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Smart thermostat choice

Hi there

Very grateful for some help with an unusual boiler setup. We moved into a house which used to be split into 2. There are two boilers which, as far as I can see, are completely independent of each other. One is a combi boiler and heats / waters half the house and it doesn't have a separate thermostat.

The other boiler heats the other half of the house, has a hot water tank, and a wired thermostat.

As I say, the two boilers have separate controllers and are operated completely separately. 

We now live in the house as one household and are happy for the whole house to be heated at the same time (ie both boilers coming on and off at the same time, for heating). For hot water, presumably the combi should supply hot water whenever needed, whereas the one with the tank should heat a couple of times a day. 

I was looking at a tado V3 with hot water control . Could it be wired to control both boilers for heating?

Sorry for long message! Very grateful for any help.  In Sussex area fwiw. 




  • Jamie Hall
    Jamie Hall Member Posts: 23,322
    How completely split are these two systems? If the split is really complete -- that is to say, system A with the indirect supplies all the uses in one part, but only in that part, and system B with the combi supplies all uses in the other part, and only in that part, then it seems to me that much the simplest thing to do it going to be to use two thermostats, one for each system. It is hard enough to get reasonably even heating in a house with just one system and one thermostat! Locate the two thermostats to get the most satisfactory heating in both sections.

    Now if the combi has the capacity to supply all the hot water needed for the whole house, then you can repipe the hot water system so that it is the only domestic hot water supply, leaving the indirect off. I'd include enough valving so that if you wanted to you could use the indirect only, and leave the combi off instead -- that's just a matter of some thoughtful valving and interconnecting pipes. You could also arrange things so that the combi feeds the indirect, which then feeds all the domestic hot water; the aquastat for the indirect and its boiler cold be set to kick on when the combi doesn't manage to reach the desired temperature. A little trickier to get set up really satisfactorily, but it can be done.

    Now you did ask if one thermostat can control two boilers -- and the answer is yes. However, if they are "smart: thermostats which require a power supply, you'll be much happier and installation will be much easier if each thermostat controls a relay, and the normally open contacts of the relays are wired in parallel to control the boiler. That removes any possibility of the power supplies for the two thermostats arguing with each other and damaging things. It's not ideal, however, as there is a substantial probability that one thermostat will initiate a call just after the other one is satisfied -- leading to some weird short cycling for the boiler being controlled.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Greg1990
    Greg1990 Member Posts: 4
    Hi Jamie, 

    Hugely appreciate your thoughtful reply! The two systems are fully independent - 'system A with the indirect supplies all the uses in one part, but only in that part, and system B with the combi supplies all uses in the other part, and only in that part' is correct. 

    I was thinking of getting tado radiator valves to even out the thermostat flow but actually perhaps it's easier just to get two thermostats (and sounds like there are other advantages).

    I hadn't considered getting the combi to supply the whole house and haven't looked at its capacity. It's a really interesting idea, as the hot water tank seems small...

    Thanks so much for taking the time to reply! 
  • Greg1990
    Greg1990 Member Posts: 4
    If you have any other observations I'd be very grateful for them! Thanks again 
  • Greg1990
    Greg1990 Member Posts: 4
    Incidentally, do you know who would be best placed to help with the above? I saw a plumber who said they wouldn't touch the thermostat as it was electrics, and an electrician who wouldn't touch the piping... And it feels I need some help from someone who will be responsible across the board! Thank you!