Hot water intermittently doesnt turn on / Vaillant Boiler

[M1AK]

Hi all.

I have a Vaillant thermocompact system boiler paired with a Hive setup.

We have our hot water on timer and it has always turned on reliably for years.

Past week we have noticed that sometimes in the morning the hot water has not turned on. The Hive indicates a green light on hot water but the boiler isn't on. If I turn the boiler off and on again it will automatically kick back in. Alternatively sometimes when this happens I'll turn the hot water off and on again and it fires back on.

Its as if the boiler is being lazy. I'm not sure if this is faulty Hive receiver or something in the boiler. Any suggestions appreciated please?

 

[CountryFan]

Given your description it’s more than likely the motorised valve and or head at fault.

 

[Harry Bloomfield]

When it has failed to turn on, try giving the motorised valve a clout on top, then see if it comes on.

 

[M1AK]

When it has failed to turn on, try giving the motorised valve a clout on top, then see if it comes on.

Is this located inside?

 

[Harry Bloomfield]

Is this located inside?

Look in your airing cupboard, where your cylinder and pump are located. Post a photo, and someone will ID the motorised valve for you.

 

[M1AK]

Sorry I meant my airing cupboard has a big white Vaillant box. Does this need to be opened and its inside? There isn't much outside other than copper pipes and the filling valve, from what I can see.

When i turn the hot water on I hear a click near the boiler and then I hear a buzzing noise inside but nothing fires up. I then cycle it between hot water and heating and back to hot water and it eventually fires up. Heating works fine, its just hot water.

 

[M1AK]

Look in your airing cupboard, where your cylinder and pump are located. Post a photo, and someone will ID the motorised valve for you.

So below the boiler I found two silver boxes with Honeywell motorized valve written on them - that must be it . I'm guessing I have two as one is for heating and one is for water? Both have the same P/N. Any way to determine which one is which? Photo attached. Sorry it is in a tight confinement so hard to picture...

A better description of what is happening. When I turn the hot water on I hear a click near the boiler and then I hear a buzzing noise inside but nothing fires up. I then cycle the control between hot water and heating and back to hot water and it eventually fires up. Heating works fine, its just hot water that is a bit lazy.

 

[fixitflav]

Any way to determine which one is which?

When everything is cold, call for CH and see which pipe gets hot

Sounds to me like a problem with the microswitch contacts in the HW valve. The microswitch fires the boiler

 

[Mottie]

Past week we have noticed that sometimes in the morning the hot water has not turned on. The Hive indicates a green light on hot water but the boiler isn't on. If I turn the boiler off and on again it will automatically kick back in. Alternatively sometimes when this happens I'll turn the hot water off and on again and it fires back on.

I had the same problem as you but with the heating, not the hot water. It was the zone valve. You can replace just the motorised head.

Odd problem with heating/boiler. One-off or something on the way out?

Odd problem, as above. Just got home from a weeks break. We have a sealed 'S' plan central heating system and unvented hot water. Vaillant ecotec boiler, Honeywell digital room stat, controller and motorised valves. When I went away I turned the hot water off at the controller and left the...

www.diynot.com

 

[Harry Bloomfield]

So below the boiler I found two silver boxes with Honeywell motorized valve written on them - that must be it . I'm guessing I have two as one is for heating and one is for water? Both have the same P/N. Any way to determine which one is which? Photo attached. Sorry it is in a tight confinement so hard to picture...

You have what is known as a S-plan, which uses two motorised valves, one for CH, one for HW. The HW one is either sticking or the microswitch inside it is not operating correctly. If you call for CH, the valve with the pipes which gets hot, is the CH valve - which means the other one, must be for your HW.

What you may find, if the HW one is sticking, is that if your HW is all set to be on, yet the boiler is not firing - simply giving the HW valve actuator a thump, might be enough to get it to come on.

The long term fix, is to replace the actuator, the electrical part on top, which turns the valve on and off. In some models, but not all, you can swap just the actuator - others, you have to change the actuator and valve, as a unit. Your model, is one where it can be replaced as a separate part - https://plumbinbits.co.uk/product/honeywell-40003916001-actuator-head/

Shop around, there are much cheaper options, than the one I linked to above.

 

[M1AK]

You have what is known as a S-plan, which uses two motorised valves, one for CH, one for HW. The HW one is either sticking or the microswitch inside it is not operating correctly. If you call for CH, the valve with the pipes which gets hot, is the CH valve - which means the other one, must be for your HW.

What you may find, if the HW one is sticking, is that if your HW is all set to be on, yet the boiler is not firing - simply giving the HW valve actuator a thump, might be enough to get it to come on.

The long term fix, is to replace the actuator, the electrical part on top, which turns the valve on and off. In some models, but not all, you can swap just the actuator - others, you have to change the actuator and valve, as a unit. Your model, is one where it can be replaced as a separate part - https://plumbinbits.co.uk/product/honeywell-40003916001-actuator-head/

Shop around, there are much cheaper options, than the one I linked to above.

@Harry Bloomfield - thank you for all your help. You were absolutely correct, it was indeed the motorised valve. In the end I diagnosed it by listening to which one was constantly buzzing! Found the lower one in my picture above to be the culprit! I did try to tap it moderately, as suggested, but not joy!

In the end I did some research, found that you can indeed replace just the head/actuator in the V4043H Honeywell valves. Much more desirable to me given I'm more competent at electrics. I got a compatible Corgi head that is well reviewed and much cheaper than the Honeywell. Wired it into the boiler junction box, replaced the head on the valve and now back into life

I appreciate your help my friend. Thank you!