@SR79 

HI, I've been a UK HEVAC Designer for 40+ years, so my comments are assuming you are in the UK

I've also assumed you have a "normal" domestic system you are trying to control, so may not have some fo the devices we install in commercial systems, and I don't know the extent of your knowledge, so please don't be insulted if I'm preaching to someone with the knowledge - that's the H&S BS out the way

I started investigating intelligent house controls as I'm mainly dealing with social housing at the moment and one of my clients wanted to see if this would be an option for their new builds and refurbishments - heads up, it doesn't look like it at the moment

1. The majority of UK heating systems are sealed systems - if you use your mains cold water to set a cold pressure, yours is too
2. UK Domestic Sealed systems rarely have a heat/pressure bypass valve installed, they normally rely on a Pressure Relief Valve, so please do not install TRV's to all heat emitters, one radiator (in the same room as your room stat) should have wheelhead & lockshield valves to act as a heat dump when all TRV's switch off.
3. I only had one Kasa TRV head & hub installed, up until today, which has been running great for a month, I had my TRV's installed 30 years ago and we'd recently noticed that the bedroom was over heating at night, so this head was replaced with the Kasa one & set to 16°C, I have noticed that they seem to open depending on the difference in temperature between the actual temp and setpoint, therefore if the difference is small they only open slightly and the radiator appears lukewarm - is this your experience?
4. Radiators that are lukewarm can be a symptom of various problems

    a. An incorrect design, pipework under sized or an old pump not moving enough water

    b. The system is unbalanced because for whatever reason, someone has altered the setting of the lockshield valve - rule of thumb, normally they are only open 1.5 turns from closed, let the sytem run, any cold radiators, open the valve another half turn - rinse and repeat

    c. Sludge in the system, this normally is only in older systems

    d. Air in the radiator, have you bled the radiators?

    e. The boiler isn't firing due to the room thermostat holding the boiler off - your other post is relevant here, there isn't a Kasa Intelligent Boiler Controller yet - a major oversight, hope the developers are holding their heads down in shame

5. I am considering using either a Kase light switch (once they come back in stock on Amazon) or a Kasa Smart Plug - having read the manuals for both, which warn against this, I will still try them out, I believe the warnings are aimed overly Health & Safety orientated as most newer boilers I know of have volt free contacts on the thermostat connections and are therefore won't over stress the 4 or 13 amp contacts, once boiler control is available, we can go for TRV's on all radiators as the boiler will never fire unless at least one radiator is calling for heat

My last problem is can I program Alexa to switch the boiler on when any radiator valve is below the setpoint

  @Wayne-TP 

Hi, I didn't create schedules, until there is a boiler controller available schedules will only work if the boiler is firing, all mine are set at a constant temperature - lounge at 19°C all other rooms at 16°C - the lounge has the room stat in and is also set at 19°C and controls ±0.5°C, other rooms all maintain between 16-18°C as we never shut internal doors and heat rises

  @Wayne-TP 

That wasn't my post, that was the original posters

  @Wayne-TP 

To be honest, without TP having a boiler controller to go with these TRV Heads, they are pretty useless

We'd have to install a compensated circuit to our systems with a pump costing £130 a year to run, plus buy & install a 3-port mixing valve, controller, external temperature sensor & a by-pass valve so that the pump isn't running against a dead head if all TRV's are shut, so that the heating medium is available 24/7 - which all negates the cost savings from the TRV's