Ariston Unvented (drip drip drip)

23 Jan 2006
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United Kingdom
Question to all about the above. Have unvented qualification but little experience of installing or maintaining them.

Went to one of these with a dripping safety pipe. Water coming from the T&P valve not other 8 bar PRV.
Water pressure in area v.high (above or close to the 3.5 bar pressure the pressure reducing valve is set to).

The pressure reducing valve is letting by so that will be changed over the next few days when available. Customer also wants new T&P valve too.

Expansion vessel pressure is 2 bar. A regular contributor to this site who DOES have experience of these systems has told me that the 2 bar pressure SHOULD be OK. However the data badge on BOTH cylinder & vessel state that the pressure should be 3.5 bar. Given that there are pressure related problems and I don't want to look like a tit, I think I will charge it to the 3.5 bar stated on the badge.

Could anyone help with: what's the best way of charging these to 3.5 bar, (petrol station?) and does anyone agree with me that given the high water pressure applied to the installation (3.5bar-ish) that the vessel should be fully charged to 3.5 bar?

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I don't see it matters a fat lot - think Megaflo type UV, the precharge there is 0 bar!

If the p red valve is "letting by" then the whole lot will eventually get up to full mains pressure, and a bit more when it heats. It wouldn't be common to get to the 7 bar or whatever that the T & P valve is set to though, with a decent amount of air in the pressure vessel.
So I'd change the p red valve and the T & P valve.

Don't foret to shut off the supply and open say the exp relief valve while pumping up the vessel - a bike pump will do. Builds your muscles nicely!
I did try to pump it up in-situ with my car foot pump but no dice after 2.3 bar, thought my hands were going to fall off.

Might try removing it and using the foot pump as intended (ie on the floor), save my puny arms...
is the feed hot?

if so it may be of benefit to install a 3 bar pressure reducing valve on the mains as there may be back pressure from a mixer tap.

I have had this loads on megaflows

but then I am a bit tidddly tonight (hic)
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Remember the EV and Unvented are made by different makers and NOT plated to be specifically used together. Have you seen the installation instructions for that UV ?

Unlikely to be the problem but have you confirmed the pipe to the EV is clear?

I still think you should be concentrating on measuring the pressure in the UV so you are sure you know exactly whats going on.

I know that the ev & cylinder are made by different manufacturers, but there is a data sticker on the cylinder which agrees with the ev that the pressure should be 3.5bar.

Am going back tomorow with pressure gauge to confirm what's going on inside...
T&P valve release pressure is higher than 3.5 of pressure reducing valve.

I would start by installing a gauge to monitor the reduced pressure, pump up the EV pressure to 3.5 and then uncouple the relief pipes to esablish whih valve is letting by. My money would be on the T&P valve. The seat can erode on this valve.

Many a time I have been to UV systems where additional pressure reducing valves have been fitted needlessly. Try diagnosing instead of blindly changing all the components. Think what each component does and how you would proove it.

If EV is set to 2 bar, the pressure reducing valve is putting out 3.5, the EV will not have much expansion volume as the bladder will already be at full stretch let alone have room for expansion once water is heated.

Also check the sacrificial rods (if fitted)
Your opinion on the EV pressure is exactly what I thought, as the pressure is already going to be fully taken up by the (cold) 3.5 bar pressure, and when the volume of water is heated it will be even more.

As I didnt have my pressure gauge at the time of the visit (rarely need one apart from when quoting for people interested in combination boilers) I just had to guess at the pressure on the installation.

took the outlet pipe off of the T&P and it had a steady flow coming from it when all draw off's were closed. When opened, the flow reduced. When I reduced the pressure with the pressure reducing valve, the flow reduced to a drip, maybe 20 drips per min.

From this I'm sure that the T&P is knackered. I couldnt be so sure about the pressure reducing valve, but the customer asked me to change it anyway and I'm not going to argue as it will save me yet another trip to the spares shop...

Thanks all
I cannot understand why a client wants a perfectly good component replaced. I take it you have explained how the system operates.

I get the feeling you want to change the pressure reducing valve because you are not sure. Why else would you be reducing the pressure to see if discharge is present when the working pressure is reduced.

I as a rule fit a gauge to the p reducing valve when I install UV cylinders
He's a very rich investment banker who, after I explained what the pressure reducing valve was and did, and that there was an (unrelated) problem related to the pressure in the vessel, said just to replace the lot!

I'm not going to argue with him, its in my interest to change these parts as

a) The less chance of a callback I have the better
b) If I can do the job in one visit rather than going back with my gauge and finding out that the pressure reducing valve is not working right THEN having to drive away or order another gauge, its better for me and the customer (I'm subbing to a firm who charge travelling time so he would be paying £100p/h for me to drive around and find one).

I know I should have had my gauge with me to verify the pressure reducing valve in the 1st place, but it was Xmas eve around 6pm when I went and I don't normally work on these things...
as the pressure is already going to be fully taken up by the (cold) 3.5 bar pressure, and when the volume of water is heated it will be even more.

Not fully - there will still be some space for expansion. As the air volume compresses its pressure goes up equalling the pressure of the water. If the pressure goes over the let-go pressure of either safety valve, then you're losing water, but if you do the sums starting at a couple of bar it isn't likely. Bigger PV's are better!

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