Air Source Heat Pump pressure fluctuations.

MartinAston

We had a Samsung 12kw unit fitted in August this year, and were advised that in normal operation the pressure gauges installed in our airing cupboard would read between 1.2 to 1.75 bar. In the early weeks this did not happen and we were seeing large movements between below 1 and above two resulting in a release of water via the overflow pipe. Our suppliers made a few changes to the expansion chamber and it eventually settled down. I was told that if it dropped out of the range then I should boost to 1.5 to correct it which I did for several weeks however it would recover to nearly 2 bar and we would have water coming from the overflow. During this period the system was (and still is) working OK - our radiators are at a good temperature and we have hot water. In the recent mild spell the gauges dropped again to below 1, but then recovered when the system was in operation. My question is should I be concerned about these changes if the system continues to function properly and what in practical terms are these occurrences telling me.

Verdigris

It looks to me like there is air in the system. It is probably getting drawn in through a leak on the return side of the circulating pump. I'd get the installer to have a look at it.

Reed_Richards

In my experience  (albeit with a gas boiler) with both pressurised hot water and central heating water your symptoms indicate a faulty pressure vessel.  But the need to frequently top-up the pressure would indicate a leak.

doodling

Hi,

It is normal for the pressure to vary with the temperature of the water in the system but I would not expect a variation in excess of more than 1 bar, and in most systems I would expect the variation to be less than 0.5 bar.

The large variation suggests that there is insufficient expansion volume in the system. Either the expansion vessel(s) are insufficiently sized, or one of them is defective. It is also possible that the air side of the vessels have been overpressurised.

If the over-pressure valve is set to 2 bar then you don't want the pressure when the system is up to temperature to exceed 1.5 bar.  If you're filling to 1.5 bar when the system is cold then (especially given the huge variations in pressure you are seeing) then water being released when the system get hot is inevitable.

In summary:
- The amount of expansion volume in the system needs to be checked and either defective vessels replaced, air side pressures adjusted, or the volume increased.
- With a 2 bar over pressure valve, I wouldn't want to fill the system to more than 1 bar when cold.  I'd question why a 1.5 bar cold fill is appropriate.

Air in the system would increase the available expansion volume so I wouldn't expect it to be that, unless you are finding air accumulating in any of the radiators?

The fact that you're having to top up frequently is indicative of a leak, but that is almost certainly a separate problem, albeit one that also needs to be fixed. I wouldn't expect to have to top up more than once every 6 months (and in my view that's still shoddy - my system has gone for over 8 years without a top-top).

MartinAston

Many thanks for contributions made so far - I should point out that after the initial shenanigans we are no longer boosting the pressure ourselves we just leave the pump to sort itself out. I am still liaising with our installers who have made changes to the system by replacing the "ASHP Expansion Vessel" and corrected the pressure setting which was originally at 3 bar now down to 1.5. As ours is a retro fit the bulk of the system has been in place for some time without any issues so we have seen no evidence of leaks at any point. My main concern was that the fluctuations would eventually lead to the system malfunctioning in some form but so far it is working well. 

Verdigris

You won't necessarily see water leaking out, when the pump is idle, but air can still get into the pipework when the internal pressure drops on the return leg.

My central heating loses pressure and the air collects in the bathroom radiator. I've just put my hand on the return side valve, on the radiator, and, sure enough, it is wet. No visible sign, though.

I suppose I'd better dig a spanner out and tighten the nuts up!

doodling

Hi,

If the system is stable then I wouldn't worry about it, in general the pressure fluctuations are a natural feature of the system and won't do any harm.

You only need to worry if the system pressure falls below ~1 bar when cold or nears the pressure relief valve setting (2 bar in your case) when hot.

doodling

Hi,

Verdigris said:

You won't necessarily see water leaking out, when the pump is idle, but air can still get into the pipework when the internal pressure drops on the return leg.

My central heating loses pressure and the air collects in the bathroom radiator. I've just put my hand on the return side valve, on the radiator, and, sure enough, it is wet. No visible sign, though.

I suppose I'd better dig a spanner out and tighten the nuts up!

On a sealed system, whilst I'm sure that transient effects may cause brief pressure reductions below atmospheric, the head generated by most domestic central heating pumps is significantly less than 1 bar and therefore, assuming a system pressurised to at least 1 bar, you should never get significant amounts of air sucked in through leaks.  

If you haven't got a sealed system (i.e. you have a traditional header tank in the loft) then yes, it is quite possible to get pressures below atmospheric on the pump inlet side and then air being sucked in is a possibility.

Have you checked to confirm that the gas is air and not hydrogen?  If you're topping up frequently then the system is more at risk from corrosion and hydrogen is one of the by-products.