blockages and pressure!

If a system is badly sludged up with very little circulation can this cause the pressure to increase from say 1.5bar to over 3 and operate PRV.

I always thought it would shut down (thermistors) or just OH. As it would obviously be too hot.

Just my tuppenceworth - others will know more and better!

Yes, some certainly can. E.g. a Puma with a dud pump blows its top on CH, though just about OK on HW.

Some boilers (Ideal Mexico ) also won't control themselves unless there's sufficient circulation because they put the thermostat pocket on the return. So the boiler doesn't realise it's got hot. That's one reason for a bypass.

Even if the boiler does cut itself off, the lack of sufficient flow on overrun can mean it boils.
Water expands by 1600 times when it boils, so the pressure rockets. Water from the PRV, steam from the AAV.

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There's another one which made me think - well, not for too long. If there's no air in the EV, and the boiler gets hot, it might boil. Pump it up and it won't boil. Odd huh?
What I think happens is that the boiler gets hot, say on overrun. Say 110 degrees at 3 bar. Then the prv lets some water out, and the pressure throughout the boiler instantaneously drops, so there's mass boiling of the water, and steam spurts, for a start, out of the AAV.
Pump it up, and the pressure rises on overrun, say to 2 bar. So no boiling, no sudden pressure drop, and it stays quiet.

When the prv opens I think it doesn't just hold the pressure at 3 bar, it must drop it well under, for an instant.
I just saw a Response doing exactly this - I thought it was a dud pump, but with 12psi in the can,all quiet.

Unless someone has a better theory?

rob884 said:

If a system is badly sludged up with very little circulation can this cause the pressure to increase from say 1.5bar to over 3 and operate PRV.

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Have you checked the pressure in the expansion vessel?

ChrisR said:

When the prv opens I think it doesn't just hold the pressure at 3 bar, it must drop it well under, for an instant.

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I believe what happens is that the water released when the temperature gets too high results in a pressure reduction at all lower temperatures, which is wrong for the system in question. I suspect that the more sudden the rise in temperature the more water is lost when the PRV opens, so there's a circumstantial element to problem.

have you checked the pressure in the expansion vessel

Click to expand...

its not my job, someone asked if we done powerflushing.

BG have to keep changing the PRV, and now wont do it anymore untill its flushed out.

Softus said:

I believe what happens is that the water released when the temperature gets too high

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That doesn't happen. It's a pressure relief valve not a temperature relief valve.....

Softus said:

I suspect that the more sudden the rise in temperature the more water is lost when the PRV opens..

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Ditto

ok, the boiler in question is apparantley quite new. so my original question was that surely even if there was no circulation- pressure increase correllates(sp) to temp rise. im assuming that it would cut out at 85-90oC

would static water be able to keep increasing the pressure?
unless of course there was an expansion problem.

ChrisR said:

Softus said:

I believe what happens is that the water released when the temperature gets too high

Click to expand...

That doesn't happen. It's a pressure relief valve not a temperature relief valve.....

Click to expand...

I know that Chris, but a rise in temperature results in a rise in pressure, ergo a rise in temperature results in the PRV opening, so my point is still valid.

If you prefer, just substitute the word "temperature" for "pressure" in my sentence, only part of which you've quoted.

ChrisR said:

Softus said:

I suspect that the more sudden the rise in temperature the more water is lost when the PRV opens..

Click to expand...

Ditto

Click to expand...

Ditto.

rob884 said:

have you checked the pressure in the expansion vessel

Click to expand...

its not my job, someone asked if we done powerflushing.

Click to expand...

And do you done it?