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  • #31730
    Mark Siddall
    Participant

    A question or two for the services inclined:

    SAP currently assumes that heating circulation pumps, based upon traditional technologies with a 65 W pump running for 2000 hours per year. This results in a total consumption of 130 kWh/year. Pumps are now available with variable flow rates which enables them to respond more appropriately to demand. (Advanced A rated pumps can be up to 60% more efficient than standard pumps.)

    1) Any ideas what the run time for the pump will be in a Passivhaus? (how will the reduced heating season affect pump use?)
    2) In domestic heating systems (combi and shstem) how widely spread is the boiler manufacturers use of these advanced pumps?
    3) In light of the likely reduced run time of the pump (in a Passivhaus) is the capital investment in the advanced pump (and it's control systems) worthwhile?

    Mark

    p.s. questions arise from flicking through http://www.nhbcfoundation.org/Researchpublications/Energyefficientfixedappliances/tabid/518/Default.aspx

    #38661
    Toby Cambray
    Participant

    Hi Mark

    Interesting questions. For context, The price of a Grundfos Alpha 2 (15-50) looks to be about £90 cheapest retail I can see on google, whereas the 'standard' UPS 15-50 is about £60. We often end up speccing these on houses simply because there's not anything out there that's smaller.

    We are starting to put monitoring kit in our projects so I hope in due course I'll be able to give some firm ish figures on this but here's some food for thought in the meantime.

    The length of the heating season in PHPP for the south England weather data file is 205 days (I've just been playing with this in some detail for my PH conf paper!)- however that's not necessarily the same as the number of days there is call for heat. Compared to the SAP assumption that's a little less than 10 hours a day which doesn't seem unreasonable (three hours in the AM and seven in the evening…). Not sure if there is such a defined heating season in SAP. Let's go with 2000 run hours for now.

    I'm about to make a fairly drastic assumption in order to keep this 'back of the envelope' style: say that the amount of power used by the pump ramps up and down linearly from the beginning of the heating season to the peak and back down again. The minimum rating is say 5W, and the peak is 65W, so if you drew a graph of the power demand against time in heating hours you have a 'childs house' profile (rectangle on the bottom, triangle on the top.) The rectangle is 2000h wide and 5W tall giving 10 kWh, and the triangle is 60 W tall and 2000h wide, giving 60kWh, total 70kWh. A constant speed pump would have consumed 2000h times 65W i.e. 130kWh. On a say 100m2 build, that happens to be exactly 1% of your PE criterion – not earth shattering savings…

    Difference of 60kWh @13p/kWh is £7.80, simple payback of 3.8 years – not bad, if my rudimentary assumption is anywhere near accurate. As you highlight, this annual saving reduces as you shorten the heating season, but on the other hand it might be more likely to be running at part load…

    However, the pump will also encourage your boiler to condense more because at part load, it pumps less water for the same heat output, so the return temperature will be lower. This is more difficult to put a figure on, but I wouldn't be surprised if it added a few % points to the seasonal efficiency of a typical system. I would also wonder whether the TRV's get exercised more and therefore last longer – but I really don't know about that.

    I haven't seen any domestic style boilers with a variable speed pump in them, I haven't tried asking a manufacturer if they can supply a unit with a specific pump in it.

    A couple of caveats; one is that as you reduce the pressure in the flow the valve authorities can start to look poor on some systems (usually only a serious issue on bigger systems) but this can be solved by appropriate use of differential pressure valves, but of course you'll be engaging an M&E consultant to get that bit right for you ;0)

    Finally, and probably most likely to be of concern, a priority DHW circuit has the potential to cause issues if the pump is in proportional pressure mode (constant pressure mode, which is energy saving but not as good as proportional pressure); if there's a lot of back pressure in the circuit and coil the pump will ramp back, thinking that all the rads have suddenly shut down, reducing your flow and meaning your cylinder heats up slowly. I might actually mention this to Grundfos (other pumps are available…), because it would not be beyond the wit of man to connect a control signal from the DHW engage to a bit of logic on an inverter to override the speed control.

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