digital thermostats for central heating

[Roger Mills]

Many - but not all - have that feature. But it can usually be turned off if you don't want it. This feature is turned off on all of my stats for reasons recently discussed in another thread.

[fred]

If that's what you're looking for then honeywell digital ones are what you want, they use proportional control (no choice) and can maintain temperatures very close to the demand (in the room with the stat).

In my view however they are too b'ldy clever by half and cause to much boiler and control cycling. Usually the stat feeds a motorised valve that in turn causes the boiler to fire when it is fully engaged. That means that every time you cycle the stat you cycle the valve motor causing it to wear out prematurely. Honeywell stats cycle 6 times an hour by default (programmable 3 - 12) so you could wear out a valve in a year or two (my multizone system has 8 so you'll see why I'm concerned).

I have one of their CM/CT907 wired stats here that I will be swapping out for something else soon for the reasons above but I have installed the wireless CM/CT927 at 2 locations with unzoned systems where the owner is perfectly happy with them. Note though at both of these locations I have had wireless link failures which required re-bonding of stat to receiver following boiler power disconnections. I reported these failures here but no-one appeared to have had a similar experience.

My suggestion would be a digital stat that has lower hysteresis or a programmable option to switch between proportional and bang-bang control. No model numbers to hand I'm afraid, cheaper digitals are generally bang-bang. There is a mid-ranged one that is propo/switched selected by jumper, if you're keen I'll see if I can track it down again.

Others' cautions about having the stat in a room with an auxiliary heat source are valid, sometimes controlling the hall with TRVs controlling the room temps is the right way to do it.

[charles]

not heating them at all could be false economy

[Michael Chare]

I have had a Danfoss TP9 for some years. It is a two channel programmer and lives near my boiler. It uses a remote wired sensor in the sitting room. The temperature control is much better than the previous Honeywell mechanical thermostat which used the same wiring.

The other big advantage of programmable thermostats is that you can vary the temperature throughout the day.

[Dave Liquorice]

No problems with the Danfoss TP75 or TP7000, I have both in use and both are RF versions. Power glitches haven't caused any required for either of them to be paired with their respective receivers again. And the older one should work with the new RX and vice versa though I haven't actually tested that.

The TP7000 has a DIL switch on the back to select:

5/2 or 7 day mode. On/off of the Optimum Start Control (start the boiler at the appropiate time to hit the first set temp on time) On/off of the proportional control. 6 or 3 periods for the proportional control.

The instructions warn against using proportional control on a system with motorised valves.

[Alan]

In message , Graham. wrote

My Towerstat RF has a switch setting to give a hysteresis of 1, 2 , 3 or 4 degrees C.

[Martin]

We don't heat the hall or the bedrooms unless it is very very cold.

[Martin]

Why?

[fred]

Thanks for the reminder, yes, that was the one I had in mind but pretty they aint. Good to see the propo warning in the instructions (I had missed that).

Honeywell are the prettiest but it's a shame that none have the option to turn off propo. Maybe it's to support their replacement valve market.

I decided to look again and the Siemens Rev13, 17 and 24 have it switchable too and have tight temp accuracy but prices are variable. Touchscreen model Rev100, 200 too but overpriced IMV.