Vaporstat in a Vacuum system

StudyingHard

Hi, I just purchased a vaporstat, and am having nagging second thoughts; have not installed it yet.  My system is a two-pipe 1920's Trane vacuum vapor system that is still working with Vacuum.  Boiler is a Slant/Fin.   Vents are Hoffman 76 vacuum vents, still working well.

When the t-stat is satisfied, the system can go into minus 15 psi. For example, when I do a 5 degree setback for the evening.

I have been unable to locate specs for the Vaporstat stating how much negative pressure it can handle without losing its innards.  Does anybody have any idea?

nicholas bonham-carter

extreme vacuum system

firstly, i would make sure that your gauge, and all of its readings are correct. the -15 psi sounds like the vacuum of deep space! how long is that vacuum maintained, between cycles? if the gauge is the same age as the rest of the system, then i would suspect the bourdon tube to be work-hardened and inaccurate. if the vacuum never persists until the next cycle, then you may want to drop the vacuum features and go with positive pressure, ample main-line venting, etc.

don't forget to measure the high pressure as well. a ball valve isolating the new gaugestore.com gauge may be necessary to protect it in the vacuum phase.

if your boiler has been sized correctly, then it may never reach an excessively high pressure. --nbc

Jamie Hall

Also check...

the units on the gauge -- are you sure it's reading in psi and not inches of water?



The vaporstat should be able to withstand any reasonable vacuum... so far as that goes.



I would differ very slightly with nbc -- if your system is really holding a good vacuum for a while, and it heats reasonably evenly, if I did anything at all in terms of improving the venting (which I might be inclined to do) I'd create an antler arrangement and put another Hoffman 76 on it, or two.  There is no reason to convert a system which is actually holding its vacuum to an atmospheric system (I do believe in if it ain't broke, don't fix it); the main reason we tend to is two fold -- first, the Hoffman 76 is the only vacuum vent which is still sort of available, and second it isn't all that big.



This assumes that the 76 is big enough to vent the system without having the boiler cycle on pressure early in the cycle... but from the sound of your post, that's not a problem.

StudyingHard

Questionable gauge and venting

Thanks to both of you.  Yes the gauge is in PSI,  and an alternate scale in "Hg.   The -15 PSI reading shows a matching value of  -0.5 "Hg scale.   Deep space, fer sure.



nbc: as per your suggestion, I will be buying a new gauge, just 'cause.  That is, just 'cause I didn't buy the existing one, and don't know its age nor pedigree.  At least 10 years old, maybe more.



Concerning the 76's, I believe them to be adequate. 

Reasoning: There are two Main Line loops with a 76 at the end of each, then there is another 76 on the manifold where the three separate dry returns merge before dropping to the wet returns line. 

The deep vacuum will hold for about 4 to 6 hours when I set it back overnight.   Last week I noticed that, starting from a vacuum of about -8 or so, and with the system having been off for a short period of time (half hour maybe),  I had heat in the riser within 2 minutes of firing, and had warmth at the end of the mains within 5 minutes.

That seems to be pretty good venting, although the jury may not yet be in. 



I am currently running with the Pressuretrol at ON at 0.5 and OFF at 2.0  

StudyingHard

the rest of my answer

oops, lost the last two paragraphs;



I am having the read the operating pressures with a magnifying glass since the existing gauge is ranged -30 PSI to +30 PSI.   When I install the Vaporstat,  I will also get a 0-5 PSI gauge.   And I will keep the Pressuretrol as a backup at 10 PSI cut-off.



Here is the original post of my system about a year ago:



http://www.heatinghelp.com/forum-thread/129460/One-foot-in-each-century-please-advise



Jamie confirmed my suspicions about the front Main feeding into the tail-end of the dry returns.  Following your suggestion, I fixed that early this year.  WOW, what a difference in max heat in the radiators, and in the radiator heat-up times.   Quicker, Hotter.



This summer I will add a drop header and replace all of the near-boiler piping with Iron.  Will keep you guys posted on my progress.



Thanks

Jamie Hall

There's no good reason...

why a vapour system in good condition shouldn't pull a deep vacuum when the burner shuts off -- after all, it holds pressure the other way!



Purely as an experiment -- when you get the vapourstat, you might try setting it at say 8" or so as a cutout -- but set the differential at a bigger number.  Since your system will pull a vacuum consistently, it should cycle back on eventually.  If it works -- and I've never tried it so I can't tell you whether it will or not! -- that could give you a significantly longer off time if and when the system cycles on pressure, and potentially somewhat better efficiency.



Maybe.  As I say, I've never tried it so I don't know if it will work... !

StudyingHard

Working in a vacuum :-)

Will do.  I have been thinking about that also.  I think I have a perfect setup to do some experiments.  Just need better gauges, etc.



It will be a few weeks though until I can "get round to it".   I will keep you posted.



Ray

Dave in QCA

Something is wrong here

The different scale that you say are on your gauge don't make sense.  Are you sure you are reating the scales correctly?  It sounds like your guauge may have negative PSI and negative inches of H2O.  Then your numbers would make sense, but not for inches of Hg.  Here is a conversion chart.

1 pound per square inch = 144 pounds per square foot = 0.068 atmosphere = 2.042 inches of mercury at 62˚ F = 27.7 inches of water at 62° = 2.31 feet of water at 62°

StudyingHard

Gauge scales

Good Question, Dave at QCA (Davenport, Bettendorf, Rock Island, Moline, and all points west)



I am away from the house this week.  I have a new gauge on the way, and will take pics

next week of the old gauge in its vacuum state. 



I am curious also.  The inner scale reads in  bar, showing about 0.5 at around -10 psi

on the outer scale.  However, at the bottom of the gauge, away from the scales, there is

a label    "Hg    The peg on the outer scale is -30 psi as I can best remember.   All will come clear next week with close-up photos of the gauge, which I will post along about Wednesday of next week.   Thanks for your thoughts.  Stay dry in QCA :-)



Ray

StudyingHard

Dave, its in InHg

Ok, duh!  I have been mis-reading the outer vacuum scale as psi rather than InHg of vacuum.



There are three different scales on the existing gauge.  the outer scale for positive pressure is 0-30 psi,  the outer scale for vacuum is from 0-30 InHg (inches of mercury).  The inner scale reads from 1 bar at full vacuum to 0 bar at 0 psi,  then 0 to 2 bar at about 30 psi. 

StudyingHard

nbc, my apologies

to you, its only half-way to deep space!



Thanks for the tip on the ball valve; I will do that when I get the 0-3 psi gauge installed.

And, will only open the valve when the pressure is above 0 psi.



Ray

StudyingHard

I need a low pressure Compound (vacuum/pressure) gauge

and can't find one anywhere.  



Low pressure meaning -3 psi to 3 psi.

Does anyone know of a source?  Preferably under $40.



As per Jamie's suggestion I am going to try setting the Vaporstat to a low Break, maybe 4 to 8 ounces, with a 16 ounce subtractive Make.   Hopefully, I will be able to run in a vacuum state for most of  the firing cycle.

nicholas bonham-carter

pressure gauge

if you can find some mercury, you could make a manometer which would handle the range of pressures, from negative to positive.--nbc