Generic Thermostat and Cycles per hour

Hardware
1

Hi all,

I have been trying to figure out how to do some DIY smart thermostat controls but I am trying to get my head round how thermostats work and how to replicate this.

The specific thermoset in question I am trying to replace is the Honeywell CM707. (Well two of them as I have zone valves for upstairs and downstairs).

My understanding is that all it does when it wanted heat is that it closed the relay which then passes 230V to the zone valve and then this passes the voltage to the boiler (Vaillant ecoTEC 831 in this case) which starts to heat.

I have managed to wire up some sonoff zb-minis which functionally “close the relay” when the “generic thermostat” requests and I seem to be getting heat but I am not convinced its working as it did before?
When the zb switch activates I can hear the valve open and then I see the boiler start to heat and the radiators do become warm but it seems to stop after a while and it took longer than I expected for the temperature to rise.

I can see in the CM 707 documentation that there a mentions of a cycle rate. Mine is set to default on 6 which I think is standard for gas combi boilers.

Does this mean that every 10 minutes it opens and then closes the relay to start another cycle? Or this this the maximum number of activations per hour or something?

Would appreciated anyone who is in the know!

Many thanks

2

Hi,
I reconfigured a Honeywell SmartFit (Gas wet Y-plan system with LV controls) and remember changing the cycles from 10 to 6 as the boiler was being turned on and off for very short durations (not good for an old system).
ISTR using a temp sensor on a radiator to log the cycles, and yes found 6 = a possible demand for heat every 10 minutes, then a pause for over-run. It the sensor is up to temp, it might be less than 6, but no more.

Ideally you’d use something like openTherm with a condensing and compensating boiler as this allows not just ON / OFF but also levels of heating which is more efficient with gas condensing boilers.

I’ve used ESPhome to control an electric towel rail using the platform: slow_pwm function with a defined cycle time, but don’t think that’s useful for gas wet systems where some form of PID control with lots of hysteresis and multiple zone valves would be better.

Given the likely complexity, would a “cheap” “smart” control be an option, with HASS just setting the temp set point when you want heating? That way, you leave the integration and gas efficiency to a specialist control (i.e. the level of “smart” kit that ScrewFix / Toolstation sells with just enough of an integration to get by).

I wonder how multiple low-cost Z-wave rad stats work across the same zone call for heat?

James

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