Pressuretrol 101

aat879

I thought I understood what a pressuretrol does, but now after reading and after discussing with contractors, I'm confused. So I thought I'd put a 101 question for newbies and homeowners like me trying to be informed users. Most of the threads I find on HH presume this basic knowledge.

I'll enumerate the questions. Please enumerate your responses for easy reference. Please also limit information to that which is applicable to low-pressure boilers for a typical home, not larger applications with multiple pressure controls (e.g., modulating pressuretrols).

1) Terminology. My understanding is that "cut-in" means the burner turns on and "cut-out" means that the burner shuts off. That's still confusing because, of course, the burner turns on when the thermostat calls for heat even if the boiler is cold and the pressure is at zero. So what does cut-in and cut-out mean?

2) Differential. This is fairly easy to understand if #1 is clarified. There is a differential setting between cut-in and cut-out, and some pressuretrols are additive (with cut-in on the outside like the common Honeywell) and some are substractive.

3) Function. Is the pressuretrol primarily a directive device (like a thermostat) or a safety device (like a low water cut off). Put differently: Does the pressuretrol, analogous to a thermostat, provide the desired minimum pressure at which the burner should turn on and the maximum pressure, if reached, at which it should shut off? Or is it meant, like a low water cutoff, ideally never to come into play unless the pressure reaches the cut-out pressure, at which point the system shuts off until the pressure lowers to the cut-in, at which the system resumes.

4) Setting. I know the setting of the pressuretrol should be low. But I've seen different guidance, and my understanding of this depends on an answer to #3. A lot of the guidance here, including of course Mr. Holohan, says put the cut-in at 1/2 and the differential at 1 (so cut-out at 1.5). But a contractor set mine at a cut-in of 2, explaining that the pressuretrol is a safety device and, because the system is sized correctly, should never even come into play; and that, if it does, it signals that there is a problem in the system that needs to be addressed. In other words, that the pressuretrol is not instructing the system to get to 2 psi but protecting the system in the event that that it reaches that pressure.

I, and hopefully a posterity of steam system homeowners, will appreciate your responses.

KC_Jones

1. You have to reach the cut out, before the cut in matters.  So it hits the high pressure setting and cuts out, then the pressure will drop and once it hits the cut in the boiler is allowed to fire.

2. Sounds like you have this one.

3. It’s a safety device.  Some will disagree, but if a contractor is planning on setting it for 1.5 psi and having it work as a control, that contractor is sizing boilers improperly.  I have a boiler sized by current standards and use vaporstats and they never activate even set at 4 ounces.  I still think my boiler is too big.

4. I completely agree with your contractor.  

neilc

I'll add to # 1,
you mention the thermostat,
the thermostat, Ptrol, low water cutoff, etcs, are all switches, wired in series, which all need to be made, for the boiler to fire,
until the thermostat calls, (switch closes), the Ptrol can't complete the circuit on its own,
so cut in, without the thermostat, or other safeties made, doesn't matter till all switches are closed,
and then it cuts out at that pressure setting.

Jamie Hall

@KC_Jones has pretty well covered it. Except for number 3.

Unhappily, number 3 -- is it a safety device or a control device -- is, as he says, the matter of much debate, most of which is caused by two factors: first, it could be either, and second, depending on the system, the boiler may -- or may not -- need control (modulation) as well as safety.

I'm going to be a little long-winded here, and I hope this doesn't bother you -- but it is a topic which needs clarity.

First, safety (and damage control). As a general rule, most boilers are slightly to grossly oversized with respect the rest of the system which they power. Therefore, if the boiler is allowed to run continuously for a long enough time steam pressure will increase. Can't help it. Now there is a mechanical pressure relief valve on all boilers -- for residential use it is usually set at 15 psi -- but there are two problems with that: first, a steam release into a residence or small building is, clearly, undesirable. Second, that pressure will usually prevent mechanical damage to the boiler and piping, but is much too high to prevent damage to other fittings, such as traps and vents. Therefore it is desirable to have a pressuretrol device which will stop the boiler if the pressure rises above some low value, just as there is a low water cutoff to stop the boiler if the water level drops below a safe level.

This is your safety pressuretrol. In most residential systems, this is an automatic reset pressuretrol -- that is, it has a cutout pressure at which it will stop the boiler, and a lower, cutin pressure, at which it will restart the burner if another control, such as a thermostat, is still calling for the boiler to run. If this is the only pressuretrol on the system, it has to be automatic, but it is combining a safety function as discussed here with a control function, as will be discussed below, which is undesirable. Further, this leads to it being set at an uncomfortable compromise (and, for some systems, simply incorrect) setting.

Ideally, like low water cutoffs and automatic water feeders, there should be a clear separation between the control function and the safety function, and the safety function should be provide, in both cases, by a manual reset control which, once activated and turning the boiler off, requires manual intervention to reset -- the idea being that someone will check and find out why it tripped.

So -- ideally, for safety, there should be a manual reset pressuretrol set at a pressure which is safe for the appliances on the system, but amply high enough to allow the system to operate. Perhaps 5 psi. There should also be a manual reset low water cutoff, set just above the absolute minimum safe water level in the boiler (there would also be an automatic reset low water cutoff combined with an automatic water feeder in that case, which latter will control the water level in the boiler at a suitable operating level).

With me so far?

Now control. As noted way back up there somewhere, the boiler in almost all steam systems is somewhere between slightly oversize and grossly oversized. Therefore, some means of control must be provided to adjust -- modulate -- the output of that boiler to match the demand from the system.

Perhaps a brief analogy will help here, as an aside. Consider your car or truck. In most vehicles, the maximum power output of the engine is grossly oversized in relation to the actual power demand of the vehicle. Therefore, a means is provided to control -- modulate -- the output of the engine to match the actual demand; the accelerator or throttle pedal. Were that not there, the only way you could control the output would be to actually turn the engine on or off as required (and notably this was, in fact, the way many early engines were controlled).

Now most residential and small commercial size steam boilers can't be fitted with an accelerator pedal -- they are either on or off -- so in order to control the output to match the system demand is, in fact, to turn them on or off. Is this ideal? Perhaps not -- but it is what it is.

This control function is provided by an automatic pressuretrol or vapourstat -- which should NOT be the one provided to set a safe upper limit to system pressure (see above). How important is this control function? Is it even necessary? The answers there are first, in some systems but not all it is very important, and second, not always.

A brief excursion into steam heating systems. There are two broad categories: one pipe steam, in which there is one pipe to each radiator and steam delivery is always on, but air venting to allow the steam to enter the radiator is controlled by a vent, and two pipe steam, in which there are two pipes, on of which sends steam to the radiator and the other of which allows air and condensate to leave. (there is a hybrid -- two pipe air vent -- which has its own quirks). Within two pipe steam, there are two sub classes -- those in which steam is prevented from leaving the radiators entirely by mechanical devices called traps, which physically close the outlet if steam is present, but can open to allow condensate and air to pass, and those in which steam input to each radiator is controlled to a rate no more than the radiator can condense, and which may or may not have traps on the outlets.

Curiously, one pipe steam systems and the first subclass of two pipe systems have similar control considerations. In these systems, if the boiler is closely sized to the system demand, the pressure will never rise above a reasonable functioning level -- say 1.5 to 2 psi. As the boiler becomes more oversized, it does become desirable to modulate the boiler output -- by turning it on and off -- to limit the pressure to roughly that level. There are a number of ways to do this, though by far the most common is a pressuretrol used as a control device which simply turns the boiler off at reasonable level and turns it back on once the system has caught up. Other approaches include timers which set a maximum run time for the boiler, or sensors which determine whether steam is or is not present at some distant point in the system.

The other subclass of two pipe systems, however, present a different problem: they will only function properly if the pressure while the system is running is actually maintained within a remarkably narrow band of pressure. Too low, and the radiators will not get enough steam to provide heat. Too high, and steam will be allowed in by the control (orifices, variable valves, etc.) and will enter the returns, which will disrupt the functioning of other radiators. Further, this pressure band -- for a variety of good reasons -- is remarkably low: typically from about 4 ounces per square inch to seven ounces per square inch. Since system demand is somewhat variable, this absolutely requires that a pressure control device will be used which will turn the boiler off when that maximum operating pressure is reached and turn it back on immediately the system pressure drops below the minimum operating pressure. (on a side note, to keep the on/off cycles reasonable, care must also be taken to match the boiler power as nearly as possible with the system demand -- but that is another issue).

While in one pipe steam systems and the first subclass of two pipe systems it is quite satisfactory to have the same pressuretrol acting as both a control device and a safety device,, this is not true of the second subclass of two pipe systems. In that second subclass, the system will operate poorly -- if at all -- if the pressure is allowed to rise above the maximum operating pressure, and further the boiler must come back on to avoid having the pressure drop significantly below the minimum operating pressure. So in those systems it is not even satisfactory, never mind desirable, to have the same pressure control device acting as both a safety and a control upper limit.

aat879

This is all wonderfully helpful and informative. Thank you @KC_Jones and @neilc for the quick summaries.

@Jamie Hall your dissertation-quality explanation really puts things in context. I appreciate your taking the time. Since I have a one-pipe system and don't have any experience with two-pipe, the last couple of paragraphs were a little opaque for me, but that's okay. I understand the relevant portion concerning a one-pipe system—namely, that the pressuretrol can effective serve both safety and control functions.

In my case, also, the steam contractor knew what he was doing and measured my radiators to ensure that the boiler was properly sized, unlikely the oversized old one, so my hope is that the pressuretrol will never even be needed in my case.

EBEBRATT-Ed

@aat879

On a normal residential boiler you need a operating control and a safety high limit control.

On steam the thermostat is the operating control and the pressure control is a safety control.

If the system is sized properly the high limit should not have to shut the boiler down under normal circumstances.

If because of improper sizing or other system variables the high limit frequently is called on to shut down the boiler than a second pressure control should be added