Tracing tiny leaks in sealed system

[Dave]

That will because it is designed to dissolve in water.

Dave

[visionset]

Okay, okay, okay, but what of the Fernox Leak Sealer additive, anyone used it? Does it do what it says?

[Tony Williams]

Arrgh! Memories of Bronco bogpaper. As scratchy as hell, and probably even aqua regea wouldn't dissolve it.

[brian.jones]

Sorry to resurrect this one but I am hoping someone could shed some light on my current problem.

I have a seal combi. The pressure drops down to zero within 20 minutes of the ch going off. I fill it and we start again.

The previous tenant kept the system alive by having the fill valve constantly open, not good.

Now. I do have a leaky DHW Heat Exchanger which will fill a cup in a day.

Is this enough to explain the dramatic drops in pressure on the system ?

Thanks in advance if anyone is still around...

[Toby]

Sounds like it might be your expansion vessel, try topping the system up to

1 bar when cold, then fire it up and watch the pressure gauge, if it hits 3 then the pressure relief valve will dump water out.

[Roger Mills]

+1

[brian.jones]

Hi Toby

Thanks for replying.

I filled to 1 and it went hurtling up to 3. Then when the heating went orf it went back down to below 1

Thoughts ?

Brian

[Harry Bloomfield]

As the water heats up, it expands and needs somewhere to expand into - that somewhere is an expansion vessel. Basically a tank split in half by a diaphragm, the water below it and air pressure above it. The water expands and pushes against the air. Either the diaphragm has burst, or there is no air in it and it needs pressurising to 1bar - a foot pump would do.

[Andy Champ]

went back down to below 1

It's the expansion vessel.

Andy

[Roger Mills]

went back down to below 1

Classic symptoms of shot expansion vessel. If it's completely shot, it will need to be replaced, but it may simply need recharging with air.

First, find the expansion vessel - a bomb-like object, probably either close to the boiler[1] or in the airing cupboard.

Then (with the system cold) de-pressurise the water system and check the air pressure in the expansion vessel. You should find something which looks like a car tyre valve on the top. Use a car tyre pressure gauge. The pressure should be 10 or 11 PSI (about 0.7 Bar). If considerably below that, top it up. You can do this by operating a car foot pump by hand. [If water comes out of the valve when you check the pressure, the vessel is shot, so you'll have to replace it].

Assuming that you're able to top up the air pressure to 0.7 Bar, re-pressurise the water system to 1 Bar, and see how you go. Hopefully, when it then heats up, the pressure will only rise by a bit (to below 2 Bar) and won't cause the pressure relief valve to open.

[1] If it's a System Boiler, it might be inside the boiler casing - in which case it's probably pancake-shaped rather than bomb-shaped

[brian.jones]

Cheers chaps, off I go to get a foot pump with a good guage on it. I will have to replace the DHW exhanger as its leaking anyway. I will let you know how it goes.....

[Toby]

went back down to below 1

As others have already said, yes, it is the expansion vessel that is at fault here, if I were you, I would firstly press the centre part of the valve (Schrader valve, like you have on a car) and see what happens.

If you get air or nothing coming out, this is good, simply pump it up to

0.7 bar (10-11PSI) but do this with your system de pressurised - if when you pump it up, you see the pressure reading on the boiler rise from 0, then you need to let some water out of the system and then top it up again - keep doing this until you have it at 0.7 bar, and 0 on the boiler pressure gauge.

If, on the other hand, the Schrader valve lets out water when you press it, then the vessel will need replacing - you can either replace like for like, or you can just add another one to the system where convenient, leaving the failed one connected (assuming it isn't leaking!). - this is the best course of action if it is not easily replaced (some boilers have the buried at the back, that required stripping the boiler down to get to it, and some are just expensive compared to a generic one.

An external one will look something like this, often seen in bright red too.

you don't have one of these, then it is probably in the boiler, so post the make and model if you cant see it in there!

[Andy Champ]

If the schraeder valve is leaking you may be able to do something with a metal valve cap. Like these:

AKA

you'll still need to fix the vessel.

Andy