DIY Doctor

Drayton DT20rF Receiver and Wireless Thermostat Advice

Postby samtcb » Sat Jan 18, 2020 12:05 pm

Hello,

Could do with some advice, please before I pick up a sledgehammer ;)

The boiler is Worcester Greenstar 28i Junior and I have a Drayton DT20rF receiver and wireless thermostat. I can't work out of the thermostat is on the blink. ....

The pairing, signal and placement are fine - green light and four arrows.

When the thermostat is set to 20c, it calls and the boiler answers. It calls until the thermostat reads 20c sometimes 20.5c and then stops calling and the boiler stops firing. Perfect. But then a short while later the thermo (still on 20.5) calls again and the boiler responds. Feels like the boiler is always firing every five minutes! Never goes to 21c

Is it just maintaining the temperature at around 20c? Old victorian house with bare floorboards e.g draughty and not great I suspect at holding its temperature.

The boiler never burns unless the thermostat calls or the hot tap is switched on, so I suspect the receiver is ok?

The other issue is sometimes when the thermostat is not calling it sounds like the boiler is burning, but when I check the green heating light, it isn't. It just sounds like it's running but not burning. Why would this be happening?

Can't work out whether to replace the thermostat? Replace the receiver and the thermostat or whether everything is fine....

Any advice greatly received.

Sam
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Postby ericmark » Mon Jan 20, 2020 5:01 am

I will guess it is anti hysteresis software? With some thermostats as it approaches the target temperature it starts to use a mark/space ratio to reduce the heat, to stop it over shooting, come cleaver thermostats work out how long of a burn is required, my Nest does that, so it switches off before the target is reached, knowing from last time how much heat is stored in the radiator.

But the cheaper thermostats simply start switching off/on to stop the over shoot.

Only very cheap thermostats have an off and on temperature, and I found this a problem, as for the modulating boiler to work efficiently the mark/space thermostat messes up the boilers own algorithms, but if you get a really cheap one without the mark/space then they don't tend to fail safe, if something stops the signal the boiler can fail to switch off.

In theory the TRV controls room temperature, the wall thermostat is only there to stop boiler cycling on warm days, in practice having *123456 on a TRV is about as much good as a chocolate fire guard, don't know about you, but I want to set the TRV to 20°C not some random number between 3 and 4?

I have eQ-3 electronic TRV heads fitted, in my case bluetooth versions, but non bluetooth start at around £10, I paid £15, and I can set the temperature to an Eco (I used 18°C) and comfort (I use 20°C) and I set the times for each, and if I use the room before set to change to comfort one button changes it, you can set the eco and comfort to what you want, and also a window open function, so in my kitchen it auto turns off when I have door open unloading car.

There is also the Terrier i30 which does same job. I still have a Nest wall thermostat, but only reason I have Nest is I want to control central heating and domestic hot water with just two wires, and Nest Gen 3 will allow me to do that, it was not got to use the geofencing or occupancy detection, although now I have it, I use it.

Last house I got 4 Energenie MiHome heads, rather expensive around £40 each, I brought them to this house, and I can alter them while at work, but in real terms they are no better than cheap eQ-3 of which I have 5.

With some boilers you can use analogue thermostats, far better than digital, but not with Bosch, the Bosch however works well with electronic TRV heads, with gas boiler in last house it auto modulated and there was little or no hysteresis, this house is oil, so does not modulate so there is a slight hysteresis, but within 1°C of target.
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