Occasional leak as well as gush through tundish?

[Andehh]

18 months in and we have had a slowish leak for a while. During the summer I needed to top up the water pressure every few days by 1 bar or so...but during the winter im losing a bar a day. 

 

System is sealed, 2 x manifolds of UFH across 250sqm, ASHP with glycol etc in the water.

 

I can't pinpoint the failure mode, but it seems like it is a very slow gradual leak and then once or twice a day a short/sharp gush through the tundish, as I can see the gycol liquid around & outside it but have never witnessed it. Turning up the pressure of the system dumps water down tundish...to the point I have masking tape around the tundish to prevent splashing.

 

Got a really good plumber, but his girlfriend about to give birth so is out until end of January before he can come and have a look.

 

For peace of mind I am trying to work out what could be causing it. He recons a failed/failing safety valve and/or pressure cylinder needing recharge.

 

We have no evidence of leaks anywhere, including round ASHP, with the blue gycol I like to think i'd spot something!

 

 

Does a pressure cylinder recharge make sense with a slow AND gushing leak? Does a safety valve? Any hazard a guess which to 'go for' first when he does come out?

 

thanks, pic below of the setup! 

 

 

Edited December 27, 2024 by Andehh

[Temp]

Some possibilities..

 

1) Once a Pressure Relief Valve opens once they have a tendency to leak and let water out too easily at a lower pressure than its rated for. The more it leaks the more easily it leaks. Not sure why? Perhaps they are sensitive to a bit of grit or scale in the water. I've seen it a few times.

 

2)The Expansion Vessel could be faulty allowing the pressure to increase too much when the system heats up. Monitor the pressure and see what happens as it heats up. Perhaps check the valve on the Expansion vessel. When pressed air should come out not water. 

 

3) The filling loop could be letting water into the system when it's meant to be closed causing over pressure. 

 

2 or 3 could cause the PRV to open then it sticks open due to 1).

 

[Andehh]

Thanks Temp, I've kept a close eye on the pressure gauge and occasionally it gets up to 3 bar when the system is doing its thing heating and DHW, but generally hover around 2bar in operation.

 

The fill loop has two valves which you can just see in the picture dead centre, but it doesn't seem to be leaking through.

 

The pressure release valves feel like they are mimicking your comments on #1 point. There definitely still feels like a slow leak and then this random gush/water dump off at random that really kills the pressure!

 

[Marvin]

Hi @Andehh

 

The pressure relief valve (PRV) should not open in normal operation. It is there in case of excessive pressure. If any water comes out of the PRV there is a problem with the system.

 

The system should be designed to allow enough expansion of the water when heated to not reach over the PRV set point - 3bar - and therefore never even drip.

 

So you have a few possibles:

 

Additional water/pressure is being introduced into the system.  This is highly unlikely because after the the system pressure goes down it would start rising again due to some fault that would be allowing additional water in the system over time (like a valve letting by).

 

PRV faulty. This is likely now, because your description indicates this has been going on for some time, however this is a symptom not the cause as shown by the gushing.

 

Not enough room for the system when the water is hot to contain the expanded water. This is the most likely problem. This is also supported by the fact that when the system is hotter, when used in the winter, it has caused more problems.

 

Areas to check:

Volume of water in your system to assumed expansion capacity.  The comparison of the difference in volume caused by the heating of the volume to the expansion tank capacity. Will all the increase fit with extra room in the expansion tank when working?

 

The functioning of the Expansion tank (ET).  The ET has to be set at the right pressure, which would be just above normal working pressure, and be able to accommodate all expansion when the water is heated.  The ET has a valve on it that allows the pressure to be adjusted - just like a car tyre. 

 

If the ET pressure is set too low: When filling the system to the normal working pressure the ET will fill and so there will be nowhere for the expanded volume of water to go when the system is heated.

 

If the ET pressure is set too high: When heating the system to the normal working temperature the ET will not allow any water in until over the PRV pressure level causing the water to flow through the PRV.

 

The answer could be a combination of the expansion tank being slightly too small and the pressure not quite right.

 

HOWEVER

 

None of these explanations explains the gushing. What does is an overheating of the water somewhere in the system causing steam. This could suggest a pump or valve problem. All the items above would cause a gradual flow as pressures rises not a sudden gush

 

Finally, the next 20 posters will probably come up with other suggestions/guidance, as I know I don't know everything....

 

Good luck

 

M

 

 

 

 

 

 

 

 

 

 

 

Edited January 1 by Marvin
further thought

[FuerteStu]

I also vote pressure release valve, just changed mine. 

 

£9 delivered in 48 hours, 15 minutes to drain down change and repressurise. 

 

(not suggesting you do it yourself unless you're competent though) 

[sharpener]

1. My installers left the expansion vessel with the factory precharge of 2.7  bar which is much too high and I got wild pressure fluctiations bc it was effectively doing nothing. Fortunately without tripping the PRV.

 

You need first to de-pressurise the system, then set the expansion vessel to about 1.2 bar and finally re-pressurise to 1.5 bar when cold, this will allow 80% of its volume free for thermal expansion. You cannot set it correctly while the system is hot or under pressure.

 

2. If the pressure relief valve in the ASHP outdoor unit is going off it the outflow will go into the condensate drain so you might not notice it. Vaillants have a reputation for doing this, it can also get over the electrics as well, you can easily take the top cover off to check.

[Temp]

On 01/01/2025 at 07:22, Andehh said:

I've kept a close eye on the pressure gauge and occasionally it gets up to 3 bar when the system is doing its thing heating and DHW, but generally hover around 2bar in operation.

 

3bar is around the pressure at which the PRV will open so I'm inclined to think there is an issue with the expansion vessel. Faulty, not pumped up enough or too small.