Return Temperatures

Does anyone have any ideas/proposals on methods to control/limit return temperatures apart from by manipulation of manual return valves on each heat exchanger and limiting flow temperature set point when a boiler is involved?

I understand that high return temperatures are a major issue for Tado System owners in countries with District Heating Systems.

High boiler return temperatures are also not good here in the UK due to the significant negative impact on boiler efficiency.

Answers

  • Pete
    Pete ✭✭
    @Klaus_Ludwig i would suggest one or more of these:-
    - Turn pump speed down
    - Increase size of radiators
    - Reduce primary flow temperature
    - Implement weather compensation / adaptation (If your boiler is compatible)
    - Reduce/balance flow rates using lockshield valves on each radiator.
  • Klaus_Ludwig
    Klaus_Ludwig ✭✭
    edited February 20

    Hi @Pete,

    Thanks for your comments. I believe that all of what you suggest can help to a degree, but I also believe that these are no longer the ideal solutions now that we have systems such as Tado where you might have only one or two radiators in use at one part of the day and half a dozen or even a dozen or more in use at another part of the day. 

    My question was also asked on behalf of those with district heating. As I understand it the return temperature on district heating systems is critically important as returning heating water above a certain temperature is financially penalised.

    I guess the ideal would be for the outlet temp of each radiator / heat exchanger to be controlled to a desired value in the most cost effective way. 

    As far as I am aware all modulating boilers control flow temperature. I have not heard of any that modulate to control return temperature as well as limiting flow temperature???

  • It’ll only ever be a byproduct of all the factors @Pete has mentioned.

  • ....or perhaps an enhanced method of control?

    With Tado we are moving from a single point of control to having multi point control - with a control valve on each radiator / heat exchanger.

    Thinking outside the box, could that single control valve on each radiator be incorporated in a cost effective way into an even smarter control loop to also include return temperature control in addition to weather compensated room temp control?

  • In a world where these things are not intrinsically linked, yes.

    Might be more practical to motorise the windows and let Tado control those. That would allow it to preserve the desired return temperature with a means to shed any surplus energy given off by the emitters and maintain room temperature.

  • I am certain that someone at some point in the future will come up with better ways of controlling return temperatures within certain limits than we currently have for most domestic systems.

    Return temperature control is important for efficiency and is already in use in some heating systems.

  • Pete
    Pete ✭✭
    The only other way (beyond my previous post) of "controlling" the return tempnis to have a mixer arrnagement on every radiator, a little bit like what is installed on an underfloor heating system (which is there to restrict flow temp in the floor).
    Truth is, if the house needs less heat than the boiler is able to supply (and the boiler hasn't switched off) the return temp will be too high. Best way is to design the boiler to modulate down to very low output. But operating the boiler at very low output also damages efficiency (probably more than a high return temp!).
    👍
  • As we all know, boiler efficiency of a condensing boiler increases as load reduces - down to a certain point unlike a non condensing boiler where efficiency decreases as load decreases. At extremely low loads as you state then the efficiency would drop off a cliff, but if the boiler is set up correctly the turn down ratio should be limited to prevent the boiler going into that area. Not being a domestic boiler expert and disregarding weather compensation I wouldn’t know if modern domestic boilers utilise return temperatures in their control loop. Flow temperatures were always the main modulating boiler controlled variable for non-condensing boilers as far as I am aware. Do modern boilers use return temp to any degree in their control loops - as this is directly linked to efficiency? As you have implied it is very desirable to have a boiler that can operate efficiently at very low loads. Something that most of us probably overlook when shopping for a new boiler. A lot of people probably have boilers that are too big. 

    Now that we have heating systems where varying numbers of radiators can be in use at any time balancing the system no longer guarantees that the return temperatures will consistently be as low as possible. If there was a return sensor on each radiator then the SRT could take return temp into account in addition to room temp and weather compensation. Too costly and too complex probably, but there is no doubt that finding a way of having better control over return temperatures than most of us do would give increased efficiency.  

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