by ericmark »
Fri Dec 14, 2018 4:06 am
I had a look at some installation instructions to see how the thermostat is connected to boiler, one showed no wiring to thermostat, the other simple on/off wiring, since I found two different instruction sets, you need to find out for your boiler if it can support a thermostat and which type.
There are two completely different types of thermostats for modulating boilers, those which simply turn boiler on/off, and those which control flame height in the boiler, the latter are far better.
However step one is what do you want the thermostat to do? May seem an odd question, but with an open plan house the thermostat controls the house temperature, but with a more conventional house with doors on the rooms, the thermostats job is to stop the boiler cycling on/off during warm days. Room temperature is controlled by thermostatic radiator valves (TRV) and the boiler monitors the return water temperature and modulates output to suit. (modulate is fancy word for turning down flame height)
The wall thermostat should be in a room kept cool, with no outside door, and no alternative heating on the ground floor, and should not have anti-hysteresis software, as whole idea if to stop boiler cycling on/off not increase the cycling, unfortunately wireless thermostats without anti-hysteresis software also don't have fail safe so really it needs to be hard wired, however what we should do and what we actually do is not the same, and I have a wireless thermostat in the hall and a eTRV in the hall as well which all goes against all the advice given in books.
So most houses simply don't have that cool room, down stairs without any door to outside or alternative heating, so there are a number of ways around the problem.
In my case the eTRV and the wall thermostat are set to same temperature, so when front door is opened and hall cools down, the eTRV opens and starts to reheat the hall, however before it reaches 18 degs C the set temperature it starts to close, slowing down the recovery after the 16 deg C point so the rest of house can reach temperature before the wall thermostat turns off the heating.
So the eTRV in living room keeps that room at 20 deg C, but hall is kept at 18 deg C all works A1 until some one leaves the door living room to hall open.
If I paid more money and fitted EvoHome, then each eTRV reports the temperature to the central thermostat and if any room is below the set mark, central heating runs, so leaving an internal door open would not matter.
Better than what I have, but not as good as EvoHome is Nest, with Nest the wall thermostat follows the eTRV so they are both same temperature.
But point is the TRV controls room temperature not the wall thermostat.
What you need to decide is how much your willing to pay? I would not recommend my level, in hind sight I paid more than I needed, I have 4 eTRV fitted the rest are standard wax type, around £200 I have paid, I could have fitted the terrier i30 for a lot less money, the difference is with mine I can alter temperature with phone, tablet or PC, with the i30 I would have to do all the setting on the valve its self.
So to gain better control swapping the old wax heads on the TRV to electronic works, no wiring just unscrew old head and screw on new, once trimmed in mine keep rooms to within 1 degree of setting, the big thing is with mine (Energenie MiHome) there are two sensors in each head, one measures air temperature, the other water temperature, the latter connects the air temperature reading to compensate for thermostat being so close to radiator.
So instead of guessing 3.5 on dial will get room to about 20 degs C I actually set it to 20 degs C so over night living room set to 16 degs C and in day to 20 degs C, there is one in living room, one in hall, one in my bedroom and one in wife's bedroom other rooms still use old wax type. I had intended to fit Nest which will work with my electronic heads, but have found it works near enough with original thermostat so have not bothered with Nest.
Yes I can set it so as I near home the heating turns up, (geofencing) but I find switching off heating the house does not cool that quickly, so I have never bothered.
Allowing the boiler to modulate works far better than switching it off/on so since you it seems don't have a boiler OpenTherm enabled or other control direct to bus so boiler can be electrically modulated, then you want electronic heads on the TRV's and control boiler by the return water temperature.
The problem with controlling boiler with return water temperature is in the summer unless you manually turn it off, it will fire up to test if heat is required, so a thermostat in a room kept cool can turn off boiler in the summer to stop is cycling, or of course simply manually switch it off.
