Honeywell dt90e wiring problem

[Sw197]

I have replaced a dial thermostat. I have a gas boiler, 3 port valves. One programmer and thermostat for rads and water. One thermostat and timer for wet Ufh. I have wired live to a, and neutral to c, then a third wire is not used. The thermostat switches the boiler on, but when temp is reached the boiler keeps running and doesn't switch off. No other boiler problems. Do I need to use the third wire as in the wiring diagram option b on honeywells installation diagrams. Or is it the internal thermostat switch that's the issue as it's not telling the boiler to switch off?

 

[PrenticeBoyofDerry]

by installing a stat, that stat should link out the boiler stat as a on/off control.

How do you know which is the live conductor and whether a neutral is present?
What dial model, did you take off and where were the control cables terminated when you removed?

 

[ericmark]

Many new thermostats do not require a neutral they just have permanent line and switched line.

Most the old thermostats had neutral to compensate for the difference between on and off temperature.

Many DIY people have wrecked the new thermostat by connecting the neutral when not required and causing a direct short.

All to that with a three port valve the thermostat works the valve then the valve works the boiler the thermostat does not control the boiler direct.

However with UFH there may well be other methods used. One has to follow the wiring diagrams very carefully and there is no one size fits all.

Maybe with more details people can help but until we know more only the crystal ball owners can help you. Just too many options.

Your New thermostat has batteries and no neutral what is unknown is how old one was wired. Although they often should have a neutral it would often still work without one.

 

[Sw197]

I have wired the two wires on the dt90e for the ufh. Live into a, neutral into b. Two other wires were on the old mechanical stat - not sure model, but it was a dual one, pretty new as was put up for underfloor heating system, it had a switch on side with timer, moon, and sun, its a watts p2343 wfht. This used 4 wires, so I have terminated these two wires one is a black switched live the other no idea I assumed earth but I am not sure. Foolishly I did not take down what old connections were. The programmer for ufh switches on, but the dt90e whilst showing the flame heating up, does not switch the boiler on. Should I wire in the third wire somewhere on the dt90e even though the installation says two wires only. The ufh has its own separate pump, manifold, zone wiring box, programmer. Boiler fine, working with water no problems.

 

[bernardgreen]

I have wired the two wires on the dt90e for the ufh. Live into a, neutral into b.

If you had done that then Live and Neutral would have been shorted out, at best a fuse would have blown, at worst the contacts in the DT90e would have been damaged.

Hopefully the wires you connected were Live and Switched Live ( demand ) and the thermostat has not been damaged.

 

[Sw197]

Thanks for your time on this. This is the manual of the old one, with 4 wires, I am afraid I don't get what the symbols say on the clock in a/b. But this is what I have replaced.

http://www.wattselectronics.com/client/document/pplimp01210_50.pdf

Are you saying that I should take neutral out and terminate and then put switched live in its place (black wire sleeved brown). The fourth wire I think is looks like earth wire but has no sleeve green/yellow and instead is sleeved brown. It was connected to the watts thermostat. This is confusing as honeywells manual says to use n for neutral?

 

[RF Lighting]

Can you post a photo of your wires and how the existing stat was connected. The one you linked to is for underfloor heating, not central heating.

 

[Sw197]

Yes it is for underfloor heating wet. I am replacing the watts one for a Honeywell dt90e for underfloor heating. Sorry if not clear. I have a separate thermostat and programmer for radiators with a Honeywell t6360.

 

[RF Lighting]

Can you post a photo of your wires and how the existing stat was connected.

 

[Sw197]

Thank you Bernard for your advice, I wired live and switched live and it's working fine (hopefully) on ufh. So I also have a dt90e I wanted to use on the rads, which has a separate programmer (water/rads) and its own thermostat (Honeywell t6360). I want to swap the t6360 for the dt90e, so if I switch the old thermostat and wire the dt90e as I have for ufh will it work? I guess it doesn't matter what it's running. Thanks again.