Alberta Heat load calculations from utility bill

Discussion in 'General Discussions' started by Caleb7777, Jul 13, 2020.

  1. Caleb7777

    Caleb7777 New Member

    Hi Everyone!

    I recently got quoted a 10 ton GSHP for my large home (they didn't do any calculations, rule of thumb) but after I crunched the numbers I feel it would be far oversized. From what I have gathered reading this most excellent forum oversizing a heat pump is not a good idea, especially if you end up with a desuperheater for DHW. The yard out front is barely large enough for a horizontal loop for the 10 ton. It can be done if we just pit the whole yard, but there is more than enough space for a smaller GSHP.

    I have been tracking my utility bills on a spreadsheet for the past few years and used the actual GJ of natural gas used with the Heating degree days calculation to determine the heating load of my home. I have one mid efficiency furnace for the main home, one gravity dump low eff furnace for the basement, one overhead heater in the garage, and one 50 gal hot water tank also on natural gas. When I did the calculations I assumed 80% overall efficiency which is in my mind probably giving the units too much credit. Changing the efficiency value even 10% one way or the other has big effects and i did not want to be low on my estimated heat load.

    I have everything in an excel file and it has about 36 months of usage history. Sheet 2 shows avg BTUH for every month and the calculated heat load for every month. The largest usage months here are december, january, and february by far. The heat load calcs are actually quite skewed for the summer months since there are almost no heating degree days but the hot water tank still runs and it causes the calculation to be weighted unevenly and show too high a load for the home. I found it really interesting.

    I was also blown away by how much my actual gas usage dropped after the summer of 2018. That summer I spray foamed the basement and changed some more windows and the usage is down like 40%! I did not think it was that much because I was watching my utility bill costs and it didn't seem to change year to year but the insulating has definitely made a huge difference now that I graph these values. So for the heat load we should be looking at just 2018/2019 winter and 2019/2020 winter.

    OK OK, so my question is: Does it sound reasonable from the usage data that I should use a 5 or 6 ton unit? I am leaning toward 5 ton from the data but wanted to ask those who have real experience if it's wise to size for 100% of the load, or 90%, or 110% etc.

    Just wanted to discuss the heat load here... I will reserve my ground loop questions for later.

    Attached Files:

  2. Caleb7777

    Caleb7777 New Member

    You can update the efficiency on the data page of spreadsheet and refresh the pivottable data if you want to see it's effect.
  3. gsmith22

    gsmith22 Member

    look at response #12 by Dana Dorsett in the comments to this article: greenbuildingadvisor dot com/article/how-to-perform-a-heat-loss-calculation-part-1 (can't put actual link or forum administrator will hold up my response for like a week)

    I haven't looked at your spreadsheet so I don't know what it is calculating per se or if it follows the procedure outlined above in response #12. I used CoolCalc (google it) to do a load calculation on my house and it came out in reasonable agreement with the heating bill gas usage but I had two, nearly identical 65% efficient furnaces. As you note, gas usage for things like hot water and dryer skew it. CoolCalc is really easy to use and after using it I had an actual load calculation for both heating and cooling and not just a guestimate. Although arguably Manual J load calcs are guestimates depending on how good the input information is. Even more important, I could vary the R values of things and see if it was worth my time getting better windows (it isn't) or insulating basement walls (it probably will help), or reducing convection losses (it will really help).

    if I had to pick what dropped your gas usage it would be the insulation. Even if you get the fanciest triple pane, thermally broken, European windows, they are still a hole in the building envelope compared to an insulated wall or insulated ceiling/roof plane (focusing only on the conduction aspect and ignoring the potential air movement/convection aspect). The best window is like R9 (ie 3x the single pane window of R3). The worst 2x4 batt insulated wall is essentially the same (average value with wood studs included). So at most, the fanciest windows ($$$$$$) get you on par with the worst insulated wall ($).
  4. Caleb7777

    Caleb7777 New Member

    Yes I agree it was the insulation that made the difference. There was none before. Just happy to see it improve that much measurably.

    I did use the method dorsett recommended. just from another blog dedicated to it titled 'out with the old, in with the new'. I had been searching for a mwthod to calc heat load based on historical usage and not just rule of thumb.
  5. Caleb7777

    Caleb7777 New Member

    So can anyone chip in here? I am confident in the calculations I am just wondering if you recommend a 5 ton unit if the heat load each winter peaks around 5 ton or if I should go bigger. Was also thinking about having a separate 1 ton unit for water heating alone and I have some radiant floors I can pass it through as well. Then no need for a desuperheater and the heat load of the home is actually lower since hot water generation was included in my heat calcs before (nat gas).

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