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Discuss Combi boiler flow temperature with underfloor heating in the Underfloor Heating Forum area at

  1. kam187

    kam187 New Member


    I have a Worcester greenstart 440 Cdi combi boiler feeding a wet underfloor to my whole house. There’s one manifold upstairs and onedownstairs.

    The underfloor manifolds have a trv to make sure the flow temperature remains at 40-50 (or even lessbecause I have a laminate floor).

    I have nest thermostats in everyroom with a temperature probe forcut off to protect the floor etc so it’sall done properly.

    One of the seals went near one of thetrv, So I changed that only to find it’scrusted up the trv itself. I can order a new one but in the mean time I justset the combi boiler flow to 45 C and removed the TRV so my heating stillruns.

    I don’t have any radiators, and the combi feeds only the underfloor systems. I know the combi heating flow temperature usually needs to be around 70 C for it to condense and save energy when the exhaust hits 55-56 C, but is that more efficient than just running the boiler at 45 Cand removing the TRVs?

    I read about low heat energy systems but I’m not entirely sure if a combi is very efficient at 45 C since doesn’t it heat exchange to save energy heating the tap water too? If the tap water ishotter than 45 C it will end up heating the pipe directly all of the time?

    I’d be interested in your opinions onthis. Running the combi at 45 C is simpler and less prone to error, but is it efficient?
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    you will damage the boiler if you run that set up for a long time return needs to be above 50-55 dc
  3. kam187

    kam187 New Member

    Ok so I need to replace that TRV and set the flow back up. Any recommendation on a flow temperature?
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

  5. kam187

    kam187 New Member

    I just read that the lower the return temperature the more efficiently the boiler runs. This seems ad odds with other advice - although that seems to be tartgeted at radiators which they estimate will drop 20C (from 70 to 50) with TRV's fitted.

    Very confused now. I also sent an email to Worchester.
  6. Matchless.plumb

    Matchless.plumb Trusted Plumber GSR

    Well too high.
    Flow temp should not be so high, normally it should be set tbetween 55-60 max. The ideal figure would be 15dc between flow and return but in real life you will have a 10 Kelvin difference.
  7. Matchless.plumb

    Matchless.plumb Trusted Plumber GSR

    That’s quite right, the lower it is the more efficient it runs. Due to the fact that the return will be preheated from the fumes and therefore you will reduce the amount of gas usage
  8. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    totally wrong

    you run that set up and you boiler will only last a couple of years hex will internally scale up due to to low of a return temp

    the lowest i would set the flow temp is 60 (45-50 return temp)
  9. Matchless.plumb

    Matchless.plumb Trusted Plumber GSR

    Really, i am so wrong :)
    Don’t think so. The hex will scale up anyway in England, doesn’t matter how you set the temp on the boiler .

    It’s a condensing boiler and it’s designed to run on low flow temp. Let’s say you set the flow temp on 55dc return will be about 45 that’s a difference of 10 Kelvin. The flue gases will have about 55dc that will be hotter than the return temp and therefore gives it energy further and reduces the amount of gas usage. What’s the point of setting it on 65/70dc flow temp and return will be about 55/60 you will not use the heat from flue gases at all and that’s called wastage or inefficient. The reason for this is that condensing boilers also extract / take , energy from the hot flue gases. They cool it with the return water, condensing moisture contained. For this to work properly, the temperatures in the flow and return should be low.

    Scale won’t be removed due the heat :) check your kettle
  10. kam187

    kam187 New Member

    Thanks for you replies.

    Just for some added info we have very hard water here so I have a hefty water softener and a hefty filter after it to remove chlorine and salt. Those feed the entire house.

    Yes I know softened water shouldn’t go into a boiler usually but the second filter takes care of any salt etc. I have tested the water output.

    So given the above, and also that at 50 - 55C into the underfloor the return is 40-45 C what should I do:

    1) set boiler flow to 70C, and set TRV at underfloor manifold to 50C / or give me different values.


    2) set boiler at 50 C and remove the TRVs / or give me a different value.

    FYI the underfloor optimum temperature is 40-55 C (because it’s laminate wood lower is better as it doesn’t trigger the cut off)
  11. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    :D wrong hex for a start your thinking of the domestic hot water plate heat exchanger Lol

    Also you haven't taken in for heat loss through the pipes
  12. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Leave the trvs in place / replace them if there leaking and set to manufacturer spec for the floor type

    I would set it to 60 (flow ) and set it up correctly

    Eg balancing your loops so you get the correct difference Yout looking around 40-45dc for your return temp
    Last edited: Apr 22, 2018
  13. Matchless.plumb

    Matchless.plumb Trusted Plumber GSR

    really ? I’m wrong . that’s fine with me :) i will keep doing it the way I got taught. Keep setting boiler temp to 70dc it’s fine with me too but the heating costs will speak for itself
  14. Matchless.plumb

    Matchless.plumb Trusted Plumber GSR

  15. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    What temp does most boilers start condensing ?

    55 is the answer so with a 15dc difference you will condense and with the new boiler plus kit you will save more etc

    Also have you check the boiler out, it has a heat store so that heats your hot water as well
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