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,

 

[Ric2013]

0.0001l is 1/10 of a ml, not 1 ml.

 

[stormyuk]

Yeh it's a typo. Basically this dial is 1000ml's isn't it? So a full rotation is equal to 1litre. So each numbered marker 1,2,3 etc etc is 100ml?

That's how it seems to work each rotation of the small dial increases the last red number on the meter by 1litre.

I can't edit the original post. I think the figures in the list are still valid.

 

[John.g]

If the unvented cylinder has been installed properly, NR valve etc, then with no usage but cold feed still open (or shut) then a 180 litre cylinder with 165 litres of water and 15 litre expansion space pre pressurized to say 2 bar, same as mains pressure, will absorb 2.67 litres of water if heated from 15C to 60C, obviously a bit less if heating from a different base, the cylinder pressure will increase from 2.0 bar to 2.65 bar but in cooling down again this 2.67 litres will again be taken up by the system and the pressure will/should return to the system. So, IMO, the flow meter should not register anything.
The mains pressure would have to increase by 0.42bar to force 1.84 litres into the expansion space and might happen if the PRV is passing and isn't "droptight"

 

[Ric2013]

If the unvented cylinder has been installed properly, NR valve etc, then with no usage but cold feed still open (or shut) then a 180 litre cylinder with 165 litres of water and 15 litre expansion space pre pressurized to say 2 bar, same as mains pressure, will absorb 2.67 litres of water if heated from 15C to 60C, obviously a bit less if heating from a different base, the cylinder pressure will increase from 2.0 bar to 2.65 bar but in cooling down again this 2.67 litres will again be taken up by the system and the pressure will/should return to the system. So, IMO, the flow meter should not register anything.
The mains pressure would have to increase by 0.42bar to force 1.84 litres into the expansion space and might happen if the PRV is passing and isn't "droptight"

Interesting thought is what if the mains pressure varies?

 

[John.g]

A PRV is essential with a unvented cylinder because if its as high as 4/4.5 bar bar then the expansion valve will lift at 6 bar when the cylinder heats up of if the PRV is passing, it only needs ~ 3.7 litres which when added with the expansion will rise to 6 bar.

 

[Ben-gee]

Do you have a balanced cold take off after the isolation to the hot cylinder?

 

[John.g]

Even though this one has a external "air bubble" they are all pretty similar.

 

[stormyuk]

If the unvented cylinder has been installed properly, NR valve etc, then with no usage but cold feed still open (or shut) then a 180 litre cylinder with 165 litres of water and 15 litre expansion space pre pressurized to say 2 bar, same as mains pressure, will absorb 2.67 litres of water if heated from 15C to 60C, obviously a bit less if heating from a different base, the cylinder pressure will increase from 2.0 bar to 2.65 bar but in cooling down again this 2.67 litres will again be taken up by the system and the pressure will/should return to the system. So, IMO, the flow meter should not register anything.
The mains pressure would have to increase by 0.42bar to force 1.84 litres into the expansion space and might happen if the PRV is passing and isn't "droptight"

Interesting, so it looks like I potentially do have a small leak somewhere, just no idea where and why the leak changes leak rate throughout the day, I have no idea. This is what confuses me. I would expect a leak to be pretty constant (or get worse over time), this is why I wondered if it could be something with the tank and the fact its cooling.

Could a faulty PRV cause water to be taken into the tank, but at a varying rate? The first hour after water usage is always the worst and then it slows down during the day as my figures show. I will do some more monitoring next week to see if its still similar rates.

There is no dripping from the tundish or overflow of water elsewhere.

Unless of course, its not installed correctly but, interestingly, the original tank had "popping" noise issues and Kingspan themselves replaced the tank under warranty and never mentioned it being installed incorrectly as potentially they could have used that as an argument not to honour the warranty, although see the next quote.

Do you have a balanced cold take off after the isolation to the hot cylinder?

Interestingly no, non of the mixers use a balanced cold feed which is probably wrongly installed. The tank balanced cold is just capped off at the PRV assembly, but Kingspan engineer themselves never queired this when they replaced my tank under warranty and its the pre-plumbed version, so they would have known about this on inspection and replacement.

Interestingly the show homes never had the balanced cold installed either (as I have photo's of the airing cupboard installs) so I am guessing this builder doesn't install the balanced colds which is probably wrong, but cheaper. Not sure that would cause this issue though? The performance of the showers etc is very good and so not sure if and how much back pressure there is to the tank because of this. How would that manifest? Dripping at the tundish due to increased pressure in the tank? I have recharged the pressure gap on the tank and the tundish did have a weep for the following day, but since then has been dry and has been dry for the rest of the time.

The house is a 2019 new build, tank was replaced in 2020.

 

[Chuck]

A leak of a litre or more a day into the fabric of a building should be quite noticeable if it continues for a few days.

Based on your observations, I suspect that the PRV is most likely to be responsible. You could confirm this by connecting a pressure gauge to the DHW and see how it correlates with your 'leak'. The mains inlet pressure should also be checked. PRVs need to have a reasonable drop across them to work correctly. If the incoming pressure is not high enough, e.g. because all your neighbours are having their baths and showers, then the cylinder will be left at too low a pressure and will top up once the inlet pressure rises again.

You do sometimes get leaks that are temperature sensitive, e.g. open after hot water has been run due to the increased temperature and close as the pipework cools, but they are not common. And, as I said above, a litre a day from your DHW is probably going to be fairly obvious once you start looking.