Combi and thermostat fitting - failed attempt!

I've been trying to install a Honeywell DT92E Thermostat to my Baxi Combi boiler, but my first attempts seem to have failed. The two attached images show the rather sparse user guide for the thermostat receiver (it's wireless), and the boiler itself. I've gone with the thermostat wiring shown in the top right of the four options, with a L/N being tapped off from the boiler for power, and terminal 1 and 2 connected to A and B on the thermostat (obviously I removed the link from terminals 1 and 2 on the boiler).

When I power up, the thermostat has power, and it correctly links to the controller, and switches on when above the set temperature as expected. Trouble is, the boiler itself doesn't respond to the thermostat, it just comes on when you set it on manually. Furthermore, the integral analogue timer on the boiler has stopped moving on.

Have I just gone about this the wrong way or is there a more subtle problem going on? I'm hoping I've not done anything totally stupid or even dangerous!

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Barzin said:

a L/N being tapped off from the boiler for power, and terminal 1 and 2 connected to A and B on the thermostat

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Are L and N permanently live, i.e even when the clock has turned the boiler off?

obviously I removed the link from terminals 1 and 2 on the boiler.

Click to expand...

Disconnect the wires to 1 and 2 and replace the link. What happens then, i.e. is the clock now OK?

When I power up, the thermostat has power

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What do you mean by power up? Turn the electricity on (where?) or turn the boiler on (where?)?

Do you have a meter? If so. check that there is 240v between N and terminal 1 of the boiler.

Are L and N permanently live, i.e even when the clock has turned the boiler off?

Click to expand...

The L & N supply to the thermostat are taken from the same supply as feeds the boiler, so yes, they are permanently live.

Disconnect the wires to 1 and 2 and replace the link. What happens then, i.e. is the clock now OK?

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okay, this may take some work to check, might not be able to do this tonight. However, given that the wire connecting terminal 1 and 2 are effectively the only part of the boiler control I messed with, I would assume the clock would work again (unless I've blown something somewhere!)

What do you mean by power up? Turn the electricity on (where?) or turn the boiler on (where?)?
Do you have a meter? If so. check that there is 240v between N and terminal 1 of the boiler.

Click to expand...

Well, the boiler and thermostat power are both from the same feed, so when I switch on the boiler "at the plug" (so to speak), both of these will have power. At this point, if I alter the temperature setting on the thermostat controller, then the thermostat receiver relay triggers when it's supposed to, as there is an audible 'click' and a green light to indicate when the switch has operated (obviously it triggers when the room temperature below the set temperature, and switches off when above the set temperature). However, there is no boiler action at this point.

The timer on the boiler is an analogue unit, with the moveable pins to select the on/off period, and a switch in the centre with options of 'off', 'timed' or 'permanently on' operation. The only way the boiler fires up is if set to 'permanently on', or if set to a time on the dial when the boiler should be on and set to the 'timer' setting. If left on the timer setting, the boilers stays on but the clock doesn't advance. The boiler is on under these circumstances irrespective of the thermostat status.

I don't have a meter at the moment (it's out on loan!) so can't check the voltage.

Hope that all makes sense.

Cheers

I've managed a bit more investigation. It seems that if the room temperature is above the thermostat set point, then the thermostat is not 'switched' to the on position (as would be expected). Under these conditions, the timer does not advance.
However, if the room temperature is below the set point, then the thermostat is 'switched' on. Under these conditions the timer does advance. However, if the thermostat switches again, the timer does not advance.

In either case, the thermostat itself is not influencing whether or not the boiler fires, this is purely does to the setting on the boiler's own timer switch.

Maybe you need an external clock?

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I think the clue is in Important note in the diagram on the left.

This is saying that terminal 1 is live only when the internal clock is on, so the frost sat would not work if the clock is in the off position.

However that should not be a problem when you are just adding a room thermostat as you only need terminal 1 live when heating has been turned on.

So we are left with the question: why does the clock only run when the thermostat is calling for heat? The only answer I can think of is that the clock has been wired incorrectly!

The clock wiring should be as follows:

Clock Terminal 1 - Brown
Clock Terminal 2 - Blue
Clock Terminal 3 - Black
Clock Terminal 4 - Black

If that is OK check that there is 240V on boiler terminal 1 when clock is ON.

If no 240V, try swapping the black wires.

D_Hailsham said:

The only answer I can think of is that the clock has been wired incorrectly!

The clock wiring should be as follows:

Clock Terminal 1 - Brown
Clock Terminal 2 - Blue
Clock Terminal 3 - Black
Clock Terminal 4 - Black

If that is OK check that there is 240V on boiler terminal 1 when clock is ON.

If no 240V, try swapping the black wires.

Click to expand...

I agree. its more than likely the clock wiring but terminal 1 should be at 240 irrespective of the clock setting
Its likely to be either 1 and 3 swapped
or 1 and 4
check the wiring at the clock as per D_Hailshams post

Matt

I don't think not having an external clock is the problem, since the thermostat seems to have problems controlling anything at the moment.

I agree with your comments regarding the timer, it does sound like the most sensible potential explanation. I'm away and won't be able to interrogate it until Thursday now, but will report back with an update! Thanks again folks, fingers crossed.

matt1e said:

terminal 1 should be at 240 irrespective of the clock setting

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If that was true, the Important note does not make sense. Also, if you have a thermostat between 1 and 2 and 1 is permanently live, the boiler will be controlled by the thermostat on its own - the clock will be ignored.

It only makes sense if the clock is controlling the supply of 240v to terminal 1.

D_Hailsham said:

matt1e said:

terminal 1 should be at 240 irrespective of the clock setting

Click to expand...

If that was true, the Important note does not make sense. Also, if you have a thermostat between 1 and 2 and 1 is permanently live, the boiler will be controlled by the thermostat on its own - the clock will be ignored.

It only makes sense if the clock is controlling the supply of 240v to terminal 1.

Click to expand...

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I've drawn it out so you see what I mean
Important note will make sense now
the side note refers to the clocks' switch connections

Matt

matt1e said:

I've drawn it out so you see what I mean. Important note will make sense now.

Click to expand...

That makes sense, but it's not logical. The clock should be on the connection from the Reset switch to Terminal 1, i.e the supply to the external thermostat. Doing it the way you show is like putting a light switch in the neutral wire.

D_Hailsham said:

matt1e said:

I've drawn it out so you see what I mean. Important note will make sense now.

Click to expand...

That makes sense, but it's not logical. The clock should be on the connection from the Reset switch to Terminal 1, i.e the supply to the external thermostat. Doing it the way you show is like putting a light switch in the neutral wire.

Click to expand...

well first of all apologies brcause the penny's just dropped

D_Hailsham said:

matt1e said:

terminal 1 should be at 240 irrespective of the clock setting

Click to expand...

If that was true, the Important note does not make sense. Also, if you have a thermostat between 1 and 2 and 1 is permanently live, the boiler will be controlled by the thermostat on its own - the clock will be ignored.

It only makes sense if the clock is controlling the supply of 240v to terminal 1.

Click to expand...

you are on about terminal 1 at the boiler I was on about terminal 1 at the clock

D_Hailsham said:

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I think the clue is in Important note in the diagram on the left.

This is saying that terminal 1 is live only when the internal clock is on,

Click to expand...

if this is the case then my diagram is wrong
I admit its from memory but im sure i've worked out the wiring to one of these before (but now that I think it may have been a potterton
but saying rhat its rhe only way I can think off why a internal clock would stop a external frost stat from working unless the internal clock acts directly on the pcb

D_Hailsham said:

Doing it the way you show is like putting a light switch in the neutral wire.

Click to expand...

with respect I don't agree about the analagy with the neutral. although I can see your point

but I think the OP has now gone anyway

I've been away so I've not had a chance to get stuck into this again. However, a guy from BG had a quick look while carrying out a service, and couldn't see an obvious reason why it was wrong (as it was a service he had to call it quits after a half hour or so of investigating). However, it didn't seem obvious that I could rewire the clock as suggested. I'm wondering if I should try wiring it as per the top left thermostat wiring diagram as opposed to the top right

Barzin said:

I'm wondering if I should try wiring it as per the top left thermostat wiring diagram as opposed to the top right

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Are you talking about the top left diagram in box 2.3?

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The diagram you are following (top right) is the correct one. We need to find to find why it is not working.

Have you by any chance inserted any links between the terminals of the relay box? If so remove them!

Do you have a multimeter?

If so, disconnect the wires from the relay box to terminals 1 and 2 on the boiler. Turn the boiler on and set the timer to continuous.

Now measure the AC voltage between 1 and N, and between 2 and N. One should read 240V, the other 0V. If both read 0V, that would explain why the boiler does not light.

The next thing to test is the relay box. You need to measure the resistance between A and B. It should be open circuit when the green light is off, and should be approx zero ohms when the light is on. If it reads open circuit all the time, the relay box is faulty.