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Thread: Evohome / Automatic bypass / Alpha 2 Smart Pump

  1. #1
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    Default Evohome / Automatic bypass / Alpha 2 Smart Pump

    Hi,

    Three components and their interactions I've been trying to get my head around for a while now!

    I currently have the Evohome system installed on a system with a MANUAL bypass valve, along with a single radiator (large and one of furthest away on the system) WITHOUT an Evohome TRV.. the latter in part not by choice (this is the hallway where the old room stat used to be and therefore no TRV body to easily clip a Evohome head to) however actually works reasonably well as this area is by far the coldest area in the house so benefits from the extra heat. I also have replaced my old knackered pump with an Alpha2 pump (which includes the ability to auto-adapt, a useful feature for a 14 rad system across 3 floors that will often be run with only a few rads open). Lastly this is a y-plan setup with flow and return.

    Firstly, the manual bypass. While I understand its purpose (open just enough to ensure that if all the rads are closed there will still be the minimum flow required so as not to overheat the boiler, knacker the pump etc) and while I'm sure such a thing was invented by someone who knows much more about heating systems than myself (!) I simply can't see how having this left permanently in this "a bit open" state can be efficient?! Surely whenever I'm say heating water I'm wasting heat diverted through the bypass unnecessarily. Similarly when heating rads - surely I want all the heat channeled to the rads and not some escaping via a shortcut back to the return?!

    Now in my setup at the moment I *always* have a single radiator open, so I don't think I actually *need* a bypass at all as the radiator itself acts as a bypass (I'm talking practically here, not per perhaps the latest building regs. or to cover extremely unlikely scenarios that someone would access this radiator behind its boxed cover and inexplicably turn it off!), with this in mind I've previously completely closed the manual bypass gate with no ill-effect (can't remember how it is right now, might have reopened it a little).

    This said I would prefer to have control over this last radiator (fitting a TRV body and Evohome head to it) and therefore THINK I would like an automatic bypass valve fitted. I understand how these work (setup to provide minimum flow rate when all rads closed, otherwise will remain closed itself) and like the idea HOWEVER I'm concerned whether or not this 'variable' will counteract the auto adapting pump?? ie. with both on the same system will the minimum flow rate still be maintained or will they be canceling each other out / constantly trying to readjust?

    If they aren't compatible I see two choices: 1) stick with a manual bypass that is setup correctly (hopefully after someone here has reassured me the wastage I'm imagining is negligible!) or 2) go with an auto bypass and run my pump in standard fixed mode (though would seem like I'm wasting a useful feature)

    Numerous question here I know - sorry - this is hopefully my last bit of knowledge gap I need filling of my CH system!
    Last edited by SimonW500; 1st March 2015 at 06:57 PM.

  2. #2
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    I would assume when all radiators are closed, the pressure rises. So the alpha pump will start pumping faster. So even more pressure. Bypass will open. Pressure goes down. Pump speed goes down. Pressure goes down even further. Bypass closes. Pressure builds up again. Etc. Etc.

    I don't think you should use auto adapt and auto-bypass together indeed.

    http://www.newtonnet.co.uk/permanent...Alpha2or2L.pdf might be helpful...
    Last edited by erik; 1st March 2015 at 07:05 PM.

  3. #3
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    Quote Originally Posted by erik View Post
    I would assume when all radiators are closed, the pressure rises. So the alpha pump will start pumping faster. So even more pressure. Bypass will open. Pressure goes down. Pump speed goes down. Pressure goes down even further. Bypass closes. Pressure builds up again. Etc. Etc.

    I don't think you should use auto adapt and auto-bypass together indeed.
    Thanks for the prompt reply. Indeed we are thinking on the same lines: the two pressure-based adapting components might adversely effect each other!

    nb. while I know little about various pressure states within heating systems.. in laymans terms the way the pump auto adapts is the more rads closed the SLOWER it pumps.. presumably due to less pipework and thus resistance to push water through?

    In your evidently knowledgable opinion is there any real disadvantage with a manual bypass (that has been properly adjusted)?
    Last edited by SimonW500; 1st March 2015 at 07:04 PM.

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    in theory, water going through the bypass would cause your return temperature to rise, because it does not have a chance to cool down in a radiator. Higher return = making your boiler less efficient. A boiler profits from cold return water. Check the link I added to my previous post by the way.

  5. #5
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    An automatic bypass valve is normally closed and only opens when the back pressure increases. Typically factory set at 0.2 bar

    Building regulations mandate an automatic bypass if a bypass is installed, to avoid waste.

    Your Alpha2 variable speed pump can throttle back when back pressure increases, assuming the installer has commissioned it correctly. A lot of these pumps are set in fixed speed mode when fitted to older systems so won't actually vary their speed. It is fair to say that many installers do not attempt to calculate the correct pump curve.
    Last edited by top brake; 1st March 2015 at 07:28 PM.
    I work for Resideo, posts are personal and my own views.

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    Quote Originally Posted by top brake View Post
    An automatic bypass valve is normally closed and only opens when the back pressure increases. Typically factory set at 0.2 bar

    Building regulations mandate an automatic bypass if a bypass is installed, to avoid waste.

    Your Alpha2 variable speed pump can throttle back when back pressure increases, assuming the installer has commissioned it correctly. A lot of these pumps are set in fixed speed mode when fitted to older systems so won't actually vary their speed. It is fair to say that many installers do not attempt to calculate the correct pump curve.
    Thanks for the above.

    Your "avoid waste" comment does nothing to allay my suspicions a manual bypass is inefficient!

    Are you suggesting a automatic bypass AND variable speed pump can be successfully used in tandem on the same system, assuming correctly configured?

    Also, while on the subject of the variable pump, are you able to explain the various benefits/decisions between constant pressure and variable pressure settings?

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    Quote Originally Posted by SimonW500 View Post
    Thanks for the above.

    Your "avoid waste" comment does nothing to allay my suspicions a manual bypass is inefficient!

    Are you suggesting a automatic bypass AND variable speed pump can be successfully used in tandem on the same system, assuming correctly configured?
    a manual bypass (fixed) is inefficient, hence the need to fit an automatic (variable) bypass

    sorry I can't really help with the pump selection or operation, please refer to your installer
    I work for Resideo, posts are personal and my own views.

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    constant vs variable: variable will save power (electricity) and might reduce water flow noise and might increase the ΔT, making the boiler slightly more efficient. I would just go with the constant setting. Set it to lowest. See if your house still heats quickly enough. If not, turn it up a notch.

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    Quote Originally Posted by top brake View Post
    a manual bypass (fixed) is inefficient, hence the need to fit an automatic (variable) bypass
    As suspected - thanks for the confirmation.

  10. #10
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    Quote Originally Posted by erik View Post
    constant vs variable: variable will save power (electricity) and might reduce water flow noise and might increase the ΔT, making the boiler slightly more efficient. I would just go with the constant setting. Set it to lowest. See if your house still heats quickly enough. If not, turn it up a notch.
    Seems like you're the man to answer the question!

    So I'm being clear: the Alpha2 pump has three 'sets' of settings: the simple fixed speeds of I, II, III, two 'proportional pressure' curves and two 'constant pressure curves' - the latter two (or rather four) vary the pump speed to achieve a fixed setpoint.. (also with an additional 'auto adapt' which is a further proportional pressure mode only this time which also adjusts the setpoint based on what it has learned.. but let's ignore this for now not to confuse things further!).

    While I can see the power saving benefit between the fixed speeds and variable speeds, what I don't understand is the difference between 'proportional' and 'constant' pressure - or rather I don't understand in which circumstances or setups each would be better?

    Thanks in advance for taking the time to hopefully explain!

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