by ericmark »
Thu Dec 19, 2019 5:24 pm
With the modern boiler it uses the latent heat, to do this the return water has to be kept under a set temperature, so the boiler is designed to modulate that is turn up/down rather than on/off.
Two basic ways, special modulating thermostat that connects to boiler ebus, or more common thermostatic radiator valves (TRV) which by reducing flow in one radiator increases it in another until the by-pass valves starts to lift which as net result means the TRV controls boiler output.
On/off thermostats are really to stop boiler cycling in the summer, not to control room temperature.
So the setting of the TRV and lock shield valve other side of radiator becomes important, in the main lock shield valve control speed of radiator heat up, TRV controls how much it heats up.
Most homes we don't want all rooms at the same temperature, so living room maybe 19 C in the day 21 C in evening and 16 C over night, not off as takes too much to re-heat. The bedroom likely a different schedule.
The electronic TRV can do this for us, starting at under £10 the eQ-3 and terrier i30 come as basic programmable units, for £5 extra the eQ-3 can be bluetooth so can link to one phone, also can be used as a group when more that one radiator in a room.
These are still basic units, so you have to build a schedule for wall thermostat and TRV head that matches. Systems like Evohome do the matching for you, but the TRV head costs around £50 each.
I use a mixture of both, so rooms occupied as I return home automatically turn on as my phone reaches the set radius, (geofencing) but bedroom use cheap TRV heads.
Clearly the TRV head can't warm room if boiler is not running, so wall thermostat program has to be set so boiler is running.
Wall thermostat call Hive uses a "call for heat" command, so if a Hive TRV needs heat it can switch on the wall thermostat even if the room the thermostat is in does not need heat.
Nest however has built in algorithms which predict time needed so allow the boiler to have a longer burn, and also detects when people are at home.
Every time the boiler turns off, any heat in the boiler goes out of the flue, so you want boiler to turn down first before it starts to cycle.
If boiler is OpenTherm enabled then Nest will link to this, as will EvoHome and Tado, but Hive will not work with OpenTherm.
As to which is best depends on your home, and your live style. There is no best, Nest was before Google took over set to run with MiHome Energenie TRV heads, I got the heads first, did not add Nest until I moved house, idea was slowly add bit by bit, EvoHome is not designed to add bit by bit, but likely the best, Hive is designed so you can add bit by bit, but likely worst of the smart systems and by time you have bought the TRV heads likely the most expensive.
Since TRV heads with many need to match the wall thermostat some careful thought is required before starting, how far to go.
Hind sight wish had got all bluetooth eQ-3 and Nest as hardly ever adjust the Mihome TRV except when at home, and now non link to Nest.
Reason for Nest is boiler in granny flat under house and only two wires between the two, and Nest allows me do set DHW and CH plus charge the thermostat, and send all other info with just two wires. So no need to lift floor boards. It suits me, that does no mean it will suit you.
From what you say, I think eQ-3 TRV heads would suit you, all the blue tooth does is save you bending down, and allows pairing if more than one radiator in a room, at 68 don't like bending down.
But either model walk into a room and press one button swaps from Eco temperature to Comfort temperature which you can set what it is. It has open window detect, so open kitchen door and radiator turned off automatic while you unload car then auto back on again. And temp shown degs C not *123456 as on mechanical TRV heads.
