Hot water from Tundish after multiple checks.

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I have a Santon 210 litre unvented cylinder. I've had water running through the tundish for a while.
Initially, I had a very weak hot water flow to taps and showers. The gauge at the combination valve showed only about 1.2bar pressure. The santon system should be set at 3.5 bar according to the instructions and panel on the cylinder (See below)

TABLE 1: OPERATIONAL SUMMARY
Maximum water supply pressure to PRV16.0 bar
Operating pressure of unit3.5 bar
Expansion vessel charge pressure3.5 bar
Expansion valve setting6.0 bar
Nom inal storage capacity of units(see Section 2 Table 2)
Max. primary working pressure (Auxillary Coil)3.0 bar (indirects only)
Max. primary working pressure (Solar Coil)6 bar
Opening temperature of T&P Relief Valve90degC
Opening pressure of T&P Relief Valve1 bar

I removed the Santon combo valve cartridge and it was gunged up inside, and the return pressure spring had corroded and was broken in two. So I fitted a new combo valve, and at 3.5 bar, the hot water pressure was now great.

I also had a leak through the tundish going back quite a while. This would be fairly continuous, and normally luke warm. However, when the boiler was fired up to heat the hot water zone, this drip would be more constant, and the water would be hot. It would continue like this until a shower was taken or the hot water taps used for a while. I also would notice when the boiler was running to heat the water, that the cold water pipe running into the bottom of the boiler would actually warm up right up to the combination valve?
Again, on checking, its the 6 bar cold water side PRV that is venting (hot water) , not the cylinder 10 bar TPRV. I replaced the 6 bar PRV.
I then run the hot taps, turned off the water supply and let the pressure fully release. With the taps still on (no water flow of course), I filled the Expansion vessel with air to 3.5 bar to match the cold pressure through the valves to the cylinder.
There is no tundish drip when the boiler is not fired up . However, there is a drip (albeit much reduced now) when the boiler is fired and heating the cylinder water. This again goes away mostly when a tap or shower is run but has not been fully eradicated.

What could the issue be? I have verified that the 10 bar cylinder TPRV is not venting when I get the drip through the tundish (it is functional ok, as Ive tested it). Ive a brand new 6 bar PRV on the cold inlet side, and the expansion vessel is only 3 years old, and not showing any signs of malfunction. Holds the air charge and no water is coming from the Schrader valve.

Could anyone shed any light on where the issue might lie given the info above? All of the equipment seems to be ok, I can't help but think it has something to do with the inlet pressure or Expansion vessel pressure.

The combo valve is set to 3.5 bar. The empty pressure of the expansion tank is at 3.5 bar.
How critical is it to have the two exactly the same? Does it matter if the expansion tank is at 3.6 bar or 3.4 bar? Is it better slightly over or under?

Should I reduce the combo valve to 3 bar, and the expansion tank pressure to 3 bar, even though the Santon instructions advise to charge the expansion vessel to 3.5 bar, and set the combo valve to 3.5 bar?

The thermostat on the cylinder is set to "5" which apparently reflects 70 degrees. Should I try reducing it to "4"?

Both Expansion vessels (the original that came with the cylinder) and the replacement from a few years ago are 19 litre.
I've read in a few places online to allow 10% of the cylinder capacity. In this case 210 litres (so 21 litre exp vessel?)
A few sites I check on recommended a 24 litre vessel for a 210 litre capacity cylinder. Should I look at replacing the vessel to a 24 litre version is it ok to do that and would it maybe give me some extra capacity for expansion when the water is heated?

Lastly, (and its a long shot), I have a leaking mixer tap on the bath. Its been leaking with a slow drip for years. I cannot change it out, because its at the side of the bath and encased in a tiled bath frame. I've tried several replacement ceramic cartridges to no avail, as I think its a manufacturing flaw in the tap. I've read something about back flow from a leaky mixer tap affecting some heating water cylinders. Now the drip from the tap is minimal, maybe a drip every 5 seconds, Could this be an issue here, and should I maybe fit a one way valve on the hot supply pipe?

Many thanks and sorry for the war and peace post!
 
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What is the E.vessel volume?
Is the combination valve supplying the cold water via its balanced port to the leaking mixer tap on the bath, if not what pressure is this cold water at, is it supplied via another PRV?
 
What is the E.vessel volume?
Is the combination valve supplying the cold water via its balanced port to the leaking mixer tap on the bath, if not what pressure is this cold water at, is it supplied via another PRV?
The E vessel volume is 19 litres, and the cylinder is 210 litres, so it falls below 10% of cylinder volume. Santon supplied a 19l Exp tank with the cylinder, but I think its on the edge of its capacity?
The Combi valve is on the mains in cold water pipe which goes into the bottom of the cylinder. The 6 bar PRV is just after the combo valve. The hot water out exits at the top of the cylinder and feeds the showers, taps and bath. The cold water entering the cylinder should be 3.5 bar as this is what the combo valve is set to and it reflects this on the gauge. The cylinder is rated by Santon for 3.5 bar. Before I posted this, I closed the water supply, run the hot taps, then reset the combo valve to 3 bar, I then pressurised the expansion tank to 3 bar as well. The drip isn't as prevalent , but after a 30 minute boost from the boiler, it again started dripping a bit again.
 
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Even if the whole 210L was reheated from a unlikely 15C to 60C then the maximum final pressure will only be 3.87bar and 4.01bar if reheated to 65c so not your problem, can you post a photo of the combination valve to ensure that the EV is teed into the correct port, this has sometimes been incorrectly teed into the balanced cold port resulting effectively in no EV. and also showing the balanced cold take off,

1701990183902.png
 
Thankyou John, I'll post up a detailed diagram tomorrow of all the pipework. Cheers.
 
My unvented system has an air gap inside the cylinder and every so often has to be "recharged" when water is seen dripping through the tundish, which only happens when the hot water is on - when it needs doing.
Could that be your issue.
 
No. Not in my case. There is no balloon inside the cylinder. The expansion vessel (same thing) is external in this case. My water runs through the tundish worse when the boiler is heating the hot water cylinder. But also does it when the boiler is off. Basically worse when there is higher pressure in the system due to heating the water. Also the water leaking from the tundish is hot, but it's the cold side prv that is leaking and not the tprv on the cylinder itself. That has me baffled.
 
No. Not in my case. There is no balloon inside the cylinder. The expansion vessel (same thing) is external in this case. My water runs through the tundish worse when the boiler is heating the hot water cylinder. But also does it when the boiler is off. Basically worse when there is higher pressure in the system due to heating the water. Also the water leaking from the tundish is hot, but it's the cold side prv that is leaking and not the tprv on the cylinder itself. That has me baffled.
Ok I also have an extra expansion vessel but yes when the cylinder needs recharging the drip in the tundish is not hot and will only drip when the hot water is on when it needs doing.
 
OK. I have attached a diagram of the cylinder and pipework in the airing cupboard. The connections are correct, but I have tidied up the routing of the pipework solely to make the diagram clearer. The expansion vessel for instance is a lot higher up than shown here. I hope this provides some clarity as to how my system is connected up.
Our Ensuite shower is directly behind the wall where the cylinder is. That is why the shower hot and cold is exposed here. The hot feed to the shower is taken off the main hot supply out. The cold feed to the showers comes off a tee in the cold water mains supply after the Combo valve and PRV as shown. I presume this cold feed is also supplying the cold water to my other bathroom (shower, sink and bath), and cloakroom toilet (sink)??

Disappointingly, I checked the tundish at about 8am just after the boiler had been on for an hour to heat the water, and the leak through the tundish was as bad as ever. Does anything look out of place on the diagram?

Also for info. I have a bathroom, Ensuite and cloakroom toilet as well as the kitchen sink. Thats 4 sinks, 2 showers and 1 bath. All of these have mixer taps on them. Thanks.
 

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Another thing that might be worth adding. When I turn off the stopcock to the cylinder as per the above diagram, it turns off the cold water supply to the two showers as well as the hot water. I hadn't picked up on this previously.
However, the cold water supply to the 4 sinks and the bath remains at full pressure. I can only turn the cold water supply off to these by turning off the mains stopcock located under the kitchen sink.
I presume that should mean that only the shower mixers x 2 are supplied by the balanced cold feed tee from the cold water inlet beside the cylinder, whilst the bath and sinks cold supply is separated and fed directly from the cold mains coming into the house.
This being the case, would it not then true that the hot water to the 4 sinks and bath mixers is supplied at 3 bar from the cylinder, but the cold supply to the sinks and bath will be at whatever pressure is coming into the house from the mains and certainly way above 3 bar?
Could that be causing some back pressure on the heating side forcing hot water back from the cylinder back up the cold feed pipe and out through the 6 bar PRV given its hot water being expelled from the PRV even though the PRV is on the cold feed in? I have felt the cold pipe from the cylinder back up to the 6 bar PRV at the combination valve warm when the leaks are happening.
 
Yes, probably some combination of the above, the "balanced" cold, if, as per your schematic is teed off downstream of the combination valve NRV which is incorrect as it will operate at the cylinder pressure, it should be installed upstream of the NRV (in the combination vale) so will run at the PRV set pressure and won't be influenced by any expansion or whatever.
 
Sorry John, you have lost me a bit there. The balanced cold is indeed teed off downstream of the combo valve and pressure release valve. Would this not be correct as effectively the cold supply to the shower and the hot supply are both pressurised to the same pressure as the cylinder?

Are you saying the balanced cold feed should be off a tee in the cold mains in before it reaches the Combo valve and PRV? How would this contribute to the leaking from the tundish when the boiler heating is applied to heat the water.
 
The combination valve is called this because it combines a PRV, a NRV and a expansion relief valve.
The balanced cold connection should be taken off exactly as shown (schematically) here, ie, after the PRV but before the NRV, this ensures that the actual expansion has no effect on the balanced cold connection which runs at the pressure setting of the PRV. The expansion relief valve is on the other (downstream) side of the NRV so that in the event of the EV failing this will lift at 6bar.
Admittedly, once a few litres of HW are drawn down then the balanced pressure (even when incorrectly installed as above) will operate perfectly normal so I don't really see why this should cause the expansion valve to lift.
What is the make/model of the new PRV that you installed?, if this is not "Drop tight" then it can pass enough to increase EV charging pressure and cause it to lift on reheat. For example if 4.74L passed into the EV before reheat then this will result in a EV pressure of 4.33bar and cause the expansion relief valve to rise to 6.0bar when expansion of the 3.4L takes place.

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