Boiler expansion tank question

TheGreenFrog

I have an oil fired boiler and there is an expansion tank next to it. I regularly have to top up the pressure in the expansion tank by opening the water inlet valve but everything seems to work OK (it seems to drop regularly to about 1 bar when cold and I top it up to 1.5 bar when cold). But someone told me to check the schrader valve to see if it leaks air or water - but when I release the schrader valve nothing seems to come out (even when the pressure guage, which is attached to the relief valve and the water inlet, reads 1.5 bar) so I am wondering if there is something wrong? I have also read about recharging the air in the expansion tank but don't know whether this is something I ought to be doing (and if so how to do it)?

ThisIsWeird

Hi GF.

From what you describe, the air side of the EV is empty. That's the bad news. The good news is that water didn't come out either. That suggests that all that's happened is that the air has leaked out over time, a non-uncommon occurrence.

In such a situation, what you can expect to notice is that the cold pressure - say 1 bar is good - will rise quite dramatically when your CH is turned on and it all heats up. That is because, with a non-functioning EV, the expanded hot water has nowhere to go. What then often happens is that the pressure approaches 3bar, and this 'blows' open a safety valve to discharge the excess water. Since your system will not have lost some water, when it cools down again, you can expect a lower-than-1bar cold pressure. So you top it up again, and it's deja-vu all over again.

You would need to be both happy and competent to do the air-topping-up task; I don't think there's any particular qualification you need just 'cos it's oil.

In essence it will involve:

1) System cold and shut off.

2) Release system pressure until it reads 'zero' - you can do this by bleeding a radiator valve.

3) Leave the bleed vale open so that water can continue to be ejected as you pressurise the air side of the EV.

4) Fit a car/bike tyre pump on the Schrader, and pump away.

5) Take air pressure readings, and stop at 0.75 - 1bar. Ideally, look up your boiler and see if there's any recommendation for this, but that figure should be fine.

6) Test the valve for leakage - a good spit will do it...

7) Close the rad bleed screw.

8) Top up your system once more - stop at 1bar.

Monitor...

There is a good chance that your safety valve will now continue to let water out, so trace where the safety discharge pipe is outside your house, and rubber-band a small clear plastic bag over the end to monitor this.

If your system doesn't have a magnetic filter on the return to the boiler, then seriously consider fitting one whilst your pressure is zero. You can then add fresh system 'inhibitor' this way.

TheGreenFrog

@ThisIsWeird - thank you for very helpful response. Will dig out my spanners and bicycle pump….

ThisIsWeird

Make sure you understand what it is you are doing, and that it makes sense. Please don't just follow 'steps' :-)

SEE UPDATE ON PART 4.