Well pump cycles and energy consumption

Discussion in 'Open Loop' started by glenski_31, Dec 4, 2012.

  1. glenski_31

    glenski_31 New Member


    First thank you for your patience. I know this has been addressed on here before and I have searched and read but wanted to add my math into things. It is looking to me like the actually energy cost for cycling the well pump is going to be minimal. Looking at it strictly from an energy use perspective.

    My ecobee Tstat says that for the first 1/2 month of November my weather adjusted run time is 0.21 hr/HDD (the accuracy of this will no doubt increase as the data points increase). So for 2011 HDD from Environment Canada for Saint John, NB I am in the range of 4500HDD. These calcs don't assume cooling for now, just heating.

    So keeping in mind heating only:

    4500 HDD * 0.21hr/HDD = 945 hr/year

    Assume: @ 5gpm my well pump runs a full cycle from full to pump on to full in 230 seconds or

    945hr * 3600 sec/hr = 3,402,000 sec/yr

    (3,402,000 sec/yr) / (230 sec/cycle) = 14,791 cycles per heating season

    Assume: Of the 230 seconds I have timed it and the pump runs for almost exactly half of that at 115 seconds therefore

    14,791 cycles/year * 115 sec/cycle / (60 sec/min) / (60 min/hr) = 472 hrs of pump run time per year

    Assume: I have a 1/2 hp pump which I do (will prob upgrade to 3/4hp when the existing finally has enough to get the GPM up when the HP is running)
    Assume: At best a 40% efficient pump/motor assembly (this is the part that is the real assumption as I don't know what the actual values are, but I am guessing that this is conservative)

    472 hrs/yr * 1/2hp * 0.746 kW/hp / (40% efficient pumping) = 440 kWh per heating season of pump run energy

    Assume: Energy rates in NB are $0.0985/kWh therefore

    440 kWh * 0.0985$/kWh = $45/year

    So the additional pumping energy cost for the year is only going to be $45 for the heating season, seems really low.

    Any thoughts, did I miss something. I know we can discuss the ethics of pump and dump, environmental impact etc. but I just want this one to be a pure number crunch and make sure I am crunching the right ones.

    I know there are capital costs associated with pump replacement for a heavily cycled pump like mine so those need to be factored in and I will consider that as things go along. Anyhow based on what I calculate you would be hard pressed to have a VFD well pump result in any kind of reasonable payback at these utility rates? Also the driller in our area says that it a pump replacement is a service call only so basically $100 max.

    Thanks again folks and sorry to load you up with math.

  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    I've never heard of weather adjusted run time. Had to google it and it came back to the ecoBee. So I actually don't know what that constant of 0.21 is defining. I'm guessing that it is something like the HVAC equipment only runs for 21% of the predicted HDD.

    But, HDD are (number of days) x (degrees). So if you want to work with hours (as in your initial calculation). You will need to x 24 hours. Unless of course, that ecoBee constant is a "magical" converting constant:).

    edit - well pump efficiency is motor efficiency x pump efficiency. I think it is around the 60% efficiency from what I recall. But also a huge range depending on quality of pump.
    Last edited: Dec 4, 2012
  3. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    I have four or five such number crunchings going around in what is left of my brain. Stephanie says my gray hair, what is left of it, is caused by my brain leaking out.

    With out getting out my phone, solar powered, big keys w/ big LCD numbers, or hitting the calculator button on this key board, I think you are very close.

    The hole I see is in pumping at 40%. I do not think what you get out of the pump is that high. If you cut it to 20% your cost is still cheap. $90.00CA.

    The thing that kills electric motors is starting, not run time. Add to that the spike caused by the PSC cap until it drops out of the starting load energy demand of the motor. Unless you need more GPM I think I would get a bigger pressure/holding tank to increase run time per start costs. Run time is good, starting and stopping is not.

    There are piping and plumbing tricks that will allow the 1/2 HP pump to live a long and prosperous life. Since I do not design/build P&D systems for moral or ethical reasons this is just internet jibberish.

    Enjoy your system it sounds to me that it works well.

    I do not currently have a passport so I am stuck in USA for now.

    Wow a math problem I could do without a mechanical arithmetic helper.

    Hope this helps,

    1-800- move- water.
  4. glenski_31

    glenski_31 New Member


    'Weather adjusted runtime' is Ecobees way of taking your total runtime hours and dividing it by the HDDs during that period as calculated using their weather data. So the units are runtime hours/HDD. Pretty useful number since they already know the runtime hours it is just one more little calc.

    Thanks again for your input.
  5. glenski_31

    glenski_31 New Member

    Thanks for your input too. Good to know I am not way off base at least. I had originally intended to go with too pressure tanks but between time and money I figured I would run it with one and see if I could tune it up but left room for a second pressure tank which may be where I go from here.

    Mine is always wondering 'how come everyone else has a heat pump and they don't spend this much time on it'

    My response: they don't have a clue what they have and they just assume that it was installed correctly, working efficiently and when it breaks at the worst possible moment out comes the credit card.

    To the ethics of Pump n Dump I don't plan on doing it again, it was a convenient retrofit on an existing house with plenty of government $. If/when I build new in my off grid home :) closed loop will definitely be considered. In fact any land purchased from here on out will have geo in mind as far as layout etc.

    Thanks again, awesome group!

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