Thermostat constantly switches boiler on and off

bitec

Hi.

Installed the tado thermostat yesterday, while I like the app and smart scheduling - I just don't understand the way the thermostat alhorithms work.

I have a Daikin heat pump with two zones. Temu is installed into one zone.

I set the desired temperature in the zone to 21 and the heat pump worked all the time until the temperature got to 21 - then it didn't stop working.

I decreased the target temperature to 20.7, then 20.6. The thermostat still kept the valve switched on though the measured temperature was 20.9. I decreased the target temperature to 20.5. Now it eventually switched the valve off, but then in 15 minutes switched it on (the room temperature is still 20.9) - then after 10 minutes - off. The outdoor unit just constantly starts and stops and even doesn't have time to warm the radiators properly - they stay cool. And it's not clear why the zone valve is toggled - the room temperature hasn't changed!

I just don't understand the thermostat algorithms at all. If the measured temperature is higher than the target - why does it switch the valve on? Why then it switches it off not letting the radiators heat? This is a waste of energy, the previous thermostat (Heatmiser) didn't have such a nice app and degrees precision but it switched the radiators for a long time and then kept switched off for a long time as well. Constant toggling of the valve (when the temperature doesn't change!) doesn't make sense to me at all.

I'm really looking for help as so far I intend to return the product back and write a negative review on Amazon

policywonk

Did you implement the Tado controls as a digital system, or as a relay system? It appears ro be the former. The previous thermostat was a simple on off switch.

Is the second zone the HW zone or yet another CH zone? What controls that?

It looks like your system is now running in opentherm or similarly controlled mode and the actual challenge is one of getting the right flow temperature though the rads. Can you confirm you set the Tado controls as digital?

bitec

Hi, thank you for your reply, it's a relay system, as far as I understand

The heat pump is Daikin EHVH08S26CB9W

Second zone is heating as well but I didn't replace the thermostat there yet, it's Heatmiser v3

My understanding is that (sorry I could use the wrong terminology as I'm not an expert) that the wiring and configuration supports only the on/off mode for both zones, the heat pump has it's own "weather dependant" configuration but thermostat may switch this off if the temperature reaches some point.

What I see in practice is that the thermostat keeps the valve "on" if the target temperature is much higher than real but starts toggling the valve if the temperature is very near (maybe dropped 0.1-0.2 degrees). AFAIK this is not a great behavior for heat pump which is expected to work continuously and support the desired output water temperature instead of switching on and off each 10 minutes (which is ok for gas boilers I guess?)

The behavior that seems more correct to me is for the thermostat to stop the heating once the temperature is reached, then switch it on for long period again once the temperature drops more significantly than 0.1 degrees (0.5 at least I believe)?

I've configured the heat pump model in the care&protect section and thought this could affect the thermostat behavior but it didn't.

bitec

what was even more strange to me was that the "toggle" happened literally every 10 minutes even before the real temperature has changed.

For example: the target temperature is set to 20.7, the real temperature dropped to 20.6, the thermostat switches on the heat pump for 10 minutes, then switches off for 10-15 minutes. The radiators are cold as the heat pump didn't have time to heat the water and the real temperature in the room didn't change - so why did the thermostat switch it off? Such "high sensitivity" doesn't make sense to me, I'd expect the long pauses for "on" mode