Hot water high pressure problem

[Patrick Walpole]

Hi.
I have a hot water high pressure problem that my heating engineer (and other engineers that he has spoken to) cannot fix. I’m hoping someone on the forum can explain what is happening and how to resolve it.

My hot water system uses an air bubble at the top of the hot water tank for expansion.
The air bubble slowly shrinks and after a few weeks it needs to be replenished when I see the pressure gauge climbing above it’s normal position.
After performing this task for years I decided put an end to the repetitive job and arranged for a heating engineer to fit an expansion vessel.
A few weeks after the vessel was fitted I noticed the pressure gauge was showing high pressure and the high pressure relief valve was releasing water outside.
The heating engineer advised that a mains supply pressure reducing valve was required and fitted one. It made no difference.
I’m back to replenishing the bubble every few weeks and the heating engineer has left me to sort out the problem myself.

I setup several test situations. The most significant result (to me) was that the pressure in the system increased if the hot water heating was turned off and the tank was allowed to cool.
Here is a recent test I made...

20/10/22.
The hot water tank expansion bubble was replenished. This now provided approximately 73L of expansion in the hot water tank plus an additional 18L from the new expansion vessel.
21/10/22.
The hot water heating was switched off. Normal hot water usage continued.
22/10/22.
Baths were taken. The water in the tank was now only very warm. The gauge read 1.8 bar.
The shower mixer tap and the mains supply were turned off. The house was vacated.
24/10/22.
I returned home 55 hours later. The gauge read 3.63 bar and the high pressure relief valve had released water outside and filled, to overflowing, the empty 1L bottle I had placed under the pipe.

Any ideas or thoughts would be much appreciated.

 

[Devisham]

Is the vessel being heated off electric immersions or indirect feed off from a boiler? What temperatures are these sitting at?

What is the pressure reducing valve set to? What is the safety valve rating on the hot water storage vessel, does this match up to the data plate on the front of it?

What is the expansion vessel charged at? The 'bubble' and expansion vessel shouldn't be losing charge.

Is it on a HWS return system? If so, what is the pump set at?

Are check valves in place? Is their a check valve sitting in front of the expansion vessel so it cannot expand?

Are you able to post a picture of the installation in situ with the pipework/fixings?

 

[Patrick Walpole]

Is the vessel being heated off electric immersions or indirect feed off from a boiler? What temperatures are these sitting at?

What is the pressure reducing valve set to? What is the safety valve rating on the hot water storage vessel, does this match up to the data plate on the front of it?

What is the expansion vessel charged at? The 'bubble' and expansion vessel shouldn't be losing charge.

Is it on a HWS return system? If so, what is the pump set at?

Are check valves in place? Is their a check valve sitting in front of the expansion vessel so it cannot expand?

Are you able to post a picture of the installation in situ with the pipework/fixings?

Hi. Thanks for your reply. To answer your questions...
The hot water tank is heated by a gas boiler.
The tank’s thermostat is set to 55’C and the boiler’s flow temp is set to 57’C (both raised to 65’C once a month).
The new pressure reducing valve has ‘3 bar’ on its side.
Next to that appears to be a new relief valve which has ‘4,5 bar’ on its top (see photo).
The safety valve, on the side of the tank, has ‘4 bar’ on its top.
There is neither a data plate on the tank nor any other identifying information (it is about 32 years old).
The new expansion vessel is 18L.
The expansion bubble is in contact with the hot water (no membrane) and therefore it is slowly absorbed.
It is not a HWS return system.
Not sure what you mean by ‘check valves’ unless you mean the relief valves. There are no other devices connected to the pipes that I can see.
I can take some photos but the boiler cupboard is small and restricted. They might not make a lot of sense.

 

[ShaunCorbs]

How are you seeing the pressure rise ?

 

[Devisham]

The expansion vessel should be on the cold supply to the hot water storage vessel not on the hot supply coming out. I would relocate that first and make sure it's charged correctly.