Used a sealant, now pressure too high...

15 Nov 2005
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United Kingdom
Moved into a small house 2 years ago with a Worcester 28CDi combi boiler. It works fine, but I have noticed that the pressure would very slowly go down. There are no leaks to be seen, so I assume there is a small leak at a join under the floorboards. Sort of forgot about it this summer, but now the weather has turned, it came up again.
I have just injected a sealant specifically for sealed systems and it has done wonders for the pressure, but here is my problem...

When the heating is on, pressure will now go just above 2.5 bar before it stops firing, and will remain there while heating is on. But when not in use, it drops to zero.

After using sealant, I bled air out of top rad and repressurized the system to 0.5 bar as per intructions. When it first started to rise above 2.5, I got worried and drained some water to release pressure...but it still wants to stay around 2.5.

Please, any ideas whats happening?...or what I need to do?

Thanks - Ian
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ukbeachbum said:
After using sealant, I bled air out of top rad and repressurized the system to 0.5 bar as per intructions.

Had another look at manual and I should've checked that the expansion vessel is charged to 0.5 bar, not the system. Gotta go find a tyre pump and a gauge now.

Thanks for the pointer.

I would like to double check one thing. In my manual it says to fill system to 2.5 bar, charge expansion vessel to 0.5, then drain system down to 1.5. I have looked at other threads here and have seen people recommend charging expansion vessel when system pressure is at zero. Does this differ depending on boiler make? Is one way better than the other?
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For what it's worth to anyone else who reads this, thought I would share my final experience in the hopes that it might help someone.

Even though I had only one response to my query, it was, luckily, spot on. It was the expansion tank. Completely out of air.

What I did may have been the slow way to do it, but the result was the same:

1. Get a foot pump with a pressure gauge.
2. Pump up expansion tank to 0.5 bar
3. Go to top rad and drain water pressure till no more water is forced out.
4. Foot pump pressure now registers zero, pump up again to 0.5 bar.
5. Repeat steps 3 and 4 until no more water is forced out of top rad (zero water pressure) and pump pressure messures 0.5 bar. (Took about 10 trips up and down stairs, but finally worked)
6. Refill system water pressure to 1.5 bar.
7. System happy.

Filling the expansion tank was a simple task, no different than putting air in a tire. Now, when system is cold, system pressure reads just below 1.5 bar. When hot, about 1.7 bar.

As long as the expansion tank holds this new air pressure, I'm looking forward to a good cold winter.


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