Central heating controller calling for heat when it shouldn't

[james74]

I have a Danfoss TP9000 heating & hot water controller which is behaving oddly, I'm hoping someone here can tell me whether it's faulty or not.

When the heating is set to come on according to the timer, the controller does what it should and activates the boiler. The house warms up to the target temperature, and then the controller shuts the boiler down again. All good so far. But then roughly every two to seven minutes, while the house is at the right temperature, the controller will switch the boiler on again for about ten seconds, just long enough for the boiler to fire up, then shut it off. All the while, the room temperature is the same as the target temperature, so I don't think there should be any need for the controller to be calling for heat. To my mind it's just wasting gas and putting the boiler through unnecessary ignition cycles, but I'm not a professional and I've never had this kind of controller before, so maybe it's a feature.

I've taken a video of it, which is here: https://drive.google.com/open?id=1--n_vkLVe6qBH1VjZ9mwFD3p1LhiXMqX

There's a whole lot of nothing happening at the start of the video but if you scroll forward to about 04:45, you'll see the following:

Throughout: target temperature (small number) is the same as actual temperature (big number), so as far as I can tell there should be no need for the controller to call for heat.

04:53: Controller calls for heat
05:20: Boiler can be heard firing up
05:24: Controller shuts the boiler off

It'll carry on doing this every few minutes for the whole time that the heating is set to be on.

The house was built in 2011 and the heating system is original to the house. The boiler is a Potterton Promax 15 SL and the room temperature sensor is a Danfoss TS2.

Any advice gratefully received.

 

[Alec_t]

Could be a faulty room-stat, dithering.

 

[james74]

Could be a faulty room-stat, dithering.

I'd ruled out the room stat because if the room stat were faulty I'd expect to see the figure for actual temperature change on the display, whereas it stays constant at the same value as target temperature. That's why I suspect the controller rather than the stat.

 

[ianmcd]

You have a faulty room stat , try new Good quality batteries, if it persists new stat time

 

[james74]

You have a faulty room stat , try new Good quality batteries, if it persists new stat time

The room thermostat is hard wired and is just a little sensor on the wall in the lounge. Setting the target temperature etc all happens at the controller, which is in the kitchen under the boiler. Pics below.

 

[dilalio]

Check the resistance of the sensor.
Should be 10KΩ±2% @ 25ºC

Or call Danfoss tech.

 

[Mottie]

My Honeywell DT90E room stat fires the boiler up roughly every 10 mins for a minute or two when the stat is showing that the room target temperature has been reached. It is designed to do that to maintain the room temperature. It can be altered in the settings if required but I've left it on the default settings as it keeps my living room within .5 of a degree at all times.

 

[D_Hailsham]

When the heating is set to come on according to the timer, the controller does what it should and activates the boiler. The house warms up to the target temperature, and then the controller shuts the boiler down again. All good so far. But then roughly every two to seven minutes, while the house is at the right temperature, the controller will switch the boiler on again for about ten seconds, just long enough for the boiler to fire up, then shut it off. All the while, the room temperature is the same as the target temperature, so I don't think there should be any need for the controller to be calling for heat. To my mind it's just wasting gas and putting the boiler through unnecessary ignition cycles, but I'm not a professional and I've never had this kind of controller before, so maybe it's a feature.

This could be the TPI (Danfoss call it 'Chrono-proportional') control algorithm working. If you have the Installation Manual, it's options 34 and 35 in the Installer Advanced Programming mode. The purpose of the algorithm is to prevent overshoot (temperature continuing to rise after target has been reached) and maintaining the temperature within close limits once it has been reached.

 

[james74]

This could be the TPI (Danfoss call it 'Chrono-proportional') control algorithm working. If you have the Installation Manual, it's options 34 and 35 in the Installer Advanced Programming mode. The purpose of the algorithm is to prevent overshoot (temperature continuing to rise after target has been reached) and maintaining the temperature within close limits once it has been reached.

I've just found this in the installation manual. There's a long thread on it here:

https://www.diynot.com/diy/threads/what-exactly-is-chrono-proportional.35622/

It could certainly explain the behaviour I've been seeing. I've never come across or even heard of this function before which may be why it seemed like a fault.

I'm going to try turning TPI off and seeing if that makes the controller behave more like I'd expect. If it does, then I'll be satisfied that the controller was working normally the whole time and I'll put it back to the factory default settings.