Unvented Water Cylinder Problem?

[dpw1986]

Hi. Im new so please bear with me. I will try to explain in as much detail as I can. I have a fairly large 3 bed house. We have a standard gas boiler downstairs (not combi) with an unvented water system upstairs. We have the main water tank with the red expansion vessel which is located just above it.

When the boiler kicks in in the morning to heat the water up, after about 10-15 minutes, water starts dripping into the tun dish where the pipe then runs out side. Also, while the water is heating up, the hot water taps feel as if they have a LOT of resistance behind them: when you initially use the hot water tap a big surge of water them it calms down. When the hot water is used for the first time after its started heating up, there is a noise which sounds like escaping air, which comes from the hot water cylinder area. Im not sure exactly where it comes from.

Previously, when we had a similar fault before, our gas engineer drained the system, re-filled it and reset the air bubble in the tank. This was fine for about 7-8 months untill it happen again. A new engineer done the same and it lastest for about another 4-5 months and it has gone again.

The latest engineer we've had out was here for about 10 minutes, didnt even touch the system and said he would order a new safety valve (the valve closest to the expansion vessel, there are 3 on the system). Im not a qualified plumber, but personally i think he may be wrong. Surely, if the valve was faulty it WOULDNT let any water out? And if it was faulty in the way of it staying open, there wouldnt be any pressure behind the hot water?

Any advise would be greatly appreciated!

Thanks!

 

[oz-plumber]

Personally I would get the plumber to change the PTR Valve and also install a new and probably larger expansion tank.

 

[dpw1986]

He said he was going to replace a valve, i dont think he said what valve though. With regards to the tank, this system was installed when the house was and the house is only 6 years old so surely they should have installed the correct size tank? If they haven't I will have have to get in touch with the company who installed it. Thanks

 

[Robert Tyrrell]

I would hope that the plumber is properly qualified to work on your unvented system (They should have a G3 qualification) but, taking this into account, they should know which valve is which and will replace the one causing the problem.
It does sound as though the TRPV is operating and this might be the valve they are changing. When this valve fails, the water reaches too high a temperature and/or the water expands too much for the cylinder to cope with.
In a vented system this would just expand up the vent pipe and, possibly, return to the F&E tank but in an unvented system, to avoid explosions, the valve operates and the expanded volume of water just disappears through the pipework and you see it at the tun dish so that you can register that there is actually a problem and get it fixed.
As for a larger expansion vessel - the correct volume should have been worked out at the installation stage so,unless you've had a larger boiler fitted, or more rads added to the system, the existing one should be of the correct volume. It might need replacing if the diaphragm so damaged though and you can usually tell this by looking at the pressure gauge; if the pressure keeps falling, after the plumber has recharged it, and you have to keep topping up the system pressure, then it could be in need of replacement.
Hopefully this has helped a little bit

 

[dpw1986]

Hi. Thanks for the reply. What you have said sounds exactly like what is happening. We haven't had a larger boiler fitted and we haven't had any extra rads fitted, the only thing we have done is replace a standard rad with a towel rad. As for the pressure, I do keep having to top it up now and again, approx 1/2 - 1 bar every 4-6 weeks. With regards to the temperature valve, I had the similar kind of theory (its similar to the way a cooling system works on a car, if the pressure gets to high in the cooling system, then the expansion tank cap will release the extra pressure). But as I said, im not qualified in anyway in this type of industry so any information is greatly appreciated. I will wait for the engineer to come (swalec are the company who are dealing with my fault), on Tuesday to replace the valve and see what happens. If that doesn't cure it then they will have to come back and try something else. Im glad I got the full cover as this could have started to get costly!
Thanks again!

 

[Riley]

Expansion vessel is for individual cylinder and is for the domestic water not the central heating expansion

 

[dpw1986]

Hi. Yes I understand it is for the hot water. I didn't mention anything about the heating did i? If i did sorry, the fault i am on about is only present when the boiler kicks in to heat the water.

 

[Riley]

My point was that the need to top up pressure is not related to your unvented cylinder as obviously the heating and dhw are on two separate circuits. The lads are probably right re the replacement valve needed on the cylinder, however your loss of pressure on the heating side could be down to a leak or a faulty expansion vessel. If you have an expansion bubble within your cylinder then the red expansion vessel can only be for the heating side

 

[Chalked]

Quick question.
How may people live on your house?
This is a regular problem when the cylinder is correctly sized for the house. But only two people live there! So not much hot water usage.
You probably havea bubble top type cylinder like megaflo..? Both of your plumbers are doing the right thing. The first recharged the air gap and as safety valves don't like dripping, your second one will be replacing it and recharging the air gap.

 

[Robert Tyrrell]

Expansion vessel is for individual cylinder and is for the domestic water not the central heating expansion

Yep You're correct. I was just trying to cover all bases, whilst having a discussion with the wife, and it's Sunday morning. That's my only defence(s).

 

[chris watkins]

You can recharge the bubble of air at the top of your Megaflo hot water cylinder yourself (instructions are on the side next to the T&P safety valve).
If you are not comfortable in doing this then you could ask for another potable expansion vessel to be installed to the hot water, this will work along side the bubble so it will not need re-charging so often.
Get a couple of quotes from engineers who understand these system not just those who have the ticket.

As others have said it can be problem with these systems, the dissolved oxygen in the fresh water was supposed to maintain the bubble but unfortunately it can equally be re-absorbed after heating.

 

[joni os]

Lets hope that the red expansion vessel is for heating side and not for DHW. From information given I am unsure.