Vaillant 618 boiler - won't heat cylinder, stuck on S20

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I have a Vaillant 618 boiler and Unistor cylinder. There are two heating zones, controlled by a VR61, with a VRC470 (installed in October; replaced a VRC430) and a VR81.

Yesterday evening, and also this evening, the boiler has failed to reheat the cylinder after a bath, despite there being a timed demand for DHW.

The boiler status is stuck on S20 ('warmstart demand'), with the fan running and does not progress through to S24 ('burner ignited') as it usually would.

I managed to kick the system out of this status yesterday by doing a factory reset on the VRC470.

This is an entirely new and sudden fault on a system that has been working perfectly. No recent work done.

Any suggestions?

thanks

Jon
 
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I take it you are using the Vaillant NTC cylinder sensor with the Unistor. Press the + button on the boiler display, that will give you the cylinder temperature, It should obviously be quite cold (say 15-25C) . Its strange it starts the sequence at S.20 and does not fire. Check you have the dhw temperature control on maximum on the boiler control and your choice of temperature (55C-60C) set on the VRC470 correctly.

A few more questions: Is D.70 set to 0? Are you using a three port or two two port valves? and what is the boiler temperature display when its trying to heat the cylinder? What happens when you press the +&- buttons together?

Hope that helps... :confused:
 
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Thanks for replying so promptly.

Yes, I am using the Vaillant NTC

The cylinder temp is indeed well below the DHW target (which is 57C)

The DHW temp dial on the boiler is full on as usual.

Yes - D.70 is at 0 - just checked - it certainly hasn't been changed recently either

It's a 2 * 2 port valve system

Interestingly, the boiler will try again to fire to S24 every 5 mins or so when 'stuck' at S20. It will 'last' for about a minute, during which the flow temp rises to 82C, before shutting down (via S26) to S20.

I'm pretty sure I have the answer, anyway. The DHW two port valve is sticking shut. When I flick it over to manual, the cylinder primary flow gets hot, the boiler proceeds immediately from S20 to S24 - stays there - and charges the cylinder as usual. When the cylinder is charged, the valve lever moves back to auto, but with a nasty grinding noise from the motor.

That all points to the valve motor sticking, doesn't it?

The DHW two port valve is the cheapo (?) unbranded one supplied with the Unistor - installed in 2008.
 
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Change the dhw valve to a Honeywell and your problems will be over :LOL:

The motor is obviously goosed, you can put it in manual using the lever till you change it. S.20 was showing because there is no 'time delay' as with heating mode. The boiler just runs the fan & pump till the flow temperature drops, in your case it couldn't because of the shut zone valve.

Well done on spotting it
 
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Why does the boiler give an S20 as if it was a combi with warmstart ?

Tony
 
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Why does the boiler give an S20 as if it was a combi with warmstart ?

Tony

When you use a system boiler with a VR65/61 box the cylinder is basically seen by the boiler as the plate exchanger is in the combi. The cylinder NTC takes the place of the 'warmstart' NTC on the plate. The boiler will therefore heat the cylinder using hot water criteria in the software not heating. The HW temp is thus controllable then on the boiler HW knob or by ebus controllers.

Max primary temperature, Kw output (most useful) and pump over-run time can be adjusted in software for cylinder reheating. There is no time delay on cylinder reheating and the software takes into account the temperature drop and rise when adapting itself to reheating the cylinder. i.e. it won't stick 28Kw into the coil just to raise the stored water 5C.

The VR65 box connects to a standard 240V zone valve(s) and operates them as a DV would be used in a combi. If 2x2 ports are used the grey and orange wires are not used/connected.
 
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thanks for this explanation.

My trusty installer swapped the DHW 2 port valve for a new Honeywell one, all sorted as you predicted.

Is 4 years' life usual for a cheap unbranded valve, or was I unlucky?
 
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