What is the current best-practice for bypassing a boiler which has an external variable speed pump?

Discussion in 'Plumbing and Central Heating' started by BlueLoo, 1 Dec 2021.

  1. BlueLoo

    BlueLoo

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    Assuming:

    1) A simple boiler without integrated pump or bypass circuits.
    2) A multi-zone system with a separate hot water circuit.

    I have a typical S+Plan heating system with a basic boiler and separate circulator.
    I have replaced the knackered constant speed pump with a modern variable speed pump (because i couldn't get a fixed speed pump).

    I am wondering how to use the variable speed option of this pump whilst maintaining a functional boiler bypass, which in my system, is currently via a ABV on the HW circuit.

    My understanding is:
    Modern codes of practice recommend the use of variable speed pumps to reduce energy usage.
    However, variable speed pumps do not ramp up the pressure as the flow decreases, rendering the standard boiler bypass method of using an ABV pretty pointless. Bypasses are not an issue where the boiler has internal compensation (secondary HE/EX's, etc) but there are a lot of boilers out there which do not do this and as i found out, fixed speed pumps are on the wane.
    I've read many posts on this forum, nothing seems conclusive. I've read the paper from the Grundfos products manager stating that variable speed is not possible without a fixed rate bypass. So, it seems i have a few options:

    1) Ignore the variable speed modes on the pump and use the fixed speed modes, keep the system (with ABV) as is and take the hit on the electric usage.
    2) Follow the Grundfos advice and use the variable speed with a fixed bypass in place of the ABV.
    3) New boiler with integrated bypass (!)
    4) Something else.....


    Option 1 doesn't let me utilize the energy saving of the pump.
    Option 2 (i think) effectively de-rates the boiler output and introduces its own losses (heat) on the constant bypass.
    Option 3 $$$$$
    Option 4 ?

    I think its a toss up between option 1 or 2. I have no idea which is best, i suspect option 1 as i can't think that bypassing the boiler constantly is good for efficiency and reliability?

    Option 3 will have to wait a few years.....

    Option 4, i was toying with using a control valve in place of the ABV. Perhaps a 2 port valve wired in series to the normally-on wiring of the existing 3-off 2 port valves such that should, any of them be open, a 2-port bypass valve would change state from normally open, to closed (thus ensuring a fail safe condition). A bit of a faff but not something too difficult?

    Or, is option 2 not such a big deal? The boiler needs 9l/min minimum flow @bypass and that seems quite a good %age of the rated flow of the pump on its DP setting.

    As per the system noted, leaving a radiator open isn't possible. I don't want the unnecessary heating and this wouldn't provide a bypass should the HW circuit only be calling for heat.

    So, (sorry for the long post), What is the current best practice for the utilization of variable speed pumps in a typical CH system?

    Or does everyone just wing it?

    Thanks for any answers.
     
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  3. oilhead

    oilhead

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    If your system was adequate on the fixed speed pump, then run your new pump on fixed speed. How much do you think it will save to run on variable? Your system design may not be conducive to run on variable.
     
  4. BlueLoo

    BlueLoo

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    I'd save about 66% or the 40KWh/year it uses....... (£5?)

    I'm more bothered about the principle than the money, hence the question. It is bugging me that there isn't a clear method of doing this.
     
  5. oilhead

    oilhead

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    There are many things that have been introduced into the world of central heating that promise the earth but cost more than the savings. If your system was 'designed' and has worked for some years, a modulating pump without an adequate pressure sensitive bypass, the cost of modification will be a lot more than the savings. It would need a very competent heating engineer to look at your system and advise accordingly.
    That would be a lot of £5's.
     
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  6. BlueLoo

    BlueLoo

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    I don't think my system was "designed" any more than trying to shoe-horn a cut-paste setup into the house.
    The very competent engineers who did it originally want shooting.....

    Anyway, costs aside, dynamic pump control is a good thing. less noise, less motor wear, less valve/rad noise etc. It is a good idea, but not much use if someone hasn't done a suitability study before putting them on the market.

    My opinion is that these pumps are not suited to older installs and the industry/sellers etc. should be made aware of that before fitting. Grundfos themselves have a few papers on why variable speed pumps can be a worst rather than better solution if installed incorrectly.

    For my situation, i have done some study, head calcs, etc and settled on the following:

    For systems requiring a boiler bypass, a variable speed pump and a Automatic (pressure operated) bypass valve is pointless.
    I want to keep the variable speed settings for the reasons above, so, i will need to use a manual bypass as i haven't come a control valve set up that would work correctly (yet).
    I can't use proportional pressure settings as the bypass flow rate increases with demand, not decreases.
    I have set the pump to a constant pressure setting (1) and set the manual bypass rate to a minimum acceptable to the boiler.
    The boiler is well sized, so net output loss is not a concern.

    I shall see how this goes. I may just revert back to a fixed speed setup.

    I was rather hoping the industry had figured it out. Seems to be not the case reading the threads on this and other forums.
     
  7. BlueLoo

    BlueLoo

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    As an update:

    I ended up having to have my manual bypass valve quite some way open to stop the boiler from tripping on overun.

    However, i can't figure out exactly what the bypass flow rate actually was (manf figure not withstanding) as the system has no pressure guage and the pump doesn't show actual flow rates.

    I know i can work it out but theoretical values seem to have some inaccuracies.

    I'm not happy with the constant bypass approach. Grundfos use a low q rate (0.25 m3/hr) on their paper but my boiler needs double that it seems. The flow noise through the manual bypass is not inconsiderable.

    So, I'll do some more study and see if any new tech pops up or i find something but I've switched back to a ABV approach with the pump now set to fixed speed and the manual valve opened fully.

    Any ideas gratefully accepted.
     
  8. fixitflav

    fixitflav

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    Probably just wording, but the bypass bypasses the rads etc, not the boiler. In order to maintain boiler flow when the controls reduce the rads flow.
     
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