Boiler pressure problems.

[Robson77]

Hi there,

I'm really hoping someone can help me on this one.

I've just moved house and the boiler's started playing up (typical, when it's so chilly out).

I got home from work yesterday and the house was freezing. On checking the boiler it would fire for about a minute then cut out.

It was running for hot water though, and if I ran the hot bath tap and then turned on the CH it would run for a couple of minutes and then cut out.

Eventually, I got round to checking the pressure guage underneath the unit and it was borderline in the red (about 3 bar).

I checked the forums and read a couple of articles where people had opened the valve at the bottom of the house to run of some of the CH water and hence reduce the pressure.

So I did this, but ballsed it up and let out loads of water. When I checked the guage it was pretty much at 0 bar, so I left it alone and called the plumber who said he could come round first thing this morning.

Anyway, when I woke up, the pressure was reading just above 1 bar (10 hours or so after I'd flushed it).

Low and behold, the boiler fired and the house heated up, although the pressure did seem to rise quite quickly. I called the plumber, and he pretty much said there was no point in him coming over, but to call him if things went up the swanny again.

I turned off the boiler and went to work, but when I got back home, the pressure was back up above 3 bar. I bled off the water as before (bringing the pressure down to 1.5 bar). Hey presto, boiler worked, but within an hour the pressure was up to 2.5.

I realise that I'm gonna need to get the plumber in on this one after all. If anyone can advise in the short term whether bleeding it off is ok so I can keep me and the Mrs. warm, I'd be really greatful.

Cheers,

Rob.

 

[John506]

Make sure your filling loop is turned off and disconnected after topping up as this may be passing slightly and over time filling your CH up.

 

[Robson77]

Thanks.

I double checked the filling loop, this is definitely closed.

The pressure builds gradually when the boiler is off, but builds rapidly when the boiler is running.

Could it be the expansion vessel that needs reseting? If so I'll give that a go tonight as per the FAQ on this site.

Cheers

 

[dcawkwell]

The expansion vessel in the boiler has lost pressure.
Try pumping it up to 1 bar with no pressure in the system.
Then fill up. If the expansion vessel looses pressure again new expansion vessel required.

 

[MANDATE]

The system will be designed so the initial pressure will only rise about 1bar when CH is on and the water expands as it rises in temperature.
So 1bar going up to 2bar or 1.5bar going up to 2.5bar is about right.
If the rise is 2bar or more the PRV will open, but this tells you the expansion vessel is the cause of the problem. Most likely to have lost some of its 'air' charge and the volume of 'air' very much reduced.
Now if the pressure is increasing when CH is not on it has to be coming from the mains supply.
If the filling loop is dis-connected, it can't be that leaking water across to the CH water.
But there is the HW to consider. Basically mains pressure cold water goes into boiler and comes out hot. the heat being transferred by a 'heat exchanger' If this heat exchanger is leaking, then you have mains water leaking across and increasing the pressure.
This would explain how you leave it at '0' at night and by morning it's gone to 1bar.
If it does the same during the day you could end up with pressures being 2.5bar to 3.5bar instead of the 1.5 to 2.5.
By all means check and re-charge the expansion vessel, but don't be too surprised if its not cured the problem.
You could try turning mains off at night and open hot tap, that way instead of water leaking across heat exchanger A to B it will reverse and go from B to A. This would mean your 1bar at night would go down during the night.

 

[Robson77]

Cheers - I think this is the case.

Emptied the system last night, recharged the expansion vessel (heard sloshing as I pumped out some of the water) and it was all hunky dorey for the evening. Woke up in the morning and the pressure was on the rise again (although chance that this was just because of the heating being on, although the wife said it had gone above 2.5 so I asked her to turn it off).

I guess I'll find out tonight. Gonna get someone booked tonight to come and sort it.

 

[Robson77]

I've just had another thought.

From the first day that we moved in, the supply of hot water was limited.

For example, if running a bath (which is right next to the boiler), the water supply is hot initially, but then soon runs luke warm. Could this be a sign of a perforated heat exchanger.

Also, if I turn off mains water and run central heating (i) if there is no pressure increase will this indicate that the heat exchanger is knackered and (ii) is this safe!?

Cheers.

 

[MANDATE]

Somebody with more experience than me may be better qualified to answer.
My thoughts are:
If the hot water is only luke warm I would think the heat exchanger is not transferring the amount of heat as it should. I think the heat exchanger may have a coating of scale which would act as a partial insulation.
It would also depend on the setting of the boiler stat. The hotter the water the better.
On the second point, I don't see any safety concern. I think mains pressure is around 4bar.
The pressure in the heating system could in theory go quite high depending on the volume of air that gets compressed in the expansion vessel.
That's why the pressure relief valve is there, it's a 'safety valve'
In a satisfactory working system the cold mains which comes out of the boiler hot is separate from the system water, so turning the mains off will have no effect.
The fact is that cold water when heated expands and its volume increases.
The extra volume has to go somewhere. If there was no expansion vessel the easy way out then would be by the PRV, (set at 3bar) unless of coarse there was air in the radiators which could be compressed and act in the same way as an expansion vessel.
A holed heat exchanger would allow water to pass, the direction which it passes depends which side has the higher pressure.
Mains at 4bar system at 1bar and pressure gauge increases.
Or Mains at 0bar (off) system at 1bar pressure gauge decreases

 

[Robson77]

Nice one, thanks.