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Thread: Issues with return temp too high

  1. #1
    Automated Home Sr Member
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    Default Issues with return temp too high

    My boiler really doesnt play nice with Evohome.

    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.

    I have an auto-bypass and a bypass radiator so no idea what else i can try.

  2. #2
    Automated Home Sr Member
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    The auto-bypass may need adjusting. Remember, though, that it maintains the minimum flow required by the boiler, so it can't just be set higher, otherwise you may end up with too much differential between flow and return (on Vaillants, this is an S53).

    It may just be a case of the boiler is unable to modulate down low enough given the low heat requirement from your CH circuit, and hence it cycles.

  3. #3
    Automated Home Sr Member
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    I have a bypass radiator as well, so turning the auto-bypass up shouldn't be an issue, its set to about 0.25 and goes up to 0.5 by the look of it.

  4. #4
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    The radiator may or may not help you as a bypass. It depends on your pump and the minimum flow rate required. The ABV is usually situated close to the boiler and so supplements the flow rate in your CH circuit to quickly return water to the boiler. It's always a compromise because ideally that water would go around your CH circuit. If the ABV is required to be more open to maintain the min flow rate, then alternatively you either need to increase your CH pipe size or you need to buy a bigger pump.

    A low loss header allows you to maintain the minimum flow rate through your boiler regardless of your CH layout.

  5. #5
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    All pipework is 22mm and 15mm to each radiator. As its on evohome all rads are fully open at the lock shield to keep restrictions as low as possible when only one or two rads are on.

    I will try opening up my bypass radiator some more though it gets noisy (vertical rad) and turn the auto one up a little.

    Its a combi so no control over the pump.

  6. #6
    Automated Home Sr Member
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    You should balance the system, rather than having all lockshields open. Evohome opens TRVs fully if the temp differential is more than a few degrees. As it stands, your rads will be heating up unevenly unless balanced.

    Your pump should still have some speed settings. Which boiler is it?

  7. #7
    Automated Home Sr Member
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    Just to add, I wonder if the lack of balancing is contributing towards your flow/return problem.

    If your boiler is condensing, it'll have an optimal delta of 20 degrees. This means each radiator should also be balanced to have a 20 degree differential. Or as close as you can get to it. Perform this with the TRVs off.

  8. #8
    Automated Home Sr Member
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    I dont think balancing is the issue. And not sure why evohome would require it.

    None of my radiators are all on at the same time even at start up so what am i balancing, pointless balancing so they all heat up the same as i never know which ones will come on.
    I know that downstairs the living room comes on a good 20 minutes before the dining room. Often only two or three rads are on at a time.

    Its an Ideal Vogue Gen 2

  9. #9
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    Without balancing, you can't ensure your delta T at the boiler will be 20 degrees.

  10. #10
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    I did that via massively oversizing the rads to a delta 35t.

    For instance the master bedroom required 1363W, calculated t35 at 2164w i fitted a type 21 600X1800 at 2295W.
    Some rooms went even higher livingroom required 1557w calculated to 2472w, fitted a 21 600X1600 2038W and a 11 450X120 842W total 2880w.

    I would be open to balancing if it would fix the issue though.
    The boiler rarely goes above 50c, and often goes down to 23c flow temp.
    Last edited by robj20; 15th November 2018 at 03:54 PM.

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