Why might a central heating programmer no longer override a room thermostat?

[dumbledad]

My central heating and hot water is governed by a Danfoss FP715 Si. Using this I can set the time each day when the central heating and the hot water come on and off. When the central heating is on it is further governed by a Honeywell DT92E wireless room thermostat in the living room. This was working fine until a few months ago. Now the central heating only listens to the room thermostat. It does not matter what the Danfoss programmer is set to, if the temperature in the living room falls bellow the value set on the thermostat the heating clicks on, even at 3 in the morning.

What's gone wrong? How do I diagnose the problem? How can I fix it so that the heating only comes on when the Danfoss says it can?

 

[ollski]

Change the clock

 

[dumbledad]

Thanks Ollski, it would have been great if it was that simple. I tried knocking five minutes off the time set on the Danfoss programmer but it had no effect. The room thermostat still takes priority

 

[D_Hailsham]

Was any work (including servicing) done on the system a few months ago, i.e before the problem arose?

Does the green light on the wireless stat relay box come on when the fault occurs?

What system do you have, i.e combi boiler or separate HW cylinder?

More details about your system would help, i,e components/makes/models.

 

[D_Hailsham]

Unclip the programmer from the back plate and check if the room stat still brings the boiler on.

If it doesn't the programmer is faulty; if it does the fault lies elsewhere.

 

[dumbledad]

I'll try that.

In the meantime you're right to ask about other work. It may indeed coincide with having the radiators flushed through (they were full of sludge).

It is a combination boiler (old but faithful Baxi WM 51 / 3RS) and the hot water tank has a Danfoss ATC thermostat on it.

The green light on the relay box does come on.

 

[dumbledad]

I tried taking the programmer away from the backplate. Once unclipped the room thermostat cannot bring the boiler on. Does that mean it is a fault with my programmer? When I unclipped it the sticker inside showed its warranty ending in 2012

 

[Agile]

It is a combination boiler (old but faithful Baxi WM 51 / 3RS) and the hot water tank has a Danfoss ATC thermostat on it.

???????

Are you sure?

 

[D_Hailsham]

Certainly sound as if the programmer has had it. The CH relay has probably gone up the spout.

If you have a multimeter, here's how to check the programmer while it's removed from wall

Set programmer to CH OFF then measure the resistance between programmer (not wall plate) terminals L and 4 - should be open circuit. Terminals L and 2 should be low resistance. (for completeness, with HW OFF, L and 1 should be low, L and 3 should be open circuit).

Turning CH and HW ON (Not timed) should reverse the results.

PS Tony has saved me asking the same question.

 

[flameport]

Did the work on the system result in a blown fuse? If so, the relay in the programmer has been welded closed by the fault which caused the fuse to blow.

 

[dumbledad]

I'm no expert, so no, I am not sure. I was reading that from here:

Back to the programmer. I switched both the CH and HW to off. Then removed the programmer from the backplate. L and 2 (and L and 1) are low, but L and 4 (and L and 3) have no connectivity. (It's reversed from the wall plate right, 4321LN rather than NL1234?)

I am not aware of a fuse blowing, but I've ordered a replacement unit which should arrive tomorrow.

 

[ollski]

Np, if the stat is turning it on and off and nobody has linked a permanent live to the heating switched live the clocks faulty.

 

[dumbledad]

Darn it.

Following all the help here (thanks folks) I diagnosed that the programmer was broken and so I got a new one off Amazon and installed it. Perfect - the heating clicked off at bed-time just as programmed.

Then a week later it has stopped working. Same fault.

Am I unlucky and two Danfoss FP715 Si's (one old one new) have broken on me, or is there something in my setup that's causing them to fail?

 

[Dean744PiN5]

just unlucky it seem's , i had the same problem last year when i replaced that exact unit for a customer, the third one done the job.