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Thread: Heating Partial Load

  1. #21
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    Quote 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; 17th January 2020 at 10:12 PM.

  2. #22
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    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.

  3. #23
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    Quote 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?

  4. #24
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    Quote 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; 18th January 2020 at 12:45 AM.

  5. #25
    Automated Home Legend paulockenden's Avatar
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    Quote 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; 18th January 2020 at 09:47 AM.

  6. #26
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    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.

  7. #27
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    Quote 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.

  8. #28
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    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).

  9. #29
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    Quote 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.

    Quote 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.

  10. #30
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    Quote 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.

    Quote 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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