No heating flow. Pump and diverter working, what else could be wrong?

[Stupot]

Hello all,

I have a WorcesterBosch Greenstar 42CDi classic combi, which is providing DHW fine, but not heating. Was working fine in spring, probably not been used until recently now it's colder.

When heating is called for, the boiler fires up ok but soon shuts down the heater as the temperature rises fast. The pump is going but no heated water is flowing around the external heating circuit. The diverter valve is pulled down.

It seems as if the heated water is being circulated around boiler like it should for DHW, the plate heat exchanger is getting hot. But without the flow to the radiator circuit it would overheat quickly.

I don't know if the reaon there's no heating flow is because the boiler has shut it down, or because there is no heating flow then the boiler has shut down.

So what am I missing? All forum enquiries about this sort of problem point towards pump & diverter, but here they 'seem' fine. Any suggestions on what to try next?


Some thoughts
1. Pump: it is spinning. I took out the AAV, small sign of weeping, it was pretty crusty, replaced with new one, that part now working fine but not solving my problem. Maybe pump its not providing enough pressure to push the water through the radiator circuit?
2. Diverter valve: the motor seems to actuating it upwards for DHW and downwards for heating correctly. I took out the rubber valve & paddle which was pretty crusty and marked, replaced with new one, but not solving my problem. Maybe its not actuating fully, so water hot water is finding a lower resistance path through the plate heat exchanger?
3. Blockage: maybe I have a blockage in the flow/return circuit or even in the plastic manifolds inside the boiler?
4. Bypass valve: not sure exactly how this operates but maybe if its stuck then that could explain why water isn't flowing around heating circuit when diverter valve suggets that it should be.

Some observations
a. The pump seems to run faster when providing DHW but slower after failing to provide hot water. Maybe this is a normal recirculation cool down speed?

Some History
A. The pump siezed last winter with kettling. It was tapped and normal operation resumed.
B. At end of last winter I decided to flush system, put in Fernox F3 to clean for a week, thoroughly flushed then Fernox F1 inhibitor. Had a slight pressure loss, couldn't find leak so added Fernox F4 leak sealer which didn't nothing, I eventually found leak and repaired.

 

[ShaunCorbs]

Valves open underneath for heating ?

 

[Stupot]

Yup, all openend after drain down for recent part changes. It wasn't touched between working in spring to not working now

 

[Stupot]

I just tried manually operating the diverter valve, in case I could make a better seal. No change, it looks like the diverter motor does just as good a job as me trying my hardest.

The reason I thought there was no flow because there is a only a little bit of heat that quickly appears on the flow pipe but has faded to room temp after 2-3 ft. My thinking being that even a small amount of flow would push the heated water further than that.

 

[Stupot]

Maybe I should ignore what I'm observing after the heating has stopped, and concentrate on why the heating stopped. When there is a call for heat, it starts up but stops a few seconds later, fan still spinning and the displayed temperature carries on rising. If the heating dial is low then it'll creep up to 80/90 before dropping, if I have the heating dial on high the temperature will overheat reachin U9 before dropping.

Is this observation normal for an aborted heating attempt?

 

[ShaunCorbs]

No that’s an overheat eg u9 means 109c

 

[Stupot]

No that’s an overheat eg u9 means 109c

Yes, if I have the heating temp high then it reach an overheat code. If I have the temp low, it'll get high but not overheat high.

So not normal, so whats causing overheat for heating but not on DHW with pump and diverter looking ok?

 

[ShaunCorbs]

No not normal operation sounds like a valve is shut or a blockage as it’s not flowing round the heating

 

[Stupot]

No not normal operation sounds like a valve is shut or a blockage as it’s not flowing round the heating

Thanks for helping. Thats what I was thinking, so it's good I suppose that I'm not alone. How do I go about locating or proving a blockage, other than taking it all apart?

Could the bypass valve cause this?

 

[ShaunCorbs]

Cdi doesn’t have a bypass to my knowledge eg flow to return

Easiest was would be link the flow and return at the boiler to see if that pipe becomes very hot that proves if it’s a boiler or system fault

 

[Stupot]

Cdi doesn’t have a bypass to my knowledge eg flow to return

Easiest was would be link the flow and return at the boiler to see if that pipe becomes very hot that proves if it’s a boiler or system fault

You're probably right. I was looking in the manual which shows the bypass valve in plastic return manifold, but now looking inside I can see there's a blanking plug fitted on the left hand flow manifold where the bypass tube should be. So that's one suspect gone!

 

[Stupot]

Problem found, it was a blockage... mostly in my head.

I had fitted an Altecnic 22mm Fill & Flush Valve earlier in the year so I could do my own power flush, fitted on the return just before the magna clean. So I thought I could close this and check pressure either side, with my theory being that a blockage would show as a pressure difference, and so it did, one side loads of pressure, the other side blew a little water out and stopped.

So the blockage was in the return between the boiler and the flush/fill valve, a short span of about 3ft with a magnaclean in the middle ... can magna cleans block? Went to close the magnas valve's to open it up and discovered one of it's valves was already shut!

Then I realised what I had done. When I last did the flush and checked the heating and everything looked good, I finished it off by cleaning the magna and mustn't have opened all the valves for some reason. I remembered thinking that I wouldn't see the benefit of all that work until winter as it was too warm for the heating to come on then, so wouldn't have noticed the problem until now.

Very relieved to now have heating working. Big thanks to ShaunCorbs who was spot on with "sounds like a valve is shut or ..."