@HDreturns 

PLease would you explain the smart plug method.  From what I ahve understood, the plug would be triggered to 'on' when the temperature sensor hits a pre-defined threshold.  Conversely, when the sensor is out of the range then the plug will be 'off'.  and here's the issue - when the plug is off, the boiler is COMPLETELY off whereas the requirement is to trigger the boiler to 'fire'.  PLease would you explain further how the boiler can be triggered when a signal is received from any of the TRVs.  

Thank you

  @HDreturns 

The only way I've found to trigger the central heating switch on the boiler is to use a zigbee switch which unfortunately currently means you cannot make a rule that your boiler fires only when a trv valve is open. I agree that this is a painful omission by Tapo from the range and would hope that they include a similar simple device in the future. 

The only way to get it close to working is to include a zigbee thermometer in each room and create a matching rule for the boiler to fire as the valve in each room has. It means duplicating the system and hoping the thermometers have the same calibration. 

Fortunately zigbee items are much cheaper. Unfortunately you get what you pay for and tapo are much better quality when they exist. 

Luckily, no. You can add these valves into Home Assistant with Matter (currently broken in my opinion) or unofficial integration. Then it is relatively simple to automate boiler on/off with any HA-compatible NO/COM smart switch (no voltage, just connecting the wires) based on valves' heat demand status. Works quite well.

  @Trobalabau 

Please share how you did this.. I am tearing out lumps of hair with frustration at Kasa !! Great device but yo can't integrate it into 'Smart' Even Google Home Scripting has a frustrating feature whereby you can only trigger an action based of the temperature from the TRV being >,< = etc etc and exact value e.g. 25C, Why they wouldn't add the functionality to say "If room tem < set temp then Start boiler" How hard could it be?

  @richrobuk 

I use Home Assistant running on cheap but low-power x86 hardware, in my case HP T530 terminal I bought for about 30 euro (4GB RAM, 32GB SSD). Better choice than Raspberry.

TP-Link integration is available in the main repository, with three possible statuses of the valves: "OFF" (gray in the app), "ON (idle)" when valves are fully closed with no heat demand, and "ON (heating)" when they're really open (at least partially). This integration reflects the reality accurately. Via Matter, idle/heating status is missing and TP-Link ignores my requests to investigate this bug: https://community.tp-link.com/en/smart-home/forum/topic/706792

My gas boiler turns on when two control wires (with no voltage!) are connected and turns off when they're not, so I purchased a simple NO/COM switch based on ESP chip with Wi-Fi - in this case with Sonoff firmware but it can be Tasmota or something else. It replaced the old thermostat device which used the same principle. Remember it must not apply any voltage to the control wires!

OP's idea to use smart plug is wrong. You should not cut off electric power from the boiler as the startup checks would take at least a few minutes and it is probably not healthy for the device.

The automation rule runs every minute. It checks if one (or more) of the TRV's are in "ON (heating)" HVAC status i.e. with temperature lower than target and therefore heat demand. If yes, the boiler switch turns on. If no, it turns off. There are additional conditions like outside temperature as well as some delay after switching to prevent sudden changes. There are other ways to implement this, probably better.

In terms of heating schedule you may use one in Tapo app, or set up something in Home Assistant. For example "Scheduler Card" (+ Component) available in HACS repository.

In addition, using Tapo T310/T315 thermometers is a good idea because they can directly replace the TRV's internal sensor value right on the hub level which is invisible for external services (they read it as internal sensor). It makes automation and scheduling much easier in HA if you don't need to include external temperature sensors and can use only TRV values.

  @Trobalabau I think i have figured this out I sacrificed a smart plug and tailored the output to operate a N/O relay. The relay is put across the boiler thermostat terminals (no voltage). When the plug is powered it will close the relay and turn the boiler on.  The temperature is controlled by a smart thermostat routine powering the plug.  This will all be packaged in a proper electrical enclosure.