Just replaced expansion vessel but system still over pressures

[Pharcycle]

Hi all,

Thanks for having a great forum, I've found many a valuable tip over the years from here but I'm now up against a conundrum.

I've just replaced the expansion vessel of my inherited Saunier Duval Thema Condens F24 E combi but the pressure is still going very close (if not over) the 3 bar limit. Any thoughts what else could cause this?

The new expansion vessel was manufactured in 2016 and is a genuine part so while not impossible it's also faulty it seems unlikely. It came pressurised correctly (0.75 bar).

To me the only logical explanation is a blocked pipe between the central heating circuit and the expansion vessel.

I've had lots of woes with this boiler and can't wait to replace it but the new one will be going in with an extension later this year and I also plan to renew the entire central heating system so don't want to mix old with new and / or have to move a new boiler or have to work around the new one.

I've only noticed the high pressure issue since doing a load of work on it:

Good clean of pressure sensor - was caked up with magnetite and was stuck reading 0.4 bar
Replaced pressure release valve (leaking)
Replaced diverter motor (wasn't driving)
Replaced PCB (turns out it was not driving the diverter motor correctly)r
Good clean of the secondary heat exchanger (was caked up with magnetite)

After all that it was working like a champ but then noticed the pressure was going too high. A small spit of water came out of the valve of the expansion vessel too so all the symptoms seemed to fit with this being knackered but after changing it the over pressure still remains.

I plan to check the pipework to the expansion vessel is clear tomorrow but thought it worth asking if there's anything else people thought it could be?

Many thanks,
Dave

 

[John D v2.0]

Assume you've checked the system volume, initial pressure and even the design temperature are correct?
If it's a system designed for low temperature water it will have a lot more of it, so it could get to 3 bar when turned up to max.

 

[Pharcycle]

Assume you've checked the system volume, initial pressure and even the design temperature are correct?
If it's a system designed for low temperature water it will have a lot more of it, so it could get to 3 bar when turned up to max.

I've not checked them specifically but it's been running for 3 years with the same volume and at the same temperature as it is now and always been in the 1 - 2 bar range cold to hot. When I said inherited it I meant it was here with the house when I bought the place, not that someone gave it to me and I've had it fitted. However there are many examples of things not installed right in this house I'm putting right (copper pipe for the condensate for example!)

Thanks for you reply, I'll pull it apart again tomorrow and check flows between the circuits

Oh and when I re pressurised the system after fitting it I filled it to 1.5 bar.

 

[John D v2.0]

Also are you sure there weren't any dead legs or radsa with air in absorbing the excess pressure?
Just thoughts and ideas off the top of my head to help.

 

[Pharcycle]

I've been able to follow most of the heating circuit and I've not seen any capped pipes and I bled all the radiators reasonably frequently but it's an interesting thought.

I've had to drain the boiler maybe a half dozen times while working on it and the only way to provide an air inlet just for the boiler is to open the auto bleed valve on the pump - which is right next to hose for the expansion vessel. Given the amount of metallic crap that comes out every time I drain it, I could well imagine some crud blocking that pipe. A solid layer had fused itself over the pressure sensor and I had to chisel that off (I thought I broke it at first when it came away!)

Speaking of the pressure sensor, its been raining the last few days so I've not been out to see if the PRV is opening but I wondered if it could just be a faulty pressure sensor.. it does show a rise and fall as expected but I have no way to validate the measurements. Any experience how pressure sensors tend to fail? It's an electronic sensor rather than a gauge. I also tend to shut the heating down when I see it getting above 2.7 bar but maybe I should let it get to 3 bar and see what it does. I think the boiler shuts down at 2.8 though.... not sure what my mains water pressure is, maybe I could try over filling it and see if it dumps some water - not ideal though!

Cheers,
Dave

 

[dilalio]

Just check the flexi hose to the EV, and the T where it is connected. With all the crud you’ve been mentioning, the rubber lined hose is a common culprit.

 

[Pharcycle]

Just check the flexi hose to the EV, and the T where it is connected. With all the crud you’ve been mentioning, the rubber lined hose is a common culprit.

Thanks for the suggestion, I’ll investigate it further today or tomorrow.
Cheers

 

[Hason]

It be the hose

 

[Pharcycle]

It be the hose

Yup, hose was completely blocked at the tee end, including the tee (so a 1 hour job turned into 3!) . Just got it all back together, will blast the heating for a few hours and see what happens but so far it's looking ok... A filter will definitely be going on the new heating circuit.

Thanks for the suggestions everyone, turned out to be quite simple, just time consuming.

Is there an easy way to test if the old expansion vessel is goosed or not, could be worth £20 to someone I guess if it is serviceable, I'm not going to put it back in though. Maybe pump the air side to 3 bar something and see if that drops over a month or so?

Cheers,
Dave