Issues with return temp too high

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  • robj20
    Automated Home Sr Member
    • Aug 2017
    • 76

    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.
  • fezster
    Automated Home Sr Member
    • Dec 2017
    • 72

    #2
    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.

    Comment

    • robj20
      Automated Home Sr Member
      • Aug 2017
      • 76

      #3
      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.

      Comment

      • fezster
        Automated Home Sr Member
        • Dec 2017
        • 72

        #4
        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.

        Comment

        • robj20
          Automated Home Sr Member
          • Aug 2017
          • 76

          #5
          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.

          Comment

          • fezster
            Automated Home Sr Member
            • Dec 2017
            • 72

            #6
            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?

            Comment

            • fezster
              Automated Home Sr Member
              • Dec 2017
              • 72

              #7
              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.

              Comment

              • robj20
                Automated Home Sr Member
                • Aug 2017
                • 76

                #8
                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

                Comment

                • fezster
                  Automated Home Sr Member
                  • Dec 2017
                  • 72

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

                  Comment

                  • robj20
                    Automated Home Sr Member
                    • Aug 2017
                    • 76

                    #10
                    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; 15 November 2018, 03:54 PM.

                    Comment

                    • fezster
                      Automated Home Sr Member
                      • Dec 2017
                      • 72

                      #11
                      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.

                      Comment

                      • robj20
                        Automated Home Sr Member
                        • Aug 2017
                        • 76

                        #12
                        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; 15 November 2018, 05:43 PM.

                        Comment

                        • fezster
                          Automated Home Sr Member
                          • Dec 2017
                          • 72

                          #13
                          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.

                          Comment

                          • robj20
                            Automated Home Sr Member
                            • Aug 2017
                            • 76

                            #14
                            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.

                            Comment

                            • fezster
                              Automated Home Sr Member
                              • Dec 2017
                              • 72

                              #15
                              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.

                              Comment

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