OpenTherm with Y plan setup


We have an Ideal Logic Max Heat 24 boiler which has OpenTherm compatibility that I'm looking to take advantage of. We have a Tado wired thermostat working in wireless mode after we invested in the wireless add-on receiver a number of years ago. Both the thermostat and receiver appear to have low-voltage terminal connections that suggest these pre-date the removal of OpenTherm from UK devices.

We have a system boiler (gravity-fed, Y plan setup with hot water tank, pump and 3-way valve) so we're currently using the switched live wiring configuration in the receiver. We're able to control heating and hot water using Tado. 

Is there a way to get Tado to speak to the boiler using OpenTherm and maintain existing functionality, either by connecting the OpenTherm connections between either the thermostat or receiver to the respective connection on the boiler and removing the switched live between the receiver and the boiler?

After reviewing the install guides, they show either OpenTherm or switched live to support the tank stat, pump etc.

Any advice would be greatly appreciated!

Thanks in advance,


  • No, relay or digital. Gap in the market.
  • If you want to use Opentherm to control your system boiler, then you will need an alternative to Tado. As well as replacing the wall thermostat you will need to replace the cylinder thermostat and then there needs to be a relay box to control the motorised valve. The valve operates on 230V and cannot be connected directly to the low voltage communication bus, hence the requirement for a relay box.

  • Is your y plan using a gravity feed for hot water? Or do you mean it’s got a small header tank, not a pressurised system?

  • @Hunter, @GrilledCheese2, Thanks for the comments, much appreciated. They generally confirm my thinking that it's not possible.

    I was thinking about removing the receiver from the setup and just going back to a wired thermostat. I think this will allow me to see if the OpenTherm benefits are actually worth pursuing, albeit as the expense of Tado controlled hot water.

    Another option, although I'm not sure I'll ever find the time, would be to use Arduino and relays to control the valve separately.

    @wateroakley, We have a header tank (F&E) in the loft for the central heating, nothing is pressurised. Just a typical 1960's setup!

  • Does your boiler support weather compensation?

    If so, go WC and relay, which will work with Y plan.
  • @DM932187, thanks. That's not a bad shout! It has WC connections available and I see the kits are fairly cheap. I'll investigate futher.

  • @ipw781 To properly use WC with relay on a system boiler you will need to convert your heating system to HW priority. This means you cannot run the CH and HW at the same time and a call for heat from the HW cylinder will turn off the heating until the HW has achieved the required temperature, usually 60°C.

    The reason to opt for HW priority is due to WC reducing the flow temperature out of the boiler. On a mild day that temperature might be 40°C or less. Obviously this won't heat your cylinder to 60°C. When the boiler knows the switched live request is for HW it will ignore the WC module and raise the flow temperature to maximum. I think your boiler does have two switched live inputs so you should be able to do this.

    HW priority is usually used with modern unvented cylinders where the heating coil is designed to heat the water as quickly as possible. Perhaps a 20KW coil that can reheat the entire cylinder in 20 minutes. With a traditional copper vented cylinder the heating coil will be 5KW at best; meaning the total reheat time can be quite slow. You can check how long it takes to reheat your current cylinder and then consider if you can be without heating for this duration of time.

  • @ipw781, @GrilledCheese2 is, of course, correct.

    We run relay and WC, with DHW priority. There is some drop in room temps during the HW heating period (maybe 0.5C at most), but the much longer running duration of the weather comp (not on/off) results in the house structure holding sufficient heat, so it's not particularly perceptible.

    Our HW takes about 40mins to heat. Long primaries run and traditional tank. Not ideal, but not a problem.

    I'm not sure, but you may also need to replumb to S plan (separate valves rather than mid-position) for DHW priority to work. Not a big job.

    Have had tado and WC running together for over 10 years. Only really use tado for geolocation and ease of programming. No smart TRVs (just dumb). WC does the rest.

    If you want to learn more about WC, see

  • Thanks @GrilledCheese2 & @DM932187, this is great advice!

    The boiler does have 2 switched live inputs so that's the first requirement met.

    I changed the cylinder to a stainless steel one during some improvements we made before moving in back in May, as the previous copper one was ridiculously small and we wanted to install a shower pump. As a result, I think we're in a good place KW wise. The Gledhill site states 23KW and the label on the cylinder says 22.4.

    I will do some further reading, but it does look like I'll need to add another valve to separate the HW. If I do go down this route, according to a couple of articles, OpenTherm might even be possible along with WC....I haven't got my brain round to thinking if Tado could cope with that!

  • OT will override the WC so not compatible.