Connecting Honeywell CMT927 Thermostat to Vaillant Turbomax

[journeyon]

My daughter's flat has a Vaillant Turbomax Plus 828E combi boiler. I want to connect a Honeywell CMT927 Programmable wireless thermostat.

The installation instructions for the thermostat's receiver show that the "N" and "L" connections on the receiver must be connected to a power supply, and the "A" and "C" connections must be connected to the "1" and "2" connectors on the combi boiler.

Turning to the Vaillant instructions, they say terminals "3" and "4" must be connected to the thermostat. So far so good (there are no "1" and "2" terminals on the Vaillant, so I simply assume that "3" and "4" should be used).

However, on testing terminals "3" and "4" on the Vaillant with a voltage tester I was surprised to find that these provide around 210v of current. My problem is this: If the receiver is already getting a power supply from the seperate connection via its "N" and "L" terminals, why should it also get 210 volts from the combi boiler. Is it not getting twice the power that it should be and won't I blow it up - or am I missing something. I would have thought that the connections from the combi boiler would have no power output whatsoever.

I would be most grateful if someone can please advise the correct connections in a simple way that I can understand.

Many thanks

 

[D_Hailsham]

The installation instructions for the thermostat's receiver show that the "N" and "L" connections on the receiver must be connected to a power supply, and the "A" and "C" connections must be connected to the "1" and "2" connectors on the combi boiler.

They are only general instructions; there's not room to deal with every boiler

Turning to the Vaillant instructions, they say terminals "3" and "4" must be connected to the thermostat. So far so good (there are no "1" and "2" terminals on the Vaillant, so I simply assume that "3" and "4" should be used).

If they say 3 and 4, then that's what you use

However, on testing terminals "3" and "4" on the Vaillant with a voltage tester I was surprised to find that these provide around 210v of current. My problem is this: If the receiver is already getting a power supply from the seperate connection via its "N" and "L" terminals, why should it also get 210 volts from the combi boiler. Is it not getting twice the power that it should be and won't I blow it up?

The A, B and C terminals are "voltage free" terminals; they only have a voltage if one is supplied from an external source, in your case from the boiler. The voltage supplied to the receiver is to power the receiver and nothing else.

Have another look at the wiring diagram Here and you will see that the Direct Control option shows a link from L to A, but the Combi option does not. This is because the combi is providing the voltage directly.

By the way you should be using terminals A and B, not A and C.

 

[journeyon]

Many thanks for the feedback! Thanks too for pointing out that I should use B and not C as I incorrectly stated.

I still have a problem understanding why there is power going from the boiler to the receiver, given that the receiver already gets power from the mains connection. Are you saying that the receiver merely acts as a relay, and that the power coming from terminal 4 in the boiler merely passes through the receiver without any effect on it, before returning back through terminal 4 in the boiler?

As you can hear, I'm very nervous about damaging the receiver by connecting a live connection to it and just want to be absolutely sure not to do anything that would damage it. What I understand you are saying is that I can go ahead and connect A to 3 and B to 4 and all will be fine?

By the way, I see that the boiler/mains connection requires a 3 amp fuse while the receiver/mains connection says <10amps. I have used a junction box to connect both the boiler and the receiver to the same mains connection. Would it be OK to use a 3 amp fuse, given that there are now two devices using the mains connection?

 

[Chris_J]

Are you saying that the receiver merely acts as a relay, and that the power coming from terminal 4 in the boiler merely passes through the receiver without any effect on it, before returning back through terminal 4 in the boiler?

Yep, exactly right. My setup was exactly the same - it had 240v across the thermostat terminals, but all the receiver does is open and close that circuit anyway. The whole point in the receiver having a relay is so that it won't interfer with any of the workings of the receiver itself. That's what relays are for.

With regards the fuse, keep the 3A in there.



EDIT: By the way, I take it you wired your "junction box" AFTER the FCU?

 

[journeyon]

Thanks for the input Chris - I connected it up and it works a treat!

You ask whether I wired the junction box after the FCU. Let me tell you what I've done just to be sure we're on the same wavelength: The main supply goes into the switched box containing the fuse. I've taken a lead from this switched, box down to the junction box and have taken the leads to the boiler and the thermostat from there. Presumably this is all in order?

Again, many thanks for the assistance.

Paul

 

[journeyon]

I've now got the thermostat connected and programmed. I'm really impressed by its functionality and ease of use, and it seems to be keeping the temperature at or near the required settings but some rather peculiar things (to my mind at least) are happening:

1. When the thermostat sends a signal to the receiver/relay the green light comes on and the boiler soon kicks in. However, some 5 minutes or so thereafter the green light goes off and so does the boiler - even though the desired temperature has not been reached. The process is repeated some minutes later. I was wondering if the boiler is cycling on and off because the radiator heating control on the Vaillant is set to around 2'o clock (as advised by them to minimise scale build up) and the radiators have heated up to their maximum temperature, causing the boiler to cycle. If this is the case, I can't understand why the reveiver is being switched off too.

2. Sometimes the green light stays on and only the boiler switches on and off.

3. I have the thermostat set back to 17 degrees at night. I awoke last night to find that while it was indeed maintaining the temperature at around 17 degrees the receiver, and consequently the boiler, were switching on and off very, very regularly. The green receiver light would switch on for no more than 45 seconds or so before going off again. The radiators hardly had time to have the chill taken off of them. During the time I was lying awake in bed listening, this seemed to be happening every 5 or 7 minutes.

I'm wondering if this is normal - my biggest concern being that all of this cycling on and off is bad for the boiler. Questions that occur to me include:

a. Surely the green light on the receiver should stay on the whole time the thermostat is callling for heat? If so, is it the boiler that's switching it off?
b. If the boiler is switching off the receiver, is this OK or is there a problem somewhere?

Some important notes:
My Vaillant boiler has a programmable clock fitted, but I have disabled this by switching it off, so that shouldn't be causing any problems. It never had any effect on the simple manual thermostat previously fitted.

I have double-checked my wiring and everthing seems to be as it should.

I have checked the factory settings on the thermostat and nothing appears to be out of the ordinary.

I would be grateful for any input/advice as I'm at my wits end with this, not knowing where the problem - if there is indeed one - could lie.

 

[namsag]

Going to go for basic the temp displayed on the screen is room temp you press button to see target temp is this higher than room temp.
Other than that if stat is demanding the green light should stay on is the live neutral earth to receiver direct from fused spur and not interupted by switching anywhere

 

[D_Hailsham]

1. When the thermostat sends a signal to the receiver/relay the green light comes on and the boiler soon kicks in. However, some 5 minutes or so thereafter the green light goes off and so does the boiler - even though the desired temperature has not been reached. The process is repeated some minutes later. I was wondering if the boiler is cycling on and off because the radiator heating control on the Vaillant is set to around 2'o clock (as advised by them to minimise scale build up) and the radiators have heated up to their maximum temperature, causing the boiler to cycle. If this is the case, I can't understand why the reveiver is being switched off too.

Do you have Optimization turned on?
Is the Controller set to Auto?

The green light comes on to indicate that the room unit is calling for heat.

It is normal for the light to go on and off as the controller is trying to maintain the temperature to within ±0.5°C of the set temperature. When the temperature is more than 1.5°C below the setting (this is called the proportional bandwidth) the boiler will run continuously. The controller then starts to cycle on and of in proportion to the difference between the actual room temperature and the required temperature. It does this over 10 minute periods (6 cycles per hour). So if the temp is 1°C below the boiler will go on for 4 minutes and off for two. When it is 0.5°C below it will go on for 2 minutes, off for 4 minutes.

2. Sometimes the green light stays on and only the boiler switches on and off.

In this case the roomstat is still calling for heat, but the water in the boiler has got up to the temperature set by the boiler stat; so the boiler goes off.
This is to be expected. What you do not want is for the boiler to continue running when the green light is out.

3. I have the thermostat set back to 17 degrees at night. I awoke last night to find that while it was indeed maintaining the temperature at around 17 degrees the receiver, and consequently the boiler, were switching on and off very, very regularly. The green receiver light would switch on for no more than 45 seconds or so before going off again. The radiators hardly had time to have the chill taken off of them. During the time I was lying awake in bed listening, this seemed to be happening every 5 or 7 minutes.

Shows the controller is doing it's job. I would sugest that 17°C is too high for an overnight temperature. We have ours set to 5°C but the house never drops below about 18°C, so the boiler never comes on at night.

I'm wondering if this is normal - my biggest concern being that all of this cycling on and off is bad for the boiler.

Perfectly normal. Will not harm the boiler

Questions that occur to me include:
a. Surely the green light on the receiver should stay on the whole time the thermostat is callling for heat? If so, is it the boiler that's switching it off?

See explanation above.
The green light is controlled by the room unit not by the boiler

b. If the boiler is switching off the receiver, is this OK or is there a problem somewhere?

The receiver turns the boiler on and off, not the other way round.

I would be grateful for any input/advice as I'm at my wits end with this, not knowing where the problem - if there is indeed one - could lie.

You will be pleased to learn that there is no problem. THe programmer is just a teeny-weeny bit more sophisticated than you are used to with a mechanical thermostat.

 

[journeyon]

Firstly, I want to say how incredibly useful I have found this forum in the short time I've been using it - I'm indebted to folk like D Hailsham who have gone to incredible lengths to assist me in such a willing way!

For those who are interested, I have to say that I think the Honeywell thermostat is fantastic, with lots of very useful features. Unfortunately, their attitude towards the end customer leaves a lot to be desired in that they refuse to deal with queries from members of the public. This is where people like D Hailsham come in - I don't know what I would have done without their input - other than to have called in a heating engineer at heaven knows what cost, only to find that there is nothing intrinsically wrong with the system.

Where I come from we have heat pumps and air handlers that move heated air around the house. Over the past few days, I've been forced to learn about combi boilers and radiators and how to bleed and balance these etc. Using what I've been able to learn on the internet I've now got a much improved system.

Yes, I now have Optimisation set to on and the controller set to auto and everything is working just great. Importantly, I am no longer concerned about short running times as the reason has been clearly explained to me and I know that it's not going to damage the boiler.

In the spirit of trying to give a little back, and in the hope that it may help others, I thought I should share the following info I found elsewhere on the net after having had my interest piqued by D Hailsham's invaluable input. It's a fairly detailed explanation of how the thermostat goes about it's business (can't understand why Honeywell and others don't include this in the manual).

This from John Phillips on UK.d-i-y in reply to a question about why the green light on the receiver keeps on going on and off
This is because the Honeywell's controller is a PI (Proportional - Integral) type.

The "proportional" part means that in a temperature band a degree C or so either side of the set temperature it varies the heating demand from:

- 100% (when the actual temperature is at the bottom end of the band, or
below); to

- 50% (the actual temperature is at the set point); to

- 0% (the actual temperature is at the upper end of the band, or above).

Proportionality is obtained by the relay box switching on the boiler for the right proportion of the controller's cycle time. E.g. when set to 6 cycles per hour this means a 10 minute cycle; so 50% demand means the boiler is turned on for 5 minutes out of each 10.

You will probably have noticed that a fixed 50% demand at the set point is not necessarily going to maintain the set temperature. That's where the "integral" part of the controller comes in. This bit monitors the actual temperature and if it's consistently above (or below) the set point with 50% demand, it reduces (or increases) the demand targets listed above until the temperature settles correctly.

PI controllers usually give much more constant temperatures than simple oon/off ones.

My thanks to all! (PS Wish my daughter's flat could be kept at 18 degrees at night without the boiler running. It seems to lose around one degree an hour when the thermostat is set back. It must have very poor insulation!!!

 

[tonybhoy]

I've been fitting these controllers-CM 67 and the now upgraded CM927-for over two years and they really are excellent.

I have one in my own house that I take to bed with me every night however the wife's not too pleased about that

Thanks for the info BTW.

 

[aharrold]

Sorry for dragging this old post up but I've just ordered the Honeywell CT927 to fit to my Vaillant Turbomax 824e combi boiler.
My boiler currently has the manual timer with the plastic pegs.
Needing some assistance to get it fitted.
1.Does current timer get removed?
2.i take it the boiler connector is in the gubbings and I need to remove the outer shell.
3.take it I need 2 core cable to attach
4.can you supply instructions on the fit. Seen on above someone mentions 2 separate power supplies?

Cheers