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Thread: Balancing and delta T with a Modulating Pump

  1. #41
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    Quote Originally Posted by DBMandrake View Post
    The link you posted shows the ABV defaulting to 0.2 bars not 0.1. In any case you were originally referring to a built in bypass in the boiler itself - I was saying I doubt that was set to 0.1 bars, if it has a safety bypass internally it will be set quite high and only open under adverse conditions to protect the boiler.

    As for the graph you're referring to - without knowing what speed the pump is set to and therefore what its maximum head is and what its load lines look like (pressure vs volume flow for the given speed setting) then taking one point on the graph as you have is a bit meaningless.

    Achieving minimum flow for the boiler when you have an ABV is mostly about the adjustment of the pump not the ABV - when all radiators are closed all the flow will go through the ABV regardless of whether its set to 0.1 or 0.6 bars, the only difference is that at a higher setting the higher differential pressure will put a bit more load on the pump and slow down the flow somewhat, but not by a huge amount. That's where the load lines for the pump will help you.

    Meeting the boiler minimum flow is not the primary criterion for setting the ABV pressure, you do that with the pump speed and there are a number of other conflicting requirements that dictate the optimal ABV setting, but its almost never going to be 0.1 bars unless you had a really low resistance pipe network in your house.

    Some of my radiators are microbore (which does of course have more flow resistance than 15mm) and even with all radiators in the house turned fully on the differential pressure is still about 0.3 bar even with no flow through the ABV. If I were to turn the ABV down to 0.1 bars it would drastically reduce the flow through the radiators and send most of the flow through the ABV and straight back to the boiler. (I set my ABV to about 0.4 bars after weighing up conflicting requirements)

    Ideally the ABV should not be flowing at all if at least a couple of radiators are fully open, but I can't achieve that on my system due to the flow resistance of microbore. With 15mm radiator feeds around the house it should be possible though.
    I am really sorry its factory set at 0.2 thats if Ideal haven't changed it anyway to get the minimum flow rate through.

    Its frustrating as I am pretty sure some of the water is going back through the boiler, only getting a 9c drop. The boiler has a modulating pump, I have balanced all the rads up too and I have one that is fully open and its standard 15mm pipework in the house.

    Is there a quick way to adjust this valve? Given its self modulating the pump would slow down if it sensed resistance I would have thought?

    I have read on forums that you do this by turning all TRV's off, along with the bypass being set at its highest then turning the bypass so that its bypassing?

  2. #42
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    For what it's worth - My ABV was set at the default (as left by my plumber in 2010) at .2 setting. When Evohome was installed Nov 2015, both CH zone valves were latched open and un-wired (as advised by installer).
    I began to notice the Worcester Bosch Greenstar 40CDi boiler periodically went into its "gradient limitation" routine. I then started monitoring the ABV/feed/return pipe temps and realised the ABV was definitely passing a goodly portion of flow back to the boiler. This would then trigger the boiler into a re-cycling shutwond (WB call it the gradient limitation). I have read elsewhere that WB can replace the control board to prevent the gradient limitation which isn't catastrophic but does increase running costs and operating down-time.

    Having asked DBMandrake (and per other comments) re ABV setting, I changed this to a fraction under .6 (thus greater pressure required to open the ABV). I am still seeing sporadic gradient limitation (cycling) activity.
    I am also aware that the Grundfos UPS 15/60 pump is running on the highest setting. I suspect that because my boiler is nigh on a commercially sized system boiler, it will cycle from time to time unless the majority of the 23 radiators + 3 towel rails in our house are calling for heat. I suspect I could turn the pump down a notch and see what happens.

    The boiler manual does not indicate the minimum flow rate so I am not certain what the recommended pump speed/ABV setting should be - other than guesswork.

    I hope some of the above might help the recent posters on this topic.

  3. #43
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    Not even sure if the Ideal Vogue has the same ABV but going to have a look at it tonight. I only have 7 rads but its got a self modulating pump.

  4. #44
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    Quote Originally Posted by killa47 View Post
    For what it's worth - My ABV was set at the default (as left by my plumber in 2010) at .2 setting. When Evohome was installed Nov 2015, both CH zone valves were latched open and un-wired (as advised by installer).
    I began to notice the Worcester Bosch Greenstar 40CDi boiler periodically went into its "gradient limitation" routine. I then started monitoring the ABV/feed/return pipe temps and realised the ABV was definitely passing a goodly portion of flow back to the boiler. This would then trigger the boiler into a re-cycling shutwond (WB call it the gradient limitation). I have read elsewhere that WB can replace the control board to prevent the gradient limitation which isn't catastrophic but does increase running costs and operating down-time.

    Having asked DBMandrake (and per other comments) re ABV setting, I changed this to a fraction under .6 (thus greater pressure required to open the ABV). I am still seeing sporadic gradient limitation (cycling) activity.
    I am also aware that the Grundfos UPS 15/60 pump is running on the highest setting. I suspect that because my boiler is nigh on a commercially sized system boiler, it will cycle from time to time unless the majority of the 23 radiators + 3 towel rails in our house are calling for heat. I suspect I could turn the pump down a notch and see what happens.
    Is that an old UPS pump with the rotary switch or the new electronically controlled UPS2 with the push button and optional variable speed modes ? I would definitely try dropping the pump speed down to medium to see if it matters improve in relation to the gradient limitation. It's not going to harm the system.

    If the pump speed is too low you might see the boiler short cycling due to the flow of water not being sufficient at the minimum modulation of the boiler, but you'd probably only see that happen on minimum speed, and it wouldn't trigger a gradient limit.

    On the other hand if the pump speed is too high you're more likely to trigger a gradient limit warning because the faster the flow the closer (hotter) the return temperature will be compared to the flow, both because it will force the ABV to flow more, (thus increase the proportion of hot water mixing directly back into the return without passing through radiators) and also because it will reduce the temperature drop across the radiators due to the faster flow through the radiators. (less time for the radiators to extract heat per litre of water flowing through them)
    Last edited by DBMandrake; 28th November 2016 at 03:59 PM.

  5. #45
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    Quote Originally Posted by DBMandrake View Post
    Is that an old UPS pump with the rotary switch or the new electronically controlled UPS2 with the push button and optional variable speed modes ? I would definitely try dropping the pump speed down to medium to see if it matters improve in relation to the gradient limitation. It's not going to harm the system.

    If the pump speed is too low you might see the boiler short cycling due to the flow of water not being sufficient at the minimum modulation of the boiler, but you'd probably only see that happen on minimum speed, and it wouldn't trigger a gradient limit.

    On the other hand if the pump speed is too high you're more likely to trigger a gradient limit warning because the faster the flow the closer (hotter) the return temperature will be compared to the flow, both because it will force the ABV to flow more, (thus increase the proportion of hot water mixing directly back into the return without passing through radiators) and also because it will reduce the temperature drop across the radiators due to the faster flow through the radiators. (less time for the radiators to extract heat per litre of water flowing through them)
    The older manual UPS with 3 speeds - running on 3. Not sure whether Evohome installer increased speed to 3 from previous setting. I temporarily changed it down to 2 and it seemed to behave but was reluctant to leave it in case of other side effects. Agree your comments about trying it on 2 - I thought likewise but somehow chickened out on trying it for a few days. Have not been able to find a minimum flow rate for the boiler anywhere so graphs don't help me.

    Many thanks for input as always. Where the heck do you find the time to solve everybody's troubles.
    Last edited by killa47; 28th November 2016 at 09:30 PM. Reason: correction

  6. #46
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    The vogue doesn't have an adjustable bypass from what I can see. It says it's built in on the literature but no mention of it in the service manual other than one needs to be fitted if using 2 port valves.

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