Replacing Sunvic TLX 2259

22 Mar 2006
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United Kingdom
Our room thermostat (Sunvic TLX 2259) has been playing up, it doesn't activate until the temperature gauge is turned up over 20C (even when room is 15c) and then doesn't turn off again until it is reduce to the lowest setting 15c, like it sticks once on.

I've also noticed that it makes a fizzing noise and you can see a green glow between the two sides of the electrodes on the metallic strip occasionally, I assume this is as it's not quite closing the circuit.

I've tried replacing it with a Honeywell T3630 (told by Plumbing shop it was compatible) but when I wired it according to instructions, live, neutral and earth it didn't turn the boiler on and off. From reading around it appears some heating systems have three wires (4 including earth) and some have 2 (3 including earth) mine being the latter.

The current wiring is as below:

while i had the cover off (with power off) I noticed the electrodes have an adjustment screw on them which alters the gap between the two electrodes. Is this something that can be adjusted or is this thermostat just worn out?

I noticed online that it's possible to buy the same thermostat as I currently have and have thought of that as an easy option. While I am replacing it I was wondering whether there were any better makes/models out there?

We currently have a Gloworm Revival2 BBU back boiler with a Danfoss 3060 Timer for the heating and hot water control.
Ideally I'd like to have the heating on during the day but at a lower temperature than in the evening.

While I was replacing the thermostat I was thinking of using an electronic one that allows for this. The only issue I can see is I would need to co-ordinate the heating control times with the times on the thermostat and the Danfoss has limited configurations:

Heating + Water On
Heating + Water Off
Heating Twice + Water Once
Heating OFF + Water Twice
Heating Twice + Water Twice
Heating Once + Water Once

I also noticed that unless you have a combi boiler it's not possible to have the heating on without the hot water?

I also thought that I could replace the Danfoss and the Sunvic with a programmable control unit with a wireless room thermostat and controller but have heard the wireless connection can be problematic???

When I looked at these I also noticed they seemed to only mention combi boilers and not the traditional boiler type I have.

Looks like three options:

1. Replace the existing thermostat with direct Sunvic replacement.
2. Replace the existing thermostat with another similar model.
3. Replace the existing thermostat and Danfoss timer with an electronic controller with a wireless unit (The current controller is in upstairs airing cupboard and the thermostat is in the hall so not easy to run cables)

Would be interested to know peoples opinions?

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If you had only 3 wires, one of which was earth, then your old thermostat would not have worked correctly. It would have given a wide switching band such as you have had because the accelerator heater was not wired in. You need four wires if one is a true earth; a live feed, a switched live, a neutral and earth. Sometimes, the 'earth' was not used as earth, but as a conductor. You need to examine the wiring in the junction box to establish what each lead does. Heating circuits often do not conform with regular electrical circuit colour codes. The Honeywell thermostat you have been sold is the equivalent of your old sunvic.
In the picture of the old Sunvic, blue is neutral, brown is permanent live and the green/yellow is the switched live.

There is no earth. The g/y should be the earth, but it is not, due to the installer using the wrong cable.
If you connect those three wires correctly to the new thermostat, it will work.

The settings on the Danfoss timer suggest your hot water is on gravity circulation, with a pumped heating circuit. Changing the timer will NOT allow you to have the heating only - changes to the piping is required and some additional components will be needed.

A combi boiler has nothing to do with it - they are fairly useless devices which attempt to heat the hot water as it is required. You don't have one, don't buy one either.
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Thanks for the advice. It's as mentioned the yellow/green isn't an earth wire (although there is a terminal on the case for an earth wire).

I found a PDF with the actual wiring online:

Thanks again.


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