Help me understand vaporstats

delcrossv

So i understand the basic pressuretrol- it's a pressure based hi-limit. Go over the limit and the burner shuts off. Gotcha.

What I don't get is the differential on a vaporstat. What I get is it's some kind of lo-limit, but what I don't get is how it does anything.

Thermostat calls for heat, boiler fires and as the water boils and the vents close the pressure goes up- but that pressure is dependent on pipe drag and rate of condensation for that cycle. I don't see how a control will change that. It's not like the vaporstat will make the boiler keep firing after the thermostat is satisfied, yes? And if the thermostat isn't satisfied, the boiler keeps firing anyway and raising the pressure.

I may have this all wrong, but I'd like to know what the differential is supposed to DO.

Thanks for answering what must be a "duh, obvious" question to the pro's here.

ratio

The vaporstat is nothing more than a pressuretrol (allegedly) built to trip at much lower pressures than a pressuretrol. It's job is to turn off the flames if pressure starts to build, indicating that steam isn't being condensed at the rate it's being produced.

EBEBRATT-Ed

As @ratio said it's the same as a pressure control it just operated at a lower pressure.

Maybe the confusion is in the scale and how to adjust it.

All controls have a "cut out pressure" ....pressure burner shuts off at

a "cut in pressure".......pressure burner starts at

a "differential"...........difference between the cut out and the cut in

Some controls are marked "Main" & "differential"

So on those "Main" = Cut out pressure

"cut in= Cut in pressure

There is no scale or setting for diff. it's the difference between the two


On the other type of controls You set the "cut in pressure".....pressure burner starts at

And you set the "differential pressure".........The difference between cut in & cut out..........sometimes this setting is inside the control

There is no setting for "cut out" it is cut in plus the diff

Jamie Hall

To add to that. Somewhere on a pressuretrol or vapourstat there will be a note as to whether the differential is subtractive or additive. Almost all vapourstats which have a clear front panel the differential is subtractive. I'm looking at a Honeywell L408J as I type this. There are two scales: the main scale (on the right) and the differential (on the left). In this vapourstat -- and any others like it -- the main scale sets the pressure at which the burner should shut off. The cutout. The differential sets how far the pressure must drop before the burner should turn back on. The cutin pressure, then is found by subtracting that differential from the cutout main scale value.

Note that pressuretrols of the more conventional blue grey box with one scale are completely different: the value shown on the scale on the face is the cutin pressure, and the differential is controlled by the dial inside the box, so the cutout pressure is the value on the face scale plus the differential.

So to really answer, yes the differential on a vapoustat sets the low limit at which the burner should start again, but only indirectly.

Neither a pressuretrol no a vapourstat will keep the boiler firing after the cutout pressure is reached at least as they are used in most residential applications. In fact, the whole purpose of the control is to shut the burner off when the set pressure is reached. Unhappily in most residential applications there is just the one vapourstat or pressuretrol -- and in that setup it is acting both as a control device intended to keep the steam pressure within the correct operating range for the system and as a safety device, intended to keep the steam pressure from getting too high.

The control device is needed in most systems because the boiler steam output is greater than the capability of the system to condense the steam. Thus the boiler output has to be modulated so its output matches the system. There are many ways to do this, and there have been many and long discussions on The Wall about this -- but the simplest way to do it is to simply turn the boiler off when the pressure gets a bit high and turn it back on when it drops some. That's the job of the vapourstat (or pressuretrol) when used as a control device. This cycling on and off is entirely normal and will continue so long as the thermostat is calling for heat.

You say that if the thermostat isn't satisfied " the boiler keeps firing anyway and raising the pressure:" That should not happen if the vapourstat or pressuretrol is properly adjusted. The system should cycle once the cutout pressure is reached the first time.

The amount of cycling -- and how soon it begins on any given system -- is entirely dependent on how closely matched (or badly mismatched!) the boiler and the system are. Ideally, of course, it would never happen -- the boiler's steaming rate would exactly equal the system's condensing rate and the pressure would never rise over a few ounces (that pipe friction you mentioned).

I could add at the risk of becoming long-winded that the ideal operating pressure range is different for different steam systems. Most one pipe systems and many two pipe systems seem to operate best with the pressure held between around 0.5 to 0.6 psi and 1.5 to 1.6 psi. What are termed "vapour" systems, however, require a narrower and lower pressure range, typically with a low (cutin) around 3 to 4 ounces per square inch and a high (cutout) around 6 to 7 ounces.

delcrossv

Very helpful guys!

So let me see if I get this, using the vaporstat as a control only (separate pressuretrol)

Call for heat. Burner fires, pressure climbs to ?WHAT do I set this for?- less than max pressure on the cutout pressuretrol?) then when it drops to 2 times the calculated drag (per LAOSH) it clicks back on and gives another burst? OK now it makes some sense.

So with a vaporstat and a pressuretrol do I wire these in parallel at the pressure terminals at the gas valve? or in series? Thinking series as both would have to prove for it to fire.

Last run on the boiler it was running at 3 ounces or so. 40's and windy with an old leaky building. Present pressuretrol is set at 1psi.

FWIW, I'd be looking at the Honeywell one.

Jamie Hall

Well, yes -- less than the cutout on the pressuretrol, which is going to be your safety.

The two are wired in series -- along with all the other safeties.

You can try to estimate the proper pressures mathematically and with tables. However, there is a better way to do it -- if you have a low pressure gauge (0 to 3 or 0 to 5 psi) and some patience. It will also tell you, in the process, if you have adequate main venting.

Here's how. Start with the system cold or at least cool. Start the burner by raising the thermostat to some outrageous value. Now. Watch the low pressure gauge. What you should see is ... nothing. Until the boiler starts to make steam. At that point, you should see the low pressure gauge rise to perhaps a few ounces -- maybe as much as half a pound (8 ounces) in some one pipe systems. Now the speed of the pressure rise should slow way down or even stop. It should just sit there at a few ounces. For quite a while. This is where patience comes in. In a well matched system it could be quite a while (for Cedric the system I care for the "quite a while" is around 45 minutes to go from 2 ounces to 5 ounces), but it could be as little as say 10 minutes or even less). Once all the radiators are really full and the vents are all closed, the pressure will start to rise again. Let it rise a few ounces -- and that is the pressure at which you want the burner to cut out, so that is where you set your cutout. If you are prompt, you can even adjust the main dial at this time -- just lower the setting until it just cuts out (this gets around the somewhat doubtful accuracy of the pressure settings to which @ratio alluded -- vapourstats are remarkably precise, but not so accurate). The differential should be set to about half of what you set the cutout for if the cutout is low -- say 7 ounces -- or for a higher main setting, at about whatever it takes to bring the cutin pressure to around 3 ounces or so.

Except for waiting for the system pressure to start the second rise, it takes longer to write than to do...

The pressuretrol is now a safety and nothing more. Set it for something reasonable like a pound or two for a cutin.

Now.

I mentioned main venting. Let us suppose you start as above, but instead of the pressure rising to some low value and then sitting there for a significant time it continues to rise. That is an indication that you don't have enough main venting. Helpful (if it does stablise, equally helpful -- you have enough main venting!).

Don't forget to turn the thermostat back down...

delcrossv

Pretty stable at 3 oz. I have a 0-18 oz gauge as the low pressure one. The 0-5 psi one was way too high a scale.

I imagine the lower I make the cutout, the more it will cycle? The cut-in pressure should be the lower "stable" pressure? Or lower than that?

KC_Jones

If you’re only getting to 3 ounces, why do you want to add a vaporstat and cause it to cycle?

Just to be clear any of these devices is technically a safety not an operating control.

delcrossv

KC_Jones said:

If you’re only getting to 3 ounces, why do you want to add a vaporstat and cause it to cycle?

Just to be clear any of these devices is technically a safety not an operating control.

Figure it would save fuel it have it cycle when it gets cold rather than run until the t-stat shuts it off. If I set it for 85 degrees it'll run all the way up to 1 psi.

Jamie Hall

KC_Jones said:

If you’re only getting to 3 ounces, why do you want to add a vaporstat and cause it to cycle?

Just to be clear any of these devices is technically a safety not an operating control.

delcrossv said:

Pretty stable at 3 oz. I have a 0-18 oz gauge as the low pressure one. The 0-5 psi one was way too high a scale.

I imagine the lower I make the cutout, the more it will cycle? The cut-in pressure should be the lower "stable" pressure? Or lower than that?

I would set the vapourstat at around 6 to 7ounces, @delcrossv . Differential at 4 -- but be sure that it will, in fact, cut back in.

In principle, @KC_Jones , you are right -- why bother. However, unless your system is blessed by being exactly on balance between steam creation and steam condensing, eventually -- as I say, it may take a long time -- the pressure will start to rise again. Now. If that system is one pipe steam, or two pipe with only traps for control, it may not matter. However, if you are running a vapour system, as I noted in a previous comment above, the system will only run properly within a remarkably narrow pressure band -- in the case of Hoffman Equipped systems, with which I am most familiar, only between around 4 ounces and 7 ounces pressure differential between the steam mains and the dry returns. This is true of any system which depends on calibrated valves or orifices.

This has nothing to do with safety. The system will be perfectly safe up to around 5 to 7 psig. No problem. It just won't run well if at all. It has to do with control. I apologise for being a bit pedantic, perhaps, but in these systems the vapourstat is a control device, not a safety, and mustn't be thought of or treated as one (an interesting comparison is in switched AC to DC voltage controlled power supplies -- there the control necessary for the controlled DC output is achieved by switching the AC on and off with the correct duty cycle to achieve the necessary DC output)(or, if you are mechanically inclined, the power output of most diesel engines is controlled by exactly how long an injector is open on each ignition event -- same principle)(or another -- one doesn't control the mood lighting in the dining room with a circuit breaker..)(or yet another -- early aircraft engines were either on or off. To control your speed or rate of climb you switched the thing on or off as required).

I will happily grant that the required control function would be better served by modulating the burner firing rate in response to the system pressure. To the best of my knowledge, on smaller systems such as we are usually dealing with this is simply not feasible for oil, and I don't know of any modulating gas steam boilers which respond to pressure (I would hasten to note that on large power boilers, this is in fact how the output pressure is maintained -- modulating the burners -- but that's a very different ballgame.).

I would also note, on an historical basis, that in the days of coal many vapour systems did in fact control the firing rate in response to pressure, and were able to maintain a very even pressure. This was done by controlling the draught in response to the pressure, and was entirely mechanical. Unhappily combustion efficiency at low fire was pretty awful (it wasn't all that great at full song, comes to that).

And finally, it is not wise to attempt to take a device functioning as a control element -- in the present case, a vapourstat -- and also ask it to function as a safety device. You absolutely should have at least one pressuretrol set at a higher, but safe, pressure, as the safety. I am frankly horrified at the number of boiler installations which I see where there is only one device -- usually a pressuretrol -- which is trying to be both a safety and a control. I am even more horrified when I see two or more devices -- a control device, perhaps, and a safety -- piped on a single pigtail creating a single point failure scenario.

delcrossv

Jamie Hall said:

KC_Jones said:

If you’re only getting to 3 ounces, why do you want to add a vaporstat and cause it to cycle?

Just to be clear any of these devices is technically a safety not an operating control.

delcrossv said:

Pretty stable at 3 oz. I have a 0-18 oz gauge as the low pressure one. The 0-5 psi one was way too high a scale.

I imagine the lower I make the cutout, the more it will cycle? The cut-in pressure should be the lower "stable" pressure? Or lower than that?

I would set the vapourstat at around 6 to 7ounces, @delcrossv . Differential at 4 -- but be sure that it will, in fact, cut back in.

.

@Jamie Hall
And if it doesn't cut back in, reduce or increase the differential? Since it's subtractive, Increasing it lowers the cut in pressure, yes?

I can take a second pigtail off the relief valve tap with a street tee, so I'm good there.

Jamie Hall

"And if it doesn't cut back in, reduce or increase the differential? Since it's subtractive, Increasing it lowers the cut in pressure, yes?"

You got it -- so if it doesn't cut back in, you need to reduce the differential. The new vapourstats with the mechanical switches can be a bit sticky about switching at very low pressures, like 1 or 2 ounces...

ChrisJ

@Jamie Hall Are you suggesting my system is blessed?


Do you have to pay more for a Vapourstat than a Vaporstat?

ratio

ChrisJ said:

Do you have to pay more for a Vapourstat than a Vaporstat?

Well, there's certainly import duties.

Jamie Hall

old habits die hard, folks...

delcrossv

Last question. Set this as a hi-limit so a 1009? or as a lo limit a 1025? Break on pressure fall doesn't seem to make sense in my application (???) -Pressure drops so boiler shuts off doesn't seem to make much sense.)

EBEBRATT-Ed

@delcrossv

It must break on a pressure rise to shut down the burner

ChrisJ

delcrossv said:

Last question. Set this as a hi-limit so a 1009? or as a lo limit a 1025? Break on pressure fall doesn't seem to make sense in my application (???) -Pressure drops so boiler shuts off doesn't seem to make much sense.)

The L408J1009 would work, but I feel a L408J1017 is better as a "normal control".

KC_Jones

No matter what, if you get one, bench test it first, they are notoriously inaccurate.

I have 2 on my boiler, and when I bench tested them, neither was correct and they didn't even agree with each other.

I ended up just setting them to what I wanted based on the gauge and ignoring the scale on the front.

delcrossv

ChrisJ said:

delcrossv said:

Last question. Set this as a hi-limit so a 1009? or as a lo limit a 1025? Break on pressure fall doesn't seem to make sense in my application (???) -Pressure drops so boiler shuts off doesn't seem to make much sense.)

The L408J1009 would work, but I feel a L408J1017 is better as a "normal control".

0-4 psi is way too coarse a control. I already have a pressuretrol set at 1psi that I'm keeping as the safety. (I may kick that up to 1.5 psi for some headroom) This would be for low pressure modulation only- on at 2oz, off at 6oz.

Thanks!

pecmsg

KC_Jones said:

No matter what, if you get one, bench test it first, they are notoriously inaccurate.

I have 2 on my boiler, and when I bench tested them, neither was correct and they didn't even agree with each other.

I ended up just setting them to what I wanted based on the gauge and ignoring the scale on the front.

The dial on any mechanical control is a Guide only. You always need an accurate gauges to determine where you are.

Jamie Hall

delcrossv said:

ChrisJ said:

delcrossv said:

Last question. Set this as a hi-limit so a 1009? or as a lo limit a 1025? Break on pressure fall doesn't seem to make sense in my application (???) -Pressure drops so boiler shuts off doesn't seem to make much sense.)

The L408J1009 would work, but I feel a L408J1017 is better as a "normal control".

0-4 psi is way too coarse a control. I already have a pressuretrol set at 1psi that I'm keeping as the safety. (I may kick that up to 1.5 psi for some headroom) This would be for low pressure modulation only- on at 2oz, off at 6oz.

Thanks!

Good choice, @delcrossv . As a more general guide, you want to have the adjustment and sensitivity of a control or measurement device appropriate to the values you want the device to operate or measure at. Hence, for your application -- 2 to 6 ounces -- a 0 to 16 ounce vapourstat is correct.

There is a reason for this. Regardless of the accuracy problems mentioned above, there is another aspect to control which is often overlooked: precision. Precision is the measure of how variable a given measurement is -- that is to say, in this case, if you want the control to open at, let's say, 6 ounces, but it operates at 6.8 ounces, that's accuracy. You can adjust for that. But you want it to operate at the same value every time -- if it's 68.8 ounces, so be it, but if it's 6.8 one time, 7.4 the next, 6.0 the next, and so on, that's not much use. Very imprecise. And the closer the range of the device is to the meauured parameter, all else equal, theJ better the precision. The L408J1017 is a fine device, but it's precision is about one fourth that of the L408J1009

delcrossv

Yep. As my Dad used to say: " You don't measure bearing clearances with a yardstick".

ChrisJ

I am curious,

Why are you setting this up @delcrossv?

This is a standard single pipe system, no?

Sure, you don't measure bearing clearances with a yard stick.
But you also don't frame a house using a micrometer. The correct tool for the job is important in both directions.

ChrisJ

Sometimes I go too far it's true..

But I try not to.

delcrossv

I said it above, I hope to see some fuel savings / improved comfort if I modulate the output rather than running one long burn. I notice some rads get steam after the burner shuts down so shorter "pulses" should even things out, yes? I'm already highly vented on the mains, so it isn't that. There is a lot of thermal mass in the system (big cast rads)

ChrisJ

delcrossv said:

I said it above, I hope to see some fuel savings / improved comfort if I modulate the output rather than running one long burn. I notice some rads get steam after the burner shuts down so shorter "pulses" should even things out, yes?

If I was a betting man, I'd bet cycling the burner in that manner will use more fuel, not less.
I shut my system down for 10 minutes if it hits 4 ounces. But I do this for noise reasons and to keep things "normal" when too many TRV's aren't calling for steam. I've yet to see it actually happen, but in theory it can.

I'd say my system is most efficient when doing a long recovery as the burner runs for a very long time and pressure stays very low, below 1 ounce.

The claims are every time you start and stop a burner a small amount of fuel is wasted. I haven't seen proof of this but that's the theory.

pecmsg

delcrossv said:

I said it above, I hope to see some fuel savings / improved comfort if I modulate the output rather than running one long burn. I notice some rads get steam after the burner shuts down so shorter "pulses" should even things out, yes? I'm already highly vented on the mains, so it isn't that. There is a lot of thermal mass in the system (big cast rads)

Seeing how most units need 15 minuets run time before stabilizing I don't think your going to see savings!

delcrossv

pecmsg said:

delcrossv said:

I said it above, I hope to see some fuel savings / improved comfort if I modulate the output rather than running one long burn. I notice some rads get steam after the burner shuts down so shorter "pulses" should even things out, yes? I'm already highly vented on the mains, so it isn't that. There is a lot of thermal mass in the system (big cast rads)

Seeing how most units need 15 minuets run time before stabilizing I don't think your going to see savings!

So why do people install these then? If not to provide a low pressure cutout and modulate on pressure?
Got me confused again...

@ChrisJ
Mine is at 3 oz after 10 min (?) Longer runs? Then, after the burner shuts off, far rads continue to heat. So if I can drag out the thermostat call time I'd hope that'd even things out.

Jamie Hall

"Mine is at 3 oz after 10 min (?) Longer runs? Then, after the burner shuts off, far rads continue to heat. So if I can drag out the thermostat call time I'd hope that'd even things out."

Quite true... but the radiators don't begin to heat for quite some time after the burner fires. On most systems it's pretty close to even, but folks don't seem to take into account that startup lag. Don't know why.

delcrossv

So, at the end of the day, does it make sense to hook this up to a 1 pipe system?  Or just run it with the present  1 psi pressuretrol?

Jamie Hall

Much as I like fancy ... I personally think that on a one pipe system which is perfectly comfortable at a maximum pressure of around 1.5 to 2 psi, I probably wouldn't bother with a vapourstat. What I would do is have two pressuretrols. One of them -- the one you have -- with a cutin set as low as is feasible (don't go all the way down the scale -- they sometimes fall apart!) -- say 0.6 psi -- and leave the differential at 1.

Most of the time it won't do anything. Probably only on a very long run.

However, I would have another one, different pigtail, manual reset, set to cutout at around 5 psi. That one will be your real safety (see above comments on safety vs. control!). Why manual reset? Because if it trips something is decidedly wrong, and you want to be obliged to actually go down there and stare at the boiler and figure out why it tripped.

From your comments on pressure, I would say you are probably in a very good place as far as main venting goes.

delcrossv

@Jamie Hall
Thanks, makes sense. So where do people use these?

gfrbrookline

On 1 pipe systems that are very dialed in with venting, main and rads, and can run on pressures under 16oz. That being said if you can't find an old mercury vaporstat don't waste your time and money. The new snap units aren't that good.

Jamie Hall

delcrossv said:

@Jamie Hall
Thanks, makes sense. So where do people use these?

Vapour systems -- where tight control of pressure differential is critical to the correct and balanced operation of the system.

delcrossv

gfrbrookline said:

On 1 pipe systems that are very dialed in with venting, main and rads, and can run on pressures under 16oz. That being said if you can't find an old mercury vaporstat don't waste your time and money. The new snap units aren't that good.

I'm pretty much there. System is usually running at 3 oz or so. But no way I'm dropping 6 beans on a Mercoid switch.