Replacment Digiital Stat/Control for Existing Honeywell CM921


Postby MickySparker » Wed Jun 13, 2018 4:46 pm

Hi all,
Our Worcester Greenstar combi gas boiler has a wireless Honeywell CM921 'stat/controller which works well - apart from the display now being barely legible (a common issue with them I believe). This started losing segments after less than 3 years and it's now been in for about 8 years.
Looking to replace it with a similar BUT NOT Honeywell device.
Worcester versions seem to be extremely expensive, can anyone recommend a sensibly priced reliable unit please?
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Postby ericmark » Tue Jul 31, 2018 6:35 pm

I am not a plumber, however I think the Bosch boiler has to use it's own thermostat, it is not it seems OpenTherm so you can't use other makes.

There are two completely different reasons for fitting a thermostat. One is to control room temperature and with a modulating boiler the thermostat has to connect to the ebus.

The other is to stop the boiler cycling, the TRV controls the return water temperature which in turn modulates the boiler, however once the minimum flame height is reached then the boiler start to cycle, and either you manually turn it off, or you have a thermostat in the coldest down stairs room without a outside door or alternative heating.

Be careful with the name digital, you can get a digital display with an analogue control, I looked at analogue thermostats however I also have a Bosch boiler, with an opentherm boiler you can use EvoHome where the TRV electronic head talks to the central thermostat (hub) which in turn tells the boiler how high the flame needs to be. However with the Bosch thermostat it will not connect to the eTRV heads, so unless used in an open plan house it does not really work.

I got around it by using two thermostats in parallel, however cheap wireless does not have fail safe, and expensive has anti hysteresis software which is not what you want, idea is to stop it cycling not have a thermostat that uses cycling to stop the hysteresis.

So you want the cheapest hard wired programmable thermostat you can find, around £30, I have one in kitchen which gives me the morning boost as the eTRV heads turn up the temperature, and one in hall which maintains the temperature, however it took a lot of fiddling before I got the non electronic TRV in hall to work with the wall thermostat.
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