Boiler Pressure

mighty2022

Hi 

I have a Baxi Duo Tec Combi 33 HE boiler which i believe is around 6-7 years old. I understand boiler pressure should sit around 1 - 2.5 bars but this is the pressure when the boiler is operating ie central heating on/heating water or should still be within that range when the boiler is off/not operating? 

Mine appears to drop below 1 when it’s not operating but rises up within the green range when it’s operating. Wanted to check if this is normal behaviour? 

Thanks! 

wedge69

It should sit in the green range when cold, and the pressure should rise when in use, so it won't do any harm topping it up a bit

Bendy_House

Some variation is ok, and it'll always be that it rises as it heats up if anything.

Many systems will actually not really fluctuate much at all.

Ideally, you want a system that sits at around 1 bar or just above when 'cold', and barely climbs at all when hot, but there is a lot of acceptable variation around that.

If it drops to just below 1bar when cold, then it should be fine, but by all means tweak it up slightly to be just within the green zone if you like. Then monitor what happens as your system heats up; if it rises but remains within the green, then that's fine. If it gets to the top of the green or even over, then consider having your EV looked at when the boiler is next serviced - chances are it just needs a slight recharge.

mighty2022

Thanks @wedge69 and @Bendy_House

The boiler is cold now, and I’ve topped it up to sit within the green zone. Will report back tomorrow morning when the heating is on

mighty2022

Hi, an update from this morning. 

I was up before the heating schedule kicked in so thought I double check the pressure again - it had dropped overnight to just above 0, way below 1 that I initially reported! Only thing that can explain this is that I had checked the pressure about 90 mins after it went off last night so the system wasn’t completely cold. So I increased the pressure to 1 again before heating came on. 

An hour after the heating has been on, the pressure increased to 3.5 bars (red zone). Just before it went off, an hour later, it stayed at 3.5 bars. 

I’m out all day today, back early evening, which gives enough time for the boiler to cool down again. Will check the pressure and report back. 

Is this an issue with the expansion valve? 

Thanks

shiraz99

mighty2022 said:

Hi, an update from this morning. 

I was up before the heating schedule kicked in so thought I double check the pressure again - it had dropped overnight to just above 0, way below 1 that I initially reported! Only thing that can explain this is that I had checked the pressure about 90 mins after it went off last night so the system wasn’t completely cold. So I increased the pressure to 1 again before heating came on. 

An hour after the heating has been on, the pressure increased to 3.5 bars (red zone). Just before it went off, an hour later, it stayed at 3.5 bars. 

I’m out all day today, back early evening, which gives enough time for the boiler to cool down again. Will check the pressure and report back. 

Is this an issue with the expansion valve? 

Thanks

Possibly, and now that it's gone over to the red it's possibly blown the PRV which would lead to a loss in pressure. Check the pipe outside that leads off from the PRV to see if it is, or has been, dripping.

Bendy_House

Yes, usually an EV issue - it either needs recharging, or replacing.

As Shiraz says, the safety discharge valve will have blown open at just above 3bar, so that explains why the pressure has stopped at that point. You'll have a 15mm copper pipe outside your house near the boiler, either pointing down at the ground, or else bending back at the wall. You can safely rubber-band a small bag over this to check for water coming out - there should be non, but almost certainly will be in your case.

That would also explain the pressure loss overnight - once these valves are blown open, they rarely seal fully closed again. So, they tend to keep on dripping...

As well as the EV being sorted, you'll most likely need a new valve too.

If a replacement EV is super-costly (some can be hard to access in the boiler, so labour-intensive), then an option is to fit one remotely, if you have room in a cupboard.

I think I'd call up a couple of local GasSafes and explain the symptoms - pressure soars, and water comes out the discharge pipe (once you've confirmed it does...). Then ask for a ballpark cost for the worst-case scenario, which is new EV and new safety valve. With luck the EV will just need a pump up.

How DIYish are you? It is possible to carry out a couple of simple checks on the EV.

mighty2022

Late response, busy day. Thank you for your responses.

So I checked the pressure when I got back and it was exactly 1 (green zone). Pipe is definitely dripping as the ground is wet. Looks like I need to call someone out, I love DIY'ing but not confident on a gas boiler! 

Just out of interest, what damage will it cause if I left it like this until May? In that the pressure is close to zero (red zone) when boiler is cold but rises up to green zone (and maintains green zone) whilst boiler is on and hot. Does lack of pressure mean I use more energy or does it cause damage to the boiler.

The only reason I ask is because I plan to replace 3 old radiators in the house and had it planned in for May.

Thanks,

Bendy_House

In theory, no harm.
Obviously 3+bar pressures impose more stress, but systems are designed for at least 10. Many boilers will also shut down for safety reasons at pressures below around 0.6bar, but yours ain't one of them!
I'd be inclined to top it up every day or so so it's at least 0.5bar cold, to be sure to be sure. 
As long as the pressure is green or above when it's properly running, it should be happy enough for a couple of months, but really best not longer.
 And if the boiler starts making noises, then best shut it off. 
When your rads are replaced and the EV fixed, don't forget fresh inhibitor. Also, if you don't have a mag filter, I'd have one put in at the same time - little effort, and around £100.
The DIY test on an EV is to locate the 'tyre' valve on it, and to check the pressure. Also briefly press the 'pin' and see if water comes out or just air. But it does mean taking the cover off, so best left.