DIY Doctor

Main navigation

Can I use my line voltage thermostat as volt-free?

Postby alan hr » Tue Jan 01, 2013 6:59 pm

I've got a ProCombi HE 85 boiler ( ... ca8b240c86) and a Honeywell T6360B 1028 thermometer ( ... 7r1204.pdf and ... 28.txt.htm).

Boiler was installed before I moved in and the thermostat is still on my wall but no longer wired to it (wires lead to boiler but are not connected). I wondered if I can wire it up rather than buying a new thermostat.

Boiler manual requires volt-free thermostat, whereas the current one is a 230V line voltage thermostat. I've read that the thermostat can be used volt-free but will lose the use of the Anticipator, so will be less accurate but apparently won't be noticeably so. I have to questions... Firstly, may attempting to use the thermostat as volt-free damage the boiler? Secondly, if I can be done, how should it be done ie how can I wire it?

The wiring diagram for thermostat (page 4 of pdf link, figure for T6360B) suggests that I might be able to use it as a volt-free switch by feeding boiler control circuit (RATHER than the intended 230V circuit) in series to thermostat terminal 1 and returning to the boiler control circuit from thermostat terminal 3. However, I don't know if there is any significant resistance/inductance inherent in the temperature-sensing-element switch which may in fact mean that it won't work with/will damage the presumably ultra-low voltage control circuit. Also, not sure if using the thermostat (without Heat Anticipator) may damage the boiler control circuit with voltage swings which might otherwise be dampened in designed-for-purpose volt-free thermostats?

NB The wiring diagram on page 33 of the boiler manual shows how a volt-free external room thermostat can be connected in series with an external timer or, as in my case, the "internal" factory configured inbuilt timer. Note, though the wiring shows live and neutral 230V line being sent to the volt-free thermostat, I presume this apparent contradiction is because though the thermostat switch operates volt-free, modern thermostats have an electronic control panel which needs power. In my case, I'm suggesting I wouldn't connect those two wires; instead I would only connect the other two wires (to the non-230V, presumably ultra-low voltage, control circuit).

Thanks so much in advance for any help! If it's possible it will damage the boiler, I'll just buy a new thermostat, but if I can reliably use the current one, that would be brilliant!
alan hr
Rank: Apprentice
Progress to next rank:
Posts: 5
Joined: Mon Jun 13, 2011 9:27 pm


Simply Build It

Postby stoneyboy » Wed Jan 02, 2013 10:50 pm

alan hr,
Try connecting the boiler control thermostat terminals to connections 1 and 3 on the thermostat. Remove all other connections in the thermostat. This way you will be using the thermostat purely as a switch. If it doesn't work then you will have to buy a volt free stat.
Rank: Project Manager
Posts: 5509
Joined: Wed Dec 10, 2008 6:44 pm

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides
  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!

  • Related Topics