VaporStat vs. Pressuretrol

Kentucky_Steam

I've adjusted and made sure it's level (in case that matters) my Honeywell 404 (additive) Pressuretrol to a cut-in of .5 and a differential of 1.  As I watch it carefully, it cuts out at 3.3 and then back in at 2.3, so the differential portion of this works perfectly. But, I can't get the cut-in set any lower on the scale than the .5 minimum before it wants to slip off the threads of the setting screw and then I watch & see it operate (cut out) repeatly at exactly 3.3 PSI.  I've read (here and many places) that these are notoriously inaccurate instruments.  Will it make that much difference in my system's (2 pipe steam on 1 year old WM EG75) operation?  It heats quickly and nicely, but I wonder (from what I read) if I can do this with less natural gas being used.

I've got a new 1-10PSI pressure gauge installed, so I'm comfortable with it's reliability and accuracy.  Is it worth it to ask for (Christmas gift, maybe) a Honeywell VaporStat to get this thing operating at a lower pressure?  Does it really matter?  Those VaporStats are pricey...

Joe V_2

it made a difference for me

because at one psi I had water coming out the wet return vent and minor hammering.  At 6oz it is quiet, dry, fast and warm.  I would not have changed it if I didn't have the water issue.

Jamie Hall

Heresy...

I'll probably get roasted for this, but... if, and only if, your system is working well and quietly, and the vents are opening and closing properly, I'd be inclined to leave it alone.  As you so accurately note, vaporstats are pricey!  On the other hand, you might save enough on gas (and then again you might not) to pay for it.  Where I'd be more concerned, actually, is for the vents -- 3.3. psi is right up there at the rated working pressure of most vents.



In my own situation, I simply can't run that high -- there is a gadget on my system (a Hoffman Differential Loop) which does exactly what it is supposed to do -- at 8 ounces -- which effectively stops the system heating, burner or no burner, at that pressure.  So I have to have a vaporstat.



Christmas is coming!

Kentucky_Steam

I Do Have some hammering

House is 100 years old and the new boiler we have (WM EG75) was put in last year (one season old, now).  The old boiler we had (American Radiator) never had hammering, but it was sloooooow to heat and ran for loooong periods of time.

This one creates quite a bit of hammering on heating up (about 3-5 minutes worth) in a section of our house where we have some long horizontal runs (supply & returns).  No doubt there are some low spots out there, but our HVAC guy has not been able to locate them precisely, yet.  All our supply mains are well insulated so the steam does get to the rads very quickly now, although with some clanking to go with it.  I just assumed this was fast heat into long horizontal runs, but if this is also a pressure issue, maybe the VaporStat would help?

jpf321

just thought of this ...

there *may* be a calibration screw in there somewhere .. pop off any "silver" caps you see on the grey exterior of the box and look for a jewelers screw .. they exist on vaporstats .. maybe also on p-trol.

Kentucky_Steam

Hmmmm??

I believe this is the Micro-switch type (I sure don't see any Mercury bulb in it).  Didn't know if there were adjustment screws but I can go look and seen about that.  It hadn't occured to me to consider a "spacer" as you suggested.  I guess something, as opposed to nothing, would cause this thing to "tip over" and cut-out at a lower pressure.  I'll go look.

jpf321

speaking of tipping over ..

deleted

Joe V_2

Maybe it is just me

But I wouldn't tamper with a pressure controller.  If anything were to go wrong,  your insurance company won't cover you.  Then that vaporstat will look inexpensive in comparison.

If 3.3 is the lowest you can go, I'd consider replacing it with a new one and keeping the old as a spare.

You should be able to break on 1psi; two easily.

jpf321

joe is right..

the idea just got in my head..I shouldn't have suggested it

Oak Park Electric

Or you could...

Somebody here, maybe more than one, has a setup with two of everything.  One pigtail with a vaporstat and low pres gauge as the operating control, and a second one with the 30 psi gauge (as required by code) and the original pressuretrol as a backup high limit.  It certainly can't hurt...

Kentucky_Steam

OK, seems better now, but there's one more question...

I have adjusted my Pressuretrol now to the point where it will cut out at 1.9 lbs and kicks back in at 1.  That, I guess will have to do, as there are no more adjustments I can physically make.

My question now is that it seems to "cycle off" due to pressure frequently.  I guess it stands to reason that, with a lower cut out setting, it takes less time for the boiler to reach that pressure and then cut out and resume its task of satisfying the thermostat after pressure drops down to the "cut in point".

Is this the way it's supposed to be?  This is a sort of pulsing type of operation.  If you graphed the way it's running now (over time), it would show up as a type of pulsing... on a graph.  Is that more efficient than longer, more sustained runs (at higher pressure) to reach the thermostat's temperature?

Thoughts, anyone....????

nicholas bonham-carter

high back-pressure?

i would get some more main [not radiator]  venting on your system, which will let the air out quickly, and reduce the short-cycling/back-pressure you are now experiencing, as the burner attempts to squeeeeeeeeeeeze the air out of the constipated little openings of whatever vents you have now.

there still may be some short-cycling after the main pipes are full of steam, and the air is escaping the radiator vents, but it should be much less. a 0-3 psi gauge would give you a better "picture" of what is happening in the system.--nbc 

Jamie Hall

It's normal...

Your description of it as pulsing is rather nice; I'll remember that for the next discussion!



Changing the cutin and cutout pressures would make a difference in the time on/time off relationship of the pulses.  However, it would not make much difference -- if any -- in the overall efficiency of the system (if anything, it would be worse at higher pressures).



If you think of the way the system operates, you have on the one hand a reasonably constant load -- the condensing capability of the radiation.  On the other hand, you have a power supply (the boiler) feeding that load, but the boiler has excess capacity -- that is, it can feed more than the attached load.  Unfortunately, the boiler cannot be dialed down in an analogue way; it can only operate on and off.  Therefore, to match the load, it will operate with a certain duty cycle, where the percentage on times the capacity, minus losses, will just match the load.  Any longer on time will only increase the pressure (which doesn't increase the rate of condensation -- heat release -- in the radiators, just stresses things).  In the world of electronics and power control, it's called pulse modulation, in fact.  The trick is to optimize the frequency (time between on calls) and therefore pulse width to minimize losses to supply a particular load.  If it were possible to operate at a higher frequency the losses would be less; with burners, though, there are minimum times required to establish stable (high efficiency) combustion -- that tends to lead to lower frequency operation.  On the other hand, you don't want the boiler and the water to cool too much, as that leads to inefficiency from warming the boiler and the water back up to boiling.



I really don't think I've ever seen a study on optimizing the frequency for boilers, now that I think about it.  I have a nasty suspicion that it would be different for every combination of burner and boiler...



In the meantime, though, it is better to have the cutout pressure as low as reasonable.  I doubt that it is possible to get the frequency low enough to get into the range of a really significant drop in efficiency from cooling, unless the boiler is really badly oversized for the load.



Slightly confused.  Where's my second cup of coffee?

Kentucky_Steam

Thanks

I'm at home today (it's cold here... where ISN'T it cold today?) and I'll go down there and time this thing a bit.  I have a stop watch and my second cup of coffee in hand.  I want to see what the burn and rest times are (based on my pressure-trol settings) when the thermostat is calling for heat.

Kentucky_Steam

Back now, after my "timing run"

Stop-watch in hand, I checked it.  I watched the water level carefully to make sure the cut-out and cut-in I was timing were pressure related only... water level didn't interfer with my timing here.

It is running, I timed it as it hit cut-out and that's at 2.0 lbs.  It took 1:39 (that's minutes & seconds) for the pressure to fall from cut-out to the cut-in, differential is 1 lb.  It then took 1:57 to get back up to the cut-out point (2.0 lbs) and kick off again.  That's close to a 50% cycle due to pressure control when t-stat is calling for heat and the water level doesn't affect it.

I haven't timed it to see how long it takes to reach the 2.0 lbs point from a starting reading of 0 lbs.  I figured this on-off cycling while running is probably typical of what's happening down there in my chilly basement.  I've got great insulation on all my pipes down there.  I did that when they put in my new boiler.  The basement used to be consistently warm (old boiler, poor mains insulation).   Now it stays cool.  I'm in favor of getting the heat to the parts of the house we live in.