Vaillant ECOmax VUW 286 EH giving CH but no HW

[bweebar]

My Vaillant ECOmax VUW 286 EH combi condenser is giving central heating but no hot water.

About 8 years old, working fine until early this year when it suddenly stopped supplying HW. Demanding hot water for the shower, kitchen sink and bathroom taps the HW would become tepid but not hot and the radiators would become warm. To get HW you must close the CH valve and switch the boiler to HW only.

A diverter valve problem then? Replaced the diverter valve with a brand new one (£14 on ebay), no change.

A controller board problem then? There are 22 volts DC at the diverter valve solenoid when HW is demanded and enough magnetic force to hold a large screwdriver in place against gravity.

A faulty diaphragm? The diaphragm is new with the valve and I removed the copper pipe which connects the diaphragm to the low pressure side of the pump and it is clear of blockages.

So now I'm stuck, I can't think of what else to try (except email Vaillant, which I just did) any suggestions?

[Ed Sirett]

Is there enough flow of DHW water to make the diaphragm move the valve fully? I'm not familiar with this specific model. The problem IS the diverter valve from the symptoms you describe. The root of its problems is what is not clear.

[bweebar]

Thanks for replying.

The manual is available for download here:

1.5MB .pdf

Do you mean the flow as in the amount of water out of the taps? A bath still takes the same amount of time to fill and a hot shower is still as it always has been (with the CH service valve closed).

Yes, it all points to a problem with the diverter components but the actual valve unit is new and having replaced it, the old one didn't seem to be in a poor condition. The system has always had inhibitor in it and the radiators get hot as expected. The old valve moves freely between positions under finger pressure, no stiffness at all and the old diaphragm looks intact.

I measured 22v at the solenoid (at the solenoid terminals as well as at the connector on the main controller board) which seems an appropriate value for a working board. It switches on with demand for hot water and off when the taps are closed again.

Yet for some reason, it's not diverting and the only other thing I can see that could cause that is the pressure tube that connects the diaphragm to the inlet side of the pump. I took the tube out completely and made sure it was clear of obstructions. Does the diaphragm need bleeding or priming to work?

[Ed Sirett]

Could the secondary heat exchanger be blocked? You could simply make it Vaillant's problem they'll sort it by replacing everything till it works.!

[bweebar]

Had a reply from Vaillant earlier, they too suggested that the flow through the DHW heat exchanger might be restricted "causing negative pressure to the diverter valve" so I'll have a look when I get time.

Thanks again for your input.

[bweebar]

Had a look today, got the heat exchanger out and flushed it through. It wasn't severely blocked but there was a constant stream of small, black flakes (insoluble in water, Viakal, oven cleaner or bleach).

Hosed it through for ten minutes then put it back in and hey presto! the boiler's diverting again.