Basic method was the thermostat controls the motorised valve and the motorised valve in turn controls the boiler. However with a combi boiler you have anti cycle software and it is designed to run with thermostatic radiator valves (TRV) not a motorised valve.
There has been a move to using programmable TRV's where you replace the heads. But this can mess up the anti cycle software, the Evo system is designed to get around this problem so each TRV talks to a central programmer which in turns talks to the boiler.
It is not being a combi boiler which causes the problem it is the being condensating which causes the problem. With old boilers the output temperature was monitored and once the limit was reached the boiler switched off. But for a condensating boiler to work the return water temperature is also important, and unlikely the old on/off it varies the flame hight. So the hotter the return water the lower the flame hight, only when it can go no lower does it turn off. Each time it turns back on it tests the return water temperature, if it returns hot it switches off again and waits longer before a retry, and if it returns cold then it waits shorter on next retry.
Using a simple TRV is great, but with programmable types it depends on the system as a whole to if when time arrives it will reheat radiator or if due to boiler retry being extended it takes a long time before boiler fires up. This is were the Evo system is claimed to improve things, the central processor tells boiler to fire up NOW as the TRV head wants heat.
All this only happens if installed correctly, and it seems some installers are still installing with one radiator without a TRV and a thermostat instead in what they think will be coldest room, and also don't fit TRV to the towel rail, some boilers will still work A1 with this set up, others will not.
If the central heating is running 24/7 the TRV will ensure every room gets hot, however turn off the system and then the lock shield valve setting becomes very important, if all the hot water goes through the nearest radiator then hot water will be returned to boiler automatic turning it down, so instead of every room heating up together, the rooms heat up one at a time as as each TRV starts to work hot water is pushed to next easiest radiator, adjusting the lock shield valve allows the whole house to heat up together.
There has been some really good salesmanship, where systems designed for hot air systems are being sold to people having British hot water systems, although computers can learn and reduce the hysteresis the best way with hot water systems is to place the thermostat near to the radiator, the TRV is near to radiator so works well, the Nest and Hive systems do allow remote operation but were never designed for a wet system, the hot air system used in the Americas works very well with single thermostats, the British wet system does not.
I looked at the EVO system which seemed great, then I saw the price. As yet I am still using an old type boiler, my mother however has the new type and getting the temperature correct when one is not living there to bit by bit tweak the controls has not worked. Not tired the programmable TRV head without the central EVO home computer and would expect to find it varies from home to home, depending how open plan the house is.
