Why is the pressure vastly different on the two boilers? (oil and wood)

[MrEd]

I have a puzzling issue. On my oil burner, in the cellar (below grade), the pressure gauge shows me 45PSI more or less, even when it is cold.

I know its not stuck because when I drained it to work on it it went to zero, and if I let if fill it goes back up. Problem is, if I fill it only to about 10-15 PSI, the Tarm (in the garage about 8 feet higher), registers zero pressure.

With 45PSI on the oil burner, I only get 5-8 PSI on the tarm...and if I drop the pressure on the oil burner back even down to 30 or so, the PSI in the tarm goes to zero. I was not sure about the pressure gauge on the tarm, so I mounted another one near it, and it more or less matches the tarm pressure.

So, should the pressure match between the oil and the wood boilers? they are all hooked into the same plumbing, how can I have such a difference?

I don't want to leave that much pressure in the oil burner if I don't need to.

 

[Nofossil]

Can you swap gauges? 45 psi is really high, and I'd expect that if you actually had that pressure your relief valve would let you know. Gauge error seems a likely culprit here.

The pressure difference should be 1 psi for every two feet of elevation.

 

[MrEd]

The relief valve on the oil boiler is spitting out about 1 quart per day. But good idea about swapping the gauges...any other thoughts? They should more or less match shouldn't they?

I have a very old steel type expansion tank above the oil tank, any chance if this failed that somehow is accounting for the difference?

 

[Eric Johnson]

If boiler pressure exceeds 30 psi then either the gauge is bad or your pressure relief valve is stuck shut. The latter can be really dangerous.

You can pick up a cheap pressure gauge for less than $10 most places. Try replacing yours with a new one and see what kind of reading you get. And before you head out to the store, lift the lever on your pressure gauge and see if it works. It should squirt water. If it's stuck shut, then by all means get a new one (30 psi, not 150 psi, which is for hot water heaters).

Bottom line, if the systems are connected and the valves connecting them are open, there's no way you're going to see 45 psi in one boiler and 5-10 psi in the other one, no matter how far underground the oil boiler is.

EDIT: If your pressure relief valve is leaking, then it should be replaced in any event. Every quart of water that it lets out means one quart of oxygen-laden water that's going into the boiler (either immediately or eventually), which can contribute to corrosion.

 

[chuck172]

If indeed your pressure is exceeding 30 psi, and your prv isn't opening, get an new pressure relief valve immediately. My tarm gauge was real inaccurate. I had to talk to the owner of tarm to get a new gauge. It was no better. The gauges Tarm uses are junk.

 

[MrEd]

NEVER MIND! - My mistake.

This old man needs his reading glasses! There are two scales on the gauge, one is for 'Altitude' whatever that is for, and the other one is for 'Pressure'. Pressure is around 22, which is about 7lbs higher than the tarm (when running) and the tarm is about 8 feet higher...boy I feel dumb, but at least I am not going to blowup my system.

 

[Nofossil]

That makes sense!

'Altitude' is the maximum building height that you could supply with the boiler at a given pressure - basically, the altitude that a given pressure will raise water. About two feet for every psi.

 

[MrEd]

So then, since my pressure gauge is spitting water, it must be leaking. I'll get myself another one this weekend.

 

[Eric Johnson]

They're not very expensive ($15-20) and it's always good to know that your safety valve is working like it should.