We got our Valiant heat pump system installed at the end of last year and finally commissioned in January 2025 by British Gas.
In the main everything has been working really well and the EmonPi kit has been brilliant. However, on 5 separate occasions I have had to top up the heating water pressure using the fill valves fitted to the system. The first two or three times I put this down to air in the system escaping through some of the auto vents thus causing a drop in pressure. Latterly though I have been watching it more closely and have noticed a couple of things.
For example if I top the pressure up to 1 bar then monitor it periodically I can see it rise to 1.4 bar when the system is running. The pressure relief valve is set at about 1.5 bar. Once topped up to 1 bar the pressure does not drop gradually as you might expect with a leak but instead will suddenly drop about 0.4 overnight despite being stable at 1 bar for a couple of weeks. When first commissioned the system ran for over 6 weeks before we first saw this happen.
We were recently away on holiday for a couple of weeks and came back to find the pressure was 0 bar and that we had an F22 error message and the heat pump had shut down. I again topped it up to 1 bar and reset the system. Things have worked fine since then but last night it lost about 0.3 bar again.
On reflection it seems that the frequency of these pressure drops is increasing.
The only clue I have so far is some water on the tun dish that connects to the pressure relief valve and expansion vessel.
I have a British Gas engineer coming out to diagnose the problem next week and will post back what he finds in the hope that this helps someone else that has the same problem.
Just now my theory is there is a problem with the expansion vessel that or pressure relief valve that causes the system to dump water periodically but we will see what the engineer says…
Hi Robert,
Sorry to hear of these problems.
Regarding the water on the tundish, isn’t that pipework connected to the hot water in the cylinder, rather than the circulating heating water that runs round the heat pump primary circuit?
If so, it’s probably worth investigation, but not part of your main problem.
Hi @BongoJim,
Yes if the diaphragm has ruptured you can get quite dramatic pressure swings as the water temperature changes.
My other thought is that if you have frost protection valve(s), these have been known to dump water unexpectedly.
Thanks for the reply perhaps our system has been plumbed strangely as the tundish we have fitted is connected to both the pressure relief on the cylinder and the pressure relief on the HP expansion vessel.
Thanks for the tip I will run this past the engineer next week and see what he says
I had similar with gas boiler and it was that the air had been lost from the pressure vessel valve and the diaphragm was pushed as far as it could be by the water.
The diaphragm couldn’t take any pressure changes and the over pressure valve kicked in, dumping water out (unseen) and the pressure then dropped a lot, I would see this and top up the water.
This cycle repeated over and over until I investigated further..
All that required to be done was to reduce the water pressure to zero, if not already. Pump up the air side of the pressure vessel and repressure the water side.
This happen with both our boiler pressure vessel and hot water cylinder pressure vessel! There’s no pressure gauge on the latter. Tapping it can tell you it’s still hollow🙂
Some heating service engineers don’t seem to be clued up on maintaining the air pressure side.
When it’s failed they will simple replace the pressure vessel at higher cost.
It’s only a fairly crude Schrader car tyre type valve.
How often do we pump our car tyres up?
Probably more often than we check our heating system pressure vessels, although they are steel tanks not vulcanised rubber tyres.
I shall be watching what our first service engineer does and asking about this..
Could also be a faulty / sticking pressure sender. Less likely perhaps though.
Thanks for this for some reason I thought the gas side of the expansion vessel was filled with something special and not just air. The more I think variation in pressure I see e.g. heat pump off 0.8 bar heat pump running 1.4 bar the more I think it must be the pressure vessel.
It will be interesting to see what the engineer says on Tuesday 
Generally, the PRV on a heating system will be set to 3.0bar, occasionally, 2.5bar, and set to a minimum of 6.0bar on a unvented cylinder. Expansion vessels are supposed to come precharged with nitrogen.
hi I have a Valiant 12 kW heat pump and like yourself generally speaking. It’s been working fantastic. However a few months after installation I had to do the same. I had to top up the water in the radiator circuit. I was very worried that I had a leak somewhere, so I checked the whole house for several weeks but did not find any leaks. I got to a point when I had to top the water up daily so the heat pump company came back and checked it out. They were able to look inside the Vailant and they could see where the water was leaking. In our case it turned out to be a small rubber gasket inside the heat pump. They got to this very quickly as I was fairly obvious. In the end they opened up the connector and cleaned / replaced the rubber seal. It was dirty. I guess there was somehow dirt in the pipe after installation and it had moved to this rubber seal causing it to leak. I’m sorry I can’t remember the details of what the rubber seal was but it was inside the heat pump unit on the right hand side about halfway up after cleaned the seal the problem never happened again. We’ve had two years with the pressure of being pretty good I have had to top it up once in two years, but I think that’s kind of normal. Hope this helps.
Well yesterday’s visit did not go as I expected. I had two engineers visit to investigate the problem. They checked the pressures of each of the expansion vessels and the functioning of the pressure relief valves and all seemed good.
They also removed the cover from the heat pump and inspected the insides of the unit all was good but while doing this one of the engineers noticed that the insulation at the flow pipe from the heat pump was wet but the return was dry. Given everything else was dry they concluded there was a leak where the flow pipe entered the heat pump. They managed to tighten this slightly so think that when the heat pump was doing its hot water cycle this was sometimes enough to cause some leakage.
They could not explain the water drips I found on the pipe below the tundish.
So despite everything pointing to the expansion vessel and PRV it may have been something totally different.
I’ll monitor it for a couple of weeks and then post an update.
Thanks to all for your input.
As I posted in my general update your issue might have had a lot more in common with mine than we originally thought so thanks for the tip 