Suspected Leak in Hot Water system vs Cylinder behaviour?

[stormyuk]

Good morning,

I have a Kingspan Tribune XE (TXN210) indirect unvented cylinder in my property (installed in 2020, in a new build property) and I am trying to understand what I originally suspected was a leak from the HW system.

My cylinder is heated twice a day on 2 x 30 minute cycles, this generally gives us enough hot water for washing and showing for us, boost is used in circumstances where extra hot water is required.

I was taking a water meter reading some weeks ago and I noticed that the small meter dial x0.0001 (in millilitres, one revolution being one litre) was moving while I was taking the reading, indicating water usage when no one was actually using the water. So I started to monitor this usage suspecting I have a leak somewhere on the property.

After making sure there was no visible leaks I could see, checking all water outlets and toilets etc. I then checked that there was no water usage when the stopcock to the property was closed. This proved the meter was not faulty as when the stopcock is closed no water is used.

I then turned off the cold water feed to the XE Cylinder and this also resulted in no water being used. When the feed to the cylinder was re-enabled I had the unexplained water loss again. So this proves the loss of water is in the HW system somewhere. I then double checked I had no drips from taps, showers etc and actually did fix a small drip on the kitchen tap (although it was a very slight drip, not explaining the figures below)

Below is a typical example of the water loss, which is not consistent and not linear. It seems to follow this pattern each day with slight variation in the figures but the "loss" always gets less from the early reading, to the later reading as time progresses.

The tank is heated between 6.30 and 7.00 in the morning and then 2 or 3 showers are taken between 7 and 8.30. We then purposfully didn't use anymore hot water during the daytime (times below) over a series of a few days to monitor.

Water Meter reading; |00308|844| (960) (start meter read, in cubic meters, followed by litres in red 844, and millitres (960ml in red also, shown on the small red dial).

Monitoring over the next few hours, shows the following change, where the number in red is last number on the meter, reading in litres. The numbers in (brackets) are the "water loss" over the previous time period.

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)

Total. 1.64 litres between 8.30am and 17.16pm.

Could this be explained by the hot water tank topping up as the cylinder cools during the day, due to contraction of the volume in the tank leading to the diaphragm moving, leading to a top up of the system?

The house is a new build and I cannot see any water marks anywhere on any walls or ceilings. The HW pipes are all in the walls and ceilings, and again there are no water marks anywhere that I can see.

I am a bit baffled by this.

Thanks,

 

[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.

 

[stormyuk]

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.

No water usage at all, I've purposefully not used any water at all for that time period to get a read on the behaviour.

So it's probably likely in normal usage scenarios it's probably letting more water into the tank but, this is an almost fake scenario with no water usage over that time frame.

It's definitely something on the HW side considering if the cold supply to the tank is turned off water usage is exactly zero over the same time period.

 

[John.g]

Maybe your neighbors were using more water during this period resulting in lower mains pressure at your stopcock.

 

[IND_Nick]

Do you know what the main incoming pressure is at your property. It could simply be that in the morning as more people are using water the pressure in the mains is lower, during the day the pressure increases in the main and this pushes through the combination valve and into the expansion vessel capacity, especially if the mains pressure, in the morning, is below what the PRV would shut off on.
If you had a leak of 1 ltr a day it would normally be very apparent quite quickly.
Just a thought.

 

[John.g]

Something like that but if a PRV is working properly then it should only open if there is some demand for water, however little, and any mains pressure rise shouldn't open it as its balanced unlike a PRV (safety valve) on a boiler which isn't balanced. When used as a Auto Fill valve for system make up (and left open) they must also be drop tight.

 

[stormyuk]

Do you know what the main incoming pressure is at your property. It could simply be that in the morning as more people are using water the pressure in the mains is lower, during the day the pressure increases in the main and this pushes through the combination valve and into the expansion vessel capacity, especially if the mains pressure, in the morning, is below what the PRV would shut off on.
If you had a leak of 1 ltr a day it would normally be very apparent quite quickly.
Just a thought.

No not sure on my pressure, think I am going to see if I can buy a cheap gauge as that's something I can probably check easily with say my outside tap which is pretty much at the stopcock.

Speaking to my neighbour again with the same Tribune XE his system exhibits similar behaviour although he's not logged it like me..🤣

He is often away and after he has been away for a week (he turns his stopcock off while away) after he returns and puts the water back on his tank 'fills' for a while. 🤔