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Thread: Evohome and Auto Bypass Valves

  1. #1
    Automated Home Sr Member
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    Default Evohome and Auto Bypass Valves

    Now i have evohome i have noticed that if only one or two radiators are on the system is noisy. While the boiler does have an internal bypass i assume this is to protect the pump.

    I was thinking of fitting an external valve. To set it i was thinking of having one radiator on and setting the valve so it flows enough to quieten down the radiator. Is this the best way?

  2. #2
    Automated Home Legend paulockenden's Avatar
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    Technically, no, you're supposed to study graphs and stuff. Practically, yes, that's what most people do.

  3. #3
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    I did see the fitting instructions but they seem to have you set it based on pressures. Where I want it mainly for noise reduction. I will fit one when I next drain the system down then.
    The internal bypass will look after the pump for times when it's trying to pump with all trvs closed I would think.

  4. #4
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    A little bit more info about your system would help.

    Is it all 22/28mm headers and 15mm to radiators, or do you have some microbore ? Microbre (which I have on most radiators) has a lot higher flow resistance and therefore requires a higher differential pressure to get adequate flow and heat transfer.

    How many radiators, and are they old fashioned or mostly new convectors ? What kind of pump - is it a variable speed modulating pump or is it fixed speed, if so what speed is it set to ?

    You need to be careful about adjusting an ABV if you have a modulating pump as the ABV opening too easily (set too low) can confuse the pump. Although I suspect you have a fixed speed pump if you have noisy radiators when only a couple are open.

    Complete rule of thumb guesswork - I would start at 0.5 bars and work your way down from there. Don't set it any lower than necessary to quell the noise as if you set it too low you'll get flow through the ABV when it's unnecessary which can lead to higher return temperatures (hot water is blended into the cooler return) which will upset condensing boiler efficiency.

    You may also find the radiators struggle a bit in the depth of winter as by reducing the differential pressure you would be restricting their flow rates. There is all kinds of theory on how to set an ABV but trial and error while understanding what the symptoms are of setting it too high or too low is what most probably use. (!)

    BTW if your boiler has an internal bypass it is likely to operate at a higher pressure than the 0.6 bar maximum that most external ABV's let you go up to. So as soon as you install the external bypass the internal one becomes redundant and will no longer operate. (And doesn't need to)

    Just make sure you install the bypass before your zone valves if you have an S or Y-Plan system so that the zone valve can't take the ABV out of circuit when it closes.
    Last edited by DBMandrake; 8th September 2017 at 04:14 PM.

  5. #5
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    Has anyone actually been able to get a modulating pump to work correctly with an ABV? I see the two as being at odds with each other. Infact a modulating pump plays havoc on my boiler that requires quite a high minimum flow rate.

  6. #6
    Automated Home Ninja
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    The general advice seems to be to use a traditional manual bypass. For example: http://www.newtonnet.co.uk/permanent...Alpha2or2L.pdf

  7. #7
    Site Sponsor The EVOHOME Shop's Avatar
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    Don't confuse bad system design with modulating pumps. Many ways to overcome a modulating pump. I designed my system at Unit 51 with a fixed speed pump (Grundfos Alpha 2L) on my secondary circuit (incorporating a automatic bypass valve) and my Intergas ECO RF does its own thing with its modulating pump on the primary circuit. I use closely spaced tees to hydraulically separate the boiler from the heating circuit. Works very well indeed.

  8. #8
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    I also have closely spaced tees. I have a variable pump on the secondary side because it’s a large circuits and requires a lot of head (4m!!!) to reach the furthest rads, but that ends up being quite noisy when there’s low demand. Right now I have a towel rad with the HR92 removed and the valve slightly open to avoid the possibility of dead-heading the secondary pump. But that feels like literally wasting heat - I’d much rather have a proper bypass of some sort.

  9. #9
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    Taconova setter, looks quite nice. I wish I had known about it earlier. I have always wondered why we don't install flow meters on our systems and nor do the boilers have any flow diagnostics, when boiler flow rate is such an important measure for efficiency.

  10. #10
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    It's an ideal vogue combi boiler and it says when all rads have trvs to fit a bypass so I'm guessing the pump is fixed it's set to 70% or something in one of the boiler menus.
    22mm mains with 15mm to each radiator.
    Last edited by robj20; 10th September 2017 at 03:44 AM.

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