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Honeywell T6 smart thermostat swap-out

Postby nice_high-peak » Sat Jan 14, 2023 3:53 pm


I am looking to upgrade my heating system with a smart thermostat. I have got myself a Honeywell T6 that is Apple HomeKit compatible, etc. However, I am having some difficulty identifying hoe to swap out the wiring.

The heating system here is based on a Glow-Worm combo boiler, but is complicated by the presence of a ‘max flow’ pressurised hot water cylinder upstairs. The programmer is a Sunvic device that will allow programming of heat and hot water, although we always leave the water system on and only bother to control the heating (which may be relevant to my queries). I am guessing (possibly wrongly) that the system I have is not an S-plan system, rather it is a combination system with outboard storage in the cylinder.

There are various guides to wiring the Honeywell T6, bit not of them really match wither my existing programmer wiring or the innards of the T6 receiver.

On the programmer, there are connections to N, L 3 and 4 only (mains x2, HW on and CH on). At the thermostat end, there are connections to COM, N and DEMAND. But notably, that thermostat takes a 240V feed as opposed to the 20V feed for the T6 programmable thermostat. Obviously, I don’t want to fry the thermostat.

So what to do?

The simple part is that I connect the existing mains from the old programmer to the T6 receiver. Then, i presume that Honeywell terminal B is equivalent to SUNVIC terminal 4.

But what about the Sunvic HW terminal 3, and what about that thermostat? Is this a rewire at the level of the boiler connection box? I have seen some installers run live jumpers inside the Honeywell receiver between for example terminals L and A.

Many thanks for any advice you might have even if it is to tell me that I have a normal come setup or some byzantine S-plan system.



Current installation
Boiler: Glow-Worm 18hxi combination boiler
Programmer: Sunvic Select 207XLS
Thermostat: Sunvic TLX 9201
Other equipment: ‘max ’flow’ pressurised cylinder

Replacement equipment
Receiver: Honeywell R4H81
Programmable smart thermostat: Honeywell Lyric T6




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Simply Build It

Postby ericmark » Tue Jan 17, 2023 1:32 am

Reading the manual ... %20hxi.pdf it seems the boiler is 230 volt control, with no option for opentherm.

It seems likely your system actually turns the motorised valves on/off, and they turn boiler on off.

The thermostat you have selected does have volt free contacts, so can be used with either low or extra low voltage, but I have no idea of your skill, and posting pictures on this forum is not easy.

Personally I use programmable TRV heads, so easy to fit, but your existing base is used by Hive, Drayton Wiser and many more, if you are not good at wiring maybe better to go for easy option.
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Location: Llanfair Caereinion, Mid Wales.

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