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Thread: Bypass with Alpha Pump

  1. #1
    Automated Home Jr Member
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    Default Bypass with Alpha Pump

    I have a sealed CH system with an Grunfos Alpha2L pump. There are TRV's on all radiators but no bypass fitted. I'm wanting to fit a Honeywell Evohome system and am realising that I probably should have had a bypass fitted on my system when I converted to TRV's on all the radiators.
    However I'm also reading that the Alpha smart pump either doesn't need a bypass fitting or won't work with an automatic bypass.
    My boiler is an elderly (10 - 15 years old) Worcester Danesmoor.

    So, three choices seem to be available:

    Don't fit a bypass
    Fit an auto bypass but run the pump at a fixed output
    Fit a fixed bypass.

    Any thoughts?

    Presumably the bypass is really necessary when using a non smart system when all the TRV's could be shut down but the boiler and pump still on and working although the boiler thermostat would surely shut it down? But as I understand it, the Evohome system would work out that the boiler isn't needed and shut it down?

    I'm sure I'm missing something here!

  2. #2
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    Quote Originally Posted by Ubarrow View Post
    I have a sealed CH system with an Grunfos Alpha2L pump. There are TRV's on all radiators but no bypass fitted. I'm wanting to fit a Honeywell Evohome system and am realising that I probably should have had a bypass fitted on my system when I converted to TRV's on all the radiators.
    However I'm also reading that the Alpha smart pump either doesn't need a bypass fitting or won't work with an automatic bypass.
    My boiler is an elderly (10 - 15 years old) Worcester Danesmoor.

    So, three choices seem to be available:

    Don't fit a bypass
    Fit an auto bypass but run the pump at a fixed output
    Fit a fixed bypass.

    Any thoughts?

    Presumably the bypass is really necessary when using a non smart system when all the TRV's could be shut down but the boiler and pump still on and working although the boiler thermostat would surely shut it down? But as I understand it, the Evohome system would work out that the boiler isn't needed and shut it down?

    I'm sure I'm missing something here!
    You need a automatic bypass valve installed even when you are using a Alpha pump with a boiler as the boiler will need a minimum flow through it. The system needs a fail-safe way to dissipate heat if for some strange reason the flow from the boiler suddenly stops.

    Take action now and fit a fixed bypass (automatic bypass valve). Don't take any chances with your Heating system.
    I am totally independent writer and my writing is not skewed in any way by receiving payment for my writing. I reduce my gas bills by using Heat Genius Smart Intelligent Heating Controls with more efficient Column radiators.

  3. #3
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    You definitely need a bypass. The Evohome system will still call for heat even if the TRVs are mostly closed, and there are some time delays between the BDR91 opening and the radiator valves closing (and vice-versa).

    There are guides on how to calculate the bypass valve setting based on minimum flow rates and pump curves etc, but I simply closed off all but the radiator closest to the pump, set the Alpha 2L to PP2, and gradually reduced the differential pressure setting on the bypass until I heard it open. I then closed it one turn - this way it'll open if that radiator starts to close. TBH, though, since the 2L can still generate decent pressure even at the lowest point of the PP curve the exact setting of the bypass valve isn't essential and I've simply ensured that it'll open with all the radiators closed off.

    The long and tedious method involving a calculator, the specific heat capacity of water, minimum temperature differentials, flow rates and pump curves etc generated almost exactly the same answer and Drayton recommend the quick method anyway.
    Last edited by Fursty Ferret; 27th November 2014 at 05:51 PM.

  4. #4
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    Quote Originally Posted by Fursty Ferret View Post
    You definitely need a bypass. The Evohome system will still call for heat even if the TRVs are mostly closed, and there are some time delays between the BDR91 opening and the radiator valves closing (and vice-versa).

    There are guides on how to calculate the bypass valve setting based on minimum flow rates and pump curves etc, but I simply closed off all but the radiator closest to the pump, set the Alpha 2L to PP2, and gradually reduced the differential pressure setting on the bypass until I heard it open. I then closed it one turn - this way it'll open if that radiator starts to close. TBH, though, since the 2L can still generate decent pressure even at the lowest point of the PP curve the exact setting of the bypass valve isn't essential and I've simply ensured that it'll open with all the radiators closed off.

    The long and tedious method involving a calculator, the specific heat capacity of water, minimum temperature differentials, flow rates and pump curves etc generated almost exactly the same answer and Drayton recommend the quick method anyway.
    I've just bought an Alpha 2 to replace the heating circuit pump, measuring flow/ return temps the differential is usually much lower than it used to be now Evohome/ HR92's are managing all radiators (except hall which is bypass). The schedule I've set also means it's much more unlikely I'll ever have demand from more than about 50% of TRV's at once. Now the systems running I'm thinking of changing out the fixed valve in the hall for an HR92.

    Difference with my setup is I have a thermal store so it handles boiler overrun on a separate primary circuit/ control meaning no there is no minimum flow rate on the heating circuit. Reading the Grundfos manual it seems to indicate that I can just set the pump to AutoAdapt, PP1 or PP2 and don't need a bypass. Anything to suggest I still need a bypass?
    Last edited by chris_gla; 28th November 2014 at 11:37 AM.

  5. #5
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    Things have moved on since my original post, and the elderly oil boiler is about to be replaced by a Windhager Biowin wood pellet boiler linked to a 300 litre thermal store. The installer is handling everything up to the store and I'm looking after things after this. I'm thinking that I simply keep the boiler on 24 hours during the heating season? Anyone have experience of this?
    Ref Evohome: I'm thinking that I transfer my Alpha pump to the supply side of the thermal store and that as far as Evohome goes then this is effectively the boiler. In other words it will start this pump when the system calls for heat in the radiators. I can't see any need to install a bypass or to leave one radiator without a TRV. Unless anyone knows different! Any thoughts?

  6. #6
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    My thermal store/ boiler was originally configured to run 24x7 but I found this was not cost effective. Gas bill has significantly reduced since I changed it to running only when needed. The way I've installed with Evohome I set the hot water to come on half an hour before heating is required (allowing for the optimisation time this is around an hour), that way the store is pre-charged.
    If by supply side of store you mean the primary circuit to boiler then I don't think the Alpha is well used there- the demand/ flow is constant on that side. You also need to be a bit careful with some thermal stores as they also modulate the supply voltage to the pumps to control temperature. My store only does this on the direct hot water circuit to control water temperature, others do this with mixing valves. the Alpha seems better geared towards the variable demand and flow on the heating circuit.
    Speaking to an installer at the weekend he felt that the flow on the Alpha doesn't modulate low enough to run without a bypass on the heating circuit- but this seems to contradict the Grundfos manual (at least for the Alpha 2 I have)

  7. #7
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    Thanks Chris

    By supply side I meant supply to radiators. I probably should have used the term "demand side.

    Interesting that you came to the conclusion that 24/7 running was wasteful. The specs from the manufactureres of the stores suggest that it costs about 2 units per day but I'll have to do some tests to check, after install.

    Ref the bypass. I understood that a bypass was necessary to cope with pump overrun when the valves were all closed. i.e. Evohome sends a close signal to all TRV's, either because they are programmed to go off at a certain time or because the temp has risen and all are closed, but the pump runs for a while to cool the boiler heat exchanger. I'm assuming that I can set it so that the demand side pump (to the radiators) shuts down as soon as the valves close? I'll have to experiment with the kit when I receive it later this week.

  8. #8
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    Quote Originally Posted by Ubarrow View Post
    Thanks Chris

    By supply side I meant supply to radiators. I probably should have used the term "demand side.

    Interesting that you came to the conclusion that 24/7 running was wasteful. The specs from the manufactureres of the stores suggest that it costs about 2 units per day but I'll have to do some tests to check, after install.

    Ref the bypass. I understood that a bypass was necessary to cope with pump overrun when the valves were all closed. i.e. Evohome sends a close signal to all TRV's, either because they are programmed to go off at a certain time or because the temp has risen and all are closed, but the pump runs for a while to cool the boiler heat exchanger. I'm assuming that I can set it so that the demand side pump (to the radiators) shuts down as soon as the valves close? I'll have to experiment with the kit when I receive it later this week.
    If you wire Evohome into the roomstat/ programmer connection on the store that should control the heating pump.

    One issue I've just noticed with Evohome is that the minimum on time can't be set any higher than 5 minutes- I don't have any specs or curves from manuals, etc but allowing this value to be set to at least 15-30 minutes to reduce boiler cycling would seem more realistic where a store is in place.

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