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Thread: Issues with return temp too high

  1. #11
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    Over sizing rads is generally good, as at the same delta T you'll get more heat output from them. I've done the same.

    However, your system needs to run at the delta T it was designed for. This design includes your boiler. No condensing boiler's heat exchanger could handle the stresses of a 35 degree delta T and would most likely result in premature failure. To prevent this, the boiler usually goes to an error code once the delta T exceeds a certain threshold (on Vaillants, this threshold is 30 degrees). You should balance the system to try and get as close to the boiler's design delta T as possible.

  2. #12
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    So how best to go about balancing considering evohome wont let me control the flow temp, or the boiler firing or not. The best i can do is remove a trv and set it high and put the trv in a cold place this should run the boiler flat out.

    Reading the installer manual it does say to balance and aim for a 20c drop across the radiator. So i guess thats the thing to try.
    Last edited by robj20; 15th November 2018 at 05:43 PM.

  3. #13
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    I wrote a reply but not sure where it went.

    Take TRVs off.
    Use contact probes (rather than IR) at each rad to get delta t of 20 by adjusting lockshield.
    Start with closest rad to boiler first.

  4. #14
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    Do i do each rad at a time as this would more closely represent how its used ie when the heating comes on in a morning it heats each room individually at different times.

    Looks like TRV4s can be balanced via the trv, so when i come to decorate i can still just close all the lockshields and it will maintain my balance settings when i refit.

  5. #15
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    Start with all closed and open one at a time, starting with the closest (this will require a quarter turn or even less) and finishing with the furthest (which will probably be fully open - about 8 to 12 turns). You should find every radiator has approx the same delta 20 across it once you've finished.

    You can then adjust TRVs as you wish (if manual) or use evohome TRVs as per normal. But the result should be that your boiler shows a delta t of 20 (there may be a menu setting to measure this). Obviously, as the entire ch system gets hot, that delta t will get smaller until the boiler begins to cycle.

  6. #16
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    Hmm maybe balancing isn't the issue. Seems its already getting a 20C differential. Thats with the livingroom and dining room requesting a +1C change.

    IMG_20181116_051341.jpg

  7. #17
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    Quote Originally Posted by robj20 View Post
    Getting Flow/Return Thermistor faults again, which shut off the boiler.
    Basically looks like its when the return temp is very close to the flow temp.

    Quote Originally Posted by robj20 View Post
    Hmm maybe balancing isn't the issue. Seems its already getting a 20C differential. Thats with the livingroom and dining room requesting a +1C change.
    So your problem is solved?

    If not, then balancing should help in getting a 20 degree delta consistently, regardless of which radiator is on or not. As I said in my first post, though, there's limits to how far your boiler can modulate down, so inevitably you will eventually get the boiler cycling when the demand from your CH is less than what the boiler can modulate down to.
    Last edited by fezster; 16th November 2018 at 12:07 PM. Reason: Edited greater to less

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