Gas pressure dropping to zero when demand kicks in?!

[Jamro91]

I have an Ariston combi e on lpg, the gas pressure drops from a lovely 37mb to 0 the second gas valve opens and fan starts to run, usually would drop to around 35mb on comparable systems. Any ideas what could cause that?

I suspected a blockage in the regulator so changed that and attached the new pigtails but problem persists!

Thanks in advance. J

 

[ShaunCorbs]

Line length ?

Blockage in the line ? You tested at the cov

 

[Jamro91]

Line length ?

Blockage in the line ? You tested at the cov

The line is about 5 metres with a metre vertical at each end.

I tested the working pressure at both regulator test point and gas inlet.

Blockage came to my mind too, but that seems unlikely in such a short run? And without cutting whole thing out and replacing it, how could I ascertain if that is the case?

 

[ShaunCorbs]

Do you have pressure at the cov when the boiler drops to 0

 

[Marc Bowers]

Sounds like the UPSO valve is triggering. The Under-Pressurisation Shut Off valve will trigger if the pressure drops below limits as it assumes there is a leak on the system. Yours sounds like it is too sensitive for the big initial demand from the boiler. I’m not sure what adjustment if any can be made to sensitivity (speak to manufacturers) but my guess would be that a bigger supply pipe will have less of a pressure drop on start up so will be less likely to trigger it.

 

[Marc Bowers]

You’ll need to find someone on the Gas Safe Register with an LPG ticket to assist.

 

[Jamro91]

Do you have pressure at the cov when the boiler drops to 0

Sorry Shaun, the cov?

 

[Jamro91]

Sounds like the UPSO valve is triggering. The Under-Pressurisation Shut Off valve will trigger if the pressure drops below limits as it assumes there is a leak on the system. Yours sounds like it is too sensitive for the big initial demand from the boiler. I’m not sure what adjustment if any can be made to sensitivity (speak to manufacturers) but my guess would be that a bigger supply pipe will have less of a pressure drop on start up so will be less likely to trigger it.

Thanks Marc, this was also my feeling hence I swapped out the regulator, where the UPSO sits. So essentially we now have two full bottles of gas, a new regulator with UPSO, two new pigtails and yet the problem persists.

The boiler demand kicks in, gas valve opens, fan pulls all of the gas out of the pipe, drops to zero, fan slows down, pressure goes back up, repeats a couple times then locks out.

And I am the gas engineer with the lpg ticket, just hoping someone has seen this before and knows exactly what it is.

 

[Marc Bowers]

Cov is the Change Over Valve

 

[Jamro91]

Cov is the Change Over Valve

Ah, Yeah this is where I’m checking, both at the cov (which I mistakenly refer to as the regulator) and the inlet to the gas valve. The pressure drops to zero here too, which strikes me as being impossible as I have two 47kg bottles of lpg stood next to me full of boiling gas!

 

[Marc Bowers]

Thanks Marc, this was also my feeling hence I swapped out the regulator, where the UPSO sits. So essentially we now have two full bottles of gas, a new regulator with UPSO, two new pigtails and yet the problem persists.

The boiler demand kicks in, gas valve opens, fan pulls all of the gas out of the pipe, drops to zero, fan slows down, pressure goes back up, repeats a couple times then locks out.

And I am the gas engineer with the lpg ticket, just hoping someone has seen this before and knows exactly what it is.

Hi Jamro. I’d still say that the initial draw is causing the problem with the UPSO. As a test, I would cut into the copper pipework from the COV and solder in a length of 22mm to give your system a temporary larger reservoir of gas and see if the system then behaves. If it then works normally, you know you need to upgrade your pipe size to relieve the pressure drop on startup.

 

[ShaunCorbs]

Have you tried one or two different bottles ?

 

[Jamro91]

Hi Jamro. I’d still say that the initial draw is causing the problem with the UPSO. As a test, I would cut into the copper pipework from the COV and solder in a length of 22mm to give your system a temporary larger reservoir of gas and see if the system then behaves. If it then works normally, you know you need to upgrade your pipe size to relieve the pressure drop on startup.

This is how I’m thinking too, the only issue here is why did it work OK before a week ago. Also bottles are definitely full, I attached one new one when I changed the cov, so we at least have one brand new bottle and one which has been the reserve so should be full too.

I can’t get back in there now until Thursday so I will try some things then, and see how it goes.

 

[Marc Bowers]

This is how I’m thinking too, the only issue here is why did it work OK before a week ago. Also bottles are definitely full, I attached one new one when I changed the cov, so we at least have one brand new bottle and one which has been the reserve so should be full too.

I can’t get back in there now until Thursday so I will try some things then, and see how it goes.

Good luck.