Unvented cylinder system - repair?

[hy288]

Hi,

We have an unvented hot water cylinder for some years.
Recently we notice an issue with water dripping via the tundish suggesting a fault. This only happens when the hot water scheduler comes on to heat the water. Within the first 10-15mins we notice water dripping via the tundish, and stops once the motorised zone closes off i.e. boiler stops. The water via the tundish is cold initially, and after a period it probably feels semi warm. The discharged water is never hot from observations so far.
You can hear a slight hissing sound coming from the combination water valve thats connected in from the cold mains i.e. the combined pressure reducing valve (3.5bar), check valve and expansion valve (6bar). Based on the sound and temperature of the discharge, suspect it likely coming from this combo valve rather than the temperature/pressure release valve (10bar) on the cylinder.

We had a service on the unvented cylinder recently to check. Hoping that the issue is more simpler and either requiring a pressure charge in the expansion vessel or just requiring the pressure release valves to be opened and re-seated etc.
The expansion vessel is suppose to be 3.5bar but at the time of check was about 2.8-3bar. So was topped up. All the pressure release valves were tested, water discharges fine and re-seats properly.

However, despite after the service we're still getting water dripping via the tundish as before when hot water heats up.
So the next steps proposed is to get the entire combination valve replaced (like-for-like).

I'm able to get the exact same part (santon brand) relatively cheap at £60. So planning on doing this.
The quoted labour for replacing this is £90+VAT (not sure how long that takes and what's involved i.e. does that require draining the cylinder etc?)

Firstly, can I confirm based on the fault observed that the proposed action of replacing this combination valve is reasonable?
And can someone advise if the quoted labour cost is also reasonable for this?

Thanks!

 

[ianmcd]

undo the compression nut on the outlet of the releif valve on the inlet set and you will know if it id dripping from there

 

[Johntheo5]

If it is the expansion valve and it only drips when the water is heating up then most likely a expansion vessel problem, possible diaphragm failure and now full of water so the E.valve will expel water each time the cylinder heats up, would suggest getting the E.vessel checked out as a start, it also should be precharged to 0.2 bar lower than the PRV which normally is set to 3.0 bar.
In your case, if PRV set to 3.5bar, then precharge set to 3.3bar.
You can do a simple test by pressing the E.vessel schrader valve pin to see if full of water.

 

[hy288]

If it is the expansion valve and it only drips when the water is heating up then most likely a expansion vessel problem, possible diaphragm failure and now full of water so the E.valve will expel water each time the cylinder heats up, would suggest getting the E.vessel checked out as a start, it also should be precharged to 0.2 bar lower than the PRV which normally is set to 3.0 bar.
In your case, if PRV set to 3.5bar, then precharge set to 3.3bar.
You can do a simple test by pressing the E.vessel schrader valve pin to see if full of water.

Like I said before, i've had a service carried out on the unvented system. The expansion vessel was checked and there is no water when pressing the valve pin. It was slightly under charged 2.8bar and was then topped up appropriately during the service.
Unfortunately this did not resolve the dripping of water into the tundish.

My understanding is that the Expansion vessel is ok i.e. no water when pressing valve pin and now charged up properly.
Hence, would the next most logical problem area be the combination valve i.e. possibly a faulty expansion valve on this combo valve?

 

[hy288]

undo the compression nut on the outlet of the releif valve on the inlet set and you will know if it id dripping from there

I'm just your avg joe and not G3 certificated. So probably not a good idea taking things apart.
Other than describing observations of the system as it stands.

 

[ianmcd]

you dont need to be G3 qualified to undo that nut, you are not altering anything, you say you had the unit serviced so why did the G3 Qualified person not check that ? you say the discharge gets warm after a while , if that is the case then replacing the inlet group wont make any difference

 

[hy288]

you dont need to be G3 qualified to undo that nut, you are not altering anything, you say you had the unit serviced so why did the G3 Qualified person not check that ? you say the discharge gets warm after a while , if that is the case then replacing the inlet group wont make any difference

Yes discharged water is initially cold, then after a period its technically more like lukewarm (but not hot).
I can certainly understand why that might be the case, as I would expect water flowing in the green paths would become slightly lukewarm as the water is all connected to the cylinder which will be hot.

If the expansion valve then operates, i would expect it to discharge water within the green flow path....hence the slightly luke warm water flowing thru the green path.

I'm trying to understand what makes you think (reasoning) that replacing the combo valve not make any difference?

 

[Johntheo5]

If the EV was serviced properly then the water end should have been depressurized and disconnected to ensure the diaphragm is hard up against the water end at 3.3bar air end pressure, the pressure should then only rise to ~ 4.0 bar even if the full cylinder contents are reheated so unlikely that the E.valvewill leak at 4bar but not at 3.5bar, itis possible of course that the PRV is faulty and that there is almost full mains pressure which will have to be 5.5 bar if the E.valve is actually lifting at 6.0 bar when cylinder reheating.

 

[hy288]

If the EV was serviced properly then the water end should have been depressurized and disconnected to ensure the diaphragm is hard up against the water end at 3.3bar air end pressure, the pressure should then only rise to ~ 4.0 bar even if the full cylinder contents are reheated so unlikely that the PRV will leak at 4bar but not at 3.5bar, it is possible of course that the PRV is faulty and that there is almost full mains pressure which will have to be 5.5 bar if the PRV is actually lifting at 6.0 bar.
0

I assume depressurizing it by turning the stop valve off just below the combo valve...then opening the hot taps to take the pressure off. I believe they did that, and then check the pressure on the EV...topping up appropriately. They did not disconnect the EV physically if thats what you meaning. They did check the pressure of the EV after depressurized, and topped up.

The system has been in for 8-9 years and probably not uncommon for the PRV to be faulty?

When I checked the manual (Santon Premier Plus), it suggests likely issues are pressure reducer valve or expansion valve? Assuming that the EV is all good. If it was the T&P valve, i would expect HOT valve to be vented into the tundish (thats not the case).

So going back to my main query of the post.
Is the replacement of the combo valve the most logical step - (pressure reducing, check flow valve, and expansion valve).
And is 90+Vat a reasonable quote labour wise?

Thanks

 

[ianmcd]

Recently we notice an issue with water dripping via the tundish suggesting a fault. This only happens when the hot water scheduler comes on

Then you circle a box saying it is continuall, cant be both

 

[Johntheo5]

If water was discharging continuously from the E.valve with no HW drawoff then would be pointing to PRV, you could monitor the EV air end with a tyre pressure gauge with no HW demand and see if it rises to 6bar or whatever, that will tell a lot. Then monitor ir when reheatng cylinder.

 

[hy288]

Apologies, it's definitely only dripping or running water in the tundish when the hot water schedule kicks in to heat the water cylinder. After 10-15mins water can be seen in the tundish "continuously" until such time when the motorised valve kicks in to stop heating things. At that point, the drip slows down and eventually stops.

If the hot water cylinder is not getting heated then simply is NO water running through the tundish.
The central heating being on will have no impact. The observation on the discharge is consistent whenever the cylinder heats up as the hot water scheduler kicks in. No hot water is being used if thats relevant to you note about "HW drawoff"?

 

[hy288]

Then you circle a box saying it is continuall, cant be both

My interpretation is when the hot water cylinder heats up, within 10-15mins water "continually" drips via the tundish. Its only stops discharging when the call for heating on the hot water stops i.e. motorised valve stops and boiler halts. The tundish discharge consistently happens under this scenario and is never intermittent.

 

[Johntheo5]

Thanks, thats how I thought it was.
Assuming that the E.Vessel is OK and that the E.valve is not leaking but is only dripping when the pressure reaches 6.0bar then the only logical explaination is that the PRV is passing, the cylinder will have to reach ~ 4.7bar (cold) then the expansion will increase that pressure to 6.0 bar to lift the E.valve, the PRV will have to leak past ~ 4.8 Litres to achieve this which is not a lot over a period of time.
You can do a few tests yourself to prove this.
With heating off, Run off say 100L of hot water from a hot tap, then immediately shut off the cold feed to the cylinder and only then shut off the hot tap.
Start heating the cylinder, if no dripping from the E.valve after 20 minutes or so then the PRV, is the culprit IMO.

 

[Madrab]

If it is only dripping when the hot water is being warmed then it has to be an expansion issue, if the vessel's been checked then it can only be the pressure relief valve (PRV), their springs can get softer over time.

If it was the pressure reducing valve (control group) then it would be over pressurising all the time and it would drip all the time, and that's only if your mains is above ~ 3.5bar.

As suggested, loosen the nut on the pressure relief valve and stick a tub under it, fire up the hot water and see if the valve does let by, if so then you need a new PRV.