Suspected Leak in Hot Water system vs Cylinder behaviour?

[John.g]

Keep a eye on the boiler pressure when off, it should fall slowly and should not rise under any circumstances if the filling valve is shut and if there is no cylinder coil leak.

 

[stormyuk]

If the water meter stops when the isolation valve on the combination valve is closed and no water is being used in the property? then this points to a leak you have tempreture and pressure relief valves fitted to the cylinder I would be monitoring these especially at heat up times do you have any idea what your incoming
water pressure is ? water pressure can certainly rise over night and a faulty combination valve is quite possible, your cylinder is still under warranty but you must determine what and where the problem is and where you losing water . Kop

My only issue with I understanding if it is the combination PRV/NRV on the input to the tank, where is this water going? What goes in should come out I suppose..

IMO, a pressure gauge is the first requirement to track down the problem.

What we havn't mentioned is a holed heating coil but if the heating system is sealed then eventually if the cylinder pressure is consistently higher than the coil pressure then the PRV will eventually lift on the boiler, however if the two pressures are very close and at times the cylinder pressure may be lower than the coil pressure, in this case water will be shunted back into the cylinder thus keeping a balance.
If the boiler& system is open vented then eventually the F&E tank should start overflowing.

Yeh it's a sealed system.

To track it down it looks like I am going to have to get an expert in to check the pressures. It's finding said expert...

I am not convinced there is a leak into the heating circuit to be honest. The boiler is reading around 1.2bar on it's gauge and the gauge on the heating pipework upstairs reads slightly lower which is what I would expect. Maybe 1.1bar but then who knows how accurate they are probably not very.

If my tank is ~3bar the boiler would have been chucking water out by now I would think. Also when I had the system serviced in March / April I also had the water condition checked and it appeared ok with correct inhibitor level and such. This would probably have all been flushed out. Its conceivable the fault has just occurred but, my gut feeling is it's always been doing it and I've just never noticed.

 

[stormyuk]

Keep a eye on the boiler pressure when off, it should fall slowly and should not rise under any circumstances if the filling valve is shut and if there is no cylinder coil leak.

Yeh boiler pressure only appears to rise when there is heating or hot water demand and it's running. It does rise to around 2bar which I always thought was quite high from a standing start of 1.2bar (1.8bar if hot water demand and 2bar for the radiator circuits).

Although it's always done this and two different plumbers who have serviced the boiler two different years have said it's fine...

 

[John.g]

If its returning to 1.2 bar when cold then no coil leak, any pressure gauge on the cylinder??.

 

[stormyuk]

The only gauge at the cylinder is for the heating circuit. There is nothing for the tank itself unfortunately.

I also still can't get my head around the declining 'leak rate' throughout the day.

4.960 08.30am
5.750 09.30am (0.79l)
5.900 10.35am (0.15l)
6.080 11.33am (0.18l)
6.270 12.37pm (0.18l)
6.380 13.37pm (0.11l)
6.510 14.37pm (0.13l)
6.590 15.37pm (0.08l)
6.600 17.16pm (0.01l)

I am doing more readings today to see how it compares to the other weeks, it will send me around the bend I think.

 

[John.g]

Without any more ado I would install a pressure gauge on the cylinder circuit, there may be a blanked tapping on the PRV in which you can install a PG, wonder how was the system was set up originally?.

 

[Chuck]

I also still can't get my head around the declining 'leak rate' throughout the day.

You said you're an electrical engineer so perhaps the following analogy might help.

The PRVs used in these systems are fairly simple. As the pressure increases at the outlet port it pushes a piston against a spring that closes the valve. The top of the spring can be moved up and down to set the nominal pressure. In electrical terms, as the pressure (voltage) at the valve outlet approaches the set point the valve impedance (resistance, R) rises. The water tank is essentially a large capacitor (C) containing a mass (charge, Q) of water. So, if you consider the system to be a capacitor being charged from a constant voltage (supply pressure) via an increasing resistance (the closing PRV) you'd expect the time constant (RC) for the process to increase as it approaches equilibrium. The settling time to 99% of steady state is minutes, but the settling time to 99.9% seems to be hours.

 

[John.g]

I just think of a PRV as a pressure reducer with a sort of proportional control where the pressure loss depends on the flow rate.
The example given below tells me that this particular (1/2") valve will maintain a downstream/cylinder pressure of 2.3 bar with a upstream pressure of 8bar and a flow rate of 20LPM but the pressure will not be higher than 3 bar with no demand.
So the PRV in question, whatever its sizing/set pressure should also not rise above its set pressure with no demand except that its leaking past the seat or it may increase the downstream pressure with no flow if mains pressure rises??. Edit: no shouldn't increase with increase in upstream pressure at no flow conditions.

 

[stormyuk]

Thanks again for the responses guys.

Here is today's meter data;

Apologies for the poor photos, it's quite difficult to get decent shots.

First photo is 8.30am and the other photo is 5.32pm. The small dial and last digit don't actually line up properly (why am I not surprised).

So it's moved just over a litre today.

0.00 8.30am (dodgy line up)
0.40 9.30am (0.41l or 0.40l)
0.54 10.32am (0.14l)
0.65 11.32am (0.11l)
0.88 12.32 (0.23l)
0.95 13.59 (0.07l)
0.99 14.30 (0.04l)
1.04 15.29 (0.05l)
1.04 16.31 (0.00l)
1.04 17.32 (0.00l)

Between around 15.30 and 17.32 the meter didn't move at all.

Figures in brackets are the loss over the period I also missed a reading on the hour at 13.30.

I am tempted to just to get the PRV/NRV assembly replaced but, it seems an expensive that might not really be warranted with the volume of water if it is the PRV not seating properly. Whatever is 'leaking' has leaked a little less today.

 

[Deleted member 123817]

Yes, you could say "nothing to see here, move on" but is intriguing if nothing else but very few would notice that small change in usage anyway.
Would that period of no usage coincide with heavier cold water usage I wonder.