31 Mar 2009
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United Kingdom
Hi all -

My existing programmer is a Honeywell ST9400C for CH and HW, each circuit having a motorised valve. I wish to replace it and have been offered a ST9500C. I understand this is normally used to control 2 separate heating zones, but in all other respects it seems identical to the ST9400C.

Can anyone advise if it is a simple swap with my existing programmer or do I need to make any wiring modifications? I am thinking for Zone 1 & Zone 2 I can just substitute CH & HW (or vice versa).

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Sounds about right. They are both 2 channel programmers and the wiring is exactly the same. The difference is that the ST9400C shows settings for Central Heating (CH) & Hot Water (HW)


The ST9500C is designed for two heating zone control and so is marked Heating Zone 1, and Heating Zone 2


So while the ST9500 will work with the existing wiring as it is, when you set the times for Heating Zone 2 that will be controlling the Central heating, and the settings for Heating Zone 1 will be controlling the Hot Water.
Thanks for your advice. I hadn't looked at the wiring diagrams but assumed they would be similar (or identical) and that both control 2 motorised valves (for either 2 zones, or CH & HW). And now I know which is which! Thanks again.

I raised this query with Honeywell some time ago and did eventually receive two replies.

The first advised that I replace my programmer with the same model which is a statement of the obvious and did not really address my query.

The second reply said both programmers are 2-channels and just marked differently. If I switched one zone on and the other off I would see which zones could be used to control HW and CH. But then finally said they do not have the same wiring and advised that I contact an installer.

I am sure Stem is correct - I see the wiring diagrams above for both programmers and the terminals N, L, 1, 2, 3, 4 correspond for offs and ons so I fail to see how they do not have the same wiring? Am I overlooking something else??

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No, you're not missing anything unless Honeywell's wiring diagrams are wrong!

Both programmers provide two live outputs each designed to operate a motorised valve. The valves can be plumbed into a heating supply pipe, or a hot water supply pipe. The programmer won't know which.
Thank you again - I was 99.9% sure you were correct so there was only a small area of doubt! It just wasn't logical that there would be any fundamental difference.


PS I'm hoping a new programmer will solve my underlying problem which is I can't have CH and HW on at the same time. This is a recent problem - but either will operate alone?
Sounds unusual, I'm not aware of why that might be caused by the programmer.

Someone handy with a multimer would be able to test the outputs from the existing programmer first to make sure.
Thanks for that, you're probably right again! But I've got the other programmer so may as well try it.
I've got a multimeter but I'm not too handy with it! (I've just been looking at some info with a view to checking the coils on my car - that's initially fairly straightforward, checking the earth on one pin and the voltage (9-14V) between it and another).
Hi Stem -

You may find a follow-up of interest to the original query you answered for me.

It seems the new (ST9500C) programmer has done the trick ie CH and HW are on together! One thing I'd noticed with the old programmer (ST9400C) was that if CH was on and I pressed the "extra hour" button for HW, I didn't hear the click the button normally makes? And although the HW green light came on, the HW didn't actually come on. Conversely, when CH was off and I pressed the HW "extra hour" button, I did hear the click and HW did come on? I'm not sure what all that means but presumably it's a programmer fault?

I was worried for a few seconds when I only saw three wires to the programmer until I remembered my CH stat is wireless! I also thought I might have to rebind it to the receiver. Luckily that doesn't seem to be the case - just as well as the process is explained entirely by "easy to understand" diagrams which I always find not easy to understand!

Sound like the boost is faulty as you have figured. Probably operating the light but not the relay that is associated with it, hence no click.

Not sure what you mean about three wires. I would expect at least 4 connections to: N & L to power it, plus a connection to 3 (for hot water control) and 4 (for heating control)

You wouldn't need to rebind the thermostat and receiver as they have not been changed, so will be still talking to each other. The receiver doesn't know what's on the other end of the wires it is connected to. ;)
Yes, very strange. You've got me thinking and worried! - just as I was patting myself on the back.
Regardless of the wireless thermostat the CH must need a connection to the programmer to tell it when to come on and off? But I'm 99.9% sure there were only 3 wires to the programmer backplate - 2 on the left (L & N?) and one on the right - I can't remember which terminal but I assumed to HW. I can't check it at the moment but I'll take another look soon. It should have been an easy job removing the front of the programmer but it was placed so close to a wall rail that the two screws on the underside are very difficult to reach.

N & L are self explanatory.

The wire in terminal 3 is responsible for operating the hot water.

The wire in terminal 4 is responsible for operating the heating.

Of course it's possible that the wireless thermostat is a programmable thermostat that provides both time and temperature control of the central heating, and that the central heating control has therefore been removed from the programmer to the thermostat. In which case terminal 4 won't have a wire connected and the Honeywell programmer won't have any control of the heating. But, if it was like that before; you would have known that surely?
Hi Stem -

I'm so so sorry that I've unintentionally misled you. I've checked today and there are in fact FOUR wires to the programmer and exactly as you said - to L, N, 3 & 4. So much for being 99.9% sure! - the only (flimsy) excuse I can offer is that one wire is an earth (in colour) but of course not used as an earth. Without your advice I might not have double-checked.

One thing I am certain about is that the roomstat is not programmable - it's a Honeywell DT92E. But I'd certainly like a programmable version if it's not a lot of hassle and I could get a good plumber/installer. Without knowing too much about it, I would think it's fairly straightforward and non-disruptive?

As a matter of interest, are you a working plumber or retired? Not easy in these parts finding a good one at reasonable cost (which is why I've got a conventional boiler instead of the combi I really wanted!).

Apologies again and thanks for all your help and advice.

Sorry, I'm not a plumber, my qualifications are electrical and I still have a few years yet to go before retirement..... & it keeps getting further away :(.... I did an apprenticeship as an electrical engineer back in the early 1980's, Today I produce specifications / designs / costings for control panels for factories.

My involvement with heating systems initially came through my father who was a plumber (sadly now deceased) and a couple of friends who are RGI's that I help out from time to time when they get stuck fault finding or when their usual sparkies get stuck with a tricky installation.
Hi Stem -

One last comment and, bearing in mind your line of work, it's probably telling you something you already know? On a YouTube video a guy diagnosed the problem with his Honeywell ST9400C programmer as a failed capacitor - costing less than £1. It didn't have sufficient voltage to actuate the motorised valve.

I had a similar problem with a large screen tv that turned out to be a simple capacitor on a board. I wasn't sure it was the fault but apparently it was a common problem and worth a try. I took the soft (safer) option of replacing the entire board which cost about £20. That was a few years back and the tv is still fine (touch wood!).


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