Pressure loss on oil combi boiler

malct

Since January, I have been fighting against pressure loss on my oil grandee combi boiler and I have replaced the expansion vessel that was leaking, the 3 way port valve and another valve that allows air out of the system that was leaking. before I did these fixes, I had added a water sealer and I had 2 weeks of no pressure loss but then it got worse where I was losing all the pressure each day from 1.5 bar to 0. Since fitting the expansion vessel last week I have been losing .2 of a bar everyday which is a lot better than what it was. One thing I have noticed, if I set the pressure at 1.5 bar on cold on topping up via the filling loop, the pressure only goes up to 2 bar when the central heating is on and it falls back to 1.5 bar before the burner kicks in again and like I said, I am losing about .2 of a bar and needs topped up evey 3rd day. I did add screwfix no more leaks on Saturday ôbut it hasn't made any difference. I am planning on replacing all the valves on the 6 rads in the house but what I want to know is, can I isolate the central heating for a day and just run the combi boiler for hot water so I can rule out the central heating system or find that the fault is in the central heating. My plan is, if the leak is coming from the underfloor piping, then I will get a quote for re-piping from the combi boiler to the rads and have the pipework above the floor level and I might even renew the he rads since their 30 years old. It's just a pain trying to stop receiving he pressure loss, so basically, is it normal for the pressure to go from 1.5 bar to 2 bar when the central heating is on, I did check the pressure on the expansion vessel and it did read 1 bar but might be a bad reading.

Also is it possible to isolate the central heating so I can rule out either combi boiler or central heating . any help would be great.

Le_Kirk

The hot water system in a combi boiler is separate from the radiators circuit and does not need to be pressurised. It works with mains pressure. Have you looked at the pressure relief valve? This is the one that should be piped outside. Sometimes once they have operated, they do not reseal. A good way to test them is to tie a plastic bag around the exit of the pipe and see if you capture any water.

I don't understand your comment about the pressure dropping back to 1.5 bar before the burner kicks in again. I don't know your particular make of boiler but there is normally a low pressure switch but I didn't know there was a high pressure switch. The boiler/burner should fire on demand from the thermostat.

malct

Regards the kicking in, what I meant was, after the he burner switches off on the central heating, the pressure is at 2 bar and then it drops back to 1.5 bar due to cooling. So I he burner does kick in at 1.5 bar but this down to the thermostat . I will check the PRV over the next 24 hours, I was releasing the pressure and water by using the PRV when changing over the expansion vessel.

Regards ruling out the central heating, would I be right in saying that I would still lose pressure on the central heating side even when not using the central heating. This is why I was asking about isolating the central heating side of the system so I could close of valves and retain the pressure on the combi boiler and if I did lose pressure, then it's the combi boiler

Le_Kirk

If you look at your manual, find the page that shows the pipework connections and you will see that the DHW is completely separate from the CH. If you are losing pressure, there is a leak in the pipework feeding the radiators or one of the radiators or the valves or somewhere in the boiler (heat exchanger for example) or the pressure relief valve.

You mention that you had released pressure by operating the pressure relief valve. This often does not reseat properly. Removing it and blowing it through might remove whatever is preventing it from seating. Try the plastic bag trick first.

You might try to isolate various bits of your system by closing both valves at each end of your radiators which would isolate each radiator in turn (but surely you would detect a leak in a radiator.) You could even turn off the CH flow and return valves on the boiler which would isolate the boiler.

malct

I know there are 2 hand valves either side of the central heating pump, but if I remember the last time I touched the hand valves, they started to leak and took ages to stop the leaks. I did get a price on a new oil combi boiler and it was going to cost £3000 to supply and fit and I could still have a leak under the concrete floor. In the village where I live, we don't have gas, so limited on what we can install