Replacing 4 wire thermostat with 2 wire progammable stat

Postby wright301 » Sat Jan 01, 2011 5:44 pm


Right, have a heating system, that currently runs with a four wire honeywell room stat, I want to replace with a digital programmable room stat, that only has two wires...

Does anyone know what wires I need to connect to what?

The current wiring has four wires, blue, yellow, red and earth, all are connected to the box in the airing cuboard, and running hot water and 3 port valve

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Simply Build It

Postby ericmark » Sat Jan 01, 2011 10:14 pm

The four wires likely are:-
Earth – no longer required
Neutral – no longer required often terminal 4 likely blue.
Com – goes to new common often terminal 1 likely brown and likely goes to terminal 4 on programmer.
NO – Normally open to new NO often terminal 2 likely brown and connected to brown/white on mid position valve.
NC – Normally closed not used with heating often terminal 3.
Having said that there is no guarantee it is wired that way. I always take photos of connections before I start and note them down what wire went where.
If I explain how it works it may help. The Earth is of course a safety thing I tape up in case needed in the future. The neutral connects to a heater and the other end of heater connects to terminal 2 so once switched on the heater warms up the thermostat and so reduces the difference between switch on temperature and switch off temperature. So on original the in and out must be right way around. New switch it does not matter if com and NO are swapped.
On the tank stat both the NO and NC are used and I have on odd time come across where the hot water is wired with single on/off and the room stat uses both the NO and NC contacts. So worth looking to see how original is connected just in case wired non standard.
The Horstmann range have one called DRT2 which includes a clock at £23.25 not expensive and instead of switching on and off with clock it changes the control temperature so no need for a frost stat and instead of going completely off it changes the temperature of room according to time of day. 4 temperatures all same time Monday to Friday and two temperatures Sat and Sun with different times.
Batteries last over a year but better to change at start of each year as has been known to fail with flat batteries even though the indicator says they are OK.
If you fit that type then just set clock on continuous.
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Postby wright301 » Sun Jan 02, 2011 5:52 pm

Thanks for the reply, a lot to take in and look into... I guess trying to integrate a new timer into and old system is a tad difficult, I had hoped that the new timer/stat would have been four wires to make it easier,

I have already got the stat a Salus RT500 only has two wires, L and N ( live and switched live )

I have a big box of wires with connectors, have a boiler, pump, tank stat, Y-Valve and a four wire room stat....

I am keen to change it but am confused by all the wires..
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Postby ericmark » Mon Jan 03, 2011 9:04 pm

I would say the old central heating system is about the most complicated home wiring.
Honeywell have published their plans and they are known by letters like the Y plan. At one time you could down load them easy from web site but think it's gone so you need to register and log in now.
I looked at the instructions for a Danfoss CET B-RF when answering your first post which gave me the contact numbers.
The basic idea is the valve has three options. At rest it supplies domestic hot water, Mid way both hot water and central heating and all the way for central heating only.
In case the valve sticks the room thermostat moves the valve and then the valve once open suppliers the central heating boiler.
The valve is an odd beast the plan shows it with one micro switch but in fact it has about 3 and so is a pig to test.
On the side of the valve is a leaver to bleed the system and latching the leaver into mid position can often get central heating going in an emergency.
However if left that way drawing off hot water in the summer can start central heating.
There is supposed to be a box with a row of terminals and these should be in set order. But builders save money and use socket backing boxes and terminal strip and don't follow the order given in the book.

So way forward is to try to identify at least some of the wires. Since two are likely 0 potential and two when running at 230 volts trail and error is no good.

So to start with try to identify as much as you can. There are four types of three port valves. Those with micro switches (that you can use) and those without. Those which are two position and those which are three position.
As well as Y plan we have C plan and M plan and combinations of the plans.

Hot water can be thermosyphon or pumped and with former temperature controlled or just use boilers built in thermostat.

Unless you know wiring on original thermostat then it can be hard even for an electrician to work out.

Normally we look at old switch and see how that works to find out how to wire new switch.
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