Wiring a honeywell thermostat to a Worcester boiler

xgfx

Having a bit of a quandary. I've picked up a Honeywell CMT927 programmable thermostat and I'm attempting to wire it up to a Worcester Bosch 24i Junior combi boiler.

I've dug up the wiring instructions from the combi manual:


and here is the wiring instructions from the thermostat manual:


Now I've done my best to try figure it out myself and I think I've got my head around it. Here's the wiring diagram I've drawn up:


However - this contradicts how I've seen other people wiring it up. See the diagram below of how others have talked about wiring it. There's a video of someone doing it this way too:


and for good measure I went in to the boiler to confirm all the connections were actually there!



The main issue I have is whether I can piggy back the programmer live straight into the combi live or whether the live has to go to the programmer first and then feed back into the combi.

Anyone have any advice beyond "don't blow yourself up"? Cheers!

I_have_spoken

Won't this do it?



Mains power comes in on L and N. You continuously power the thermostat with Ls and Ns, then make a link from L to A (A is 'common' as described in combi manual) and finally B to Lr. B is 'switched live' to energise the boiler's Live Return when the 'stat is calling for heat.

N.B. in the 'stat L and L are already internally linked.

Myser

Take the receiver supply from the boiler so that there is one point of isolation.

Ls and Ns on boiler to first two L and N terminals respectively on the BRD1. Bridge the second L terminal on the BRD1 to common terminal A on the BRD 1. Connect a wire from the Lr terminal on the boiler to terminal B on the BRD1 (Normally Open contact).

Edit: As illustrated by I have spoken above.

Heedtheadvice

"don't blow yourself up", hmm I wonder from your questions if you believe you are A Competent Person, no insult intended but you do need to do things correctly and do tests for both functionality and safety!

I do not know either unit in detail so you need proper advice that I will not attempt to give. Try Worcester technical help (link on their website), they are very helpful and will tell you the boiler's needs.

Some boilers, and possibly this one, need mains power as a separate feed from the Lr (heat demand) to allow boiler circuits to work and pump overrun control to function i.e. to run the pump for a short time after heat demand ceases (to cool down the heat exchanger) so this mains power should be supplied 'unswitched' by the heating controls. Similarly the programmer also needs a supply that is 'unswitched' and these two 'unswitched' Live and Neutral feeds should come from the same supply so that when one is isolated so is the other ( helps to prevent shocks through isolation confusion). This latter common supply applies to any the other heating circuit controls too!

The LR feed to the boiler needs a live supply passing though the heating control switching (programmer/thermostats) and this should also be sourced from the same isolatable live source and via only one route so links may need to be removed.

Are there any other controls for the boiler? What do you currently use for the heat controls? I assume you are just replacing or updating? These are all factors that might affect the design/connections.

Best ask Worcester for the boilers control needs, if you are a competent person that is, and if not get a qualified heating engineer or electrician to do the work for you. If you install incorrectly at best you may end up damaging the boiler over time (assuming you get the thermostat controlling the boiler) and at worst create a hazard. The cost of an electrician is a lot less than (potentially) a new boiler and also less than a funeral!

xgfx

Cheers I have spoken and Myser. That makes much more sense.

Hah Heedtheadvice, it was only a light hearted joke. I'd never do anything I didn't feel 100% comfortable with. Destroying something is not conducive to money saving after all!

I've spoken to Worcester once a few years back and they were less than forthcoming - mainly because I was a member of the public and not a tradesmen. Which leads me on to your next question. Currently I have a DT20 programmer on the combi, which I fitted. No thermostat, only the standard fitted central heating temperature control that goes from Frost setting 0 to 5 plus a 'Max' setting.

That leads me to wonder, should I set the temperature control to 5 once the thermostat is fitted?

Myser

xgfx wrote: »

No thermostat, only the standard fitted central heating temperature control that goes from Frost setting 0 to 5 plus a 'Max' setting.

That leads me to wonder, should I set the temperature control to 5 once the thermostat is fitted?

If you mean the dial with a radiator symbol next to it, you set that based on the number of radiators in the system. If you aren't sure about the setting, try just below half-way and then increase it if the radiators do not reach temperature fast enough.

I_have_spoken

Hmm, that's a oddity having a programmer to define when a combi can supply HW!

TBH, if it was my system I'd have gone for combined 5-2 day programmer/roomstat like a Drayton Digistat +3 or the DT20RF Digital RF Thermostat with Twin Channel Programmer. With the separate DT20 7-day programmer and Honeywell CMT9277-day roomstat it'll me a 'mare to programme & you'll always be wondering which one is inhibiting the CH!

Myser

I_have_spoken wrote: »

Hmm, that's a puzzle having a programmer to define when a combi can supply HW!

No, the programmer on that model controls when the heating comes on and off not the hot water.

OP, are you going to remove the DT20 when you install the new wireless programmable thermostat?

xgfx

Well I'll be removing the DT20 as the Honeywell will program the central heating. We have never used the pre-heat function for the water, seemed a little pointless when the water is hot within 5 seconds.

The central heating temperature control does indeed have a rad icon next to it, but the manual states the following:

Min --- 40 °C
1 --- 47 °C
2 --- 53 °C
3 --- 61 °C
4 --- 68 °C
5 --- 74 °C
6 --- 80 °C
Max --- 82 °C

Nothing relating to the amount of rads in the system. I'm thinking I'll set it to 5 or 6 and let the thermostat regulate the rest.

edit: Myser beat me to it. As above, yes I'll be removing it

Heedtheadvice

I'm surprised at the lack of response to you from Worcester, they couldn't have been more helpful when I had questions about a year ago, but such is life I suppose.
Looks like you have virtually all the answers you need. What a great (not so) little forum.
Only query I would have is for your choice of temperature. Yes it will have an effect on heat up time as already mentioned but does it also control the hot water temperature? If so then beware of having that too hot and causing a burn risk at the taps. As an example, from the Kids health website "Set the thermostat on your hot water heater to 120°F (49°C), or use the "low-medium setting" — a child can be scalded in 5 seconds in water at 140°F (60°C). If you're unable to control the water temperature (if you live in an apartment, for example), install an anti-scald device, which is relatively inexpensive and easily installed by you or a plumber."

Good luck with your modifications

xgfx

Yes the forum has been awesomely helpful! It's always the way with customer service, depends who you speak with! In fairness to the operator I spoke to I think they were just concerned about someone who might not know what they're doing potching about with a boiler.

As for the temperature knob it only regulates the rad heat. The temperature of the hot water on this combi is fixed to 55 degrees. It's done the job so far