Combi boiler pressure falls to 0.5, rises to 1.9 bar when heating is on

Mrmarc

Recently had a new heat exchanger fitted, as my hot water was only lukewarm. This fixed that problem a treat, but a few days afterwards I started getting an F9 code “Water pressure sensor fault, blocked flow filter or pump fault” which the gas engineer put down to the system being empied and drained, which causes a load of gunk to get into the sensors. He cleared it out, and the error went away.

About 3 days later I came home to no heating, and my boiler on 0.5 bar pressure making sounds as if it was struggling. I switched it off, eventually it came on (after I’d ran the tap, perhaps a coincidence) and it’s worked ever since, but the pressure falls to 0.5 when cold, and rises to 1.8/1.9 when the heating is on. Sometimes when it’s at 0.5 it takes a while to “kick in” or starts, stops and then starts again.


I don’t have any obvious leaks in my radiator system, and I never experienced any pressure related issues prior to the new heat exchanger being insallted.

Any thoughts on whether this is a problem introduced by the gas engineer, or something that’s become apparent only now I have super-hot water due to the new heat exchanger? The fact that it has stayed at the lowest of 0.5 for a few days now also makes me wonder if it’s a sensor or something internal to the boiler itself, rather than an external leak. How much money should I expect to throw at this? H.E. cost me £300 already!

Any thoughts or suggestions much appreciated.

neilmcl

It's completely normal for the pressure to rise slightly when the heating is on. You shouldn't leave it at 0.5 bar though, that's too low, it needs to be 1.0 bar minimum so you'll need to add water via the filling loop.

What's the make/model of the boiler?

roddydogs

Surprised it works at all at 0.5 b, should be 1 bar min, about double when hot.

chickendipperbabe

Our boiler is set at 1 bar cold, and it rises to 1.5 – 2.0 when the heating is on. We had issues with ours over pressurising causing the PRV to drip, turned out the expansion tank had lost its pressure, our fitter pumped it up with a bicycle pump and it’s been fine ever since.

bris

It may just need the expansion vessel recharged, have it checked, this is a common problem with higher than normal pressure.


Expansion vessels lose about 20% of their air every year so it's only a matter of time before it needs topped up.

jrrowleyws

Check there is no air in the system. Top up to 1 bar.bleed all your radiators and any other bleed points you have (on mag filter, on high points etc) then re top up to 1 bar. If you've recently had the whole system drained then it may well be air trapped and this can cause pressure fluctuations (when I thought I had an issue with my expansion tank, the engineer just bled everything during the service and fixed it without touching the expansion vessel).

coffeehound

Or the expansion vessel may be leaking. How old is the boiler?

JimmyChanga

There's nothing stopping you from topping up the pressure. Usually something that the householder should do anyway. 0.7 to 1 bar is in the ballpark. But your manual will advise what is best.

Check all your radiators for air by trying to bleed them.

Mrmarc

Hi all

Thanks for the responses, very helpful indeed.

I've not topped it up myself as I'm thinking it it's gone down only 3 days after being set at the correct pressure, that must indicate a fault?
Does anyone know if the pressure should rise if heating is off, but I'm running the hot water? It does this (slightly)
The guy I had look at it fix it reckons (based on my email description) it's the filling lock passing hot water into the heating system, which caused it to overflow, and loose pressure. Reckons it'll need a new valve. I don't think boiler is that old (maybe from 2012).
Does that sound reasonable?

Appreciate all the help

Mrmarc

If I reset it and run the hot water, it goes up to 0.6, which then allows the heating to kick in. Oddly it doesn't go over 1.9 if I leave the heating running for a few hours.

no1catman

roddydogs wrote: »

Surprised it works at all at 0.5 b, should be 1 bar min, about double when hot.

I agree - I've had times when wondered why the heating wasn't on - turned out the pressure was too low - 0.5 bar! Needs to be at least 1.0 even if it rises to over two, don't worry, shouldn't be above three though.