Honeywell T40 Worcester Highflow 400 thermostat problem

[yaryl]

Hi,

Hope someone can help. Had a room thermostat fault develop on a Worcester Highflow 400 electronic boiler.

When the boiler is switched on (by the timer) and the room thermostat has reached the required temperature (in other words it is telling the boiler to stop centrally heating) rather than the boiler turning off you can hear a relay (on the left of the control panel) switching on/off every second and the indicator light by the central heating water temperature control flashes in unison. If the thermostat temperature control is turned up (so heating is required) then the boiler comes on and heating is provided as normal.

I have checked the thermostat with a meter and it appears to be switching as on/off expected as the thermostat is raised and lowered. But when the thermostat is telling the boiler to stop heating the boiler is cycling on/off every second. However if I disconnect the thermostat from the control board (either at the thermostat and or the boiler end) then the boiler remains off without cycling.

Anyone able to give some pointers to fault-finding if it's the thermostat or part of the boiler's control board?

Thanks.

 

[ShaunCorbs]

sorry to say as you have removed the boiler case and you arnt gas safe you have broken the law

time to get a gas safe engy in im sorry to say

 

[yaryl]

Perhaps you don,t know this boiler but the control unit is a totally separate unit outside of the boiler case, so no I haven't "removed the boiler case" or broken the law.
Hopefully there might be some more helpful answers.

 

[ShaunCorbs]

Perhaps you don,t know this boiler but the control unit is a totally separate unit outside of the boiler case, so no I haven't "removed the boiler case" or broken the law.
Hopefully there might be some more helpful answers.

so you dont need to remove the bottom two screws and to get at the these you need to remove the front cover?

 

[chris watkins]

Sounds like you may have a back feed on the neutral.

You sound like you have a degree of electrical knowledge, if not much decorum when seeking help from us poor illiterate Plumbers !!

As this is external from the boiler why don't you try replacing your existing room stat with one of the more modern battery power digital ones which do not require a Neutral, if that doesn't work then give a heating engineer a call, speak slowly so they can understand you & they might come round & fix it for you, of course there will be a fee.

 

[yaryl]

The front cover lifts off without screws and is designed for regular access - for example the pressure gauge is viewable only with the decorative cover removed - to top up the system for example.

Access to the facia mounted control unit (an optional accessory) is permitted under the legal regulations. I've checked with the gas safe register.

1. Decorative casing: These cases can usually be removed by the consumer e.g. can be lifted off or arehinged panels and do not need to be unscrewed. A decorative case can be removed safely by the consumeror anybody else remembering to follow any manufacturer’s instructions and warning labels and making sureyou have safely isolated any electrical parts. However, if you are in any doubt, always use a Gas Saferegistered engineer.
   

 

[yaryl]

Hi Chris,

Thanks for your response. I did test across the thermostat and got total isolation between the live and return when the thermostat was in the off position. There is a dedicated two wire feed (live and return) form the boiler to the thermostat so given the total isolation when the thermostat is in the off position could there still be "back feed"? I also measured the resistance between the send and return at the boiler end and it was fully isolated when the thermostat was off. The heating resistor (in the thermostat) is wired in this particular installation (measures 10k) so I was wondering if that might be the cause of the problem.

Before replacing the thermostat I am trying to find out if it could be something in the control unit, by detaching the thermostat connections have I isolated the issue to the thermostat?

The boiler is serviced regularly by a qualified expert as should be the case - I'm just looking to try and resolve an external electrical issue.

Thanks

 

[Dotty]

The front cover lifts off without screws and is designed for regular access - for example the pressure gauge is viewable only with the decorative cover removed - to top up the system for example.

Access to the facia mounted control unit (an optional accessory) is permitted under the legal regulations. I've checked with the gas safe register.

1. Decorative casing: These cases can usually be removed by the consumer e.g. can be lifted off or arehinged panels and do not need to be unscrewed. A decorative case can be removed safely by the consumeror anybody else remembering to follow any manufacturer’s instructions and warning labels and making sureyou have safely isolated any electrical parts. However, if you are in any doubt, always use a Gas Saferegistered engineer.
   

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