sealed system losing small amount of pressure...

[drives]

Hi all,

We have a sealed CH system with unvented cylinder (which is quite new, 3 or 4 years old).

I've been monitoring the pressure gauge for a few weeks which I try and keep at 1.3 bar. It has needed topping up every now and again, and will lose around 0.1 bar every few days.Interestingly we went on holiday for a couple of weeks, and topped it up to 1.3 bar precisely the day before. Put HW on holiday mode, came back, and needle hadn't budged.

Now we are back, and using HW sure enough the pressure has started to gradually drop again.

I've never quite got my head around the expansion vessels and which one does what. I have one right by the filling loop, and then the other which seems to be connected to CW supply pipework to cylinder. I have checked pressure today and the lower one is 1.1 bar. The one higher up is 2.79 bar.

To be honest I thought I had a small leak somewhere under our ground floor floorboards on the CH system but am I right in thinking this lower expansion vessel might need topping up a bit? I've put some tissue paper on the two tundishes we have this morning. I must confess not quite knowing precisely what I am doing other than being an indicator if these are letting water by.

Any advice much appreciated. Photos attached of the setup if that helps.

 

[Madrab]

The Expansion Vessels (EV) - White one is potable and should be for the Unvented (UV). Grey one (filling loop) would then be for the CH system. The EV's may need topped up but that wouldn't be why the system was losing pressure - if there was no water at the tundishes. If an EV needs topped up then there would usually be water at the tundish for the UV or the CH system would over pressurise (>3bar) but again there would usually be water at the tundish.

If you are losing pressure then it can only mean that water is exiting the system. That can be either through the pressure relief valve (PRV), which would show at the tundish or somewhere out on the system, either at a rad valve, pipework or connections on the system or the boiler.

 

[drives]

The Expansion Vessels (EV) - White one is potable and should be for the Unvented (UV). Grey one (filling loop) would then be for the CH system. The EV's may need topped up but that wouldn't be why the system was losing pressure - if there was no water at the tundishes. If an EV needs topped up then there would usually be water at the tundish for the UV or the CH system would over pressurise (>3bar) but again there would usually be water at the tundish.

If you are losing pressure then it can only mean that water is exiting the system. That can be either through the pressure relief valve (PRV), which would show at the tundish or somewhere out on the system, either at a rad valve, pipework or connections on the system or the boiler.

Thanks madrab. That makes sense regarding the two EVs.

I'll keep monitoring the tundishes. Seemed odd that it lost no pressure when the HW was off for a couple of weeks but perhaps the movement in the system and pressure changes are when it manifests itself.

 

[Johntheo5]

Hi all,

We have a sealed CH system with unvented cylinder (which is quite new, 3 or 4 years old).

I've been monitoring the pressure gauge for a few weeks which I try and keep at 1.3 bar. It has needed topping up every now and again, and will lose around 0.1 bar every few days.Interestingly we went on holiday for a couple of weeks, and topped it up to 1.3 bar precisely the day before. Put HW on holiday mode, came back, and needle hadn't budged.

Now we are back, and using HW sure enough the pressure has started to gradually drop again.

I've never quite got my head around the expansion vessels and which one does what. I have one right by the filling loop, and then the other which seems to be connected to CW supply pipework to cylinder. I have checked pressure today and the lower one is 1.1 bar. The one higher up is 2.79 bar.

To be honest I thought I had a small leak somewhere under our ground floor floorboards on the CH system but am I right in thinking this lower expansion vessel might need topping up a bit? I've put some tissue paper on the two tundishes we have this morning. I must confess not quite knowing precisely what I am doing other than being an indicator if these are letting water by.

Any advice much appreciated. Photos attached of the setup if that helps.

The CH EV has a 12L capacity and came with a P.Press of 1.5bar, it's more than likely hasn't a P.Press of 1.5bar now because if it had then once the system pressure falls/fell below 1.5bar then the pressure should fall almost instantly to a head equal to the highest point in the systam, usually 4.5M to 6.0M, 0.45bar/0.6bar. The P.press was probably reduced on installation to somewhere between 0.8bar and 1.0bar, you can only check this by draining/partially draining the system and then checking the air pressure at the schrader valve (air end).
If you assume the P.press at 0.8bar then a filling pressure of 1.3bar will result in a (reserve) water vol of 2.61L in the 12L EV, when/if the pressure falls to 1.2bar then there is 2.18L in the EV, theoretically a loss of 0.43L. If the P.Press was 1.0bar then the corresponding numbers are 1.57L & 1.09L, a loss of 0.48L, average of the two, 0.46L. If you are really losing 0.46L every few days then quite a hefty loss, equals to ~ 80L/annum.
I would suggest topping up to say 1.8bar (cold), then when the pressure falls to say 1.3bar (cold) note the time taken, this should give a more accurate feel for whats going on after say 2 or 3 top ups. the system will lose a average of 1.77L between top ups, so based on the above 0.46L loss, should require topping up ~ every 7 or 8 days, far more meaningful IMO.