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PostPosted: Tue Feb 06, 2018 5:31 pm 
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Hello - Im new to the forum.
I have a Combi Boiler which is about 17 years old. I just replaced the wired thermostat with a Honeywell THR840duk, and its behaving a little strange. The target temperature was set at 19.0, with the room temperature also 19.0, with heating ON. I increased the target to 20.0, and when the display returned to room temperature (19.0), the heating switched off ??? ECO is turned off, so it isn't that.
I had to increase the target to 20.5 to get it to switch on the heating again, with the display returning a room temperature of 19.5.
There are two wires going to the boiler: red connected to A on the stat, and black connected to B.
Is this abnormal behaviour or am I missing something ?

Paul based in the UK.

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PostPosted: Tue Feb 06, 2018 5:33 pm 
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:welcomeuhm: best asked in the CH section. I'll move it. Ah Mod 7 beat me to it.

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PostPosted: Wed Feb 07, 2018 3:22 am 
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The Honeywell THR840duk simply switches on/off, it is not a modulating thermostat so you have to expect the temperature to vary slightly even with some anti hysteresis software it is only switching the boiler on/off.

To keep a room at a constant temperature the air has to circulate, so with a myson radiator (fan assisted) by varying the fan speed you can get a very well controlled room, going down in price you can fit electronic heads on the TRV which both measure the air and water temperature and can compensate for the temperature of the water in the radiator and as part of a boiler modulating system like EvoHome can again keep a room within a degree or so of set temperature, however without a circulating fan I can read the temperature in the two down stairs rooms and there is often 2 degrees C difference between two thermostats placed in different parts of the room, in the main the eTRV does hold the room to around 1 degree of set temperature while boiler is running.

But wall thermostats are not normally designed to maintain room temperatures, they are to turn the boiler off once no longer required, the TRV controls the room temperature, the thermostat controls whole system, OK there are exceptions EvoHome thermostat is not really a thermostat it is more of a hub, it collects information from each eTRV and modulates the boiler to ensure the temperature is maintained in all enabled rooms.

Thermostats like Nest will also modulate the boiler, that is adjust flame height, and using IFTTT they can be linked to eTRV's with a follow command.

But with a non modulating thermostat the best it can do is use a mark/space ratio to reduce boiler output as it approaches the set temperature, this means it keeps switching boiler on/off at it get to within 2 degrees of set temperature and slowly increases the off time.

I am not sure if a Myson ivector radiator will actually save money, even when it clearly keeps the room spot on temperature set, it depends on the room, even with double glassed windows put a thermometer on windowsill and likely it is cooler than centre of the room, so moving the air means warm air is kept next to window, so more heat is lost.

My first house had hot air central heating, the whole house was maintained at the same temperature with a single thermostat, but with single glassed windows so much heat was lost it cost a fortune to run.

EvoHome is a nice compromise, not as good as fan assisted radiators and building management, but also a lot cheaper, however except for babies and old people we don't really need such a good control as even EvoHome will do, so terrier i-temp electronic heads down stairs in main rooms, standard TRV upstairs in bedrooms, and a couple of wall thermostats to tell boiler when to run is likely all we need.

So maximum temperature is controlled by the TRV, the wall thermostats ensure the boiler runs when the TRV's are likely calling for heat, idea is the wall thermostat is fitted in a down stairs room with no outside door and no alternative form of heating on an inside wall opposite to the wall the radiators is on.

Problem with both my house and mothers house there is no such room. My house is open plan so single wired programmable thermostat next to arch between dinning and living room. Mothers house two thermostats one in kitchen on side of oven, programmable so only active in morning when house heating up, and one in hall not too close to the radiator which also has a TVR to slow hall heating up so other rooms can get warm, neither set up is spot on, but both work well enough for my requirements, mothers house with doors on the rooms has two electronic TRV heads in the two main down stairs rooms.

So it is a compromise, or to put it another way, I am too much of a skin flint to pay out for EvoHome.

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PostPosted: Thu Feb 08, 2018 5:46 pm 
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Thanks for your reply.
As a novice with this stuff, most of it was over my head I'm
afraid. As you don't mention the wiring to the combi, I
trust that means I've done it correctly, although I did
find the diagrams a bit confusing.
I've never had anything other than a wired wall stat that
switches the heating on/off based on the temperature where
the stat is located.
My point was that the stat doesn't always respond to the
temperatures correctly. For example:
1. Target Temp(TT) = 19.0 C, Room Temp = 19.0 C,
Heating On.
2. Increase TT to 20.0 C, RT = 19.0 C, Heating turns off??
3. Increase TT to 20.5 C, RT = 19.5 C,
Heating turns on again
However, the stat does appear to be working, so I will
see how it goes.

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