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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Grant Combi 90 Mk 2 : wireless thermostat problems https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1283797478 Message started by apsimpson on Sep 6^th, 2010, 7:24pm

Title: Grant Combi 90 Mk 2 : wireless thermostat problems
Post by apsimpson on Sep 6^th, 2010, 7:24pm

I have a Grant Combi 90 Mk 2 oil boiler which has a mechanical timer that does provide enough heat when needed. But need to add thermostate control. So I thought a wireless programmable thermostst would be easily enough sorted. First I bought a sunvic TLX RFP afer peaking to Sunvic. As the TLX RFP only has one control wire, this failed. Grants couldnt offer any help. Does anyone know of a wireless programmable thermostat that does not cost the earth?

Title: Re: Grant Combi 90 Mk 2 : wireless thermostat problems
Post by Lectrician on Sep 6^th, 2010, 8:07pm

I am not really sure I understand what you mean.

What do you mean by "one control wire"?

Are you trying to ditch the old time clock in prefference to a stat?

Does the current clock do more than heating (water?).

Do you currently have a room stat and cabling?

Photos may help.

Title: Re: Grant Combi 90 Mk 2 : wireless thermostat problems
Post by CWatters on Sep 7^th, 2010, 6:40pm

I think I understand the problem...

The wiring diagram for the Grant assumes that the stat is a "volt free type" - See pins 4 and 5 on the terminal block page 50.

http://www.grantuk.com/userfiles/download_81.pdf

Some stats have "volt free outputs" eg two output signals that are either connected together by a relay in the stat or open circuit depending on the room temperature.

The Sunvic only provides a single "Call for heat" signal that is either 0V or 230V depending on the room temperature.

Solutions:

There is probably a way to connect the Sunvic directly to the Grant but I'd need more time to think about it.

It could be done using a relay to convert the single "call for heat" signal to "volt free".

or you could find another wireless stat that provides "volt free outputs".

Title: Re: Grant Combi 90 Mk 2 : wireless thermostat problems
Post by CWatters on Sep 7^th, 2010, 7:30pm

Ok I can't see how to connect the sunvic directly to the Grant without using a relay. Perhaps best look for a wireless stat with volt free outputs.

Caution: If you just connected the Sunvic "call for heat" signal into terminals 5 it might appear to work BUT I think that would be dangerous. It appears that doing this would bypass the internal overheat/limit thermostat and the pressure switch!

It appears the route "live" takes around the control system is...

Incoming Live Pin 3
Boiler on/off switch
Pin 12
Pressure Switch
Pin 13
Limit Thermostat
Pin 10
Relay 1
Pin 4 (Comment: live only reaches this point if boiler is safe to fire up)
Link (or via the room stat to)
Pin 5
Heating Switch
Pin 7 (or Pin 7 via built in timer depending on heating switch position)
Relay 2 and onwards to the inner workings of the boiler.

Title: Re: Grant Combi 90 Mk 2 : wireless thermostat problems
Post by CWatters on Sep 7^th, 2010, 8:26pm

Cheapest wireless programable stat I can quickly google up is the Siemens RDJ10-RF for £55 inc vat from several places including..

http://www.heatingcontrolsonline.co.uk/siemens-rdj10rf-wireless-programmable-room-thermostat-p-358.html

Screwfix also have it at £66.69. Personally I think it looks a bit ugly. Screwfix also appear to have an older version(?) of the same stat at £51 but not all the reviews are good. Some say battery life is poor.

I don't think the £60 Horstman HRFS1 is volt free.

I'd save up and get a Honeywell or Heatmiser but they are £90-£120.