Recommended setting on old pressuretrol

ansman

I have an old (I’m assuming old because of the mercury switch) pressuretrol that seems to be set completely wrong.

Firstly I want to make sure it actually works but assuming it does what is a good setting. I’ve read the discussion about it being a control device vs a safety device and this isn’t really what I’m asking.

The reason I’m asking is because ours is not very granular. The lowest it goes is 2 so setting it to 1.5 cut off and the diff to 1 doesn’t seem feasible. But perhaps you are supposed to eyeball it?

Any advice would be appreciated here.

Jamie Hall

Yours is the subtractive type, so the cutoff -- assuming this is one pipe steam or conventional two pipe -- is pretty much OK as is. That's the right hand scale. The left hand scale -- the differential -- is subtractive, so it sets the boiler to cut back in when the pressure drops to the cutout to the cutout minus the differential. A little thought will show that the way yours is set up makes no sense at all.

Reset the differential to about two thirds of the way up from the bottom to the "2" mark on the psi scale.

Now the fun part: does it work? It probably does -- the mercury pressuretrols are remarkably reliable, unless they've been abused -- but one does want to know. Ideally one would contrive a little test rig with a low pressure gauge, a few pipe fittings, and some way -- like a hand pump -- to add pressure, then use that. However, you can do the same thing (and will want to anyway) with what you have, plus some pipe fittings and a low pressure gauge (0 to 3 or 5 psi). Take the one you have off its pigtail and put a T on there. Then two nipples, one left and one right. A 90 elbow on each nipple. Two nipples up, and the existing gauge on one and the new gauge on the other. Now fire the boiler and watch the low gauge -- if the pressure gets up to the pressuretrol setting (2) the mercury bulb should tip and the boiler should shut off. Then the mercury bulb should tip back at a bit more than 0.5 psi and the boiler start up again. If the numbers are off, check the levelling of the pressuretrol -- they are very sensitive to level.

ansman

Awesome. Yes the diff is completely strange, I'm not sure our plumber knew about this at all.

I have a 0-3 gauge that I'm yet to install. Since I took the picture I've leveled the pressuretrol as it was very off.

Thanks for confirming, I'll reduce the diff.

EBEBRATT-Ed

@ansman ,

The boiler should shut off at the cut off or"main" setting in your case it looks like it is set to 2psi. That is the maximum it should be set to.

The differential is subtractive to the main setting so if the boiler shuts off at 2 psi-the differential which looks like it is set to 3 1/2 which means at those settings the boiler wont ever restart.

I would leave the main at 2 psi. and the differential at 1 3/4 but as @Jamie Hall mentioned the scale on these controls is less than accurate and bench testing is the best way to adjust.

Also, after you put on the low pressure gauge do not expect to see much pressure unless you boiler is oversized. If sized correctly and with proper venting you may not see any pressure.Completely normal

ansman

Thank you so much for all of your help. I got my small gauge and was able to calibrate my pressuretrol so now it shuts off exactly at 2 PSI and turns back on at exactly 0.5 PSI!

I also realized that my 0-30 gauge is completely shot so I've ordered a new one.

JUGHNE

IMO, it is a waste of your money to replace the 0-30 gauge.
A new one will not move to show 2 psi, if it does it will be very difficult to read.
Code required you to have a 0-30 gauge, no one will ever know that it may not work.....it is just supposed to be on the boiler.

ansman

Yeah, I thought so too but something about having broken equipment just really bugs me so I rather fix it.

Jamie Hall

Nice work. And be happy you have that old pressuretrol. As I mentioned, the ones with the mercury switch are about as close to bulletproof as you can get, and are very precise -- which is to say, once set with a low pressure gauge, unless someone knocks them off level they will operate at the same pressure every time.

JUGHNE

How did you know it was broken?

If it sat there showing 10 psi with a cold boiler then yes.

But if you never have seen it move....pretty normal.

Chris_L

ansman said:

Yeah, I thought so too but something about having broken equipment just really bugs me so I rather fix it.

And why do you think it is broken?
The first mark on your 30 psi gauge is 2 psi. Your 3 psi gauge indicates the pressure is a bit less than 2 psi. Your 30 psi gauge indicates the pressure is a bit less than 2 psi.
Looks good to me.

ansman

I've spend about an hour staring at it while calibrating and it's within about 1oz every time which is remarkable for a device that is that old.

What was nice is that you could move the scale after calibrating it too. It has a build in leveler so I can also adjust it back to the same level.

Well, I figured it should have moved when I hit something like 2.5 PSI but it was at 0 still.