Radiator balancing with combi boiler, 10C or 20C drop?

Discussion in 'Plumbers' Talk' started by ProTofik, Dec 5, 2021.

  1. ProTofik

    ProTofik Member


    I just had my house converted from oil to gas combi boiler, Ideal Logic C30. As the plumber left, I started balancing radiators and 2 days later I have achieved perfect 9-11C drop across all radiators in the house.

    I was aiming for 10C drop because that's what every youtube video or written up guides on the internet were always saying. Either 10C, 11C, or 20F.

    I got bored today and started reading my new boiler's manual and found this: https://imgur.com/a/FbqkDnN

    By the looks of it, boiler's manufacturer recommends 20C drop across radiators. Is that normal for modern combi boilers? Is it something unusual, or are guides I've read out of date? Why 20C drop and not 10C like everyone else is saying?

    I'm looking for someone to give me some logical explanation before I start running around the house again balancing all radiators from scratch.
  2. Jimbo

    Jimbo Screwfix Select

    It’s just a function of water speed at the end of the day. As long as the return temperature is sufficiently low to achieve condensing it doesn’t really matter.
  3. ProTofik

    ProTofik Member

    Which I don't think it does atm. After balancing all my radiators to achieve 10C drop, my boiler reports that flow temperature is 69C (that's what I set it to) and return is 59C. If my understanding is correct, to achieve condensation I will need to get it down to at least 55C on return.

    Reading more about it over the past hour on the internet my understanding is that 10C drop is for old non-condensing boilers, and 15-20C is what you should aim for if you have condensing boiler like I do. Oh well, I guess I will have to do it all over again.
  4. Jimbo

    Jimbo Screwfix Select

    why not just turn down the flow temperature a bit?
  5. MGW

    MGW Screwfix Select

    I started by setting a temperature drop, but this was a rough setting, next was to trim them in. There are three reasons to adjust flow with lock shield valve, one is to ensure all radiators get some water, the second is to ensure return water cool enough for boiler to extract the latent heat, and the third is to limit warm up speed to what the TRV can cope with.

    So if I look at my TRV heads, I get 4 TRVs-1.jpg giving me current and target temperatures, if the current exceeds target then I close the lock shield a small amount. In mother house I moved the electronic TRV head around, set the lock shield with the electronic head, then put the mechanical head back, once set with electronic heat the mechanical head worked spot on, except where morning sun in bay windows caused over shoot, can't blame CH for that.
  6. ProTofik

    ProTofik Member

    That's for the thermostat to decide via OpenTherm. You don't adjust flow temperature manually in modern systems.
  7. Jimbo

    Jimbo Screwfix Select

    The max CH flow should have been configured to the installation when the control was first set-up.
  8. Baxi Boy

    Baxi Boy Active Member

    I have always found it impossible to get a difference of 10 degrees on all radiators.The water gets through the small ones fast so even with the lockshield set very low 5 degrees is about it,and the flow and return pipes at combi boiler ends up at 9 degrees.(Small systems of about 6-7 kw)

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