This is a wierd one. I have a residential oil-fired boiler, with a
backflow preventer and PRV set at 12PSI, expansion tank charged to 12
psi, city water at around 80 psi, and typical temp-pressure relief
valve that relieves at 30 psi. Normally, everything works fine.

When I (the owner) leave home for more than a couple of days, I like
to turn off the main city water shutoff valve to the house to lessen
the risk of having a flooded basement. Last winter when I did this, I
returned home to find the boiler TP relieving water all over the
floor, and boiler pressure gauge reading 40 psi. I checked the
expansion tank, found it had been charged to too high a pressure, and
reduced it to 12 psig. The expansion tank was checked out later by a
local boiler serviceman, and found to be OK (half full of water).

I suspected a faulty PRV valve, and closed the makeup water valve to
the boiler, and kept an eye on the gauge for a few days. The boiler
pressure krept up and the boiler TP valve started to drip.

I suspected that I had a small leak in the system (I've found one that
I haven't fixed yet), and that when I close off the source of makeup
water, eventually I loose pressure and get air into the system, which
expands the next time the boiler fires and overpressurized things.

To test this, last weekend, I closed the city water main valve, and
turned off the boiler burner circuit when I left. I returned home to
find the boiler cold (as it should be), but the boiler TP was once
again flooding water all over the basement, and the boiler pressure
gauge read 40 psig.

I should add that I have no problem for months at a time with the
boiler TP valve as long as I leave the city water main valve and the
boiler makeup water valves open. Once one of them is shut, whether
the boiler is on or not, the pressure in the boiler system starts to
climb.

I'm confused!! I'm a heating systems design engineer, but that ain't
helping me figure this out. Any ideas?

Thank you very much!

Linc Vannah
Stratton, Maine

does your pressure tank have a bladder, that could be the problem

Linc Vannah wrote:

This is a wierd one. I have a residential oil-fired boiler, with a
backflow preventer and PRV set at 12PSI, expansion tank charged to 12
psi, city water at around 80 psi, and typical temp-pressure relief
valve that relieves at 30 psi. Normally, everything works fine.

When I (the owner) leave home for more than a couple of days, I like
to turn off the main city water shutoff valve to the house to lessen
the risk of having a flooded basement. Last winter when I did this, I
returned home to find the boiler TP relieving water all over the
floor, and boiler pressure gauge reading 40 psi. I checked the
expansion tank, found it had been charged to too high a pressure, and
reduced it to 12 psig. The expansion tank was checked out later by a
local boiler serviceman, and found to be OK (half full of water).

I suspected a faulty PRV valve, and closed the makeup water valve to
the boiler, and kept an eye on the gauge for a few days. The boiler
pressure krept up and the boiler TP valve started to drip.

I suspected that I had a small leak in the system (I've found one that
I haven't fixed yet), and that when I close off the source of makeup
water, eventually I loose pressure and get air into the system, which
expands the next time the boiler fires and overpressurized things.

To test this, last weekend, I closed the city water main valve, and
turned off the boiler burner circuit when I left. I returned home to
find the boiler cold (as it should be), but the boiler TP was once
again flooding water all over the basement, and the boiler pressure
gauge read 40 psig.

I should add that I have no problem for months at a time with the
boiler TP valve as long as I leave the city water main valve and the
boiler makeup water valves open. Once one of them is shut, whether
the boiler is on or not, the pressure in the boiler system starts to
climb.

I'm confused!! I'm a heating systems design engineer, but that ain't
helping me figure this out. Any ideas?

Thank you very much!

Linc Vannah
Stratton, Maine

(The boiler safety relief should be/is pressure only, not T&P.)

Do you have a separate storage water heater? Was it firing
during this last test while you were away? If so, is it fitted
with an expansion tank? Is the tank plumbed in such a way that
overpressure (due to expansion) can be telegraphed to the boiler
feed circuit?

Jim

How do you heat your domestic hot water? Is it a coil inside the boiler?


"Linc Vannah" wrote in message
m...
This is a wierd one. I have a residential oil-fired boiler, with a
backflow preventer and PRV set at 12PSI, expansion tank charged to 12
psi, city water at around 80 psi, and typical temp-pressure relief
valve that relieves at 30 psi. Normally, everything works fine.

When I (the owner) leave home for more than a couple of days, I like
to turn off the main city water shutoff valve to the house to lessen
the risk of having a flooded basement. Last winter when I did this, I
returned home to find the boiler TP relieving water all over the
floor, and boiler pressure gauge reading 40 psi. I checked the
expansion tank, found it had been charged to too high a pressure, and
reduced it to 12 psig. The expansion tank was checked out later by a
local boiler serviceman, and found to be OK (half full of water).

I suspected a faulty PRV valve, and closed the makeup water valve to
the boiler, and kept an eye on the gauge for a few days. The boiler
pressure krept up and the boiler TP valve started to drip.

I suspected that I had a small leak in the system (I've found one that
I haven't fixed yet), and that when I close off the source of makeup
water, eventually I loose pressure and get air into the system, which
expands the next time the boiler fires and overpressurized things.

To test this, last weekend, I closed the city water main valve, and
turned off the boiler burner circuit when I left. I returned home to
find the boiler cold (as it should be), but the boiler TP was once
again flooding water all over the basement, and the boiler pressure
gauge read 40 psig.

I should add that I have no problem for months at a time with the
boiler TP valve as long as I leave the city water main valve and the
boiler makeup water valves open. Once one of them is shut, whether
the boiler is on or not, the pressure in the boiler system starts to
climb.

I'm confused!! I'm a heating systems design engineer, but that ain't
helping me figure this out. Any ideas?

Thank you very much!

Linc Vannah
Stratton, Maine

No, my domestic hot water is heated by a "sidearm heater" (heat
exchanger that is installed in the boiler).

The boiler system (and as a result, the domestic hot water heat
exchanger) was off while I was away.

The only expansion tank is the one on the boiler. That tank is
charged to 12 psig. I set the pressure of that tank (through the
shraeder valve on the bottom) while the boiler was pressurized.
Should I have set it when the boiler is depressurized (by closing the
make up water valve and draining some heating water from the boiler?)

Thanks,

Linc

Do you have a separate storage water heater? Was it firing
during this last test while you were away? If so, is it fitted
with an expansion tank? Is the tank plumbed in such a way that
overpressure (due to expansion) can be telegraphed to the boiler
feed circuit?

Jim

(m Ransley) wrote in message ...
does your pressure tank have a bladder, that could be the problem

Yes, the pressure tank has a bladder. I don't get any water out of
the shraeder valve at the bottom of the tank though, so I believe that
the bladder is OK.

Linc

"HeatMan" wrote in message link.net...
How do you heat your domestic hot water? Is it a coil inside the boiler?


Yes, the domestic hot water is heated by a coil inside the boiler.

Linc

"Linc Vannah" wrote in message
m...
"HeatMan" wrote in message
link.net...
How do you heat your domestic hot water? Is it a coil inside the
boiler?


Yes, the domestic hot water is heated by a coil inside the boiler.

Linc

That's most likely your problem. You have a pinhole leak in the coil and
it's causing the pressure to increase inside the boiler.

You possibly can have the indirect coil replaced, but I'd recommend either
an indirect tank or a separate water heater.

For someone that really knows wet heat, check out this link.
http://www.heatinghelp.com/getlisted.cfm

Thanks for the response, HeatMan. If I had a pinhole leak in the heat
exchanger, why would I only see a rise in boiler system pressure when
I turn off the city water supply valve?

Thank you,

Linc


"HeatMan" wrote in message . net...
"Linc Vannah" wrote in message
m...
"HeatMan" wrote in message
link.net...
How do you heat your domestic hot water? Is it a coil inside the
boiler?


Yes, the domestic hot water is heated by a coil inside the boiler.

Linc

That's most likely your problem. You have a pinhole leak in the coil and
it's causing the pressure to increase inside the boiler.

You possibly can have the indirect coil replaced, but I'd recommend either
an indirect tank or a separate water heater.

For someone that really knows wet heat, check out this link.
http://www.heatinghelp.com/getlisted.cfm

For starters, I can only guess since I can't see it from here.

The 'TP' valve is a safety. That's it. If it blows off, you have a
problem. It could possibly be trouble with the expansion tank.

"Linc Vannah" wrote in message
m...
Thanks for the response, HeatMan. If I had a pinhole leak in the heat
exchanger, why would I only see a rise in boiler system pressure when
I turn off the city water supply valve?

Thank you,

Linc


"HeatMan" wrote in message
. net...
"Linc Vannah" wrote in message
m...
"HeatMan" wrote in message
link.net...
How do you heat your domestic hot water? Is it a coil inside the
boiler?


Yes, the domestic hot water is heated by a coil inside the boiler.

Linc

That's most likely your problem. You have a pinhole leak in the coil
and
it's causing the pressure to increase inside the boiler.

You possibly can have the indirect coil replaced, but I'd recommend
either
an indirect tank or a separate water heater.

For someone that really knows wet heat, check out this link.
http://www.heatinghelp.com/getlisted.cfm