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Thread: Heating Partial Load

  1. #1
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    Default 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.

  2. #2
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    Quote 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.

  3. #3
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    Quote 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; 13th January 2020 at 02:51 PM.

  4. #4
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    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.

  5. #5
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    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.

  6. #6
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    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.

  7. #7
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    Quote 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.

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

  9. #9
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    Quote 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

    Quote 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.

    Quote 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

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

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