ASHP losing pressure

[joe90]

We have an ASHP heating a buffer tank for UFH and via a diverter valve an unvented DHW tank, it’s all been working ok but recently I noticed the pressure in the ASHP pipe work dropped to zero, topped it up a few times but then disappears. I also noticed that when the ASHP was running the pressure rose. It’s 1bar normally. A plumber friend thinks that it’s rising to 3bar and the relief valve is blowing. He also noticed that we have no expansion tank on the ASHP loop, there is one on the cold onto the DHW tank and another on the UFH loop. The question is should I also put an expansion tank on the ASHP loop as well as the others?

Edited March 2, 2019 by Nickfromwales

[Jeremy Harris]

I think your plumber has nailed it.  The pressure will rise in the sealed ASHP circuit as it heats up, and there should be an expansion vessel in that circuit to allow for this and limit the pressure rise.  I see about 1/4 bar change in pressure between hot and cold on ours, but that's with an expansion vessel.  Without one I suspect the pressure is rising to the PRV threshold and dumping a tiny bit (probably only a small cupful) which is then causing the pressure to drop a lot when it cools down.

[PeterW]

Quick one but yes ..!

 

Should have an 8 litre expansion vessel in the system somewhere - not difficult to add and just set the static pressure on it to 1 bar (they normally come with a 3 bar pre-charge)

[joe90]

Thanks guys, none of the plumbing schematics I had included an expansion vessel on the ASHP loop but they also did not include a buffer tank ?

[PeterW]

You can calculate the expected size which is 10% of system volume usually but that’s due to the delta 55c used in the G3 calcs. 

 

Unless it’s a huge buffer, 8 or 10 litres will be fine. 

[scottishjohn]

yes there should be an expansion vessel in any sealed system

could be  your PRV on ASHP is leaking , 

doubt every much its gaining 2 bar in pressure at the sort of temps the ASHP is running and if you charged it up to 1 bar when system is hot ,it should only go down in pressure when not running =

so PRV sounds a good suspect 

always have run mine at 0.8bar  cold  --no problem

expansion vessel is set to 1bar 

If you going to spec it right then you need to know total volume in UFH pipes+system  - a bigger expansion tank will be no detriment if you got space 

[ProDave]

I ended up buying the 12L expansion vessel kit as it was available at the time on the bay, slightly cheaper than others were selling an 8L one.

 

I actually installed if for my first HP but when that didn't work the replacement HP I was given has an expansion vessel inside the HP, so I now have two as I never saw any point removing the one I had fitted.

[Jeremy Harris]

With no expansion room at all, then the pressure will rise a very great deal with even a small temperature change.  Water is, to all intents and purposes, incompressible under the sort of conditions in a heating system.  A 70 litre volume system will try to increase in volume by roughly 0.3 litre for a 20°C temperature change, so with no expansion vessel, and ignoring the slight increase in system volume from the thermal expansion of the buffer tank and pipes, the pressure will increase by a lot, possibly several bar unless a pressure relief valve were to operate.  My earlier guess of the PRV letting by around a cupful seems to be about right, too, at 300ml!

[joe90]

Found a short term solution (well so far!) set pressure at about .4bar and it only rises to roughly 2bar when running. Perhaps this is why it appeared to run ok up till now. I have ordered an expansion tank and will fit it this week.