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Wiring for a Honeywell CM907 Programmer

Postby cad2cam2017 » Sat Feb 10, 2018 2:06 pm

I've currently got a Honeywell DT90E wired to a Worcester Combi Boiler . It's working off a 2 wired connection with the 3rd wire not in use and running to a terminal block .

Regards the Honeywell CM907 will this function correctly off the 2 wires in use running to the terminal block Ref A & B . I require the Honeywell CM907 to give me greater flexibility during periods of the day.

Thanks in advance for advice on this .
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Postby Mr White » Sat Feb 10, 2018 9:32 pm

Both programmers you mention are battery operated, with "clean contacts"
So I do not see a problem with using either.
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Postby ericmark » Sun Feb 11, 2018 12:23 am

It is volt free contacts so it should work will most boilers, however Wave is the thermostat normally fitted to Bosch to take advantage of direct ebus connection so it can modulate the boiler output.

Old boilers switched on/off, new boilers first turn down flame height then start pulsing on/off when it can’t turn down any more, this reduces the hysteresis, there are two ways to modulate the boiler output, one is connection to the ebus the other is the return water temperature. Where you can’t control the ebus then you only have the return water temperature.

The temperature of the return water is controlled by the thermostatic radiator valve (TRV) as they gradually close first it forces more water into cold radiators then it opens the by-pass valve allowing hot water to return without going around the system, this causes the flame height to reduce.

There is a very clever system called EvoHome, where the electronic TRV heads close and also report to a central thermostat (hub) which in turn using opentherm tells the boiler what flame height is required, unfortunately Bosch do not support OpenTherm so you have to use their own system called Wave. Or use simple on/off.

With simple on/off the idea is the wall thermostat does not control the room temperature, what it does is turn off boiler on warm days, idea is the wall thermostat is mounted in a room with no door to outside, which is normally the coolest room, and has no independent form of heating, and no TRV is fitted to the radiator in that room. However such a room rarely exists, so a compromise, in my mothers case the hall is used and there is a TRV on the radiator which was carefully trimmed to match the wall thermostat.

However to match the two means you can’t really vary the temperature at different times of the day, it relies on the TRV being rather poor, so if you add an electronic head to allow changes in temperature the TRV is too good, and the TRV and wall thermostat fight each other rather than compensate each other.

The other method is multi wall thermostats, if you have two or more thermostats in the rooms which are important then the wall thermostat ensures boiler is running and the electronic heat on the TRV stops room over heating, although you would need a manual off switch for summer.

Although you can simply set the eTRV head to same time and temperature as wall thermostat, you can also link the eTRV head and thermostat together, not quite as good as EvoHome but Nest thermostat and MiHome eTRV heads use a follow command so one can follow the other.

There are some open plan homes where a single programmable thermostat will work, also with homes with hot air central heating, but for most homes the TRV controls room temperature, the wall thermostat controls whole house so to alter the heating on/off then a programmable thermostat set at either 12°C (off) and 20°C (on) will work, but set to 12°C over night, 22°C in morning, 20°C during day and 22°C in the evening will not work, To do that you need to program the TRV head.

I used MiHome but they are a bit expensive and I have never used the geofencing option so likely the pegler Terrier i-temp i30 White Programmable Thermostatic TRV head is likely good enough. The MiHome head has two sensors one for air and one for water so the air temperature is corrected by water temperature, so although right next to radiator if it reports 20°C the room is at 20°C not sure if i30 does the same.
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Postby cad2cam2017 » Sun Feb 11, 2018 7:04 pm

Regards the Honeywell CM907 would anyone recommend an alternative Honeywell programmer which will control various temperatures throughout a 24/7 period .
Looking for 2 wired hardwired options only not wireless.

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