Replacing a mechanical timer with a digital programmer/therm

[gunnag]

My heatline viso plus 24 uses an electo mechanical 24 hour timer and since the on/off switch is loose it sometimes turns itself off.

I was thinking about fitting a new timer but since they are ridiculossly expensive at around £130, I thought it would be worth seeing if I can dispense with the old timer completley and fit a digital programmer/thermostat.

Looking at the back of the old timer, there are four wires. Two wires (term 1 & 2) are 24 volt DC and are for the timer motor. The other two wires are connected to term 3 & 4 and are 240 volt AC, these are the boiler on/off wires.

Is it feasible to connect a programmable thermostat to the two boiler on/off wires? There is also a separate room thermostat which would need to be wired into the programmer as well.

I have also noticed my boiler has a facility for the "open therm" connection. DOes this simply control the boiler temp or is it also use to turn the boiler on and off ?

All info welcome.

 

[mikely]

You can connect the volt free contacts of a programmable thermostat (such as a Honeywell CM907) to the thermostat terminals described in section 9.4 and shown in Fig 7 of the manual: http://www.heatline.co.uk/assets/boilers/products/manuals/Vizo Plus 24 manual.pdf. Set the timer to permanently on.

 

[bolshy]

Open therm is an industry standard where the temperature controller (roomstat) not only sends a demand for heat, but additionally it communicates with the boiler and tells it when to modulate instead of the boiler modulating from the circuit temperature. It apparently allows finer temperature control of a zone. You would need an open therm roomstat. The claimed energy savings are 3-4%, I think you would need an extremely well insulated home to see the difference.

 

[gunnag]

Thanks for the replies, problem with setting the timer on permanently is that the timer switch is loose and tends to switch itself off.

I guess I could permanently connect the wires connected to the timer terminals 3 & 4 as these are the 240 volt boiler on/off connections. This would mean the boiler is on all the time.

If I connected a programmable thermostat as suggested, the thermostat would then control the boiler daily on/off as well as the temperature. Is this correct and would there be any harm in hard wiring the old timer connections so the boiler is always on?

From the sound of it, the open therm option probably isn't worth it.

 

[bolshy]

Just wire in the roomstat as per it's instructions. It should have volt free switching terminals. Connect these to the boiler roomstat terminals (not the open therm) and switch the clock timer to permanently on, as Mikely posted. Simplest way.

 

[bolshy]

Sorry, didn't notice the bit about the clock timer switching itself off. If there are terminals on the timer, then I can't see why you can't disconnect it's switching contacts and use a choccy block to connect them together.