OSO 20 indirect unvented hot water tank

[sintler]

I'm having strange problems with an OSO 20 hot water tank. We recently had the TPR valve and expansion valves changed for water loss from the tundish. Then we had an external expansion tank fitted as the hot water was initially flowing at extremely high pressure and temp and unstable. This was advised by OSO who said the air bubble was going.
The problem is now still similar. When turning on a hot tap we have a burst of extremely high pressure water. This lasts 10-15 secs and then drops and seems stable after. We have a pressure reducing valve fitted on the mains inflow to the house. This has a pressure gauge on it. I have noticed that as the hot water pressure builds, the gauge reads over 6 bar on the cold mains. Its usually 4 bar, reduced to 3 with the valve. This drops with the hot water being turned on.
Our plumber cant explain this. To me things are not right. The only thing I have noticed is that the hot tank pressure reducing valve is set to 0. I seem to remember it being 2+. (small blue dial)
The pressure in the expansion tank was 4.5 bar. Random check with hot water in there. It says it should be 3.5bar when cold. Could there be too little pressure in it.
Ant ideas?
Thanks for reading and in advance.

 

[Norcon]

There may be a cross flow some where in the system where hot and colds are connected. Could be through a shower mixing device, a set of taps or a porous housing in a basin tap or a twin feed washing machine although I think they are supposed to have an air gap which should make this impossible.

 

[steve32]

You have to be registered to work on unvented cylinders (G3) is your plumber registered.
As you realise you have pressure build up when the cylinder is heating it should be turned off until the fault is corrected (turn off the heat source)

This fault is usually caused by a faulty pressure reducing valve (prv) or no charge or incorrect charge in the expansion unit.

The expansion units are precharged you adjust it when fitting and check the pressure yearly when the unit is serviced.

What was the charge pressure when it was fitted it should be 1-1.5 bar below cylinder standing pressure min 0.8bar (pressure after cylinder prv) or as per manufactures instructions.
To check pressure in the expansion unit the cold water inlet should be turned off and pressure in cylinder relived (open hot taps).

 

[sintler]

Thank you.
The PRV on the cylinder is adjustable (somehow) but seems to be set to zero. I seem to remember it used to be on about 2.
The expansion tank is a 24l tank (white) cylinder is 210l. Mains pressure is 4-5 bar. Is that the standing pressure in the cylinder or does that need to be measured from the cylinder? The expansion tank says 3.5bar precharge pressure on it. I guess thats what to pump it up to with the pressure taken out of the system?
As for the plumber, no idea!! Thanks for the advice on that.

 

[Norcon]

Your plumbing may be something like this.......

But you need someone qualified to have a look and identify the problem before more parts are pulled off and changed to no avail.

 

[sintler]

Thanks
So pressure could get from the hot side to cold through a shower etc. We have grohetherm 3000's x 4.

 

[sintler]

thanks

 

[sintler]

Apologies. Its the green dial version and its probably set on 5. There is no 0 on the dial. In that case, is this allowing an initial 5bar pressure which drops to 3 bar as the mains prv only allows cold water at 3 bar?
Can I adjust this valve? how does it adjust if so?
Thanks

 

[Norcon]

Thanks
So pressure could get from the hot side to cold through a shower etc. We have grohetherm 3000's x 4.

Four.
Its doubtful as I think these valves have nrv's fitted. Do you know the plumbing layout? Sounds like a large property.
Do the showers have in fact unbalanced supplies and can you identify any where in the system that has unbalanced supplies?
Could the mains supply split at your kitchen to a utility room area and to the unvented location?
Best not to start meddling with the pressures I think.

 

[sintler]

There is no balanced cold water connection on the cylinder. The showers are thermostatic. Can't think of anywhere else that hot and cold may meet.

If the PRV is set on 5bar could this account for the 10-15secs of initial high pressure that drops to mains (reduced to 3 bar)?

 

[Norcon]

There is no balanced cold water connection on the cylinder

If thats the case then all your supplies are unbalanced between hot and cold.
The cold will be set from the incoming prv at say 4 bar and hot from the icg setting.

 

[sintler]

The cold incoming PRV is set at 3 bar. What do you mean by icg please.

 

[Norcon]

ICG = Inlet control group. Which combines a stainless steel strainer,prv ,balanced cold water connection, and expansion valve.

 

[sintler]

Thanks, thats what I thought you meant. Thats what is set to 5 bar. Its the adjustable version. (510511 multiblock) I guess this pressure needs reducing but I cant figure out how to do it.

 

[sintler]

Hi Norcon / anyone
I went to check the pressure in the expansion tank. Turned off the cold inlet and opened the hot tap in the kitchen. Initially got hot water and then it went cold and continued to flow at low pressure and didn't stop! I had a good think about your advice and went round the house listening to the shower valves. One has flow in it the rest were quiet. I am guessing that this means the valve is faulty and is how the cold system is being pressurised by the hot tank. Does this sound feasible?
Are Grohe shower valves repairable or will it need replacing.
Do I need to get the adjustable PRV on the cylinder turned down from 5 bar?
Thanks