Adding a switch in series to Honeywell T40 Thermostat

[jlew]

Over xmas, I'm fiddling with a little project to control my thermostat from my computer. I can switch anything on/off from my PC, including mains voltage with a suitable relay.

My thermostat is a Honeywell T40, a wiring diagram of sorts can be found here:

http://www.ultimatehandyman.co.uk/forum1/replace-honeywell-t40-t30722.html

I want to add in a switch to override the thermostat (in series, so that if my switch is off thermostat is off, if my switch is on, thermostat controls heating)

My question is, where should I add my switch? (I suppose my question is also, why are there 3 wires for this thermostat..?)

Thanks for any response, was expecting my thermostat to just be a switch and surprised there's actually mains voltage going through it

JL

 

[spraggo]

The wiring diagram seems to be incorrect, the T40 connections are 1= L, 2 = N, 3 = Switched Live (On), 4 = Earth not normally used. The neutral activates a resistor or heater to make the thermostat more sensitive to temperature change. If you replace it with a digital thermostat neutral is not used. To switch the room stat On and Off with the computer you will need to add the switch by breaking into the line out from the switched live.
Beware you are dealing with 240v ac.

 

[bernardgreen]

Put the over-ride OFF contact of the relay controlled by the PC in series with the switched live lead.

This contact has 230 volts on it. The relay and associated wiring will need to be be capable of carrying 230 volts without any risk of 230 vots getting into the PC's output circuitry. A 2000 volt minimum breakdown in insulation between the contacts ( mains ) and coil ( connected to PC ) is recommended for safety reasons.

Also think about what happens if the PC is turned off and the relay goes to the OFF position. You will have no heating.

 

[AndyPRK]

I can never understand why people want to control things from their computers, when there are much better ways of doing it.

 

[flameport]

If you insist on using the computer and a relay, note that the load being switched is inductive, so the relay contacts will need to be suitable for switching inductive loads.

 

[jlew]

I can never understand why people want to control things from their computers, when there are much better ways of doing it.

It's not strictly controlled by 'my computer', I'm using a microcontroller to poll a web page where I'm posting a setting to tell it whether to turn my heating on or off. The web page is accessible from any browser (like, my phone or any PC), so I can turn my heating on or off from anywhere.

I couldn't find anything off the shelf to do this for less than £300 otherwise I would have bought it.

Half doing this because it's fun, half doing it because my house is full of holes and all the heat escapes, so I can't afford to have the heating on when I'm not here!

 

[jlew]

If you insist on using the computer and a relay, note that the load being switched is inductive, so the relay contacts will need to be suitable for switching inductive loads.

Thanks for the info. Presumably if I can find the relevant docs for my boiler / timer I can get more info on what's actually being switched?

Are there likely to be any other ways to switch my boiler without messing with the thermostat?

JL

 

[bernardgreen]

half doing it because my house is full of holes and all the heat escapes, so I can't afford to have the heating on when I'm not here!

Fit a simple frost stat to the boiler that will turn it on when the temperature gets low enough to create a risk of frozen pipes.

Then when you go out simply turn the boiler off. The frost stat will overide that OFF switch if the house gets too cold. That way you do not have to keep checking the weather forecast and updating your web site page.

If you want a bit of "hi tech" in the system then use a micro-processor and PT 100 sensor to create the frost stat. TIP.... if you are controlling mains from a micro-processor look at the OPTO22 range of industrial I / O modules. Much better than mechanical relays.

 

[Chri5]

I can never understand why people want to control things from their computers, when there are much better ways of doing it.

Which tend to be logic control or mini PC type applications.

The PC, Smart phones are the obvious user interface since everyone seems to want to use them.

With wi-fi in home, much better wireless security now common it is the natural progression.

Why not control everything from Ipad, netbook, laptop, pc or smart phone ?

 

[JohnD]

half doing it because my house is full of holes and all the heat escapes, so I can't afford to have the heating on when I'm not here!

have you considered spending your time with a few rolls of loft insulation, and a can of expanding foam, to solve the actual cause of the problem?

http://www.diy.com/diy/jsp/bq/nav.j...efview=lister&ts=1293533030256&isSearch=false

 

[jlew]

half doing it because my house is full of holes and all the heat escapes, so I can't afford to have the heating on when I'm not here!

have you considered spending your time with a few rolls of loft insulation, and a can of expanding foam, to solve the actual cause of the problem?

http://www.diy.com/diy/jsp/bq/nav.j...ister&ts=1293533030256&isSearch=false[/QUOTE]

You are of course completely right, and this is what i should be doing with my time. My problem is gaps between floor boards through which cold draughts blow and old single pane sash windows

 

[JohnD]

if it is a ground floor, insulate it from underneath.

You can use a cheap plastic film and double-sided tape for the cheapest DIY way of preventing draughts and adding a double-glazing layer to old windows. Clean the frames thoroughly first. http://home-garden.shop.ebay.co.uk/...blon&_osacat=3187&_trksid=p3286.c0.m270.l1313

 

[bernardgreen]

My problem is gaps between floor boards through which cold draughts blow and old single pane sash windows

Sheets of marine plywood will cover gappy floor boards and kill all those draughts. Low cost fast fix. Give some thought to providing access for later work.

 

[ban-all-sheds]

Why not control everything from Ipad, netbook, laptop, pc or smart phone ?

A better question would be "why?"

 

[flameport]

Sheets of marine plywood will cover gappy floor boards

Hardboard does the same and is 10 times cheaper.

Or just fit laminate flooring throughout - no one seems to care about access these days, after all most people only think about electrical work after having the entire house redecorated.