I would like to replace my old (80s) Honeywell room thermostat with
the 907. It's a very common model.
On the old thermostat there are 5 terminals (top to bottom): 5 2 3 4
1. Of these, 2 (Neutral), 3 (Switch Live) and 1 (Live) are connected.
I understand that the 907 only accepts two wires.
Is my installation compatible with the 907? The installation manual
suggests current needs to by <8A; how does one check that?
Am I right that this a DIY job? I know I need to switch the boiler off
at the mains and drop the fuse while the wife and kids are out
You will only need "Live" and "Switched Live". Tape up the neutral as it is
not needed as the thermostat is electronic. Don't worry about the 8 amp
aspect unless you are switching electric heating. It means less than 8
amps - and I guess your boiler has a 5 amp fuse.
It actually *accepts* 3 - but you only need and *must* only use 2.
According to the diagram in the document you sited, the three are:
A: Common - (Use the Live from the old stat)
B: Call for heat - (Use the Switched Live from the old stat)
C: Heating Satisfied - Do not - repeat DO NOT - connect anything to that.
Your old neutral wire is not required. Disconnect it and tape it off safely.
DO NOT connect it to anything.
You have to make sure that you're not asking it to switch more than 8 amps.
If you're simply switching the boiler and pump and/or a motorised valve, all
of those together are likely to take less than 5 amps, probably no more than
3 - but check the spec of all the relevant equipment.
Your last question worries me! It's a simple job - well within the scope of
a *competent* DIY-er - but the fact that you had to ask, and couldn't work
it out from the Honeywell literature - not to mention being uncertain as how
to isolate it when working on it - makes me wonder whether maybe you should
be getting some professional help.
Also, it's a moot point whether it's notifiable under the dreaded Part P -
since central heating controls seem to come within its scope. I'm not sure
how convincingly you could argue that it's a 'like for like' replacement.
Thank you John and Roger for the quick answer.
On Mon, 22 Oct 2007, Roger Mills wrote:
Thanks for your concern; what's uncertain about dropping the fuse? :-)
Sure, I will check for current at the terminals as well, isn't that
The reason why I felt compelled to ask if it's DIY was the quote I got
from my gasman for this replacement: 70 pounds. Also, the standard set
of questions from Honeywell mentions a sub-base as a possibility; I am
not sure how that is defined, so I thought this could be an issue. In
general I thought I had underestimated its complexity.
That's important to me: who does one ask? My case would be that it's
"like for like" in the sense that the new thermostat performs the same
electrical task; it just does it in a internally different way.
According to the Pocket Guide 8 (from NICEIC July 2006), "power or control
wiring for a ***new*** central heating system" is notifiable.
This would imply that repairs/replacement of elements of an existing system
is not notifiable and therefore simply needs to meet the requirements of
You *should* have an FCU (fused connection unit) which feeds the whole of
the heating system. Turning that off will isolate the heating system without
affecting anything else in the house. But it's always a good idea to check
for *voltage* on anything you're likely to touch, even when you think you've
isolated it. You'll need a voltmeter with a 300v AC range for this - don't
rely on neon screwdrivers!
The only 'complexity' is in understanding the function of each wire, and
being able to work out what to connect to what when the new unit is not
identical to the old one. A common problem is that some people feel
compelled to connect the old neutral wire to something - and end up
connecting it to the only spare terminal, which is 'Heating Satisfied' (C in
your case). The result is that when the house gets up to temperature and the
stat switches, they then have a dead short between live and neutral -
causing a loud, potentially expensive, bang!
Just to let you know that, thanks to your valuable advice, I have now
replaced the old thermostat with the 907. For the benefit of anyone
contemplating to do the same, here is the process I followed:
I switched off the boiler at its fused switch and removed that fuse. I
checked it was below 8A. I then removed the external casing of the old
thermostat and tested that there was no voltage down to 10V AC at the
terminals. Although it's pretty standard, I noted which colour wire
corresponded to which type (Live, Neutral, Switched-Live), then
disconnected them from the old thermostat and dropped its base.
At this stage I only had three wires and the box on the wall. I
isolated Neutral using a connector, then wrapping the connector with
electrical tape. (My initial intention was to use heat-shrink sleeve,
but I could not readily source it.) I screwed the base of the 907 on
the wall, then connected the Live to terminal A, and Switched-Live to
terminal B of the base, as per the instructions. The rest was as per
instructions. I switched on the boiler at its fused box, then tested
that the thermostat drives the boiler (actually I think it drives the
pump in my case, but never mind) on and off.
I like the new thermostat and am well pleased that I did it myself, if
I may say so :-)
With a little help from my friends, that is! Thanks again.
On Mon, 22 Oct 2007, Kostas Kavoussanakis wrote:
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