Demand for Heating Not Firing Boiler

[techweb]

Hi,

I have a Vailant EcoTec Boiler with a Megaflo. These are controlled by a Honeywell controller. I have one 3 port diverter, where one of the outputs goes to a 2 port diverter prior to the Megaflo (I believe this is some safety feature).

I have run into an intermittent problem where some mornings the heating does not come on. The heating is set to be on, the thermostat is past the trigger point but the boiler simply does not fire. The only way I can get it to fire is to turn on the hot water also. At this point the radiators warm up but only until the hot water demand (tank up to temp) is met.

If I turn the boiler off (killing power to the controls, valves and boiler) and restart it starts operating properly. This is the second time this has happened in a week.

I considered it might be the 3 port diverter but then that only controls the flow of water and not the instruction for the boiler to fire (unless I'm mistaken).

Any advice welcomed 🙂

 

[Basher]

The 3-port valve does supply the power to the boiler from its microswitch (orange wire)
Google 'Y-plan circuit diagram'
It does sound as if your suspicion could be correct.

 

[techweb]

The 3-port valve does supply the power to the boiler from its microswitch (orange wire)
Google 'Y-plan circuit diagram'
It does sound as if your suspicion could be correct.

Thanks a lot!

That makes a lot of sense now... I found this and it provided a good explanation. Honeywell Motorised Valve Faults - Free Heating Advice - https://freeheatingadvice.com/articles/honeywell-motorised-valve-faults/#Honeywell_3-port_mid-position_valve_operation_V4073A1039_and_V4073A1088

I'd say it is almost certainly the diverter valve. If it happens again (which I am sure it will), I will have my wife toggle the controls whilst I observe the valve. Interesting that it has an intermittent fault though. I mean it's working perfectly now and I can switch between CH only / HW only and Both.

 

[Basher]

If I remember this correctly, the CH microswitch is at the full end of travel of the motor/gearbox. So if the physical valve part on the pipe gets stiff to turn, or the motor/gear mechanism becomes worn, it can happen that the motor doesn't have enough power to get to the end of travel where the microswitch lever is pushed by a cam on the gears.
So operating the valve a couple of times 'free's it up', and it works fine when you play with it, but leave it overnight to come on cold in the morning, and the motor struggles. Well that's my simplistic rationale anyway!

The Honeywell valve seems to be regarded as the best of the bunch, so I suggest you stick with that brand.
You should be able to take the motorised head off and see if the valve stem is stiff to turn, it will only turn about 30 degrees I think. If it's stiff you may need a replacement valve part, otherwise you could try replacing the motorised head.

 

[techweb]

If I remember this correctly, the CH microswitch is at the full end of travel of the motor/gearbox. So if the physical valve part on the pipe gets stiff to turn, or the motor/gear mechanism becomes worn, it can happen that the motor doesn't have enough power to get to the end of travel where the microswitch lever is pushed by a cam on the gears.
So operating the valve a couple of times 'free's it up', and it works fine when you play with it, but leave it overnight to come on cold in the morning, and the motor struggles. Well that's my simplistic rationale anyway!

The Honeywell valve seems to be regarded as the best of the bunch, so I suggest you stick with that brand.
You should be able to take the motorised head off and see if the valve stem is stiff to turn, it will only turn about 30 degrees I think. If it's stiff you may need a replacement valve part, otherwise you could try replacing the motorised head.

Thanks for the sound advice... Presumably if it fails to operate completely I can just manually override to CH by sliding the switch to MAN which will engage CH only. I'd then use my immersion heater on the cylinder.

I've learnt a lot this morning including why I have a second two way valve for the Megaflow next in line of the three way.

I actually have boiler and control cover with Vailant (D&G). I'd never normally go for this kind of insurance but It costs me ~£22 a month and with 21 radiators, TRV valves, boiler and control coverage including an annual service, it's just a no brainer. You might ask why I don't just get them out but I like to understand how things work and take a lot from knowing that I could resolve it myself, if I had to do so.

I'll monitor it for now and use the manual workaround if I get stranded until they come out to replace.

 

[Basher]

Thanks for the sound advice... Presumably if it fails to operate completely I can just manually override to CH by sliding the switch to MAN which will engage CH only. I'd then use my immersion heater on the cylinder.

I don't believe moving the manual lever over and locking it operates the microswitch, so that may not help.
My understanding is the lever is primarily used when emptying/ filling the system.