Combi Boiler

[awalker58]

My combi boiler pressure dropped and now when i try to repressurise the system as usual, the water just flows straight out of the overflow pipe. Can anyone give me any idea what is wrong and how to fix it?

[Hymie]

The pressure relief valve is stuck open � normally it is only accessible after removing the boiler cover. Most relief valves have a red coloured knob � give it a twist through 30 degrees or so, which will open & close the valve.

From what you say � that the water is p!issing out � the valve will need replacing � but sometimes crud can cause the valve to open partially. Opening and closing it may help.

[Hymie]

Follow the overflow pipe into the boiler and the relief valve will be close to where it is terminated.

It is fairly easy to replace � close off the central heating pipes at the valves and make sure that the central heating boiler water pressure is zero (and you are not trying to re-pressurise the system). Drain the boiler via the drain valve/plug. The pressure relief valve has a securing nut/olive, (to the boiler and to the outlet pipe) once released it can be removed.

When replacing the valve, don�t be tempted to check the operation of the new valve (by manual operation or over pressurising the boiler) � if you do, you will risk getting crud on the value seat and your problems will start again.

[spudqueen]

We've got this problem as well, but the water that comes out of the overflow is rust coloured. Would replacing the valve fix this is or there anything else we should do?

[Hymie]

Spudqueen � the reason the water is rust coloured is that it is (very) partially flushing the system. The fill loop will be connected within the system close to the pressure relief valve. Most of the mains water flowing into the boiler (in attempting to pressurise the system) will be flowing straight out of the overflow pipe. Some will disturb debris within the adjacent boiler pipery. This is normal (to discharge rust coloured water) for a boiler not flushed/cleaned recently.

Personally I would not go to the expense of having the system power flushed or professionally cleaned unless you are experiencing blockages within the system and/or radiators which have sludge build up � but a professional boiler flusher/maintenance person would probably disagree.

If you are going to attempt the replacement of the pressure relief valve yourself � you should familiarise yourself with the bits that need removing/undoing � ensure you have the correct tools for the job and only work on the boiler with the mains power disconnected.

[gucciman]

hi

hymie is perfectly correct in that the prv is stuck open but there must be a reason why the prv opened up to let the water out in the first place i.e. the system over pressurised and the prv opened up to protect the boiler.
my money would be on your expansion vessel being flat and the system will keep over pressurising until you get an engineer to sort out the expansion vessel.


spud queen, if you want a cheap way of flushing out your system (presuming its a combi). buy some system cleaner from a plumbing shop, add it to the system and run it as per the instructions on the bottle. then attach a hosepipe to a downstairs drain plug (if uve got one) open the drain plug and then turn on the filling loop, you then have a mains power flush thru your heating system.
you will then need to add some fresh inhibitor to prevent further sludge build up.

[Hymie]

As gucciman says, there could be an underlying fault causing the valve to open, if it is the expansion vessel, then you are looking at an expensive repair.

I had exactly this problem on my boiler and it was only the valve. If the boiler pressure is reading low, then the value should not be open. If you are able to pressurise the boiler and get it going, keep a close eye on the pressure gauge.

A pressure relief valve will cost around �15 and should take a novice less than an hour to fit (although on some models, access is limited). Call out a repairman and the sky�s the limit.

[gucciman]

not this repairman, our charges are very reasonable!

the prv doesn't start leaking for no reason, it must have opened up to dump water out and then hasn't reseated.

its very rare the expansion vessel actually needs replacing usually just repressurising, for this we charge �50 and we would also service the boiler for you in the price.

if the expansion vesel did need replacing then you'd be looking at around �100 depending on the model of boiler.

it is still well worth a try and clearing the prv by opening it up a few times and letting the water flush thru it.

[Steviegb]

If your prv is running its usually because you have dirt/ sludge in the system. 90% of the time its because the small pipe which connects to your expansion vessel is blocked where it comes off the boiler.This then does not allow your expansion vessel to do its job, therefore the pressure inside your boiler builds up to 3bar, hence prv starts running.Ideal combi`s are renown for this especially the classic combi, and its a pig to change on these.
Personally i would cut your losses and get some one in who knows what they are doing, could save you money and a lot of messing about, especially if you don`t know what you are doing

[gucciman]

not come across that one before steve, from what ive seen so far its 90% down to the expansion vessel being flat.

must be a nitemare trying to get at that pipe to unblock it then because most of them are hidden down the back of the boiler?