's oversized, header is undersized or it's been running for 60+ minutes and there is no longer any "pickup" remaining (system piping and radiators are fully heated).

This means it only needs enough pressure to transfer heat by condensing steam. The pressure only moves the steam through the piping and does nothing, other than make it less efficient. Industrial system need higher pressure because they want to move more energy in smaller pipes and need high temperatures.

You can design a steam system that operates under a vacuum. This is done with industrial evaporators to concentrate liquids such as sugar liquors.

My system was designed to run under a vacuum for a coal fired boiler and operates at less than 1oz now even with an oversized boiler. All steam is condensed and piping is oversized and has enough venting so it never builds any pressure.

EBEBRATT-Ed

@mike2015, I agree in a perfect world that is how all steam heating systems should be. But in reality most boilers are oversized.

PeterD2

Thanks @Chris_L, @Eberratt-Ed and @Mikeg2015 for these additional details. The thermodynamics of these systems fascinates me! Unfortunately, I don’t have the original manual or system design document, but there was a mark 1/2 way up the water level gauge when we bought the house so that is what I have tried to maintain. Unless the system runs very long and becomes fully heated, as mikeg2015 said, there is always water visible in the gauge. And when the system runs that long it builds up more pressure than is needed, anyway.

The thing that frustrates me is that, with the current pressuretrols, I can’t seem to get the system to operate at the low pressures you describe. I’ll look into adding a more accurate pressure gauge so I can determine what the pressure actually is. The actual pressure may be lower than the internal syphon gauge I have is reporting.

It’s -7 Fahrenheit outside right now (our coldest morning yet) and, after the adjustments that I made to the pressuretrol, the water level is only down about 1 1/4” when the high pressure cut-off kicks in, and the pressure relief valve hasn’t fired again, so I think things are working OK.

And BTW, I am in MA and this is a gas-fired, thermopile-driven millivolt system (no external electrical connection, which I like :-)), so the code requirements Chris_L described probably answer my original question about why I have two pressuretrols. Not to mention that it’s always a good idea to have backup. The secondary pressuretrol has no reset nor does it kill the pilot, but at least it does provide a safety.

As for the thermostat, it’s a fairly new Aprilaire 8466 which came with our A/C-heat pump system, which is independent of the steam system, but the steam system is coordinated with the heat pump as auxiliary through the thermostat. (Its way to cold for my heat pump today!) I once contacted Aprilaire technical support to try to find out how their device handles steam systems and the folks I spoke with didn’t even seem to know what I was talking about. The thermostat just runs off of the ambient temperature and, although modern firmware should be able to support a separate steam system mode, has no mechanism for controlling the heating/pressure cycles in the boiler. This is why I’m relying on the pressuretrols.

Thanks again for all the helpful information.

mikeg2015

I'm still of the belief that the pressuretrol is only a high limit and should only come into play after very long run times when radiators are fully heated and you've eliminated nearly all the pickup and the boiler is effectively oversized to just the EDR.

Thermostat is the primary control device. That determine if you need heat. Presure control is a limit device.

Think of thermostat like cruise control/speedometer in your car and the pressure control as the tachometer & transmission. So you set the cruise control at lets say 70mph and the engine computer manages the transmission so you stay under the maximum RPM limit and don't blow up your engine.

In this analogy, the boiler is like a car with only 1 gear or neutral.

If you had a multistage boiler, it would be like having a stepped gear transmission, if you have a CVT like a lot of cars do now, that's like a modulating boiler.

Shoveling coal is like having a manual transmission.... haha. Don't get me wrong, I like manuals, but it you take a step back, they are pretty crude and a throwback and automatics finally caught them in performance and function.

Chris_L

@PeterD2, it sounds like your millivolt gas valve is not wired properly. I suggest you have a pro look at it.

The two pressuretrols should not be wired in series with a millivolt gas valve. The secondary pressurtrol is wired through the powerpile, and if tripped, the pilot should go out and have to be relit before you can use the boiler.

If you have adjusted your primary pressuretrol as described in the many posts on this board (to cut out at 2 psi, and in at 1/2) and the pressure is going higher or the pressuretrol not resetting right, you most likely need to clean out the pigtail. You can find many descriptions of that on this forum, too.

Fred

@PeterD2 , Most of these Pressuretrols come from the factory pretty far out of Calibration. They can be re-calibrated to be pretty accurate. It does require that you have a good 0-3 PSI gauge on the boiler.Here is the Procedure:
Inside the Pressuretrol, right below the micro switch, there is a pivot arm. At the end of that arm you will see a screw pin that is activated by the diaphragm at the bottom of the Pressuretrol. If you look very carefully at that screw pin, you will see it actually has a tiny (I mean tiny) hex head on it. It takes a .050 hex wrench and you can turn it clockwise (Towards the bottom of the Pressuretrol to decrease the Cut-out pressure or counter clockwise to increase the cut-out pressure (which none of us want to do but who knows, your Pressuretrol may be really screwed up!). Turn the power to the unit off first. You may find the first attempt to turn that screw a little bit stubborn (relatively speaking) because it has some Locktite on it but it does turn. Don't turn too much, a tiny fraction of a turn goes a long way towards getting it adjusted where you want it (maybe 1/32 inch turn to start with) . You may need to play with it to get it exactly where you want cut out to be.

PeterD2

I do appreciate all of the input and @mikeg2015 am certainly glad I’m not shoveling coal! :-) I also like manual transmissions (otherwise I probably wouldn’t be fiddling with my boiler), but I hope to get this system back on automatic soon.

It seems that my boiler may be oversized, but my experience is that, without the limits set by the pressuretrols, my thermostat does not control the pressure properly. This can lead to unnecessary high pressure as witness the recent HPV blow-off. Since my current thermostat doesn’t seem to be able to manage a steam system, I am hoping that the pressuretrols can provide the low level pressure management. Please let me know if I am misguided in this.

Does anyone have a recommendation for a good thermostat that can manage a steam boiler as the auxiliary for an A/C and heat pump system? The Aprilaire I have handles the A/C and works well when outside temperature is suitable for the heat pump. But it doesn’t know how to cycle the boiler when the steam system is providing heat - hence the pressuretrols.

As for wiring, it does sound like my system is nonstandard, but with the difficulty and cost of finding a real expert, I plan to get things working with the current configuration. I do like the idea of a secondary reset trigger that would kill the pilot, but that would require some potentially delicate rewiring so I think I’ll live with what I’ve got. At least the secondary pressuretrol can help prevent super high pressure if the primary fails.

Fred

There is no Thermostat that can manage Pressure. They were never designed for that. They are basically on/off switches based on room temperature. Find a programmable thermostat that allows you to set the number of cycles, from 1 through 5 or the basic thermostat that allows you to set the heat anticipator, manually. Honeywell makes several good models. Pressure, if the boiler is over-sized must be controlled by a Pressuretrol or a Vaporstat. If the boiler is properly sized, the Pressuretrol becomes a Safety device, and probably never engages in day to day operations.

mikeg2015

Thermostat isn’t supposed to manage pressure. Boiler is supposed to be sized so it never builds pressure except if maybe a radiator valve or vent is closed or a very long recovery. Size the boiler small enough then vent slow enough at the radiator to control and balance.

To control heat pumps and steam boiler you need a thermostat that allows cycles per hour or custom time delays and temp differentials. I’d also recommend balance point control using outdoor temperature. My Lennox S30 uses the later. But it’s pricey. Honeywell Vision Pro will do all of this and allow sensor averaging for multiple rooms or floors. But you need the top end model for OAT limits.

The real challenge is adding relays to turn on heat strips for defrost so you don’t have a cold draft but not call the boiler. Or only run heat pumps above about 35-38f and they almost never defrost.

2nd problem is the air handler keeps blowing air when boiler is calling. But there are some work around as depending on the model and setup. Or might be able to use relays again.

Alternate option is separate boiler thermostat. This ultimately probably works the best. You can still use isolation relays and have both stats call for the boiler if you want redundancy.

Last possibility is dual fuel setup on Visionpro thermostat but you tell it that the equipment controls the Fan on the fossil fuel. So aux call would not run the fan unless there’s a Y call. Air handler board would command the fan only.

Of all the options this may work best.

PeterD2

@mikeg2015 Thanks for all of the thermostat suggestions. I do have the steam and heat pump systems configured with a balance point based on outdoor temperature with overlap of steam and heat pump between 35 and 40F degrees through the thermostat. This works fine and heat pump doesn’t try to help below 35F. But as said, the termostat doesn’t have a mechanism for cycles so tends to overheat the boiler on colder days. Unfortunately, I can’t resize the boiler so it looks like I need to manage with the pressuretrols although I will research these other thermostats to see whether one of them would help.

AnthraciteEnergetics

I wired mine so the "operating" pressuretrol is in the main gas valve circuit after the ignition module, and the other "safety limit" in the 24 V loop with the LWCO, vent damper, rollout switch, etc. When the opearating pressuretrol turns off at 1.5 psi, it cuts off the main burner but leaves the pilot lit. This avoids having the whole ignition sequence from having to happen in order to cycle on pressure.

The pressure cycling only comes into play when I have radiators shut off in unused rooms and the system is recovering from a set back temperature or just really cold weather like we just had.

mikeg2015

I wonder if a aquastat could be installed on a radiator to inhibit the call. Maybe Use a 20f differential. Use a strap on bulb type and clamp it to maybe the last section of the radiator in the room with the thermostat. Open at 180f. Close at 160f. You’d have to play with it to find a sweet spot.

Actually. I like that idea. Might use it myself. Then when I replace my boiler, use it to cycle the boiler for a hot water zone.

PeterD2

Being able to control for the temperature at the radiator would be useful, but I'm not sure whether that would replace the need to control the steam pressure. In my case, my concern is that on cold days it takes a long time for my house to reach the thermostat's set point and when the boiler runs for a long time it builds up more pressure that is needed to deliver heat to the radiators. This means I'm burning more fuel than I really need to and it concerns me that drawing down the level of water in the tank because it has been turned into steam risks damaging the boiler. I noticed this when it was zero degrees F outside. Today it's over 30 and the boiler cycles are much shorter.

Also, a key factor is that I have day- and night-time settings on my thermostat (about a 10 degree differential). The morning warm-up is when the boiler needs to fire for the longest time and probably when the greatest pressure is built up.

So much to think about. I appreciate all of the help on this!

Fred

If you use set-backs with steam, it should never be more than 3 or 4 degrees. A 10 degree set-back makes it next to impossible for a boiler to bring the house and all of its contents back up to temp in a reasonable amount of time, and when it does, what do you do? You program the tstat so that the same cycle starts over again. I use to use a 4 degree set-back, several years ago and I learned it was more satisfactory for me to set the Tstat and leave it alone.

mikeg2015

> @Fred said:
> If you use set-backs with steam, it should never be more than 3 or 4 degrees. A 10 degree set-back makes it next to impossible for a boiler to bring the house and all of its contents back up to temp in a reasonable amount of time, and when it does, what do you do? You program the tstat so that the same cycle starts over again. I use to use a 4 degree set-back, several years ago and I learned it was more satisfactory for me to set the Tstat and leave it alone.

I recovered 15f in 2 hours when it was -12f outside from a 50f cold start. Haha. Not a normal condition. Flue damper motor failed when I was out of town and since I had heat pumps to prevent freezing. service tech came the next day.

Don’t tell me what’s not possible.

I don’t like cycling on pressure because it usually short cycles. You can really store up steam capacity.

PeterD2

I don't know how much energy I'm saving by having a significant nighttime set-back (on warmer nights the temperature doesn't usually drop more that 4 - 5 degrees), but my system had no trouble recovering from a 10 F deficit in what I consider to be a reasonable amount of time -- about 1 1/2 hours during our recent cold spell when it was 0 F outside. (I don't mind waking up to a cool house as long as it warms up fairly quickly.) But in such a case the issue seems to me to be preventing excess pressure if the boiler continues to fire as the house warms up. This is where the pressuretrols come in handy and, although I don't know how to define a short cycle, with the differential set to 1 on my primary pressuretrol the cycles seemed to have 5 minutes or more between firings and the water level in the boiler didn't get too low.