DIY Doctor

thermostat problem

Postby reddevilman » Thu Nov 10, 2011 11:17 am

Having problems with my danfoss thermostat think the model is rmt 230 which has four wires 2 black and 2 red ,so decided to change it to a digital home expert THR840DUK BY HONEY WELl two wire connection, father in law tried to wire it up for me and it tripped the kitchen socket fuse now only the hot water works from the programmable switch in the utility room i have oil heating.Got fater in law to wire old thermostat back up but it not working either any ideas whats went wrong would be a great help.
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Postby stoneyboy » Thu Nov 10, 2011 11:22 pm

Possibly blown the circuit board in the programmable controller.
You really need professional help to check the controller first.
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Postby ericmark » Fri Nov 11, 2011 3:09 pm

The old thermostats had a problem in that the on temp and off temp were quite wide apart so to compensate they had a small heater which required a neutral connection.

The new thermostats don't have this problem so don't have a neutral connection. However because the same thermostat can be used for heating or cooling it often has both normal open and normal closed contacts.

So when swapping between the two types it is easy if not an electrician to end up connecting so at some point one has a direct short. If this happens it can burn out fuses, contacts, and circuit boards. If lucky it will be just a fuse or MCB that has opened but guessing as to what has blown does not really help.

It may be that the thermostat does not work the boiler direct. But works a motorised valve which has a micro-switch which in turn will work boiler. The very small micro-switches can burn out with a short circuit or it may just burn out the printed circuit the micro switch is mounted on.

However if the guys skills are not good enough to stop him causing a direct short then it is unlikely he will have the skill required to work out what he has burnt out or how the repair it. Even as an electrician I have bought whole new motorised valves as I could not get replacement micro switches on that day. With a motorised valve costing around £50 it is not worth doing a try it and see if it works method of repair.
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