Remove Honeywell 9400c replace with EvoHome

[Chris Darling]

Hi,

Just moved into a new build house where they have a Baxi boiler and an unvented hot water cylinder. Next to the boiler is a Honeywell 9400c which controls the CH and HW. There is a thermostat for the rads downstairs and one upstairs for the rads upstairs (Honeywell CM707).

In my previous house we had Evohome installed (had a combi boiler) and bought everything with me i.e BDR91 Wireless Relay Box. The Baxi boiler supports OpenTherm. I have also purchased a Honeywell Home evohome Hot Water Kit | ATF500DHW. When I look at the wiring at the back of the 9400c there are a number of wires

N - One cable comes from the boiler, one goes to the fused spur, not sure about the other one
L - One from the boiler, one goes to the fused spur, one connects to number 1 (HW off)
1 - while one goes to L, not sure where the other goes to
4 - CH on, not sure where it goes to

I have had a quick look at the manuals for the boiler and the cylinder and it did not really help. Any pointers would be greatly appreciated.

thanks

 

[CBW]

You’ll need the wireless cylinder thermostat if it works with unvented, otherwise you’ll just have to wire it up for the thermostat only.

 

[stem]

The pictures show only a small part of the system. If there are two heating zones and a stored hot water system, there will be 3 motorised valves, a pump, a cylinder thermostat and also a wiring centre somewhere, which will be the heart of the system.

Each of the 3 motorised valves will have a small switch inside. This is there to start and stop the boiler. When the valve is open the boiler starts, when it closes it switches it off. In your case, as the boiler will be supplying 3 different heating loads (upstairs / downstairs / hot water) it's difficult to use OT as the 3 different loads could require a different boiler output simultaneously. [Unlike a single zone combi boiler that just has one heating load to look after]

As you have two heating zones, it will also need two room thermostats (and for Honeywell Evohome, two BDR91's). Plus the hot water controls.

There are also some things in your post that are unusual and would require further investigation:

1. You say that there is a "Honeywell 9400c which controls the CH and HW". The 9400 is indeed a two channel programmer for CH & HW. However, you also say that you have "Honeywell CM707" thermostats for controlling the central heating. These are 'programmable thermostats' and are designed to be installed without being connected to a programmer, as they already have time control built in.

2. Back to the Honeywell 9400. You say that it's controlling the HW. But according to your photograph below there doesn't appear to be a wire in terminal 3 this is odd because........

.......as per the diagram below, terminal 3 is the one that is responsible for switching the hot water 'on'.

3. Again looking at your ST9400 photo, there is a wire in terminal 1 which is 'hot water off' this connection is generally only used with systems that have a single 3-Port motorised valve. This type of valve is only installed with single heating zones and a hot water cylinder. These valves do not work with multizone heating systems.

Your previous system was likely very simple, ie just a single room thermostat receiver (BDR91) wired directly to a combi boiler. However, due to the complexity of the system you have, of which the wiring diagram below would be a typical representation of a system like yours with 2 heating zones and hot water cylinder, I believe that it is likely that it may require considerable rewiring to change it to Evohome, something requiring a high level of electrical skill and understanding of how heating systems work, so probably too complex to advise over an online forum.

You should also consider that as a 'newbuild' any changes made to the heating controls could effect any warranty.

 

[Chris Darling]

Thanks Stem,

You are absolutely right, I know when I am outside of my comfort zone, and this is indeed such a situation. I will get in touch with the company who performed the installation and see if I can book them to replace the current installation with Evohome - I really like having the multiple zones for heating.

thanks