Like several have reported, my system sometimes generates a data error on the hot water cycle.
In my case, I have observed that although the water pressure drops slowly over time, no air gets trapped in any of the radiators. I can only assume that the air is getting vented in one of the automatic purge valves. Increasing the pressure back towards 2psi is all that is needed to get things working again for several days.
So, my question is if anyone with a similar problem has tried fitting an automatic filling loop like this one:
It would seem the simplest solution, in my case at least.
If you need to periodically top up the system, the water must be going somewhere. This usually indicates a leak somewhere on the system. It’s worth getting the precharge pressure in the expansion vessel checked.
Thanks, yes this would be the logical conclusion. However, during warmer weather when the system doesn’t fire very often the pressure holds fine. It is only with the much colder weather when the system is on for extended periods and cycling often that I notice a very slow gradual loss of pressure. The system was installed in late summer so the expansion vessel is only a few months old so maybe not so likely to be the culprit.
In any case, several others report needing to purge air from their system regularly. Presumably purging air also leads to some pressure loss. So, as far as I can see I have the same issue as others except my AAV happen to be catching and expelling the air leaving me with just a very gradual pressure loss. Once the pressure drops to around 1.5psi the ultrasound flow sensor begins to malfunction on the hot water cycle.
I am not able to understand what the mechanism of what is happening is but wonder if the issue is associated with the location of the expansion vessel. It is connected on the return to the ASHP which is itself 10m away. There is no buffer tank so the pump in the ASHP is doing all the work. I wonder if the long pipe run between the expansion vessel and the pump is contributing to the problem.
Beware of auto-air vents. If it on the ‘negative’ side of the water flow it can leak. When the pump stops it will expell the air and a few drops of water. When the pump starts it may pull air in (the float within the AAV drops). Either move it to the positive side or replace it with a simple air bleed nipple. If the top of the AAV is covered in crud then it’s your problem.
Thanks William2, good suggestion. I think I may have made some progress. I increased the fill pressure to 2psi with the water near room temperature and this seems to have improved things. The pressure has been holding steady at around 2.1psi for several days. My expansion vessel has a sticker claiming that the factory pre-charge pressure is 1.5 psi. So, it makes sense to me to have the fill pressure always above this level to try to prevent the problems you mention when the pump starts / stops. Certainly, the advise I have always heard is to have the fill pressure at 1.5 psi which I am not sure is correct. I think the factory pre-charge pressure of the expansion vessel should be considered.