Drayton Digistat RF programmer controller

[RichD1]

After about 12 years my old Digistat RF programmer decided to go awol. So I bought the new version as a replacement.

We only tend to use it in manual mode so we can take it to the area we are sitting in. At night we turn it down to about 17c and leave in the hallway.

Everything seemed to be fine with it until one day I came down first thing in the morning. The rads appeared warm but not hot so obviously had it had turned on in the night. Whilst in our utility room where the boiler is located it turned on and then about a minute later it switched off. About five minutes later it switched on again. This time it stayed on longer.

Since then I have monitored it regularly and have noticed that it will switch off even though the setting is at 18c and the display is only showing 17.5c

The new Digistat appears to have no hysteresis built in as is usual with bi-metallic thermal switches. This switching on and off must be extremely wasteful on gas and the radiators don’t get hot enough to heat the house. The old version would switch on and stay on for a good 15 minutes or so.

Do you think the controller is faulty? If it is working as it should is there any thing I can replace it with that will work as I expect.

I recently about a fan electronic controller that you can set the on temperature and also set the off temperature. Are there any CH wireless programmers like this on the market?

 

[RichD1]

Been watching it this morning and with the thermostat set at 18c and the display showing 18c it has been switching on for about 20 secs before switching off. Sometimes the boiler hasn’t even got through its start-up routine. Flicks on about every 10 mins or so.

 

[ericmark]

It seems the term "Digistat RF programmer" covers a series of devices, the instructions so in general there are a few pointers.
1) I had a wireless thermostat loose sensitivity and it progressively needed to be closer to the base unit.
2) Some boiler generate RF interference when they run, and so the receiver unit is often mounted some distance from boiler, meter or so.
3) There are a number of anti hysteresis methods, as you point out the resistor but that is only used with old non electronic hard wired, the other is to switch boiler off/on using a mark/space ratio as it approaches target temperature, as you point out, not a good method as each time boiler is turned off, any heat in boiler goes out of the flue, the other is to predict the over shoot and stop the boiler early, allowing for heat stored in radiators, but change if door left open or closed upsets the system, so the only way to get it spot on is to modulate the output rather than switch off/on.

So the cheapest most basic modulating controller is the thermostatic radiator valve (TRV) and if you google eQ-3 you can find programmable heads for under £10, as to if you want cheapest is another story, but basic theory with a modulating boiler that modulates due to return water temperature there are two methods, one a radiator with a car type matrix which controls output by varying fan speed like the Myson iVector which is rather expensive, or a series of TRV heads and a by-pass valve, so with the latter as the TRV closes it first forces more water through those still open and then the by-pass lifts so the return water gets hotter and the boiler turns down the output until at minimum then it uses a mark/space ratio to further reduce output.

There is a problem, using return water only to control boiler, it would cycle on/off all summer, so a wall thermostat is used to turn off boiler on warm days.

There are better systems which connect to the boiler ebus, these can ensure the water temperature reduces as the output falls so when it starts the mark/space there is less energy in the boiler to escape through the flue, however some boilers have very good inbuilt algorithms that do this anyway.

Since all boilers not made equal, saying which is best system is impossible. So as users we have to decide how far to go with control, I have both very cheap eQ-3 TRV heads and more expensive MiHome Energenie TRV heads, and I can't with hand on heart say which is best. With the more expensive in theory although not got it working yet, I can sit watching TV and say "Hey Google turn living room heating to 21°C" but at the moment get answer "I don't know how to do that yet." like having children again. I can change temperature with my phone, tablet, or PC, with more expensive TRV heads anywhere in world, with cheaper ones if in bluetooth range. (Note eQ-3 can only be paired to one device when using bluetooth version).

So the big question is which TRV head will suit you? I feel I made a mistake, when I bought MiHome Energenie TRV heads they were claimed to work with Nest, but today Nest has withdrawn some of the support. I like the idea of Hive, where the TRV head sends the wall thermostat a "demand for heat" so thermostat runs for ½ hour. Evohome lets you view each TRV from the main thermostat (hub) and sounds very good. If you get whole package the EvoHome is cheaper than Hive, where Hive gains is you can add a bit at a time.

If you head is buzzing, well then same as mine, there is no one best system, and I wonder if the blindfold and pin is not best way to select. Everyone says their system is best, however it does seem the TRV is the main control, and at £10 each maybe the cheap eQ-3 is the way to go, still would cost me another £50 to add the last 5, I have 5 already plus 4 MiHome Energenie TRV heads with 15 radiators one does not count as in bathroom and connected to DHW.

There is a product that tries to turn a standard radiator into a fan assisted radiator, I tried as an experiment to put an axial fan next to a radiator and a thermostat (was really to work fridge for brewing beer) to switch it on when pipes got hot, it was a failure as it cooled radiator too quick.

However it did raise the question of how, why, and when to circulate air. Old house no double glazing don't want air circulating as it will heat cold windows wasting energy, but well insulated house and if air circulated then any thermostat will better control the rooms, so ideal is radiator on outside wall, and thermostat opposite on inside wall. Reverse it and the cold wall affects thermostat, so found by trial and error about 3 foot from radiator on tea trolley worked best, this is nearly where the TRV is placed, the problem is they are slow acting so lock shield valve setting is important specially if the TRV is on the return pipe.

 

[RichD1]

Wow, a lot of info there, thanks very much. It’s going to take me a while to digest it all.

I have just done some reading up on the Honeywell EvoHome and the Drayton Wiser system. Both offer programmable TRVs. Both systems appear to have a minimum time on for the boiler, typically 5 minutes so that would be a good start.