CH pressure rises constantly

[marley]

can any of you central heating experts out there give me a clue about how to solve this problem i'm having ?

why would CH pressure contantly rise if there are no airlocks and not too much water ?

the pressure reading on my combi boiler slowly rises until about 3 bar, at which point the relief valve opens and releases some water until the pressure drops, which all seems 'normal' enough, but the the cycle immediately begins again (pressure rises, relief valve opens, pressure drops)....and so on.

this all started after i drained the whole system and refilled it (along with a quantity of corrosion inhibitor).

I know what you're thinking.......AIR in the system!!. This was my first thought, and i've been bleeding the rads (or attempting to) regularly since refilling the system, but for the last couple of months now, i'm getting no more air, so the system appears to be 'air free' so what do i do now ?

i've checked the filling loop that doesn't appear to be feeding anything in, so just where is this pressure coming from ?

i'm not an expert in CH systems so i don't know what other things could cause pressure rises.....i only know about airlocks and too much water.

[Tony Eva]

Check again. Even better, disconnect the filling loop altogether and see if the problem still occurs (you *should* have a non-return valve on the filling line so you can do this).

I'd be willing to bet that the isolating valve on the filling loop is leaking very slightly, which will slowly raise the system pressure. At least, that's what happened to me.

[Paul Barker]

Yes, what Tony said.

Additionally there are two further avenues of investigation shoul disconecting the filling loop (which in a any case should not be lef permanently connected) not solve it.

1/ water can get in through the secondary heat exchanger in which cas you need a new one. To check for this turn off the isolation where th cold water goes into the boiler and see if that stops it. Obviously yo have no hot water while you are checking this out.

OR

2/ should this only happen when the system is hot the pressure vesse requires attention. Either the air pressure needs checking, or th membrane is ruptured or it is incorrectly sized for your system in tha order.

Start with the filling loop. I usually leave these disconnected when do a service or installation. Should it be the loop get a good qualit replacement

-- Paul Barker

[marley]

thanks Tony and Paul for your suggestions, i'll check these out and post the results.

marley

[John]

Only one reason - there is water being introduced from somewhere!

Well I wasn't thinking air at all.

To check your filling loop disconnect it and place a receptacle under the fill valve from the cold water main. If it drips in at all you have a source of a slow pressure rise. Either shut off the valve properly or change it. In any case the filling loop should be disconnected while not in use specifically to avoid this sort of problem.

There is another and more costly failure which can cause pressure rise - a pinhole leak between mains and heating circuit in the boiler secondary heat exchanger. You can check this after you have eliminated the filling loop as above by turning off the cold mains inlet to the boiler and open a hot tap which will depressurise the mains water circuit through the boiler. If the heat exchanger is the culprit the pressure rise will cease. You then need to replace the secondary heat exchanger.

What model of boiler is it anyway?

[marley]

It's a Worcester Bosch 24 Si II combi, fitted summer 2003