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 2021

    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.  

  • How about fitting a temperature sensor and thermostat on the return pipe so that when the water temperature exceeds say 55deg the boiler ignition signal from the Tado controller is turned off? The pump will continue to run so as the return water temperature falls the ignition circuit will be turned back on again, thus firing up the boiler. Suitable digital thermostats are readily available from the usual web sources for little money.

    Are there any reasons why this should not be done? It seems so simple there must be an objection to doing it.

  • Perhaps you might ask a boiler manufacturer and then post the response. I would guess that in the real world, where systems such as Tado turn individual radiators on and off; unless you have a boiler that is capable of modulating down to a very low load, the boiler may cycle a lot which is also not good for efficiency. I would suggest that ensuring that the boiler selected is not oversized and has a good turn-down ratio is quite important as a precursor to controlling the return temp.

  • eezytiger
    eezytiger ✭✭✭
    edited November 2021

    I'm new to Tado and I've been rapidly increasing my understanding of heating systems from a standing start of nothing at the end of September this year. I understand the importance of return temperature regarding efficiency of condensing boilers. My boiler is a Vaillant Ecotec 831 Plus. Max output for CH is 24kWh. Minimum is 9kWh.

    I have been running it with Tado for a few weeks using e-Bus control. Tado has been keeping its end of the bargain, often calling for flow temperatures as low as 33C, perhaps even 30C. This is all well and good, but with residual heat in the system of, maybe, 31C when Tado calls for more heat the boiler hits that in 30 seconds, before hotter water even reaches the rads, and immediately shuts off the gas, wastes five minutes of energy on pump overrun and then enters anti-cycling for as long as it needs. Eventually the heat demand is sufficient to warrant a bit more temperature and a longer burn, but my boiler simply can't modulate low enough to operate this way.

    Consequently I've returned to on/off relay switching, without all the weather compensation smarts. At the moment I've dialled in a 60C limit on the CH flow temp. I have attached a temperature logging Bluetooth thermometer to the return pipe and I'm monitoring return to make sure I'm not compromising efficiency.

    Things seem to be working much better now, with the boiler restrictions relaxed a bit. Return temperatures remain at comfortable levels so I'm happy with this setup, even though it is not fully automated and needs watching.

    By the way, under e-Bus control Tado might well call for higher temperatures, e.g. 60C or even 75C if I raised the permitted ceiling on the boiler, but whatever the flow temperature it always seemed to cut the gas the instant the return hit 54C. Coincidence? I don't know. I like to think it was intelligence, but still not appropriate for a boiler that wants to heat an entire house at 9-24kW (I rated it down to 9W max, but still not good enough) and not a single radiator.

    Here is my boiler with my monitors. The three small thermometers do not log or send data, but they are quick to respond with real time temperatures and indicate - room temperature, flow and return. The larger display has a thermocouple attached to the return pipe and insulated against it. This logs temperatures at intervals (configurable) and can transmit current temperature and logged data to an app via Bluetooth. The display only updates when it logs a data point, so it is not quite real time. Hence the delta between 39.0 and 38.6.

    All were supplied by Amazon UK.

    EDIT : Here's sample output from the return logger for today. The graph rounds off peaks and troughs when you com out to a full day, but the max return temperature is listed - 45.5C today so far.


    Sorry, one more edit - The boiler has just had a longer burn, blowing past the 60C flow limit to at least 63C. Return was rising past 50C and, as soon as it reached 54C the boiler cut the burn. So maybe this boiler (or all condensing boilers?) self limit the return temperature for efficiency. Here's a better photo of the gauges as return approaches 54C....


  • I am a district heating customer and I need to keep my return temperature as low as possible.

    I suggest that Tado make a temperature sensor for mounting on return pipes, and connect the measurement (via the hub) to the thermostat on the inlet.

    Thus, the thermostat can be set to turn off the heat if the return temperature becomes too high.

    With such a dual temperature thermostat, reheating can also be performed with maximum utilization of the radiator area.

  • I used to have independent rooms calling for heat with different schedules around the day. That might work well for modulating boilers and gas based that can warm up pretty fast.. Oil fired boilers, specially condensing, it is better to have all radiators on all the time so that your boiler will cycle less. Hot air will always travel through your rooms unless you have individual rooms insulated which is unlikely. Having cold rooms with radiators on 0 or close to it, will just make your boiler work for longer with lower energy output and cycle a lot more

  • Sounds like eezytiger's boiler is monitoring return temperatures. Not all boilers do this though. I was disappointed to discover that my brand new Viessmann 111-W does not monitor return temperature at all. I believe many boilers are similar. They will modulate the burner to achieve the desired output flow temperature, but do not modulate the pump in response to return temperatures. It is pump modulation that I would really have liked and is the thing that would make the biggest difference to the efficiency of the boiler in my scenario.

    I'm currently using tado in relay mode and relying on the boiler's own weather compensation to adjust the output flow temperature in response to the weather. I might using tado to control the boiler using opentherm, but I have read other posts in the forums suggesting that this might not be any better than relay mode. The thing that really puts me off trying this is that the Viessmann 111-W (and all their recent 100 series boilers I believe) insist on running the pump and boiler fans at full speed when in any mode other than weather compensation. That makes the boiler noisy, and makes the radiators noisy because the flow rate is too high when tado switches off some of the radiators with its smart TRVs. I have to say I'm pretty disappointed the Viessmann's limitations. I can't help wondering if they have made some deliberate choices on limiting the boiler's behaviour to push people towards their own thermostats and smart controls. i.e. If you want an efficient and non-noisy boiler then you had to buy their control equipment, which is not cheap, and especially hard to swallow if you've already invested hundreds of pounds in tado equipment.

    One plus for the Viessmann boilers is they have a good low minimum burner modulation. So my 111-W can output 32kw but can modulate the burner down to 3.2kw. I think that the latest 200-W series can go even lower than that, which would result in even lower boiler cycling. However, tado in relay mode does it's own cycling in order to modulate. I wish there was as way to tell tado to modulate less. There doesn't seem to be an option to tell it that the boiler has weather compensation so I suspect it is cycling a lot more than it needs to.