Heating Partial Load

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  • rs1987
    Automated Home Jr Member
    • Nov 2019
    • 44

    #16
    UPDATE: I had a conversation with Richard from the Evohome Shop where I bought my kit, who as always, was incredibly helpful. Apart from saying that the ultimate solution is to get a boiler that's correctly sized as this one can't modulate below 6kw, which in many cases is way too much for a single or two rads and the ultimate cause of the problem, he did suggest I balance to a 20 degree drop. I did try, but the rads did not get hot when I went for a 20 degree drop. They did seem to be ok on a 15 degree drop.

    The curious thing is that even with a 15 degree differential across all rads, when they were all on simultaneously, I certainly didn't see anything more than about a 5 - 7 degree flow-return differential at most at the boiler (which is a little better than before). I presume this is because hot water is being dumped directly into the return flow via the bypass valve? @mtmcgavock is this why you suggest to close down the bypass valve to 3/4 closed? And if so, while closing the bypass should lower the return flow temperature as it's dumping less hot water directly into the return, won't the increased pump speed of 100% simply narrow the flow-return differential again, causing the behaviour to be similar?

    At this point I'm really cursing the genius who decided to install a 38kw boiler where a 24kw at most would have been more than adequate.

    Comment

    • bruce_miranda
      Automated Home Legend
      • Jul 2014
      • 2307

      #17
      I am in exactly the same position. My silly plumber got me to get the biggest boiler because he told me bigger is better! I had months/years of issues because the boiler is too big. Finally now the boiler set to 18Kw is working fine. What I noticed with the Vaillant 438 was that just set the Partial Load to 18Kw made the bottom end drop to 5Kw making it behave like a 418. You could try that on your boiler and see if you are seeing the same thing. However unless you have some way of reading the Relative Modulation of the boiler, you will never know if the lower end of the modulation band has changed or not.

      Comment

      • rs1987
        Automated Home Jr Member
        • Nov 2019
        • 44

        #18
        Originally posted by bruce_miranda View Post
        I am in exactly the same position. My silly plumber got me to get the biggest boiler because he told me bigger is better! I had months/years of issues because the boiler is too big. Finally now the boiler set to 18Kw is working fine. What I noticed with the Vaillant 438 was that just set the Partial Load to 18Kw made the bottom end drop to 5Kw making it behave like a 418. You could try that on your boiler and see if you are seeing the same thing. However unless you have some way of reading the Relative Modulation of the boiler, you will never know if the lower end of the modulation band has changed or not.
        Thanks Bruce. Yes, I've done that already - it's set to 12kw, but still the problem comes in when only one rad calls for heat. Since the minimum heat output is 6kw on my boiler, it's way too high for a single or even two rads.

        The 938 has an LCD display and the burner level is displayed graphically. It certainly appears as if it has modulated to the minimum

        Comment

        • rs1987
          Automated Home Jr Member
          • Nov 2019
          • 44

          #19
          So the property I'm in at the moment is actually a rented property. If I owned it, I'd have no hesitation in replacing the boiler.

          That said, the heating performance is so miserable that I have two choices: replace the boiler or move house. if I compare the cost of moving house, it's way more than the cost of replacing the boiler, so I'm considering proposing a better boiler to the landlord, at my cost.

          I think the landlord would be unlikely to agree to downsizing the boiler as it's a large house with more than one bathroom, and if a large family is staying here, the boiler needs to be able to run multiple hot water taps / showers at once, which probably explains why he put the 938 in - it's sized for hot water generation and not for heating.

          The solution seems to be to find a boiler with a very low minimum heat output, so I did a search for the boiler with the lowest minimum heat output on the market and it turns out that the Viessmann Vitodens 200w, while being a large boiler, can actually modulate down to 1.9kw at minimum (it has a 1:17 modulation range), which means it's unlikely to cycle since I have two unmanaged towel rails; together with a single rad, these two towel rails are likely to be more than 1.9kw at any one time.

          This Viessmann boiler has identical dimensions to the Vaillant 938 I have, so it'll fit directly into the space with no modifications needed. And given that it's maximum output and capability is the same as the Vaillant 938 and it has a whole bunch of other nifty features like inbuilt weather compensation, the landlord shouldn't really have much room for objection on the grounds of its technical features and specifications.

          So first question for all of you experts: based on the tech specs of this boiler, would it be likely to improve my heating performance (ie., eliminate the constant cycling) and is it compatible with Evohome?

          Comment

          • bruce_miranda
            Automated Home Legend
            • Jul 2014
            • 2307

            #20
            Viessmann doesn't play nicely with Evohome and Opentherm. Do you have OT?

            Comment

            • rs1987
              Automated Home Jr Member
              • Nov 2019
              • 44

              #21
              Originally posted by bruce_miranda View Post
              Viessmann doesn't play nicely with Evohome and Opentherm. Do you have OT?
              No. The Vaillant doesn't play nicely either. I've just got the BDR91 wired directly into the boiler at the moment. Apparently using the OT interface sold in the Netherlands will void the warranty on the boiler, so I am not using it

              Wouldn't it be a greater priority to have the boiler able to modulate to a very low level instead of prioritising Opentherm compatibility?

              Also this Viessmann has integrated weather control, so it will automatically modulate the flow temperature and doesn't theoretically need Opentherm if I understand things correctly
              Last edited by rs1987; 17 January 2020, 10:12 PM.

              Comment

              • paulockenden
                Automated Home Legend
                • Apr 2015
                • 1719

                #22
                Why not go Intergas?

                Your landlord will love it because they tend not to go wrong, so fewer callouts.
                Your heating engineer might hate it for the very same reason though!

                Plays well with Evohome using OpenTherm.

                P.

                Comment

                • rs1987
                  Automated Home Jr Member
                  • Nov 2019
                  • 44

                  #23
                  Originally posted by paulockenden View Post
                  Why not go Intergas?

                  P.
                  Because I'll still have the same problem and be out of pocket by a few thousand pounds. Intergas boilers all have minimum heat outputs in the 7kw - 8kw range, which is even higher than my existing Vaillant 938. It's this high minimum rating that is causing the boiler to go into anti cycle mode constantly on low heating loads, as even 6kw is too high in most cases.

                  The Viessmann has the lowest minimum rating on the market, able to modulate 17:1 down to 1.9kw.

                  The Viessman has very good inbuilt weather control, which obviates the need for Opentherm surely?

                  Comment

                  • blowlamp
                    Automated Home Sr Member
                    • Apr 2017
                    • 98

                    #24
                    Originally posted by rs1987 View Post
                    Because I'll still have the same problem and be out of pocket by a few thousand pounds. Intergas boilers all have minimum heat outputs in the 7kw - 8kw range, which is even higher than my existing Vaillant 938. It's this high minimum rating that is causing the boiler to go into anti cycle mode constantly on low heating loads, as even 6kw is too high in most cases.

                    The Viessmann has the lowest minimum rating on the market, able to modulate 17:1 down to 1.9kw.

                    The Viessman has very good inbuilt weather control, which obviates the need for Opentherm surely?


                    The Intergas Xclusive and Xtreme models modulate down to ~ 3.5 kW.

                    OpenTherm is a two-way comms protocol that can control many boiler functions. If you want to closely control CH flow temperature via a separate thermostat as you would with a weather compensated system, then you need a protocol like OpenTherm to do it.
                    Last edited by blowlamp; 18 January 2020, 12:45 AM.

                    Comment

                    • paulockenden
                      Automated Home Legend
                      • Apr 2015
                      • 1719

                      #25
                      Originally posted by rs1987 View Post
                      The Viessmann has the lowest minimum rating on the market, able to modulate 17:1 down to 1.9kw.

                      The Viessman has very good inbuilt weather control, which obviates the need for Opentherm surely?
                      Nope. Without OpenTherm it'll be a BDR91 cycling your boiler on and off. There's no way for it to know that the heating loop needs, say, 30 degrees to keep your house warm. So nothing that's going to tell it to modulate down to that 1.9kW. You'll just have pulses of very hot water wandering around the system.


                      Plus if you google 'boiler anti cycle' you'll see it's the owners of just one brand of boiler that seem to have a problem with it.
                      Last edited by paulockenden; 18 January 2020, 09:47 AM.

                      Comment

                      • mtmcgavock
                        Automated Home Legend
                        • Mar 2017
                        • 507

                        #26
                        No matter what you'll still have the same issue - a Green IQ Vaillant will modulate 1:10, but that still will only modulate to 3.5kw Which is more than just one rad running at once.

                        Have you actually tried doing what i've said? Unbalance all the rads and put the put the pump into 100% mode, leave the Partial Load on Auto, turn all the rads on. Run the system from cold and let it heat up and modulate down. I'd also close the bypass a bit.

                        The issue here is flow, you've restricted all the rads thus the boiler isn't getting enough flow for it's output. If you are saying it's Anti Cycling before Evohome was even fitted then you've got issues with the system. A 938 should easily do 10 rads without any issues without Anti Cycling.

                        The boiler to me sounds like it's been correctly sized - what others are referring to on here are more conventional systems with either Heat Only/System boilers where yes this will make a difference. A large boiler on a normal system is never going to work - most normal houses only need 15-18kw for the heating side thus fitting a 38kw boiler is always going to give problems.

                        Comment

                        • rs1987
                          Automated Home Jr Member
                          • Nov 2019
                          • 44

                          #27
                          Originally posted by mtmcgavock View Post
                          No matter what you'll still have the same issue - a Green IQ Vaillant will modulate 1:10, but that still will only modulate to 3.5kw Which is more than just one rad running at once.

                          Have you actually tried doing what i've said? Unbalance all the rads and put the put the pump into 100% mode, leave the Partial Load on Auto, turn all the rads on. Run the system from cold and let it heat up and modulate down. I'd also close the bypass a bit.
                          I was initially reluctant to try this as it took me so long to balance the rads. That said, I've now done as you suggest (although I have not touched the bypass). It's only been running a few hours, so I need to monitor it over a day or two to see how it behaves, especially since it's rather cold today. However, initially I can say that

                          • With all lock shields open and all rads on, the two rads at the furthest extremity do not get hot. They barely get lukewarm. This is why I balanced in the first place. However, in normal use it's unlikely that all my rads will be on at once and they do seem to get warm once a few other rads are closed down
                          • With heating partial load on auto and pump set to 100%, it did not go into anti-cycle mode and I didn't get an hourglass cursor. Once everything was hot, however, it did turn off the burner without going into anti-cycle mode once or twice over a hour and a half
                          • I then reactivated evohome with all zones set to 30 degrees so that all the rads were fully open and then restored each zone to it's scheduled temperature one by one, waiting about ten minutes between changes. I have not seen the boiler anti-cycling yet, although I have not been watching it all the time, so I might have missed it. However, it definitely anti-cycled a lot more before I did what you suggested above. The boiler did switch the burner off once or twice without anti-cycling; I'm not entirely sure why this behaviour occurs as the radiators are still calling for heat (unless it is actually cycling and I return just in time to see the anti-cycle timer expired and before the burner turns on again; I suspect not though as I've not seen the hour glass cursor and normally in anti-cycling mode the rads cool down a lot)
                          • The delta between my return and flow is around 5 degrees so I can't put my heating flow much higher than 57 as it overshoots by a degree or three sometimes and I need to keep the return flow at or below 55 degrees for condensing to occur, otherwise suffer a higher gas bill


                          I have a radiator upstairs in a dressing room off the hallway which I never normally turn on and it is not managed by evohome. I've taken the manual TRV off this radiator and opened the lock shield fully as well, so it's constantly dumping heat. This should help dissipate more heat at the expense of a higher gas bill, although some of this heat bleeds out into the hallway which should cause the hallway radiator not to need to go on as frequently.

                          Comment

                          • rs1987
                            Automated Home Jr Member
                            • Nov 2019
                            • 44

                            #28
                            Ok, so it's still anti-cycling, but not nearly as much as before, so that seems to be an improvement, although when the weather gets warmer I suspect it might anti-cycle more frequently.

                            You said to leave the partial load on auto. Is there any reason for this? As far as I understand it, setting the partial load to a figure caps the maximum but doesn't do anything to prevent the boiler modulating down to the minimum, right?

                            So if this is correct, is there any reason why I should not switch the heating partial load back to a capped figure (12kw is sufficient for my average load).

                            Comment

                            • mtmcgavock
                              Automated Home Legend
                              • Mar 2017
                              • 507

                              #29
                              Originally posted by rs1987 View Post
                              I was initially reluctant to try this as it took me so long to balance the rads. That said, I've now done as you suggest (although I have not touched the bypass). It's only been running a few hours, so I need to monitor it over a day or two to see how it behaves, especially since it's rather cold today. However, initially I can say that
                              • With all lock shields open and all rads on, the two rads at the furthest extremity do not get hot. They barely get lukewarm. This is why I balanced in the first place. However, in normal use it's unlikely that all my rads will be on at once and they do seem to get warm once a few other rads are closed down
                              • With heating partial load on auto and pump set to 100%, it did not go into anti-cycle mode and I didn't get an hourglass cursor. Once everything was hot, however, it did turn off the burner without going into anti-cycle mode once or twice over a hour and a half
                              • I then reactivated evohome with all zones set to 30 degrees so that all the rads were fully open and then restored each zone to it's scheduled temperature one by one, waiting about ten minutes between changes. I have not seen the boiler anti-cycling yet, although I have not been watching it all the time, so I might have missed it. However, it definitely anti-cycled a lot more before I did what you suggested above. The boiler did switch the burner off once or twice without anti-cycling; I'm not entirely sure why this behaviour occurs as the radiators are still calling for heat (unless it is actually cycling and I return just in time to see the anti-cycle timer expired and before the burner turns on again; I suspect not though as I've not seen the hour glass cursor and normally in anti-cycling mode the rads cool down a lot)
                              Yes so it's basically behaved really as I expected - part of the issue is your restricted flow when you've balanced them down. I'd expect the boiler turn the boiler on and off as you describe.

                              Originally posted by rs1987 View Post
                              Ok, so it's still anti-cycling, but not nearly as much as before, so that seems to be an improvement, although when the weather gets warmer I suspect it might anti-cycle more frequently.

                              You said to leave the partial load on auto. Is there any reason for this? As far as I understand it, setting the partial load to a figure caps the maximum but doesn't do anything to prevent the boiler modulating down to the minimum, right?

                              So if this is correct, is there any reason why I should not switch the heating partial load back to a capped figure (12kw is sufficient for my average load).
                              Yeah there was a reason - just to determine if it would run OK left on Auto. Which seems it did behave as it should. The furthest two rads will probably need the bypass closing some more before they will work, however you are quite correct now you have Evohome i'll balance itself out better than it did before.

                              You say it's now Anti Cycling again since your first test - what has changed? Have you turned EvoHome back on to their normal set points?

                              In terms of your flow temperature, the boiler will still condense at a flow temperature of 75oc, albeit not as much. It doesn't really mean it isn't as efficient though. Personally I'd conduct two tests, one at a flow temperature of 75oc and one at 55oc to see what your gas usage is. The issue probably is that the radiators in the property aren't sized correctly enough to give you a big enough drop in your flow temperature, and as you say as the system gets up to temperature the issue will become worse.

                              In terms of the Partial Load Setting i'd now calculate the total output of all the rads and then set the Partial load to this Output and take it from there.

                              I'd leave the rads unbalanced for now, as when you're balancing them down your creating other issues. You can address the balancing at a later date, once you've solved the Anti Cycling issue.

                              Comment

                              • rs1987
                                Automated Home Jr Member
                                • Nov 2019
                                • 44

                                #30
                                Originally posted by mtmcgavock View Post
                                Yes so it's basically behaved really as I expected - part of the issue is your restricted flow when you've balanced them down. I'd expect the boiler turn the boiler on and off as you describe.



                                Yeah there was a reason - just to determine if it would run OK left on Auto. Which seems it did behave as it should. The furthest two rads will probably need the bypass closing some more before they will work, however you are quite correct now you have Evohome i'll balance itself out better than it did before.

                                You say it's now Anti Cycling again since your first test - what has changed? Have you turned EvoHome back on to their normal set points?

                                In terms of your flow temperature, the boiler will still condense at a flow temperature of 75oc, albeit not as much. It doesn't really mean it isn't as efficient though. Personally I'd conduct two tests, one at a flow temperature of 75oc and one at 55oc to see what your gas usage is. The issue probably is that the radiators in the property aren't sized correctly enough to give you a big enough drop in your flow temperature, and as you say as the system gets up to temperature the issue will become worse.
                                Yes, I turned Evohome back to its normal set point. It only anti-cycles, and infrequently, when the rest of the zones are up to temperature and there is only one rad open. It's a big improvement.

                                You're correct that the rads are undersized in some rooms. So I have a double problem: oversize boiler and undersize rads. The main hallway rad is on pretty much permanently just to maintain the zone at 19 degrees. The living room rad is too small for the room but eventually manages to get the room to 21 degrees after a few hours, although it then is also on pretty much permanently in order to maintain the temperature. The conservatory (double glazed) which serves as a dining room has a woefully undersized radiator that cannot even maintain a 15 degree set-point in this weather, so I leave it off and heat with a 3kw electric radiator during the times I need to use the room.

                                I'll experiment with different flow temps to check what the gas usage as. I really need to run at a higher temp to compensate for these undersized rads. I'm a little fearful of the result though, as I recall before I put in Evohome I had a gas bill of £200+ for 30 days usage in one month when running a flow temp of 65 degrees and only a few rooms in the house were actually being heated.

                                Originally posted by mtmcgavock View Post
                                In terms of the Partial Load Setting i'd now calculate the total output of all the rads and then set the Partial load to this Output and take it from there.

                                I'd leave the rads unbalanced for now, as when you're balancing them down your creating other issues. You can address the balancing at a later date, once you've solved the Anti Cycling issue.
                                Great, I'll do this. So next step it seems is to have an engineer come and close down my bypass valve from the default to about 3/4 closed, if I recall your instructions correctly.

                                Thanks for help and advice.

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