Thermostat hysteresis

Hi. I'm looking to replace my current setup - programmable timer for CH and HW (Potterton EP2002), with a wired thermostat for CH (Salus RT500). Separate HW tank (Megaflo). Boiler is an old Alpha system one (non-condensing). I'm hoping (funds allowing) to change the boiler and tank next year.

I've spent ages looking at all the Smart options - Drayton Wiser, Hive, Nest, Netatmo, and Tado. To begin with, I want something that will work with my current boiler. The Hive is okay, but defaults to ITLC, and isn't configurable without contacting support - and support seems a tad bad, according to what I've read.

Nest seems cool, but has a really bad score on trustpilot, and I don't like the fact that the minimum on time for HW is 30 minutes.

For the Tado, I like the look of it, and the ability to add radiator thermostats. However, it's hard to find out how it handles the call for CH. Can it be configured as a simple on/off, or does it insist on using a form of load compensation (so, will keep on switching the boiler on/off as it thinks it's approaching the temp - which I don't want with my current boiler)?

Also, if I set the temp to 20 degrees, when will it call for heat, and stop calling for heat? - i.e. call at 19.5 and stop at 20.5, or some other values.



  • You’ll need the wireless version. UK product is now relay only (as apparently, few people used modulation).

    I have relay. Uses a form of TPI. Heats to 0.1 above target, fires back in at 0.1 below. Moves around +/- 0.1 around there and then settles to hold steady with intermittent firing. That said, I try to keep it just under target as I have a weather comp boiler.

    So in terms of hysteresis, much tighter than the old school +/- 0.5 degrees.
  • johnnyp78
    johnnyp78 ✭✭✭
    edited December 2022
    Tado will simulate modulation on relay by cycling the boiler to get to target temp. It can’t be used as an on/off thermostat, so if you’re looking for that you’re better off with a different system.

    If you were to set a target temp of 20c it would probably cycle the boiler to about 19.5c and then leave the temp to rise through rad heat, but it has also been known to heat over target temp, presumably as a means of maintaining target temp and avoiding having to fire the boiler for a while.
  • Thanks for the answers. To be clear: if I set a schedule such that the set temperature moves from, say, 17 degrees to 19 degrees, will the Tado first call for heat and let the boiler cycle of its own accord until the temperature hits 19.1(so, no modulation of the boiler) and then cycle the boiler to remain in 0.1 of 19 degrees - or, will it modulate the boiler whilst the temperature increases from 17 to 19?

    At the moment, I'm tending towards waiting until I upgrade the boiler, as I don't want to have my current (20+ year old) boiler being cycled too often during winter, and risk an issue.

    Once I upgrade to a modern condensing boiler, the Tado will make more sense.

  • The former. It’ll most likely call for heat to 19/19.1 then start cycling on and off to keep it at 19. It may start the cycling as it approaches 19, to avoid overshoot. As you have an older boiler, the flow temp may be higher, so overshoot is more likely than with a lower flow temp. Tado should learn how your house heats up. That’s the theory, anyway.

    You could find your savings with Tado are greatest before you replace the boiler as the modcons are inherently more efficient.
  • Thanks. Must say that this forum is a lot more proactive than others.

  • Cheers @cholma. Unfortunately, I guess we're just a bunch of heating nerds.

  • :)

    So - I've decided to go with the approach of using the Netatmo to replace my current wired room thermostat, and then expand to radiator valves later. See how it goes, and hopefully upgrade the boiler to Opentherm and switch to that Netatmo version if everything is going okay.

    Before I take the plunge, just to check: the thermostat is wired into a standard s-plan - i.e. it wires into a box in the airing cupboard, along with the programmer, two port valves (HW and CH), and the tank thermostat. The boiler is a Alpha system boiler.

    So, all I want the Netatmo to do is replace the on/off for the CH into that s-plan.

    I presume that should be fine?

  • Yep. That will set up Netatmo as a wired room stat. Just check the existing wiring to the current wired stat will work for the Netatmo.

    When you replace your boiler (and assuming you stick with gas over a heat pump) if you can, get one with advanced weather compensation. You won’t regret it.