Pressuretrol set low, pressure gauge reads high

Hen

Hello,

Two boilers in two family house.  One Crown, second Peerless.  Each about 100,000 BTU.  I have the Pressuretrol 'cut in' set on both all the way down and the 'differential' set on both at 1.  I understand the'cut out' should result in 1 1/2 lbs. BUT, the Crown cycles between 3 1/2 and 4 1/2 lbs.  The Peerless cycles between 4 and 6 lbs.  I have a low pressure 1-3 lbs gauge on order.

Qestion 1: Will I damage the low pressure gauge if the system actually does cycle at high  pressure, like 4 or 6 lbs?

Q 2:  Could I have bad Pressuretrols?  The gauge on Crown returns to 0 when the boiler is not operating; the gauge on Peerless returns to "  -2 lbs. when the boiler is between cycles or OFF (the Peerless gauge has  negative numbers below zero, aside from a scale up to 30 lbs).

Thank you.

Brad White

A number of things

To your first question, will the gauge be damaged? Depends on the gauge. A good ANSI Grade A gauge can take 50% over-pressure without damage. What happens is the spring which reacts to the bourdon tube to turn the needle, gets over-extended and can lose its resiliency.



As to the negative numbers, you may in fact be pulling a vacuum, especially if vents are closed and the steam collapses between cycles. (Not that likely with most normal vents).  It may be your gauges- if the system is off and cold, of course they should read "zero". You may be able to turn the "zero" screw on the back of the gauge to bring that back to calibration.



To your second question, you may have bad pressuretrols but my first inclination would be to check for a plugged pig-tail/siphon loop. Shut her down, remove the instrumentation and clean the piping. The piping should be brass (I specify red brass) hard tubing, so if it is iron piping now, you may want to replace it for good measure.



It may also be that your differential is additive and not subtractive as another thought.

Hen

The 'differential' is set at 1

I have edited my initial post.  It is THE  'differential' that is set at '1' on both boilers. Sorry and thank you.

Both pigtails are brass.  I will check both. I am not sure if they are 'red' brass.

Hen

Settings on Pressuretrol

Is it OK for the 'cut-in' to be set all the way down, or does 'cut-in' have to be set at '0.5 lbs' ?

Jamie Hall

There is a limit...

to how low the cut in can be set, and in the pressuretrols I've seen it is around 0.5 pounds.  Two problems.  First, that's the limit of adjustment, and things inside can fall apart if the adjustment is turned too far.  Second, the repeatability of a pressuretrol isn't all that great, and if the cut in is set too low it is possible for it to not reset and cut in when the boiler turns off.  Leaving you with no heat...



Use a vapourstat if you want better control at residential operating pressures!

Brad White

Red Brass

Red brass has a higher copper content and is more ductile than yellow brass. Yellow brass has a higher zinc content and about 65% copper. Red brass has about 85% copper, plus zinc and tin, so really is a bronze, technically.



But if you have regular yellow brass, that is fine, lose no sleep. Either are less prone to blooming corrosion than steel pipe is, but any pigtail can clog.

Hen

Is it OK to install a valve before low pressure gauge??

Hello,

I want to install a low pressure gauge.  Is it OK to install a valve, like a ball valve, to protect the gauge?  Thank you.

Brad White

Yes

Perfectly OK and in fact desirable to install a ball valve upstream of a pressure gauge.



Just no valves upstream of (between boiler and) operating controls.

I mentioned in another post that the MA Large Boiler Code requires a tee and valve at the ready below the operating gauge, to install a temporary gauge of know accuracy to check the primary gauge. I think that makes sense.



Keep in mind though, that the valve does not protect the gauge (the pigtail or siphon loop does that). The valve protects YOU.

nicholas bonham-carter

installing low pressure gauge

i think the wika gauges we all like are ok for 5psi over their range, but you can ask when ordering. you could install a valve before the gauge, but do not put the valve in front of the pressuretrol. it is best to have the gauge, and pressuretrol on the same tee so you can see what's happening.--nbc

Hen

How does the valve protect ME?

My thinking to install a ball valve before low pressure gauge was, so that I can keep  it closed (except when taking readings), to protect the low  pressure gauge in case the system, for some reason, operates at much higher pressure than desirable.  I cleaned the pigtail on  Crown boiler last night.  The cut out pressure reading went from 4 1/2 to ...it never cut out, until the thermostat was satisfied.  The highest pressure reading after cleaning was less than 1.    I thought, too high a pressure could blow the low pressure gauge (I hope to get to the Peerless pigtail today).  How does the valve protect ME?

Brad White

If you have no valve

and no gauge installed, you get a face full of hot steam.



Seriously, the valve is a convenience to remove a gauge to check it and isolate it to re-zero it, but it does not protect the gauge itself.

Hen

On pigtails; Crown and Peerless

Hello. 1. Does length of a pigtail matter?.  The one on Crown boiler was about 4" ; the one on Peerless was 6".  Both were installed atop the LWCO and each had only the Pressuretrol living on the other end. 

2. Is there a top and bottom to a pigtail?  I read on The Wall, that pigtail has to face the boiler certain way.  It looks that the pigtail 'belly' should be perpendicular to the boiler wall.  Does it matter, if 'the belly' is toward the boiler wall, or away from it, as long as the pigtail loop is perpendicular to the wall?

3. Does the pigtail have to be installed at a certain height?  The Crown pressure gauge once lived on top of the boiler, directly in a hole in the boiler jacket, as installed by the plumbers that  put the boiler new, years ago .  At some point when replacing a broken gauge, I elevated the new gauge on a brass nipple.  Let's say,  I installed a pigtail there, would the pigtail still maintain the water seal and function as intended, or does the pigtail need to be installed at the height of LWCO?

Thank you.

Rod

Pigtails

Hi- Pigtails all have "the loop" to trap water and the trapped water protects the controls from Live Steam.  The length and configuration (straight, 90 degree) are just to save having to use nipples and fittings to get the installation desired.



The loop or "belly" of the pigtail should be installed directly facing or looking away from the boiler.  (Standing in front of the boiler and in front of the pigtail, you shouldn't see the loop.) The reason for installing it this way was that if the pigtail loop was installed in a parallel plane to the face of the boiler, when heat expanded the pigtail piping, the control could slightly rock to the side. This isn't as important now as it was with the old pressuretrols  (They had mercury switches which were very sensitive to be tilted) but it is still (IMO) good practice to do so.



Height - Ideally the pressure controls (gauges, pressuretrol etc.) should be installed at a reasonable distance well above the boiler's waterline and in a manner so that excess water (not the loop's) will immediately drain back to the boiler. If there is a extra port in the top of the boiler, a lot of the pros prefer to install the controls there. If convenient, it's also not a bad idea to use separate boiler ports and individual pigtails fo the pressuretrol and gauges as this helps determine if one of the pigtails is clogged up. The benefit of using a bronze (red brass) pigtail has already been mentioned. If you can't find the pigtail you want locally , try Mc Master Carr on the internet. They have a large assortment.

- Rod

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