Minimum flow temperature

Do you know how can I identify what is the minimum flow temperature the boiler can maintain ?

As per its datasheet it can modulate down to 3.5 kW, however I have the feeling that Tado requests too small temperature and what is happening is that the boiler heats the water to 42 Celsius, then turns off the burner and just circulates the water in the system until it is back at 32 Celsius, then repeat. This results that at every 1-2-3 minutes the burner is turned on then off. When the temperature was colder outside it could produce a stable 38 Celsius water.


  • It depends on the return temperature, which in turn depends on the heat demand.

    At 3.5kw your boiler already has a very good minimum output for modulation but will still inevitably cycle when the heat demand is lower.
  • The return temperature is basically the same as the flow. Only a few radiator request minimum heating.
  • If there is only one wavy line for the radiators in question it would not surprise me if the valves remain closed and your flow water is heading straight back to the return or circulating within the boiler's bypass.

    One wavy line is no guarantee that the valves have opened far enough to actually pass water. I've discovered this more than once on more than one radiator, and sometimes it is the case even with a radiator that has actually forced the boiler to fire. I'm not alone in this discovery.

  • If this is true then basically the pump is running for no reason… Hopefully will not die earlier then expected.
    I have some radiators in my system without smart thermostat and they are always open so the water goes through them always whenever someone requests heating.
  • If there is heat demand, but the system cannot transfer the heat to satisfy the demand, it sounds like poor system design.

  • @eezytiger would you say its better for the radiators that are set to independent (no zone controller), to remove the tado trv's and have normal trv's as they are always open?

  • @Ervcrt100 Hard to say. It's personal choice depending on exactly how YOU want to operate YOUR system.

    FWIW I have nine radiators and eight Tado SRTs. The radiator in the hall is my bypass radiator and it has no thermostatic valves, but it is balanced.

    My setup at the moment (and I have tried several, but this one is looking like a keeper) is to have the master bedroom as my only room connected to the zone controller. All other rooms are set to independent, with a fixed temperature setting of 21C.

    So the bedroom gets to choose when to turn the boiler on. The bedroom is set to 19C 24x7 and the other rooms will take whatever heat they can whenever the boiler turns on. Due to the pipe run, radiator sizing and balancing of all rads this usually results in appropriate temps in every room.

    It's worth mentioning that we are a retired couple and the house is usually occupied, so no need for scheduling around work, school etc. Basically the aim is to maintain steady temperatures so that the boiler can run at low flow/return temps instead of ramping up to raise the temp by 5C or whatever. This is very much the "heat pump" style of operation, which I'm trying to simulate in advance of the day when the boiler needs replacing.

    So with my setup I could actually remove all the Tado SRTs and simply have a wireless temperature sensor in the bedroom - although the SRTs are often cheaper, so one SRT would function equally well. No actual need to mount it on the rad. You could place it anywhere in lieu of a WTS.

    Effectively I am following the philosophy (inspired by Heat Geek) to heat the whole house rather than individual rooms. So when one room (the bedroom) needs heating, every other room (I.e. the whole house) gets heated too. The only purpose served by my Tado SRTs currently is to let me monitor temperatures in each room and to set a maximum temperature of 21C rather than a target temperature of 21C. This means that all my valves are always open, except the bedroom, which continues to open and close depending on its heating demand.

    But you may have more advanced needs for scheduling different temperatures in different rooms, turning some rooms off etc. etc. So the solution I've ended up with may not suit your needs.

    Here's how my rooms are looking right now....

    I'm happy with that, and it's only Bed 1 controlling the show.

    By the way, the Hall is set "off" because that's where my Wired Smart Thermostat is located. It is the Zone Controller, but I don't want it to call for heat, only the bedroom. And since it is my bypass radiator it is always open and accepting heat when the boiler is on.

  • p.s. Example video from Heat Geeks about heating all rooms for greater boiler efficiency (condensing boilers only) by increasing the radiator surface area (more radiators in play) and thus allowing lower flow/return temps.

  • thank you for the detailed reply.

    i guess i need to try and balance my system so i can get even temps across all rads which are set to independent.

  • @eezytiger : with this setup, what are your flow and return temperatures ?

    My target would be to operate the condensing boiler constantly with low flow temperature (30-40 C), for this purpose I have partially UFH and oversized radiators. The idea is that after all rooms reached the target temperature, then maintining the level can be done via low flow temperature and it does not matter which room is calling for heat until the current indoor temperature is maintained by producing a steady low temperature flow in the system.

  • Cont...

    When it was little colder outside (-2 C) the boiler produced a constant a 37-38C flow temperature to maintain the indoor temperature. Now it is not cold outside (7-8 C) and what is happening is the boiler turns on when the flow temperature is 32 C, heats the water to 40-42 C, then turns off the burner and just circulates the water in the system until it goes back to 32 C. Basically this is happening during the whole day and I do not understand why it is beaving in this on-off fashion and not just creating a steady e.g. 34 C flow temperature. Maybe for the 3.5 kW minimum output it is capable the 32-34C flow temp is too small ?

    BTW with this setup I had ~7m3 gas usage/day when it was -2C (excluding DHW production), now with the more mild weather it is 3-4 m3/day, in a 150m2 row-house, 21 C in rooms where we are actively and 19 C during night, non-used rooms are set to 16 C.

  • eezytiger
    eezytiger ✭✭✭
    edited December 2022
    @Valaki1234 With overnight lows of 5C my flow temp has maxed out at 50C, but only briefly.
    Return temp has maxed out at 41C, again only briefly.

    Boiler is operating in relay mode with a 55C limit + potential 3C overshoot. 55C not required today. :-)

    The problem I have is that my minimum boiler output is 9 kW, so with the whole house only needing 1-2 kW in these temps it is impossible to lose all the heat through the rads, so flow temp will continue to rise until either Tado turns it off or it shuts off due to reaching the set flow limit.