Fitting 2 Wire Thermostat to a 4 Wire Main Combi 30 System


Postby PCK10 » Tue Sep 08, 2015 8:18 pm

Hi,
I have a 'main combi 30 he' boiler which has a 4 wire built in timer/ thermostat. The 4 wires inside are 2 black a blue and a brown. I need to install a thermostat outside of the room that the boiler is in so I am going to run a cable to a new one. The new thermostats available are mainly 2 or 3 wire installations.. Any idea which config is best to use or which colours are not necessary??
Thanks
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Postby ericmark » Wed Sep 09, 2015 8:29 am

Step one google "main combi 30 he wiring diagram" as you say there are four wires plus earth on the mains input block.

It states:- "Important: If an integral timer is fitted to the boiler an external frost thermostat wired as shown will not operate correctly. Only external timers may be used in such installations as in the diagram."

The diagram shows a link wire and a switch the latter in dotted lines I would guess where timer is fitted then there is no link. I can only guess as not worked on that boiler but I would expect if you want the timer to still work you will need a connector block and put your thermostat in series with existing clock. That on page 31.

Again I can only guess as diagram on 47 shows Optional Timers with four wires but not how it interconnects on the circuit board. But guess is end wire is one to remove and put thermostat in series with.

Some points to remember. Modern boilers should not simply turn off. The flame will go out then after the heat exchanger has cooled it will stop the pump. Over heating can happen if pump does not run on.

In the main modern boilers do not use electric room thermostats. They use thermostatic radiator valves you can get digital heads for these with timers built in.

The theroy is the boiler monitors the temperature of the return water and adjusts the flame hight to keep the return water cool enough for the moisture in the flue gases to condensate when it can no longer reduce the flame any more it switches off. At this point a timer starts and once time has elapsed then the boiler tries to fire up. If after the set delay water returns still too hot then the re-try timer time is increased. And if it can fire up and run then time is decreased this is called anti-cycle software.

So on the water side as each TRV closes the return water gets hotter finally the by-pass valve will open allowed water to circulate without going through any radiators this as shown above will shut the boiler down.

Fitting electric thermostats with a open plan house i.e. no doors is still an option hence instructions how to do it. However where there are doors so each room is heated independently then an electric thermostat messes up the system. Only real way is have the electric thermostat work a motorised valve.

The TRV is nearly a motorised valve anyway and the modern system involves replacing the heads either simply thermostatic with timer or with WiFi link to central controller.

The latter normally uses a data link with boiler rather than simple on and off and is both expensive and complex.

The problem I have found is plumbers may pass their gas safe but some times don't seem to understand the modern system and connect the hydraulics as they would have done 30 years ago. They still seem to think one radiator in the coldest room should have not TRV and an electric thermostat. Mind you not only plumbers my own son does not seem to have worked out the system and he's an electrical engineer.

In my mothers house I have a problem. One she has an electric thermostat fitted in the hall but keeps the doors closed so temperature of rooms depends on wind direction the TRV can allow hot water to flow if the hall thermostat has turned the boiler off.

The other problem is setting the TRV as it takes time just a little at a time higher or lower until room is at right temperature unlike the electric version you can't simply set at 20 degs C unless you swap for electric heads.

There was also a problem with the lock shield valves these are other end of radiator to TRV and they do nothing when all is running but are very important on start up. Unless set so there is about 2 degrees between inlet and outlet when TRV is fully open then instead of all rooms heating together they heat up one at a time. All hot water goes through one radiator until the TRV starts to work then it starts on the next. This means hot water returns to boiler and turns boiler down. So it takes ages to warm up the house but once warm the TRV takes over and it works well.

Every time the electric thermostat turns off and back on the process starts again and again one room at a time is heated unless the lock shield valve is set correctly.

My mother now only uses 3 rooms so it still works reasonably I intend to buy electric TRV's heads but as yet not got around to doing it and also want to fit a TRV to hall radiator.

Not sure if we can put in links but this http://www.rapidonline.com/electrical-p ... aqxu8p8haq shows a head at under £15 each what I am not sure of is if all heads are interchangeable. But it would seem much easier than fitting an electric thermostat to the boiler.
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Postby PCK10 » Thu Sep 10, 2015 9:21 am

Hi,
Thanks for the informed reply.

I have looked at the diagrams online and having looked in the system, the optional timer shown on page 47 is wired up and does work, the problem with it is it doesn't keep its time very well. my plan was to disconnect the optional timers 4 cables (as you said, unknown where these connect to looking at diagram) remove the timer all together, remove the link wire between terminal 1 and 2 in the main block and use a 2 wire system to go across terminal 1 and 2 to my thermostat. The thermostat I was planning on using was a simple dial, drayton rts8 was one I looked at for example, or a Honeywell of the same style..is this the type of thermostat that can be fitted to the system?
Thanks again
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Postby ericmark » Thu Sep 10, 2015 9:26 pm

I used a thermostat with a clock built in. It has worked well as long as batteries changed every year. Rather than switch off it reduces temperature.

However my boiler is not condentating and house is open plan.

Mothers house with doors the thermostat is a pain. It has never worked well. What is really annoying it was hard wired and when new boiler was fitted wires were snipped and RF link one used. So to use one with a timer is rather expensive for RF version hard wired they are cheap.

I used an old version of the Horstmann DRT2 Programmable Digital Room Thermostat which at £27 is cheap enough and has volt free contacts so would work with your boiler.
ericmark
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