by ericmark »
Wed Oct 02, 2019 6:01 pm
Thermostats that are hard wired are cheap, for around £35 you can get a programmable type, but wireless the price goes up, with wireless if there is not fail safe a simple flat battery can mean it will not switch off, some do have a fail safe so if it goes wrong it always fails off, but looking at around £100.
Depends on boiler, house design and many other factors, but in general there are two types of wall thermostat, off/on or modulating, the latter is clearly better, but depends on boiler.
With the off/on thermostat with an oil boiler they can have anti-hysteresis software, what it does is start switching off/on before the target temperature to stop it over shooting. But with a modulating gas boiler this wastes energy as every time it switches off it upsets the boilers built in sensors that turn down boiler as the TRV close.
More expensive thermostats remember how long it took last time to heat up room and turn off before target temperature so it will reach temperature without turning off/on.
The design of the house can mean the TRV will control temperature well and all the wall thermostat does is turn off heating as summer comes.
So if you fit the likes of EvoHome or Tado the TRV heads sense the temperature in each room and tell the thermostat which in turn tells boiler what to do, and if boiler has opentherm it turns the boiler down rather than off so house temperature is really well controlled.
Nest has OpenTherm and will link to Energenie TRV heads, but wall thermostat tells the heads what to do, rather than the TRV heads telling the thermostat, but it does work out in advance what heat is required so even without OpenTherm works well.
Hive does not have OpenTherm but with hive the TRV head tells the wall thermostat to keep boiler running.
Now you could use a £35 wall thermostat and some £10 programmable TRV's carefully set to temperatures that may do the same thing, but it's not as automated and needs some skill setting it all up.
The main point is of course the TRV sets room temperature and the wall thermostat stops boiler cycling when there is no demand.
With an open plan house you may not need TRV's down stairs the single thermostat may do the job, but fit doors and the TRV becomes the main control.
So set one is look at the boiler, I would in google enter name of boiler followed by "installation instructions, then once you have a copy look for opentherm or ebus and see if the boiler will work with high end thermostats, if not look for the output, if output is 24 kW then off/on thermostat is fine, but if output 8 kW to 28 kW then that means the boiler modulates, if so then it controls output by measuring return water temperature, so main control is with TRV's not wall thermostat, all the wall thermostat does is turn it off in summer.
Also some boilers use a 24 volt control and other 230 volt control, Hive duel control will not work with 24 volt control, Nest will, so you need to know. Does the control need to control the domestic hot water? if not then Hive single channel is volt free.
Some boilers like Bosch do their own system, think called wave, so may be limited to what will work, in some cases depends on how old the boiler is.
Even radiator types matter, most have TRV (thermostatic radiator valve) but some are fan assisted so turn fan up/down, off/on to control output.
I fitted Nest not because I wanted to control with phone, but only two wires go to boiler, and Nest only needs two wires. Many wireless thermostats need a neutral where there is no neutral then Nest e is one of the few wireless thermostats that is battery operated both sender and receiver.
