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Nest TrueRadiant algorithms and low-temperature heating

Community Member

Hello, i've been using my Nest Gen 3 learning thermostat EU version for a few years now. I've come to a few conclusions that I want to share with this community, maybe they are valuable to someone.

The Nest TrueRadiant algorithm seems to be developed with floorheating in mind. Over here in the Netherlands, regular iron water-based radiators are much more common. These require a certain minimal water-temperature to radiate their heat into the surroundings. The TrueRadiant algorithm though, seems to go as low as 30 degrees Celsius for the last section of it's heating schedule.

This means that with a typical radiator setup you will never get your room to the right temperature, and either the Nest will keep on staying in heating mode, or your central-heating boiler will go into a stop/start cycle, which is not good for energy-consumption and wear. 

It doesn't matter what kind of "lead temperature" you configure in the Nest Pro settings, this is only the initial temperature that the TrueRadiant is using. The only work-around for this is, is to configure your boiler to ignore the lowest temperature request from the Nest OpenTherm (TrueRadiant) commands, and overrule them with a higher temperature that will achieve proper heat-transfer into your room(s). Not all boilers are able to do this though. So a much better solution would be to have this a configurable setting in the Nest settings itself. 

Another interesting thing i've encountered: now with the increasing gas-prices there is a big push for consuming less gas by heating at lower temperatures for longer periods, "go low & slow". If you do this, with TrueRadiant, you get into the same issues as described above: either the room never getting to the required temperature, and/or your boiler stopping/starting all the time because of internal overheating. Also, modern heating systems have a smart high-efficiency modulating boiler, which will automatically modulate the wattage and pump based on outgoing and ingoing water temperature. The more autonomously you let the boiler do it's job, the better it performs.

The only good solution, i've found out, is to disable TrueRadiant, and go into a good old On/Off algorithm. Then set your lead-temperature in the Pro settings to the low temperature you want to use on a continuous base. F.e. i'm currently heating with an outgoing water temperature of 45 degrees Celsius. Disabling TrueRadiant also makes my Nest behave much more predictable, and there are no big over or undershoots, even better, the hysteresis is about 0,1 degree Celsius, wich is much better than the 0,5 degree Celsius i've noticed with the TrueRadiant algorithm.

It's a pity I cannot adjust the pre-heating temperature based on outside temperature and other weather conditions, especially eastern wind, clouds and rain have a much bigger impact on my heating requirements than a cold but clear weather (due to much more sun coming through our windows and giving natural heating).

I hope this info is of some use. TBH i'm kind of disappointed that Nest never gave us the options to customise the TrueRadiant algorithm in more detail for our heating requirements, this would make the Nest a much smarter device. Currently i need to have it running in the least "smart" way to get the optimal results.


Community Member

Hello, first of all, thank you for your feedback according your experience with Nest Gen 3 learning.
I purchased one of these few months ago, and I testing it now. Well, i'm not completely satisfied... I bought it for driving an Opentherm combi boiler with a wireless link from the thermostat head and the heat link. Unfortunately Opentherm mode is non modulating very well; I have to fix the maximum heating water temperature sended to iron radiators and the system doesn't reduce the water temperature one time reached the set point ambient temperature... but it turn off the boiler! If this is modulation...
Without this system i ran the boiler for some years setting the appropriate climate curve, the temperature gradient and the maximum water temperature on the boiler. This one modulate the heating temperature water acording to the external temperature (readed by probe) by itself. It's a bit tricky to find optimal settings for your building/apartament; you need to try some times but it works pretty well.
Nest thermostat would be a step more for me, also for a smarter managment, and adding an internal ambient temperature feedback to the system.
But all the "advanced algorithm", (Pre-heating time, Early on, Eco temp, Home/Away) are completely unnecessary for me.
Opentherm users need a PID regulation, where set point user value is holded with minimum amount of energy, and 24/24h working time.
Also the DHW control is unless with a combi boiler! Water is heated only when requested!
Al the rest of options are basically On-Off or surrogates of it.

Forget the reporting system: simply it only reports the hours of operation which mean nothing, because the Opentherm system runs at minimum power, therefore it does not consume gas like one running at maximum power.

Let see if Google/Nest think to implement better logics about Opentherm.