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Thread: Balancing and delta T with a Modulating Pump

  1. #11
    Automated Home Legend paulockenden's Avatar
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    I suppose an internal bypass won't have the desired effect during over-run (losing heat from the system). Hence the requirement for something external, or an open rad.

  2. #12
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    Quote Originally Posted by DBMandrake View Post
    That was me. The standard Honeywell V4043 2 port zone valve only opens less than half way when on the filling latch, so if you want to bypass it my recommendation is to wire it to the power supplying the pump, if possible. This will make sure it fully opens whenever the pump is running but not leave it permanently on wasting power or seizing up.

    This is probably even more important with a variable speed pump configuration as the pump will "sense" the extra restriction and may be confused by this.

    Side question for DBMandrake

    You said that the Honeywell V4043 2 port zone valve, when latched, only opens part way. When my installer fitted Evohome last year, he latched open both zone valves. The flow differential temperatures at the ABV by-pass are usually less than the 10deg C differential that the Bosch Greenstar boiler expects to see so it often goes into a "Gradient Limitation" routine which then runs the condensate process, drops flow temp down and re-fires the burner.

    Could it be that the [half] open latched valves create an increase in pressure causing more flow to redirect via the ABV and thus not having dissipated sufficient heat, triggers the gradient limitation.
    If so, should I follow (for different reasons) the recommendation to hardwire the zone valves to a permanent live rather than leave them latched open?
    Last edited by killa47; 15th November 2016 at 10:42 AM. Reason: correction

  3. #13
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    Quote Originally Posted by killa47 View Post
    [B][U]
    If so, should I follow (for different reasons) the recommendation to hardwire the zone valves to a permanent live rather than leave them latched open?
    Is there ever a recommendation to connect them to permanent live? It tends to make the spring less springy, the motor fail, and the valve stick.

    I'm new to the EvoHome way of doing things, but took the view that whatever the old 2-channel controller was doing via S-plan was good enough so no reason not to put the EvoHome bits in the same way - ie controlling zone valves rather than boiler. It also made installation simpler since I only had to change the connections in the 5-core cable to that programmer. Happy to rewire it if there is a reason, but missing what that reason might be. Can someone explain it?

  4. #14
    Automated Home Legend paulockenden's Avatar
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    You get more control options.

    I'm half tempted to pair a spare BDR91 so that my controller /thinks/ it has a boiler relay, but leave the actual control hanging off the back of the two port valves. Just to open up those extra parameters.

    Can anyone see a flaw in that?

  5. #15
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    I don't see why that wouldn't work Paul.

  6. #16
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    Quote Originally Posted by killa47 View Post

    Side question for DBMandrake

    You said that the Honeywell V4043 2 port zone valve, when latched, only opens part way. When my installer fitted Evohome last year, he latched open both zone valves. The flow differential temperatures at the ABV by-pass are usually less than the 10deg C differential that the Bosch Greenstar boiler expects to see so it often goes into a "Gradient Limitation" routine which then runs the condensate process, drops flow temp down and re-fires the burner.

    Could it be that the [half] open latched valves create an increase in pressure causing more flow to redirect via the ABV and thus not having dissipated sufficient heat, triggers the gradient limitation.
    If so, should I follow (for different reasons) the recommendation to hardwire the zone valves to a permanent live rather than leave them latched open?
    Yes, the half open valve would cause a flow restriction that would tend to cause more of the flow to go through the ABV as the loop to the radiators becomes a more difficult path compared to the ABV.

    And as you say, any flow that goes through the ABV will not drop in temperature like the flow through a radiator, so too much flow through an ABV will raise the return temperature, which will be a blend between the hotter water coming through the ABV and the cooler water returning from the radiators, in proportional to how much of the return flow comes either way, just like adjusting a mixer tap.

    I think it's also fair to say that setting an ABV too low in pressure is not good for condensing boiler efficiency as you're raising the return flow temperature unnecessarily by having the ABV flowing most of the time. Ideally you only want the ABV to flow when there is only one radiator flowing or a few only slightly open, to limit the maximum pressure and avoid the radiators that are on getting too noisy. But if you have at least two radiators fully open there shouldn't really be any flow through the ABV - that's the point of using a pressure relief valve design instead of the old fashioned slightly open gate valve that passes some flow all the time.
    Last edited by DBMandrake; 15th November 2016 at 01:36 PM.

  7. #17
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    Quote Originally Posted by scut0 View Post
    Nope. Can see graphs of most things but not gas flow/modulation. Only flame on or off. Can see fan speed so maybe that would be a guide. Ref my two way valve I've actually got that open with a permanent electrical (rather than the mechanical lever) supply so no issues there.

    Interestingly I turned on another zone this afternoon (office) and while that was getting up to temperature (40 minutes or so) the pump was running at 100 percent again and the delta T at boiler was very low.

    That radiator is quite small (approx 0.75kw) but i had left the lockshield quite wide open as it is one of the furthest from the boiler. I wonder if, while the hr92 was wide open on this rad and the rest of the open hr92s were quite closed (as their respective rooms were already at temperature) maybe the office circuit was flowing the majority of the water acting like a bypass and thus leading to a low delta T at the boiler.

    The thing that I can't work out is why the pump would run at 100 percent, I would of thought it would slow down to increase the delta T.

    Once the office got up to temperature (and presumably the hr92 started closing off) everything reverted to good pump modulation and good delta T.

    Makes my head hurt.
    I wonder if your pump is oversized for the system ?

    I came across this:

    http://www.modbs.co.uk/news/archives...ve_energy.html

    In Zurich, legislation was brought in to force owners of large buildings to change to variable-speed pump drives. At face value, this appeared to be a sensible piece of legislation. However, when comparing year-on-year consumption adjusted to degree days it was found that many variable-speed drives installed in response to legislation to reduce the energy used by heating pumps in Zurich caused electricity consumption to increase.

    A fundamental error was made; nobody checked the design and actual duty points before the variable-speed pumps were installed. Put simply, the contractors had matched the new pumps with the old, which of course were considerably over-sized.

    If variable-speed pumps are over-sized, the computer chip in the pump cannot see small changes in system resistance when working at the end of the pump curve. Consequently, the pumps will always run at maximum capacity and maximum power consumption; a pump with maximum power consumption will always run excessively hot, causing failure of the motor windings. Measurements were taken and most buildings were found to be over pumping. Considerably smaller pumps were then fitted, resulting in a significant reduction in power consumption. The message from many other pump manufacturers is that a variable-speed drive on a pump will solve all problems but this is only applicable if the pump is correctly sized.
    Food for thought anyway - if the normal operation point of your system is at one extreme of the range that the pump can deal with (pump under or over sized) then it may not modulate the speed in an appropriate manner.

  8. #18
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    Quote Originally Posted by DBMandrake View Post
    Ideally you only want the ABV to flow when there is only one radiator flowing or a few only slightly open, to limit the maximum pressure and avoid the radiators that are on getting too noisy. But if you have at least two radiators fully open there shouldn't really be any flow through the ABV - that's the point of using a pressure relief valve design instead of the old fashioned slightly open gate valve that passes some flow all the time.
    As we have two upstairs towel rails - the one in our ensuite constantly on as it is a manual valve plus a landing radiator always on, I suppose I could almost shutdown the ABV or leave it set on minimum.

    The ABV is a Honeywell DU144 type which sets from .1 bar to .6 bar so I presume I should probably now set this at say .1 or .2 bar at most so it is almost closed.

    Am I correct on this setting or should it be .5 or.6 bar for almost closed?

    I presume I could temporarily close it fully and see what happens?
    Last edited by killa47; 15th November 2016 at 05:50 PM. Reason: correction

  9. #19
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    A higher setting is more closed as it requires a higher pressure differential to overcome the spring that holds the valve shut.

    Be careful not to adjust it to more than 0.6 - some brands of ABV (like the Corgi one I use, which I think is identical to the Honeywell one) don't take kindly to being turned past 0.6 as the dial jumps teeth and gets out of calibration.

  10. #20
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    Quote Originally Posted by paulockenden View Post
    You get more control options.
    Thanks, that helped me track down some other discussions and realise that the S-plan wiring I'd adopted from the Installation Guide was for "simple" conversion of S-plan without adding radiator valves. I'd assumed they'd just missed off the HR92s for clarity / brevity.

    So I think I stumbled into a configuration that works for me at the moment but has hidden some of the detail of what controls what away from me. I'm still hazy on some of the configuration terminology, but...

    1) We originally had standard 2-channel programmer controlling CH and HW via S-plan zone valves.
    2) HW has stayed on its original programmer channel.
    3) CH is now on EvoHome, with BDR91 now controlling the CH zone valve. I think this is installed as a boiler controller.
    4) Evo finds a chilly room, opens that rad valve and calls for heat from boiler via BDR91. It happens to work just fine because the CH valve is effectively passing that request to the boiler.

    Maybe this works because I'm only using CH. Am I right in thinking that if I added HW control as well then Evo would trigger two BDR91 - one that it thinks is connected to the boiler, and one that is controlling the HW zone valve? If so then this would be semi-bad in my case as it would cause the CH valve to open as well as the HW valve. However only "semi" since if all the HR92 are closed it won't actually create an extra flow path. And maybe Evo wouldn't do this anyway because it knows that simply triggering the HW valve should fire the boiler without an explicit call from itself.

    Quote Originally Posted by paulockenden View Post
    I'm half tempted to pair a spare BDR91 so that my controller /thinks/ it has a boiler relay, but leave the actual control hanging off the back of the two port valves. Just to open up those extra parameters.

    Can anyone see a flaw in that?
    This threw me more. It seems to be describing what I have, and I do see editable values for Cycle Rate and Minimum On Time if they are the ones you mean.

    Clearly I'm a little confused, but thanks for making me think about it .
    Last edited by Little Tinker; 15th November 2016 at 11:56 PM.

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