L408A 1157 vaporstat behavior

SusanC

My L408A 1157 Vaporstat has been installed in the same manner since 2007-2008.  At annual maintenance each year the pipe connections are checked and reconnected as are those to the pressure gauges..  Starting in the 2011-2012 heating season and increasing in this 2012-2013 season the vaporstat exhibits this strange behavior.  For clarity I will define my terms:  heating session means the whole time of boiler operation including offs and ons for pressure; heating cycle refers to off on pressure (top psi reached), then on at the lower make psi.  My vaporstat is set to break at 1.5 psi and make at 0.5 psi.  In a heating session (e.g. coming up to temperature from 2 degree setback)  the vaporstat makes at too high a psi in the first 1 or 2 or maybe even 3 cycles; after that it is right on the money.  It always breaks at 1.5 psi by both pressure gauges, but in the first cycle it makes already above 1 psi; the next cycle may be the same and by the next or the one after that and thereafter it makes when it should.  It seems that somehow the mercury flops back when the pressure is still higher than the make pressure (like it is not heavy enough at what I will call the off (far left) end of the mercury-containing tube) or somehow on early cycles the pressure doesn't stay steady on the diaphragm but does everywhere else.  It's not the pressure gauges which are on a connection off the same pipe (all above the siphon, of course) and, as mentioned, the set-up has been the same for about 6 years.  One way I can tell that the problem isn't a general system early pressure drop, in addition to observing the psi on the gauges, is that the period that the boiler is on or off on pressure is much shorter on the cycles when the vaporstat makes way too early. I am very familiar with the system cycle timings since I have observed, not only make and break psi's but also boiler on and off time periods between make and break cycles since installation of the new system in 2007.  What has changed is the make psi in early, but not later, cycles.

I am loathe to get an equivalent non-mercury-containing vaporstat because, from all I hear, they are even worse than those using mercury.  Does anyone have any idea why my vaporstat always breaks exactly as set and is fine on make after the first 1-3 cycles, but makes way too early on the first 1-3?

Rod

Tilting Vaporstat?

Hi- Is your pigtail oriented properly? If you are standing in front of your vaporstat, you shouldn't be able to see the pigtail's loop. It's important that Mercury vaporstats are level especially from side to side.(on the newer non mercury units, this isn't as important)

If the loop is oriented the wrong way, what maybe happening is that when the loop heats up and expands, this tilts the vaporstat, changing the setting. On a pigtail oriented the wrong way, If the level and settings were checked when the pigtail was hot they would change when the pigtail cooled.

- Rod

Jamie Hall

As Rod said

leveling the mercury type vapourstat is really critical (but do keep it -- quite right; the new ones aren't as precise).  It could be the pigtail oriented a little off; it could also be that the vapourstat isn't set quite level to begin with.  There is a little pendulum with an arrow in the back, on the right, and an index on the case.  Ideally the arrow should be right on the index when the unit is level, and that should be right.  However, if you have a very accurate low pressure gauge you can double check by noting the pressure at which it breaks -- it should, of course, be what it's set for!



The return seems to be a little flaky sometimes -- mine does exactly the opposite of yours; on the first cycle it is very slow to return, but on later ones quicker.  No, I don't know why...

nicholas bonham-carter

Strange behaviour

If the main venting were bad, it might take 3 cycles to get the air out of the system. Each time the burner cuts off, the resulting vacuum will be diluted by any remaining air in the system, effecting the pressure reading (especially low pressures). Do you have a low-pressure gauge, and if so what is the back-pressure of venting. Put on enough venting to show 2 ounces during the air escape phase.

This is why I think the dry returns should always be vented, and not merely the mains past the last riser, ( excepting a counter flow system).--NBC

SusanC

Vaporstat L408A

The pigtail is oriented correctly (no donut hole to look through) and, as I said, this exact set-up was perfect until the last 2 heating seasons 2011-2013. 

SusanC

L408A

The pigtail is oriented correctly (no donut hole to look through) and, as I said, this exact set-up was perfect until the last 2 heating seasons 2011-2013. All is checked at annual maintenance.  I would think the leveling might be relevant here because we have to have it off a little for the vaporstat to work correctly, which it did in 2007, 2008, 2009, 2010 and last half at least of 2011.  In fact in those years it was extremely consistent in making and breaking behavior and, as noted, the break is extremely consistent now, right on the money at 1.5 psi; it never fails to break exactly at that pressure.  I would believe the leveling was the problem if this behavior occurred in 2007-2010+ but why would it start after 5 years?  Also, why would it occur only on the first 1 or 2 break and make cycles. 

SusanC

L408A

Not sure if I have to reply to each individual or just to the thread so I've included relevant parts of the whole reply to different responders.  This is the whole response.  As for venting, please note that both pressure gauges are reading consistently and vaporstat breaking is extremely consistent -- the weird behavior is noted only in the vaporstat and only on making and only on early cycles.  If the problem were venting and thus more rapid pressure fall, wouldn't that show on the pressure gauges also?

----------------------------

Whole response:

The pigtail is oriented correctly (no donut hole to look through) and, as I said, this exact set-up was perfect until the last 2 heating seasons 2011-2013.  Venting hasn't changed and all is checked at annual maintenance.  I would think the leveling might be relevant here because we have to have it off a little for the vaporstat to work correctly, which it did in 2007, 2008, 2009, 2010 and last half at least of 2011.  In fact in those years it was extremely consistent in making and breaking behavior and, as noted, the break is extremely consistent now, right on the money at 1.5 psi; it never fails to break exactly at that pressure.  I would believe the leveling was the problem if this behavior occurred in 2007-2010+ but why would it start after 5 years?  Also, why would it occur only on the first 1 or 2 break and make cycles.  If the venting were the problem, wouldn't that also show on the pressure gauges?

Rod

Vaporstat Problems

Hi- If you answering multiple posts just put the answers together in a single reply.

      Okay... It's good to know the pigtail is properly orientated as that eliminates that possibility.  It would sound as though some how the problem is heat related as the vaporstat doesn't seem to work properly until it is warmed up.  You might just try fiddling around with making it level. Given the amount of on /off cycles over the years it could be that the vaporstats parts are just plain worn out.  Has the pigtail been removed from the boiler and cleaned  recently?

- Rod

SusanC

Vaporstat behavior

Thanks.  Your diagnosis makes sense to me.  Probably the vaporstat is wearing out; I dread replacing it with a non-mercury successor -- at least with the mercury the cut-out (break) psi is always correct.  I think I will wait for annual maintenance, which my company does in the spring, for us (tech. and me) to fiddle around with the leveling; then I will have a professional here to deal with any consequences.  I am assuming that, since the pressure gauges always agreed that in the first couple of cycles the vaporstat was making at too high a psi and the boiler on/off timing also indicated that, the problem was not the venting problem suggested as a possibility in this thread.  Do you agree?  Thanks again to everyone.  Soon I will start a thread on an entirely different matter, so stay tuned.

By the way, since I see a lot of steam questions in the Main Wall, should I be double entering steam questions so they appear in Strictly Steam and the Main or are the Strictly Steam threads also included in the Main? I don't want to clutter the Wall up any more than I already have.

SusanC

Vaporstat -- I forgot to answer 1 question

Yes, the pigtail is removed, checked and if necessary cleaned every year at annual maintenance.  Also the vaporstat and both low and required pressure gauges are on brass fittings (also checked) above the pigtail and the pressure gauges are very consistent. (Yes, I know the required pressure gauge has too broad a scale to judge accuracy but, interestingly enough, it agrees very well with the low pressure gauge.)