Over Pressurisation of Heating Circuit - Filling Loop Off

9 Aug 2010
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United Kingdom
This one is driving me mad.

Water was dripping outside through the discharge pipe, so checked the boiler pressure guage and it was indicating 3.5 bar! (Well over the 3 bar maximum) Initially suspected the Pressure Reducing Valve. It was looking wet also. Pulled the cartridge out and it was in a bad shape internally. Fitted a new one after cleaning out the valve casing. Repressurised the system checking the external expansion vessel had 3.5 bar of air pressure inside, as stipulated. Pressure rose again to 3.5 bar inside the boiler without turning on the boiler (i.e. water still cold) !! Grrr

Next thing, is the filling loop small isolating valve damaged as this was still connected? On disconnecting it, noticed that non-return valve was leaking slightly and some water exiting the boiler, but no sign of water passing the isolating valve into the boiler. Changed the NRV anyway and fitted a new filling loop and isolating valve as I had one.
Repressurised the boiler to 1.5 bar, shut the filling loop valve, disconnected the filling loop braided pipe and pressure again rises slowly to 3.5 bar!! Grrr

Checked the boiler safety valve and this is functioning correctly. Quarter turn and it dumps water and pressure quickly through the discharge pipe. Leave it to opperate normally and at 3 bar, it opens slightly releasing pressure to a drip, drip, drip outside through the discharge pipe. This suggests the pressure is building slowly somehow.

So how can the system be repressurising with the filling loop disconnected? The mains pressure is higher than 3.5 bar, but the cold water feed has the pressure reducing valve to limit the pressure to 3.5 bar. Ironically, this is roughly what the boiler guage is reading.

How can water enter the heating circuit with the filling loop disconected?

Could the Vaillant Vantage 120/150/200 cylinder have a cracked coil and water is entering the boiler circuit from the hot water side? If so, how can I test this and more importantly, can these be repaired? They are the best part of £700 to buy and I don't think they make these models any more!

Any advice appreciated.
P.s. The boiler is a Vaillant Thermocompact VU GB 282-5
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No the coil has almost certainly gone in the cylinder but the expansion vessel pressure shouldn't be that high
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I don't know where you are getting this 3.5 bar for the expansion vessel from. It should be about 1.0 bar.

Even so you are telling us the pressure rises with the boiler cold.

Based on what yo0u have told us the heating coil in the cylinder is leaking!

You can test this is the cause by filling the boiler to 0,5 bar and then isolate the cylinder and depressurise it by opening a hot water tap and then see if the boiler system pressure continues to rise.

But before doing this you have to also isolate the cold supply to the filling loop.

I think the OP is referring to the Hot Water Expansion vessel NOT the heatingone the hot water vessel should be charged to 3 - 3.5 bar (same as the reducing valve however the HEATING vessel of course should be at 1 bar or what ever the cold fill pressure is - it does sound like the coil in the un-vented cylinder is the culprit! ;)
Okay with no pressure in boiler circuit and tap side of the tank (i.e. taps run dry), open system isolation main valves, but kept filling loop valve closed and pressure builds on the boiler guage! It builds slowly to 3.5 bar! Taps run again and boiler circuit is full with safety valve working again discharging water outside, drip, drip, drip!

Close off both main isolation valves and pressure falls very slowly. Open a hot tap and pressure guage falls more quickly!

I believe this suggests the inner coil of the cylinder is split. :(

Is this a replacement job or can the coils be removed and replaced?

These cylinders are a massive £700 and I'm guessing a few hundred pounds in labour costs too. :cry:
Massive £700 ?

That's a very typical price for a smaller unvented cylinder.

I used to charge about £460 to first time fit one.

But as a replacement about £230.

You don't have to get the more expensive Vaillant one. I rather prefer the Santon brand. A second name for Megaflo but they are a better design with separate expansion vessel.

This just came in from another source:

"Those cylinders rust through on the coil. So the hot water is pressurising the boiler. It will need replacing. Get a Santon Premier Plus, or a Stelflow. All pipe work will need altering as the Vaillant cylinder is a short dumpy type. The heating will need altering to two motorised valves instead of a single three prt that you have at present."

I guess this matches up with what we are thinking here.
Does anyone make a replacement cylinder that closely matches the space and pipework connections of the Vaillant Vantage 150? Not that I plan to do the job myself, but to place the cylinder in the same space under the Vaillant boiler and make the job easier for the engineer to do the pipework?

The Vaillant Vantage 150 holds 150 litres (Obviously) and is 95 cm high and 60 cm wide. (Diameter)

Perhaps Vaillant make direct replacement cylinders to replace old models.
You have already been told ( elsewhere ) that the Vaillant cylinder is short and dumpy.

As far as I know here are no others with similar dimensions.

Most would prefer the slimmer designs where most are only 505 mm diameter.

There is only around 8 inches between the top of the Vaillant Vantage cylinder and the bottom of the boiler and a much taller cylinder looks like it wouldn't fit!

I had a look at the RM Cylinders and Santon websites this morning and the cylinders all appeared to be much taller and as you say, narrower for a similar 150l volume.

Also, the Vantage conections enter and exit from the top, whereas the other RM and Santon cylinders connect to the cylinder sides. Trying to fit a square peg in a round hole springs to mind!

Here, a NewTeam power shower died last year and luckily the company that bought the old NewTeam company, Bristan made a later shower model with exactly the same dimensions and hose/pipe and wiring points, making an exchange simple.

I was kind of hoping someone made a cylinder that matches up with the size and, shape of the Vaillant Vantage too, but perhaps not. Really don't want to move the boiler and may have to settle for a smaller volume cylinder, to sit under the boiler.

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