Worcester Heatslave 20/25 Overpressuring & red hot radia

[Philjeff]

Help,
Our Worcester Heatslave 20/25 oil fired boiler has been playing up again recently. Over the past 14 months we have had a new burner, diverter valve, expansion vessel plus 3 bar valve and an auto air vent replaced.

The latest problem started with low pressure on the boiler (discharged through the PRV). System was topped up to 1 bar, and expansion vessel checked - it still has air in it (1 bar), so we assume this is ok.

Since this, the boiler is now overpressurising when the heating is on (not when cold, so can't be filling loop / heat exchanger problem) and the radiators are getting hotter than normal.

We have the central heating on twice a day. When we open the upstairs bathroom hot water tap in the morning (20 mins after heating's been on) there is a groaning pipe noise from the tap to the boiler - sometimes with a bang, as if there is air pressure building up.

The boiler pressure (heating circuit) increases towards 3 bar, but if we run the hot water tap, this reduces to about 2 bar and appears to settle. Next time the heating is switched on, the same scenario occurs.

All radiators have been bled and there is no air.

We have spent £560 on plumbing call outs in the past 14 months.

Any suggestions please - is it likely that the divertor would have failed within 12 months, or could it be something else?

Thank you

 

[Dan Robinson]

What is the starting pressure (when cold) of the expansion vessel?

The noise is not necessarily the boiler and could be a loose pipe somewhere.

 

[wilhelm]

I would go for heat ex, Also, there is a check valve on inlet to h.e from div valve, that you have to swap over when fitting a new h.e. maybe that is forked?

why not whip h.e out and check ...errr.... the check valve

I say take the thing out cos, ive found them check valves in so freeekin tight you wont be able to do in situ.

 

[oilman]

System was topped up to 1 bar, and expansion vessel checked - it still has air in it (1 bar), so we assume this is ok.

If you did it in that order the expansion pressure reading is not valid. The air pressure has to be checked with no water pressure.

 

[gtsinax]

Hi Oilman, this is Phil's daughter. If the expansion vessel was shot, surely there would have been water coming from the Schraeder valve when tested (and there wasn't). Also why would the pressure drop (and stabilise) when hot water is run? If it was the expansion vessel, wouldn't the pressure continue to rise when the heating was on until the PRV valve releases? This doesn't happen.

Wilhelm, if it's the heat exchanger, would this not cause a pressure rise when the boiler is off - i.e. mains water passing from the DHW circuit into the heating circuit? Or is it not that simple?

Changing heat exchangers/ checking check valves is a bit too techy for us, but nevertheless it would be useful to have a diagnosis from this forum before we have to call out the plumber again. Bearing in mind how much we have already spent on repairs, it'd be nice to tell the plumber what's wrong, rather than him taking a guess and replacing yet another expensive part.

Any help much appreciated.

 

[holty]

could be the braded hose to the EV is blocked but that would mean a lot of carp in the system. but by sounds of it ,bet inhibitor has not reached it in a long time.

 

[oilman]

If the expansion vessel was shot, surely there would have been water coming from the Schraeder valve when tested (and there wasn't).

The expansion vessel doesn't have to be "shot", there may just be insufficient air in it to cope with the expansion.

Also why would the pressure drop (and stabilise) when hot water is run? If it was the expansion vessel, wouldn't the pressure continue to rise when the heating was on until the PRV valve releases? This doesn't happen.

This makes diagnosing the problem difficult, as the OP stated:

The latest problem started with low pressure on the boiler (discharged through the PRV). System was topped up to 1 bar, and expansion vessel checked - it still has air in it (1 bar), so we assume this is ok.

Since this, the boiler is now overpressurising when the heating is on (not when cold, so can't be filling loop / heat exchanger problem) and the radiators are getting hotter than normal.

......it would be useful to have a diagnosis from this forum before we have to call out the plumber again. Bearing in mind how much we have already spent on repairs, it'd be nice to tell the plumber what's wrong, rather than him taking a guess and replacing yet another expensive part.

Any help much appreciated.

Try here for someone who may be able to help. Or you could get WB in on a fixed fee deal.

 

[Jonnie]

Help,
Our Worcester Heatslave 20/25 oil fired boiler has been playing up again recently. Over the past 14 months we have had a new burner, diverter valve, expansion vessel plus 3 bar valve and an auto air vent replaced.

The latest problem started with low pressure on the boiler (discharged through the PRV). System was topped up to 1 bar, and expansion vessel checked - it still has air in it (1 bar), so we assume this is ok.

Since this, the boiler is now overpressurising when the heating is on (not when cold, so can't be filling loop / heat exchanger problem) and the radiators are getting hotter than normal.

We have the central heating on twice a day. When we open the upstairs bathroom hot water tap in the morning (20 mins after heating's been on) there is a groaning pipe noise from the tap to the boiler - sometimes with a bang, as if there is air pressure building up.

The boiler pressure (heating circuit) increases towards 3 bar, but if we run the hot water tap, this reduces to about 2 bar and appears to settle. Next time the heating is switched on, the same scenario occurs.

All radiators have been bled and there is no air.

We have spent £560 on plumbing call outs in the past 14 months.

Any suggestions please - is it likely that the divertor would have failed within 12 months, or could it be something else?

Thank you

Check Expansion vessel hose for blockage

 

[Bambergaspipe]

What is the starting pressure (when cold) of the expansion vessel?

The noise is not necessarily the boiler and could be a loose pipe somewhere.

Behave Mate.....

 

[gtsinax]

Thanks for the responses so far.

How do we check the flexible hose to the expansion tank - is it a plumber job - will things need isolating? As the boiler is floor mounted, we can't really put a bucket underneath to catch any spills.

I'm almost sure that my parents have already had the expansion tank replaced twice, and it was pumped up with air by the plumber in Dec 08...which is why we originally thought it can't have failed AGAIN! Is it possible that the expansion tank is not big enough for the job - my parents live in a large three bedroom house with about a dozen radiators?

Any opinion on fitting an external expansion tank for £20-£30 (Screwfix) in addition to the one in the boiler. My parents' plumber has said that the diaphragms in the expansion tank fail more frequently now due to changes in legislation about what material they can be made of - is this true?

Finally, is the pressure drop when hot water is run a red herring then? The DHW tank will be topped up with mains cold when the tap is run, and presumably this picks up the heat via Heat Ex and therefore reduces pressure on the heating side?

Thanks again for your help

 

[oilman]

If the hose is blocked, the pressure will rise quickly every time the boiler fires.

The tank may not be big enough, but still should net be failing that quickly. If the pressure starts at 1 bar, it shouldn't rise to more than 1.5 bar.

You can never have too much expansion space.

The cold water flows through one side the heat exchanger then to the tap. Hot system water flows from a tank in the boiler through the exchanger and back to the tank.

When the pump turns on, the flow past the pressure gauge or its sensor may drop, but if it rises again, it is best ignored.