Wiring help

27 Nov 2019
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United Kingdom
Hi all. Appreciate any help in understanding wiring set up in new build home. We have a glowworm ultracom 24 boiler. Honeywell st9400 programmer downstairs. 2 Honeywell cm901 thermostats -- one upstairs and one downstairs. We also have a tank in the airing cupboard. I would like to understand how this all works. The 9400 programmer has a wire attached to L,N,HW off and Ch on. I haven't been able to locate a diagram matching this that helps me understand how it works. I am not convinced the HW is actually been controlled by the 9400.
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From your description it would appear to be a rather unusual set up on several levels.

1) First of all, unless my memory is mistaken (& it's quite possible), a Glowworm Ultracom is a combi boiler. If so, it would be a bit unusual to have a hot water cylinder connected to it. Combi boilers produce hot water on demand when a tap is opened, and so have no need of the time control required by systems with hot water cylinders where the boiler has to heat up the hot water cylinder before you can use it. [Systems with stored hot water usually have a heat only, ie not a combi boiler]

2) For a two heating zone + hot water cylinder, that would require three x 2-Port motorised valves for control (Sometimes called an S-Plan Plus). However, a 2-Port valve for control of hot water does not require a connection to the programmers 'HW off' terminal.

3) Also the CM901 is a programmable thermostat which means that it can control both time and temperature settings. Therefore it isn't normally connected to a separate programmer as well.

Normally I would expect to see:
Heating Zone 1..... Programmable thermostat 1 connected to motorised zone valve 1
Heating Zone 2..... Programmable thermostat 2 connected to motorised zone valve 2
Hot water cylinder..... A single channel programmer connected to zone valve 3, via a hot water cylinder thermostat

Sometimes where there isn't a hot water cylinder, I have seen a dual channel (heating & hot water) programmer used to control the 2 heating zones, except you do have a hot water cylinder, and it still wouldn't explain the HW off connection

I have also seen a dual channel programmer used with two non programmable room thermostats connected to the Heating channel, and the hot water cylinder to the Hot Water water channel but that wouldn't explain the HW off connection either.

I wondered if the system had once been a single heating zone with a 3-Port valve that had been converted, which might would explain the HW off connection, but it would still need a HW on connection and it's a new build so won't have been converted anyway. All very odd. :confused:

So because it's a new build I would suggest that you get the builder to come and explain how it works, or if your neighbours have a similar installation you could ask them if they know.
Might be a Glowworm Ultracom Hxi, open vent boiler., with a y-plan with possible 2 port? Unvented cylinder perhaps? Photo of cylinder cupboard and wiring of st9400 might help. Why are you not convinced it’s controlled by the programmer?
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@The Novice. Thanks for the info re the boiler, I wish manufacturers would give their combi's and heat only boilers different names!

If there are two heating zones and a hot water cylinder, a 3-Port valve isn't suitable. This is because it's not possible to close both outlet ports of a 3-Port simultaneously. Which means that if there were an additional 2-Port valve, whenever it opens and triggers the boiler, the boiler would also be supplying one outlet of the 3-Port valve whether it was required or not.

Although having said that, if the OP thinks his water isn't controlled by the timer...Hmmm... not on a new build surely?
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@stem, they are a nightmare, but they’re denoted by the last letters (OP should be quoting all of it):
Hxi heat only, Sxi system boiler Cxi combi
Thanks all for the responses. It is the ultracom hxi model. I will take some airing cupboard photos when home – to answer the question around why I didn’t think the HW was controller by the timer – I turned the timing mode to OFF last night on the HW side, however the water was still perfectly heated when I needed It this morning.
on the sticker on the ST9400 it says it is -- i guess im not clear enough to know if this can be altered etc.
I'm not recommending that you go altering things in an ad-hoc fashion, without finding out where wires go and what they do first. But the hot water control wire would normally be in (3) 'Hot Water On'.


There are some systems that use the Hot Water off (1), but even they still have a connection to (3) Hot Water on as well. But you say yours doesn't have a connection to terminal (3).

Hence why I was checking to make sure what you said was correct.
once again - appreciate all the inputs. there is definately no connection to 3 (HW on). there is a black wire to 4 (CH on) and brown wire to 1 (HW off). the wire from 1 looks to go into a wago 221 connector and link to the L wire. airing cupboard is per the attached.


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Are you able to upload a photo of ST9400 wiring?
here it is -- there is a wire in L,N,1 and 4


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There is indeed a wire in (1) 'hot water off' and In that case, unless you know what is on the other end of the wire, I can't guess why it's there, especially when there isn't a connection in 'hot water on' (3) unless it has simply been put in the wrong terminal, but I would have thought if it was, that you would be aware of that. I'll explain.

With this type of system, normally there would be a wire in terminal (3) 'Hot Water on' that goes first to the hot water cylinder thermostat and then on to the Hot Water motorised valve. When Hot Water is required by both the programmer and the thermostat, the motorised valve receives 230V from them and it winds open. When it's open a small internal switch starts the boiler & pump.

There isn't a connection to 'Hot Water off' required because when the programmer or thermostat switch off the hot water, the motorised valve simply looses its power supply and a spring pulls the valve back to the closed position. (accompanied by a whirring sound)

If it were simply a matter of the wire being in the wrong terminal, the Hot Water would be working in reverse, ie 'on' when the programmer was 'off' and vice versa. You could try a simple test:

1) With the heating set to 'off' and the hot water set to 'off' at the ST9400. Turn the hot water cylinder thermostat to maximum and wait for a few seconds. Is the pump running? It shouldn't be.

2) Now, leaving the cylinder thermostat on max, turn only the hot water on at the ST9400 and wait. Does the pump start up? This time is should.

If it's working in reverse to the above, then the wire in (1) should be moved to (3)

It also looks like there are a couple of wires in terminal 2 (Central Heating off) as well, or is that an optical illusion?


Not that it helps at all, and if so the plot thickens.
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stem and others -- thanks for all the advice. i will perform the test above tonight and report back. i can confirm there is no wire in terminal 2, it is an optical illusion.
this has turned into a real bug bear - i cant work it out. even with the testing above. i recognize its hardly a bad problem in the current environment though. if i turn the heating zones off - the valves make a sound, and the pipes post the valve slowly cool down. i dont find anything immediately happens when i take the test above on the HW. should the valve at the top do something? or only the pump? or both

thanks to anyone who has the time



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