Hot water hot/cold/hot/cold....

[chrisgr1962]

Hi,
We've just had out combi fixed as there was no hot water and the rads got hot. The 3 way valve was repaired which has now stopped the rads getting hot when the hot water tap is on.
However, when the hot water adjuster is set to ECO (recommended setting which worked fine originally. Its a Glow worm 30ci) the gas burner comes on and heats the water but then dies down and goes out. After a few seconds it comes back on and does the same thing. This goes on forever in approx 10 second cycles. By turning the hot water temp up to nearly max (the booklet says you should only do this occasionally) the gas burner does not go out but if you watch it it starts off on full burn, then dies down (but doesn't go out now) and then flares up again. The actual temp of the water varies as the gas burner varies.
Any one any ideas?
Thanks
Chris

 

[gas4you]

On the face of it, this is a classic case of the plate HE being blocked and the boiler over heating, but I cannot think why the engineer who repaired the DV did not check and assess why this was giving this fault

 

[chrisgr1962]

More info: Would this cause the water temp to be below normal as well? The temp has been slowly decreasing over a period of time. The only way we can get a decent temp is to keep the taps on a slow setting.
The CH works fine if the hot taps are kept turned off but starts cutting out as previously mentioned as soon as the hot taps are turned on.
Also it didn't do this before the 3 way valve was repaired.

 

[ChrisR]

HW would typically go cool then back up to VERY hot if the plate heat exchanger is blocked.
Does tie up with being more noticeable with a new diverter.

Could be a sensor, too.

The HW turns the CH off, by the way.

 

[Agile]

The diverter valve was leaking to the rads and masking the other fault with your blocked plate HE.

Previously the heat had somewhere to go but now it doesn't and the burner has to turn off.

I hope there is plenty of space on the left of the boiler?

Tony

 

[ChrisR]

I hope there is plenty of space on the left of the boiler?

No need, it comes out of the front!

 

[chrisgr1962]

Hi,
Further update. The gas man has been and they seem to think it is the Flow Rate valve. Does this sound correct as it was diagnosed over the phone?
Thanks
Chris

 

[chrisgr1962]

Further info: Before the gasman changed the DV the pressure in the system was always kept at about 1 Bar. After the DV was changed it was pressurized to approx 1.5+ Bar. Would this cause the problem?
Thanks
Chris

 

[Gas2Air]

No pressure is correct. It is a partial blockage in the plate to plate with sludge/scale, that caused the 3 way to not fully close giving the initial fault. Needs a good and thorough flush hooray for you.

 

[chrisgr1962]

Would you expect it to sludge up so quick, the combi was only fitted approx 6 years ago although the central heating system is somewhat older.

 

[spacethegas]

yes, if not flushed & chemically treated correctly when installed.

 

[Agile]

You are lucky it has lasted so long!

When a new boiler is fitted the system should be powerflushed.

With a dirty system which is not treated it can block up in days!

To Chris, its a lot more work getting it out from the front!

Tony

 

[chrisgr1962]

OK, many thanks for all the responses. All I need to do know is convince the gasman what to do. I thought the central heating system was drained & refilled when the boiler was fitted but obviously they didn't clean it. Is it a big job to clean it out?
Regards
Chris

 

[spacethegas]

don't be surprised if you are quoted £500 (i'm not saying you should pay that sort of figure).

 

[chrisgr1962]

OK, further update.
The gas man has been and changed the flow rate valve with little or no difference made to the problem.
He has said the system may need cleaning (which you have all suggested, many thanks) but has made no mention of changing the heat exchanger. From what the other half has said I get the impression they put something into the CH system to clean it (but this is not effective if CH not run for a few hours each day) and then wash it out, etc and all should then be OK.
Will this clean the HE as well or will it still need replacing?
Many thanks
Chris