Heating Partial Load

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  • rs1987
    Automated Home Jr Member
    • Nov 2019
    • 44

    Heating Partial Load

    I have a Vaillant Ecotec 938 plus, which is a 38kw boiler. It appears to be grossly oversized for the property. Total radiator wattage is 20.2kw at delta T50. If I leave the heating partial load on auto, it goes into anti-cycling mode all the time and also my gas usage is completely off the charts.

    I have a total of 11 radiators, of which two are towel radiators. Of these eleven radiators, two are switched on to anti-frost level permanently, as one is a dressing room which gets warm enough from the ambient air coming in and the other is a guest bedroom that rarely gets used. Some of the other rooms only get used at certain times of the day when other rooms are not in use.

    So all in all, I usually have an absolute maximum of 7 radiators on and calling for heat at the same time, including the two towel radiators. This gives a typical maximum of 12.5kw used at any given time (unless of course the heating has been off for several days and the whole house needs to be heated, in which case it's likely to approach the maximum 20.2kw) as they get all the rooms up to temperature.

    What recommendation do you experts have for where I should be setting my heating partial load? At 20kw - 22kw for the maximum possible use, or at about 13kw - 15kw for my typical maximum use?

    I'd like to avoid the constant anti-cycling as it causes the heating flow temperature to cool down considerably (by about 10 degrees or more) before the boiler cycles back on and also reduce my astronomical gas bill.
  • mtmcgavock
    Automated Home Legend
    • Mar 2017
    • 507

    #2
    Originally posted by rs1987 View Post
    I have a Vaillant Ecotec 938 plus, which is a 38kw boiler. It appears to be grossly oversized for the property. Total radiator wattage is 20.2kw at delta T50. If I leave the heating partial load on auto, it goes into anti-cycling mode all the time and also my gas usage is completely off the charts.

    I have a total of 11 radiators, of which two are towel radiators. Of these eleven radiators, two are switched on to anti-frost level permanently, as one is a dressing room which gets warm enough from the ambient air coming in and the other is a guest bedroom that rarely gets used. Some of the other rooms only get used at certain times of the day when other rooms are not in use.

    So all in all, I usually have an absolute maximum of 7 radiators on and calling for heat at the same time, including the two towel radiators. This gives a typical maximum of 12.5kw used at any given time (unless of course the heating has been off for several days and the whole house needs to be heated, in which case it's likely to approach the maximum 20.2kw) as they get all the rooms up to temperature.

    What recommendation do you experts have for where I should be setting my heating partial load? At 20kw - 22kw for the maximum possible use, or at about 13kw - 15kw for my typical maximum use?

    I'd like to avoid the constant anti-cycling as it causes the heating flow temperature to cool down considerably (by about 10 degrees or more) before the boiler cycles back on and also reduce my astronomical gas bill.
    Does it do S.53? How old is the boiler; there's probably another issue at play rather than the Output being too large. It's a 38kw boiler but it'll do only 30kw on heating, within that it'll modulate 1:6 so down to 5kw which should really prevent anti cycling mode.

    Comment

    • rs1987
      Automated Home Jr Member
      • Nov 2019
      • 44

      #3
      Originally posted by mtmcgavock View Post
      Does it do S.53? How old is the boiler; there's probably another issue at play rather than the Output being too large. It's a 38kw boiler but it'll do only 30kw on heating, within that it'll modulate 1:6 so down to 5kw which should really prevent anti cycling mode.
      It's a two year old boiler, so it's pretty much brand new.

      S53? Do you mean D053? If so, no - it jumps from D051 to D058.

      That said, if I match the heating partial load exactly to the radiator output then it never cycles. So currently I have 7 rads running, the partial load set to 18kw and it hasn't cycled once in the last two hours. Normally, it cycles every few minutes if the output is considerably above the radiator maximum draw. Setting it to "auto" causes it to cycle constantly. I have these 7 rads all open to max right now as I'm balancing them (I've been upping the load as I bring each new rad online so that it doesn't cycle and keeps the flow temp constant).
      Last edited by rs1987; 13 January 2020, 02:51 PM.

      Comment

      • DBMandrake
        Automated Home Legend
        • Sep 2014
        • 2361

        #4
        5kW minimum output without cycling is still a lot for spring/autumn. Even if anti-cycling doesn't kick in in the middle of the winter I'd expect that you'd see it in the warmer months when the house just needs a little bit of heat. Probably not much you can do about it thought.

        I have 12 radiators of various sizes across 9 zones and a 100 litre hot water cylinder all on a 24kW boiler which I don't think is even turned up all the way and it seems to manage to reach the set point with all radiators on full so 38kW or even 30kW does seem a lot.

        Comment

        • bruce_miranda
          Automated Home Legend
          • Jul 2014
          • 2307

          #5
          My Vaillant 438 has been set to 18Kw and that is plenty. I 38Kw will need such a large pump to shift that heated water away from it.

          Comment

          • rs1987
            Automated Home Jr Member
            • Nov 2019
            • 44

            #6
            Anyone aware of any optimization I can make to the boiler settings that might improve things? For example, setting the pump to a fixed and higher speed rather than the auto speed springs to mind (although that probably will make the boiler less efficient as it's likely to increase the temperature of the return flow).

            My boiler is in it's default settings. The installer never set it up for the property, so it's likely that some of the settings are not optimized.

            Comment

            • mtmcgavock
              Automated Home Legend
              • Mar 2017
              • 507

              #7
              Originally posted by rs1987 View Post
              Anyone aware of any optimization I can make to the boiler settings that might improve things? For example, setting the pump to a fixed and higher speed rather than the auto speed springs to mind (although that probably will make the boiler less efficient as it's likely to increase the temperature of the return flow).

              My boiler is in it's default settings. The installer never set it up for the property, so it's likely that some of the settings are not optimized.
              The pump probably want's adjusting and the internal bypass adjusting too, have you got an external bypass?

              How quick does the anti cycle kick in when the Partial Load is set to Auto? Anti Cycling shouldn't really kick in, even when modulating down to 5kw....

              S.53 I was referring to the Status code when the boiler is running, So Menu then Live Monitor. This will give you the S codes when the boiler is running in normal mode.

              Comment

              • bruce_miranda
                Automated Home Legend
                • Jul 2014
                • 2307

                #8
                On a boiler that big, you are going to have to run the pump at full at a constant speed, I would have thought.

                Comment

                • rs1987
                  Automated Home Jr Member
                  • Nov 2019
                  • 44

                  #9
                  Originally posted by mtmcgavock View Post
                  The pump probably want's adjusting and the internal bypass adjusting too, have you got an external bypass?
                  No external bypass as far as I am aware. I've got an engineer coming to fit my evohome system tomorrow, so I'll ask him to check.

                  What do you recommend for the pump setting? Putting it up to what %?

                  I'd be loathe to touch the bypass without asking an engineer to validate the setting though

                  Originally posted by mtmcgavock View Post

                  How quick does the anti cycle kick in when the Partial Load is set to Auto? Anti Cycling shouldn't really kick in, even when modulating down to 5kw....
                  In minutes literally...it spends most of its time on anti-cycling and also costs me a fortune (one month I used 4,200Kwh of gas; setting the partial load drops that by 50%). When set to a much lower partial load it still cycles occasionally, but much less frequently. It's like the auto mode is not modulating down at all, as the heating flow gets up to temperature very quickly.

                  My radiators are all balanced properly, with a 11 degree flow drop across all of them.

                  Originally posted by mtmcgavock View Post
                  S.53 I was referring to the Status code when the boiler is running, So Menu then Live Monitor. This will give you the S codes when the boiler is running in normal mode.
                  No, it only appears to have one option: S.04

                  Comment

                  • bruce_miranda
                    Automated Home Legend
                    • Jul 2014
                    • 2307

                    #10
                    Check the fault log and see if you have any F.53?

                    Comment

                    • mtmcgavock
                      Automated Home Legend
                      • Mar 2017
                      • 507

                      #11
                      Originally posted by rs1987 View Post
                      What do you recommend for the pump setting? Putting it up to what %?
                      Put it in 100%, and close the bypass some more. Not all the way shut but just open enough - get your installer to do this for you. You can't as it means removing the front of the boiler and you can't carry out 26.9 checks.

                      Originally posted by rs1987 View Post
                      In minutes literally...it spends most of its time on anti-cycling and also costs me a fortune (one month I used 4,200Kwh of gas; setting the partial load drops that by 50%). When set to a much lower partial load it still cycles occasionally, but much less frequently. It's like the auto mode is not modulating down at all, as the heating flow gets up to temperature very quickly.
                      There's something a miss then, it shouldn't do that. Even if the boiler was running at 5kw from system cold it shouldn't go into Anti Cycling mode.

                      Originally posted by rs1987 View Post
                      My radiators are all balanced properly, with a 11 degree flow drop across all of them.
                      Have you balanced them yourself? Why did you balance them, did you have radiators that weren't hot?

                      Originally posted by rs1987 View Post
                      No, it only appears to have one option: S.04
                      How do you know it's Anti Cycling then? S.04 is burner on - which it'll modulate in that mode. What does it do when it does Anti Cycling? (What status code). S.53 and S.54 are a common problem when the system isn't set up right.

                      Comment

                      • rs1987
                        Automated Home Jr Member
                        • Nov 2019
                        • 44

                        #12
                        Originally posted by bruce_miranda View Post
                        Check the fault log and see if you have any F.53?
                        Nope. No faults showing.

                        Comment

                        • rs1987
                          Automated Home Jr Member
                          • Nov 2019
                          • 44

                          #13
                          Originally posted by mtmcgavock View Post
                          Put it in 100%, and close the bypass some more. Not all the way shut but just open enough - get your installer to do this for you. You can't as it means removing the front of the boiler and you can't carry out 26.9 checks.
                          Well, this isn't a definitive test yet, but prior to reading your reply I set the pump to 70% and the heating partial load back to auto. It hasn't cycled once in one hour. Normally it's done so multiple times by now.

                          Right now it's keeping the temperature at exactly the set point with the burner on low (looks like 5% at a guess)

                          The only disadvantage to setting the pump speed higher is that it's clearly increasing my return flow temperature. Measuring on the pipes going directly into and out of the boiler, at a 55 degree flow temp, the return temp is just on 50 degrees, so increasing the flow temp to 60 or above would increase the return temp to above 55 and so the boiler won't work in condensing mode. I can live with a 55 degree flow temp though as the radiators are still pretty warm when fully open.

                          Is it ok to leave the pump permanently set at 70% - 100% (D.014) ? I'm not going to break anything, right?

                          EDIT: it has actually cycled now once. The behaviour was a little odd. It kept the burner going even after the temperature went past 55 and got to 59 (despite the flow temp being set at 55). I then got the hourglass cursor and the anti-cycling timer started counting down.

                          Originally posted by mtmcgavock View Post
                          Have you balanced them yourself? Why did you balance them, did you have radiators that weren't hot?
                          Yes, I balanced them myself. They'd never been balanced obviously as all the lock shields were fully open on all the radiators. The problem was that the two radiators at the extreme opposite ends of the house were not getting warm. Now they're toasty and they all get warm at roughly the same speed when starting from cold.


                          Originally posted by mtmcgavock View Post
                          How do you know it's Anti Cycling then? S.04 is burner on - which it'll modulate in that mode. What does it do when it does Anti Cycling? (What status code). S.53 and S.54 are a common problem when the system isn't set up right.
                          Because it shows the hourglass cursor on the LCD display and when checking the time on the anti-cycling countdown timer, it's counting down. The heating icon shows as off and the flow temperature reduces slowly by about ten degrees while the hourglass cursor is displayed. If I then go and reset the anti-cycling time, the hourglass cursor goes off, the boiler turns on again and the radiators heat up. Or alternatively if I just wait until the countdown timer hits zero, then the boiler turns on again.
                          Last edited by rs1987; 15 January 2020, 01:53 AM.

                          Comment

                          • mtmcgavock
                            Automated Home Legend
                            • Mar 2017
                            • 507

                            #14
                            Originally posted by rs1987 View Post
                            Well, this isn't a definitive test yet, but prior to reading your reply I set the pump to 70% and the heating partial load back to auto. It hasn't cycled once in one hour. Normally it's done so multiple times by now.

                            Right now it's keeping the temperature at exactly the set point with the burner on low (looks like 5% at a guess)

                            The only disadvantage to setting the pump speed higher is that it's clearly increasing my return flow temperature. Measuring on the pipes going directly into and out of the boiler, at a 55 degree flow temp, the return temp is just on 50 degrees, so increasing the flow temp to 60 or above would increase the return temp to above 55 and so the boiler won't work in condensing mode. I can live with a 55 degree flow temp though as the radiators are still pretty warm when fully open.

                            Is it ok to leave the pump permanently set at 70% - 100% (D.014) ? I'm not going to break anything, right?

                            EDIT: it has actually cycled now once. The behaviour was a little odd. It kept the burner going even after the temperature went past 55 and got to 59 (despite the flow temp being set at 55). I then got the hourglass cursor and the anti-cycling timer started counting down.



                            Yes, I balanced them myself. They'd never been balanced obviously as all the lock shields were fully open on all the radiators. The problem was that the two radiators at the extreme opposite ends of the house were not getting warm. Now they're toasty and they all get warm at roughly the same speed when starting from cold.




                            Because it shows the hourglass cursor on the LCD display and when checking the time on the anti-cycling countdown timer, it's counting down. The heating icon shows as off and the flow temperature reduces slowly by about ten degrees while the hourglass cursor is displayed. If I then go and reset the anti-cycling time, the hourglass cursor goes off, the boiler turns on again and the radiators heat up. Or alternatively if I just wait until the countdown timer hits zero, then the boiler turns on again.
                            The place i'd start is open all your lockshields full. Put pump into 100% mode and close the bypass some so it's 3/4 closed. Set flow temp at 70oc. Run the system from cold and see if all the rads get hot without the need for balancing. If they're all good run the boiler and see what it does, whether it starts to anti cycle or not and leave running till system is up to temp. With all the rads on it shouldn't anti cycle and modulate down to 5kw.

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                            • rs1987
                              Automated Home Jr Member
                              • Nov 2019
                              • 44

                              #15
                              Originally posted by mtmcgavock View Post
                              Run the system from cold and see if all the rads get hot without the need for balancing
                              They didn't all get hot. The two rads at the extreme ends of the house were not getting warm. That's why I balanced the system in the first place. I balanced it with a constant 85% pump speed. I'm reluctant to open the lock shields fully again as it took hours to balance the system.

                              Originally posted by mtmcgavock View Post
                              . If they're all good run the boiler and see what it does, whether it starts to anti cycle or not and leave running till system is up to temp. With all the rads on it shouldn't anti cycle and modulate down to 5kw.
                              Once balanced, with all rads on it didn't cycle much at all when I set a heating partial load to match the radiator load and kept a constant pump speed.

                              Now, however, since the Evohome system was installed yesterday, it's gone back to anti-cycle mode frequently. It's worse with a higher pump speed when only a small number of rads are calling for heat, as the higher pump speed causes the delta between the flow and the return flow temp to be too small and so the boiler turns off the burner very quickly and the anti-cycle timer kicks in even though Evohome is still calling for heat and some zones are not up to the target temperature.

                              The only thing I can't really touch is the bypass. I'd need to open the boiler to change it and there are dire warnings in the installation manual. It seems you're suggesting that this may be the solution, if I understand you correctly? The installation manual says:


                              8.4 Setting the bypass valve
                              Caution.
                              Risk of material damage caused by incorrect setting of the high-efficiency pump


                              If the pressure at the bypass valve is increased (by turning it clockwise) and the
                              pump output is set to less than 100%, the product may not operate correctly.

                              ▶ In this case, set the pump output to
                              5 = 100% using diagnostics code D.014.
                              However, if I do this then won't I have the same problem, ie. delta between flow and return too low because of the high pump speed, so the burner switches off quickly?

                              I have two towel radiators not under control of evohome and opened fully, so there is an external bypass.
                              Last edited by rs1987; 16 January 2020, 10:14 PM.

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