Low boiler pressure when OFF

Funky-Melon

Funky-Melon

Funky-Melon

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Hi all,

I've not long moved into in my first house and I'm still discovering all its issues...

Latest is the boiler, I've noticed over the last few days that the pressure is 0 when it's not being used but when the heating comes on gradually it rises up to 1.3bar and stays there.

The sticker on the boiler says the system pressure should be between 1bar and 1.5bar but from what I've read online it should be at this pressure all the time, even when it's off?

Hot water works fine on demand when pressure is 0 and the pressure gradually rises when the heating comes on to 1.3bar.

I know nothing about boilers so this is all new to me. The boiler is a Heat Line Monza 28 combi.

Is this a problem?

Side note: large radiator in living room doesn't get hot, I've bled it and it only gets warm at the top, bottom still cold so I assume its sludged up, don't know if this is relevant to this boiler issue? It's a 4 bed house and all other radiators work fine.
 
Yes, it is a problem. The pressure should be 1-1.5bar when the system is cold and increase somewhat when hot. I think mine is around 1.5 bar cold and 2 bar hot.

Inside the boiler there is an expansion vessel. When water in the central heating system warms it expands, taking up more space. The expansion vessel holds the "excess" until the water cools down and shrinks.

When the expansion vessel is broken there is nowhere for the water to go so pressure keeps rising until the pressure release valve (PRV) kicks in and dumps the excess water (I thinks this is around 3.5 bar). Now when the water cools there is not enough water to maintain pressure so it falls to 0.

The fault could also be the PRV opening at too low a pressure.

I would suggest getting a plumber in to service the boiler as it probably hasn't been done in years, if ever. He can diagnose the fault. If you're lucky the expansion vessel will just need repressurising, which is done the same way you'd pump up a car or bicycle tyre.

This problem is separate to your cold radiator. The radiator could be sludged up or it could be a valves on the radiator are not open enough to allow enough water flow. If you do want to fiddle with this look up radiator balancing and note down the number or turns you open or close each valve so you can return things to how they were if you make things worse!
 
ideally 1.0 bar when cold but 0.1bar more for each 1 meter that is the highest point of system to a max of 1.5bar.

So say your boiler is downstairs in kitchen and highest radiator is upstairs about 3 meters above boiler than it should ideally be 1.3bar cold.
 
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I'd start by topping up the system when cold so that is it up 1bar - and then see how it goes before assuming the expansion vessel is faulty.
 
If the expansion vessels is knackered (on my boiler it is a big rubber balloon inside a metal case) the pressure will go sky high when boiler is on fully heated so you may then vent water through the high pressure relief valve this would lower pressure in system and be really low when stone cold.

They red line on my boiler pressure gauge is 3.0bar and ideally it does not go over 2.5bar in normal use.
 
GeX said:
I'd start by topping up the system when cold so that is it up 1bar - and then see how it goes before assuming the expansion vessel is faulty.
Click to expand...
A wise course of action. I did jump the gun by not suggesting topping up and seeing what happens.
 
At what point should the PRV close again though, back down at 1bar? My thinking here is to try and avoid doing something that might leave the OP without heating.
Given there are sludged up rads, it's fair to assume any drain points could be in poor condition and OP might not be able to let that much pressure back out?
 
I had the same issue with my boiler and I kept topping it up to 1.5 bar from 0 with the tap. This would cause the PRV to open and spit out some of the water until it was down to the right pressure again. I noticed after a while it didn't really matter and the boiler has actually been running fine like this for a while now. Its stay on 0 unless its on and then its around 1.3 or so. I have not topped it up again for some time and it works without any issues, i made the assumption a while ago that the gauge on my boiler is faulty. I mean how can it work without any pressure? :)
 
Don't top your boiler up! Leave it as it is until you get someone qualified to look at it. Tell them the vessel needs recharging before recharging the system pressure.

Just because the needle is on zero doesn't mean the system is empty. You know the system cannot handle the expansion, so it's not ideal to add more water, you only risk activating the prv, which will normally result is replacing the valve if the boiler is more than a few years old .

If there wasn't enough water in the boiler for it to operate then, it will stop, so nothing to worry about while it's low pressure :)
 
Try bleeding the expansion vessel, if water comes out the membrane is split and at that point you might as well replace the boiler (if the vessel is behind everything).

It's like a car tyre valve so just press the middle of it and see if it hisses with air.
 
Take care in what you're doing if you decide to go tinkering inside the boiler !

The expansion vessel is at the side of the primary heat exchanger, and swapping them is quite easy, certainly not anywhere near the cost of a replacement boiler. Even though the heat line is a cheap model. ;)

Mick
 
FlyingFish said:
Take care in what you're doing if you decide to go tinkering inside the boiler !

The expansion vessel is at the side of the primary heat exchanger, and swapping them is quite easy, certainly not anywhere near the cost of a replacement boiler. Even though the heat line is a cheap model. ;)

Mick
Click to expand...
Mine was behind my boiler, so the whole thing would have to come off the wall anyway.
 
Thanks all, had a heating chap out today and he's replaced the gauge, re-pressurised the system and it seems OK now.

I expect these gauges are about 50p and not particularly good. Either way, problem solved......For now.

Onto the next one!

FlyingFish said:
Even though the heat line is a cheap model. ;)
Mick
Click to expand...

Yeah, I was quite happy when the solicitor said the boiler was only 2 years old but when I Googled the make and model the happiness faded. It only cost him £500 - Doubt I'll get years of life from it.

He was a cheap sod the last guy, spent all his money on fags and not on the house :D
 
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