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Thread: Heating Partial Load

  1. #11
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    Quote 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.

    Quote 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.

    Quote 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?

    Quote 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.

  2. #12
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    Quote Originally Posted by bruce_miranda View Post
    Check the fault log and see if you have any F.53?
    Nope. No faults showing.

  3. #13
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    Quote 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.

    Quote 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.


    Quote 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; 15th January 2020 at 12:53 AM.

  4. #14
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    Quote 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.

  5. #15
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    Quote 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.

    Quote 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; 16th January 2020 at 09:14 PM.

  6. #16
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    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.

  7. #17
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    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.

  8. #18
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    Quote 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

  9. #19
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    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?

  10. #20
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    Viessmann doesn't play nicely with Evohome and Opentherm. Do you have OT?

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