Combi losing and gaining pressure (expansion vessel or filling loop?)

[BenFrearson]

Hi, I've got a Glowworm ultra com 30 cxi that has been losing pressure. The PRV drain was dripping so last week I replaced the PRV and repressurised to 1bar.

However as the central heating came on I noticed the pressure rose to around 2.6 bar. Clearly something is causing pressure to rise and then open the PRV. There is still water on the outside pipe, and each morning pressure is down to 0.4 bar.

Having researched a little, I figured this is likely to be the expansion vessel or possibly the filling loop which was left attached by the installer.

I disconnected the filling loop and both isolation valves appear to be passing. However I'm not sure what part specifically needs replacing. It also seems odd to me that both valves might have failed at once. The valves I'm talking about are shown in diagram 12.2 (D and E).

Also I checked the expansion vessel - there was water on my screwdriver when I touched the valve the first time, but I've repressurised and there doesn't appear to be any water there now. Is it possible there might be a slow leak? If so, I would expect to see a gradual drop in pressure as air entered the system, and then a rise once all of the air gets vented and the water heats up right?

Thanks
Ben

 

[ianmcd]

read about re-pressurizing your expansion vessel on here , and follow all steps , you can not just pump air into it

 

[BenFrearson]

Oh. I guess not all DIY advice is born equal, as I followed instructions that suggested doing just that. Forgive me if I'm being dense, but surely there must be some water pressure in the system when doing this, as the manual says it should be charged to 0.5 bar. If the system is empty and I open the valve, surely I won't be able to get it below 1 bar as that's atmospheric pressure?

The water pressure was 0.3 bar as the filling loop had been off and passing back from the CH return for a while (but certainly not 10 litres as suggested in the FAQ!). What are the implications for not doing this properly?

 

[ianmcd]

No you have to drain your boiler , leave a drain point open, attach a pump to the shraeder valve on the expansion vessel, with the drain point still open you pressurise the expansion vessel to the pre -charge pressure stated in the MIs then close the drain point and pressurise the system water to between 1.0 and 1.5 Bar, it can not be done any other way you must leave a drain point open while you re-charge the expansion vessel

 

[BenFrearson]

Yeah, I read the instructions in the FAQ - Do I need to fully expel the air from the expansion vessel by opening the schrader valve before I drain the boiler? Otherwise I can't see how I can get below 1 bar.

Just to check - I can do this with the boiler isolated from the rest of the system right?

 

[ianmcd]

No , the reason you have to leave a drain point open , is that while you pump up the expansion vessel with air it will expell all the water inside the expansion vessel, once it is expelled then you can start to charge the expansion vessel , then you ad water to the system after closing the drain valve

 

[BenFrearson]

Right, but just checking - before I start, should I completely let the air out of the expansion vessel, or does the air in there not matter?

 

[ianmcd]

No it does not matter , leave a drain valve open then charge the expansion vessel with air, once the vessel is empty of water it will accept an air charge , once that is done , close the drain point then pressurise the water system. how many times do I have to repeat this ????

 

[BenFrearson]

Yeah I understand the process. My question was what needs to happen before I begin. There's already air in the expansion vessel because I (obviously incorrectly) put it there. If the pressure is higher than it needs to be, as far as I understand it I'll only be able to lower it (by depressing the schrader valve) to 1 bar as then it will be equalised with the open drain valve and the air outside.

 

[ianmcd]

NO you do not understand the process, the pressure on your boiler guage is the pressure of your primary system water, not the pressure of the air in your expansion vessel, sorry I am not going to answer you again, you have to read what you have been told, the air pressure charge in your expansion vessel and the hydraulic pressure in your primary system are completely different, they are not the same , one is air one is water ?????????

 

[BenFrearson]

I know that, sorry if I wasn't being clear - I was talking about the pressure on the air pump I was using to pressurise the expansion vessel. I realise that the water pressure is irrelevant, however if the system is open, then then water is still under atmospheric pressure.

The volume of air inside the expansion vessel absolutely matters once the system is resealed. I know that I need to drain the boiler, leave a valve open and then pump air into the expansion vessel to 0.5 bar as per the manual, that's fine, but I still don't know whether to:
a) start the process with the air that I've already pumped into the expansion vessel
b) depress the schrader valve in the expansion vessel first to ensure that it's filled with water and I start at 0 bar.

Not sure why that's a difficult question to answer..

 

[DP]

Ian, do you feel you are talking to a brick wall?

OP, why not do what Ian suggested. Forget trying to understand the process.
Else get a heating engineer in to carry out a proper service as what you are attempting would have been done during the annual service

If you want to understand the process, here goes.......
When you have pumped up the expansion vessel, air side will be at 0.5 bar and water side will be at 0 bar. When job done, water side is pressurised to 1.25 or whatever.

 

[BenFrearson]

Ok, I'll put my hand up and say that maybe it's me not being clear enough. I fully intend to follow Ian's advice, there's no issue with that and those instructions are clear. But I'm not sure it's a good idea to tell someone to stop asking questions and start a process when there's still ambiguity about preparing to do this. If the things that I'm asking make no difference, then let me know that!!

There were two questions I asked about it that still haven't been answered that I wanted to check first:

1. Do I need to isolate the boiler from the rest of the CH system?

2. Does it matter how much the expansion vessel is already filled with air?

Once I've got answers to those questions I'm happy to get started!


Also as a separate issue, as noted in my first post, the filling loop is still passing and I need to address that too. I can't seem to find any replacement valves despite having the P/N from the manual.

 

[DP]

1. No, does not matter
2. With CH side at 0 bar maintained for the duration whilst pumping, vessel charged to .5 or .6 or whatever

 

[BenFrearson]

Great thanks. I'll get onto this when I get home.

What happens if I don't do this btw? The boiler's been working at a steady pressure of about 1 bar cold for the past 24 hours and there's no problems at the moment?