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Community and Q&A

Evaluating Heat Pump Needs for Cold Climate

masincalgary | Posted in Energy Efficiency and Durability on

Before I repair my gas furnace, I am deciding on whether to replace it with a heat pump now, given that I upgraded my home building envelope (of 1800 sq ft) on the path of vision net zero. My last furnace was oversized and loud. I have one year + of energy use data and 1) I’m wondering if I can use that to help work out a proper size of furnace. 2) I’m guessing that over sizing also reduces efficiency (right?).  3) I’m wondering if its size would factor into the requirement of increasing the amp service to my house. Penultimate question:  4) does anyone know which specs compare efficiencies of the Daikin and Mitsubishi brands?  And lastly, 5) I’d like to know how to convert the GJs I consumed into electricity consumed to power a heat pump (I expect I could do this roughly, e.g. reducing the COP during the three coldest months of the year).   Hopefully I am not unrealistic in asking these questions. Thank you so much in advance! I’ve learned quite a bit from reading this site over the years and helping me make decisions. I am not a trades person but I like to understand.  Also the number of days of -20C and below average 17/ yr over the past decade. It can get to -30 C and colder for a week.

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Replies

  1. Expert Member
    Akos | | #1

    Oversizing on gas furnace doesn't effect efficiency all that much, it is mostly a comfort issue. Right sized units are cheaper to buy, need smaller ducts, create less noise and provide much better comfort.

    Most heat pumps for a reasonably sized house need very little power, even with a reasonably sized backup strip heater, you should not significantly increase your house power use. Something like a 36000BTU hyper heat unit uses around 20A at full power, less than an EV charge station.

    I would browse through here for comparing efficiency of units:

    https://ashp.neep.org/#!/product_list/

    Most cold climate hyper heat units deliver 100% rated output at -15C and usually 80% down to -25C. Usually the best is to size the unit to handle your design temperature and add in a bit of electric strip backup for those polar vortex days. You can also always install a small wood stove, great for warming the spirit as well as heating the house.

    GJ*277/HeatPumpCOP=kWh electricity consumption. Most cold climate units average around a COP of 3 over a heating season.

  2. masincalgary | | #2

    Thank you for your reply. I found that my energy audit gave me the info needed for sizing, much the same as in your reply. They also estimated utility costs under two scenarios (gas vs electric) and electric is coming out a bit cheaper (despite our super low natural gas prices).

    Re neep.org, the product list is great for listing all the brands. I'm not sure how to use the the two columns of heating capacity. E.g. don't I want to look at heat pumps with a 36000 BTU capacity at -30 C?

    As far as a wood burning stove, cozy yes but I won't be putting another smoke stack in (got rid of the fireplace's chimney). Your reasons for a right sized gas furnace unit transfer right over to a heat pump (right?) - all important to me. Living with a furnace that was almost 4x my needs has really made me pay attention to this.

    Can you tell me how to use the heating capacity indicators on the heat pump list? Thank you.

    1. Expert Member
      Malcolm Taylor | | #3

      masincalgary,

      By contemplating going all electric in Alberta, you are a disgrace to your province and should he banned from the stampede for life :)

    2. Expert Member
      Akos | | #5

      Only a few units on the site have specs for -5F, you definitely won't find anything for -22F on there. You'll have to do a bit of digging on the manufacturers site for the engineering data for actual low temp data. Start from a slightly oversized hyper heat unit (you can tell these as their capacity does not drop off at 5F) and check the engineering data.

      If you are looking for 3ton capacity at ultra low temps, this might be a good starting point:

      https://ashp.neep.org/#!/product/29006

      This would deliver around 38000BTU at -25C, so probably pretty close 3ton at -30.

      Also double check with the manufacturer that it won't shut off at very low temps (some do) and that it has a pan heater (or available as an accessory). Without a pan heater in snow country the outdoor unit can easily ice up.

      The good thing that you had a 3x oversized furnace, it means you have oversized ducts. This sometimes it means you can get away with a right sized low or mid static pressure ducted unit. These are significantly cheaper than ones that look like standard air handlers.

      P.S. Just check that Calgary has an outdoor design temp of -25C, make sure to design to that. If you want additional buffer capacity, add in a strip heater for polar vortex days but do not oversize your unit.

      1. masincalgary | | #28

        Super helpful. As we move off gas, as we retrofit our homes, I hope Neep and the manufacturers will post data for -20C to -30C to help us make the move to electric sourced heat via ASHP. I did not know what hyper heat unit meant; I thought it was a model, but from your response I take it to be 'oversized' so that at low temps it still puts out the required BTUs. Let's face it, those temperatures are precisely why we need a furnace!

        I don't know what is meant by right sized low or mid static pressure ducted unit. I'm told that the air handler needs to be sized for the unit (outdoor heat pump). I'm very motivated not to have a bigger fan than I need as I hated my last furnace every time it was on.

        1. Expert Member
          Akos | | #35

          Since you are replacing an existing furnace, you are looking at a one-to-one unit. This means a single outdoor unit and indoor unit. These usually come pre-packaged as a pair, so you don't need much sizing outside of BTU output.

          Where you do need to match these is selecting an a pair with an indoor unit that can deliver enough pressure for your existing ducting. Most manufacturers offer three flavors of these, low static (0.15 to 0.25 in w.g.) mid static ( 0.6 in w.g.) and high static (1.0 in w.g.).

          The best is to measure the pressure through the duct using of the existing air handler and see what the pressure loss will be with the correctly sized unit. If the existing furnace is way oversized, chances are you can use an air handler with lower pressure capacity.

          If unsure, you can't go wrong with a high static unit. These look like a standard furnace and the simplest install for the HVAC tech as it can be installed with no/minimal duct modification. They are bit pricier but when you factor in labor costs, it is probably a wash.

          In climate with occasional lows of -30C, there is need a backup furnace. Modern hyper heat units can work without issues.

          1. masincalgary | | #37

            Thank you. My current system is 3x the size I need and actually too big for the ducting. Proper sizing of air handler will really help noise reduction. I'll ask about this. And yes, my focus is on BTUs and after that, what electrical upgrade I'll need. (Hopefully not ripping up my yard and reaching under a sidewalk to the box, and ripping through my super nice cedar deck!)

          2. Deleted | | #38

            Deleted

  3. masincalgary | | #4

    :0 That's OK with me!!

    And maybe I'm not a total disgrace! My father was a geophysicist and constantly fed a message in my ear that we (as a consumer society) cannot continue in the hyper growth model, and it was very WRONG (!!!). He said we should be evaluating ourselves on sustainability (as in what the earth can provide sustainably)--he worked for Mobil Oil! I wasn't sure about Dad's views at the time, but . . . turns out he was an intelligent, nature loving scientist. And maybe there are more like him, who don't take it personally when their discoveries are not shipped as fast as possible around the world. :)

    1. Expert Member
      Malcolm Taylor | | #6

      Let's hope so.

  4. Shawn Baldwin | | #7

    Masincalgary,

    As someone who relies on a 18k Mitzubishi ducted heat pump for heating in zone 6, I can tell you they are great. I get a constant circulation of air through a mere 13 filter. With -20f for multiple days in the winter, I've never had any trouble with the heat load being maintained. All of the old-school people in the professions will tell you that you can't do it, but listen to the people in this post and they will steer you in the right direction. Good luck.

    1. masincalgary | | #9

      Also, can you tell me the BTU size of the 18k? Thank you.

      1. Steve Grinwis | | #11

        18k is shorthand for 18,000 BTU!

        I myself have a 30,000 BTU mitsubishi Hyper-Heat unit in Zone 5, that has held me just fine on a polar vortex day of -30C in Ontario. You will not find more comfortable heat than an inverter heat pump. Nice, even, quiet heat that just perfectly matches load (assuming it's reasonably well sized). No hot blasts when it fires up, no getting cold before it turns back on. Just, nice, even heating.

        I love it.

        1. masincalgary | | #29

          I realize that it's not just temp, but insulation and size of house that factors into the BTU size needed. Last year we had 7 weeks of below -20, about 3 or 4 of less than -25 and a few days of less than -30. We are zone 7. Ma and Pa Google have their limitations. One is the temp range for zone in Canada. Nice to hear your feedback on your mitsubishi.

    2. masincalgary | | #10
  5. masincalgary | | #8

    Thank you! -20 F =-28 C is COLD. We'll get -20 C and below for almost three weeks a year. I am definitely listening to people like you. I need a certain amount of understanding the product and install. It seems to me that heat pump is basically a reverse a/c. Is the install about the same? Apart from the increased electric load (bc I need so much more heat than I do a/c), what is the difference for the installer?

  6. masincalgary | | #12

    I'm really curious about how a heat pump basically differs from an a/c regarding the install.

    1. Steve Grinwis | | #13

      The install is virtually identical, except for the thermostat, and one of the wires.

      The heat pump has a reversing valve internally, that when energized switches the flow of the coolant from A/C (high pressure off of compressor directed outside to dump heat outside), to heat pump (high pressure directed inside). This is built into the unit, you just have to wire the thermostat to it, and your good.

      More advanced heat pumps with variable output are a little more involved, but identical between AC and heat pump modes, since they 'talk' to each other digitally.

      1. masincalgary | | #24

        Hope you see that I asked you another question below. Thank you.

  7. Expert Member
    Akos | | #14

    Install will be the same but if it is anything like my local HVAC folks, as soon as you say mini split, price goes through the roof. Make sure to price the equipment you spec independently so you can compare the quote. A good installer should give you comparable install cost to a standard AC unit.

    About the only difference between them is that a lot of mini splits have a bunch of installer options you can configure. About the only one that must be set is the external static pressure of your ducting, the rest are fine 99% of the time. Most installers won't know about any of these, so you might have to do a bit of reading and set them yourself.

    1. qofmiwok | | #16

      Why does the price go up with mini-split? In my experience people consider it a cheap heating system.

      1. Expert Member
        Akos | | #17

        Here mini splits are sold as an alternative to high velocity AC system which was the only option for older homes with hot water heating. Since these systems are not cheap, contractors pitch mini splits as a cheaper alternative but still in the same price range.

    2. masincalgary | | #30

      Could you suggest search terms for the reading, or where to start? I'm still working on the size of the unit, whether I need back up, and electrical requirements, but next will be to understand this too. Thanks. (re the static pressure)

    3. masincalgary | | #52

      Are the air conditioner units also not mini splits? (with one unit outside and an air handler inside?)

  8. masincalgary | | #15

    Thank you so much. I won't be having mini splits. I'm sure that the time I take now to make a good decision will be worth it. I'm not done yet, but just want to say I appreciate all the replies here.

    1. PBP1 | | #18

      As with Steve #11, have a 30kBTU Mitsubishi hyper heat in zone 6, Montana for 2100+ sq ft, no back up heat but a small direct vent natural gas fireplace. Mini split is a term be careful - I have the single outdoor air source heat pump split with three ducted air handlers - noting that wall mounted air handlers are more efficient. Mini split is a viable zone 6 option. If insulated ok with low ach50 mini split should be fine. But nice to have some alternative if electric goes out. If you contemplate solar, mini split can be a good option.

      1. masincalgary | | #20

        I do hope to attain solar one day. People in Calgary with no gas have turned off their places to test out how fast the temperature falls. . . and we've never had a power outage for more than a few hours. I feel pretty safe. If we had a days long outage in a deep cold spell, we'd all be in deep trouble. We don't have the type of winter storm that knocks off power. I have 1818 Sq ft. I'm looking at the BTU/h during -20C weather to estimate my size need (it must achieve 34000 output). Does that sound right? Calgary is north of Calispell, Montana by about 7 hrs. Don't know temperature zones.

        1. PBP1 | | #21

          Maybe one of the experts will weigh-in? I had a Home Energy Rating System (HERS) guy help out - along with the HVAC installers, who had experience with Mitsubishi systems in Montana winters (load was around 28 kBTU, the "30kBTU" heat pump was rated around 28.6 kBTU). Things I've noticed, if ducted (Mitsubishi SEZ air handlers), keep ducts/air handler in conditioned space (not unconditioned garage or unconditioned attics/crawl) - and even in conditioned space, insulate at least the ducts (supply "hot" side). Keep the fluid lines short between the heat pump and air handlers. Zone appropriately. Try to use gravity drain for summer/AC water condensate from air handlers, the condensate pumps can buzz (but not too loudly) and they run constantly when set the system is set to "cool". With gravity drain, the internal water condensate pump is switched off (jumper settings), might save some energy too. Don't mount the heat pump where snow will accumulate. Keep filters clean. Don't use large temperature set backs (e.g., don't set back 10 F at night). If ducted, be aware of duct cross-sectional area, length and filter size (and not too high MERV on filters) - the units are "low static" (pressure) so you don't want to eat up blower motor energy with long/curved ducts, high MERV, etc. Generally, higher flow past the heat exchanger in the air handler, the better the performance. They also make higher static air handlers too. Be aware of how much throw Feet Per Minute (FPM) from (each) a supply vent and how much volumetric flow Cubic Feet per Minute (CFM) from a supply vent(s) you want/need in a room(s). Adjustable louver vents can be helpful. I have an anemometer I can mount on a tripod to "measure" throw/flow. Good to have some air flow near windows to reduce condensation of indoor humidity (at -20 C, even 0.21 U double pane windows can fall below dew point temperature). Try to estimate kWh per Heating Degree Day (HDD-60F or HDD-65F) to get an idea of how much electric you'll consume in a cold month with 1300+ HDD-65F (I see CA uses HDD 18 C, which is 64.4 F but one degree C is not one degree F!) https://calgary.weatherstats.ca/charts/hdd-yearly.html. I've seen numbers reported by others over 1.5 kWh/HDD-65F, which means 1950 kWh in one month for heat pump. Some passive/net zero homes (very tight, relatively small) have reported 0.2 kWh/HDD-65F. Again, I'm not an expert and only have experience with my Mitsubishi system.

          1. masincalgary | | #26

            Also, what does "low ach50 mini split " mean? Is that a fan size (air changes/hour)? My fan will need to match the size of the heat pump, I do believe (I've asked if I could have a smaller one).
            I"m curious how you and Steve get by with a 30K unit. I your home super well insulated and is it super rare to get temps that are between - 20 and -30?

      2. qofmiwok | | #22

        "noting that wall mounted air handlers are more efficient"
        What do you mean by that? As opposed to what?

        1. masincalgary | | #25

          Maybe they meant that a person can heat in one room, and not in another? As opposed to the Canadian norm of housing which typically uses a central fan blowing warm air through ducts to all rooms.

        2. PBP1 | | #31

          Sorry for lack of clarity. As opposed to a ducted air handler, the data are usually included by Mitsubishi for multi-zone ducted versus multi-zone wall mounted. Having the fan/heat exchanger, supply/return all within the compact space of a wall mounted translates to greater efficiency.

          As to "low ach50" (or ACH50) that meant blower door test result (or other equivalent test result): Air Changes per Hour at 50 pascals (Pa) pressure. The blower depressurizes the home and checks air flow at 50 Pa (pressure differential I believe). The total volume of the home is then used to determine air changes per hour. As mentioned in various posts, it's easier to get a low ach50 with a small home (less volume) noting that for high ceilings, the "sq ft" (or sq meters) does not equate too well with ach50 goals. A big home (large volume) can be challenging to get ach50 less than 1 (code here is 3 ach50, not sure what it is in Calgary). I did see for Canada Natural Resources Canada (NRCAN) and the Office of Energy Efficiency (OEE): "The only specific performance standard required for certification is an air leakage level of 1.5 air changes per hour at 50 Pascal (ACH50)". So certified in Canada is apparently 1.5 ACH50.

          Less leakage, can mean less heat load. But too tight means make-up air, usually through an air to air heat exchanger that exchanges heat in outgoing "stale" air with incoming "fresh" air.

          As to getting by with 28.6 kBTU/h max at 5 F: (1) spray foam insulation (seals and insulates) mostly "flash n batt", (2) hunting down leaks and sealing them, (3) insulating the air handlers and the ducts, (4) painting the two-car garage door gray (it was white) as it faces south and absorbs heat during the day (soaked up a bit by all the concrete), (5) windows/doors at 0.3 U to 0.21 U (I have 6 sliders, all six foot wide - we like natural light), (6) clean low air resistance air filters, (7) heat pump dryer (does not vent to exterior and hence does not exhaust/waste conditioned air), (8) drapes (but be careful, don't overdo or else you can get condensation issues) just need drapes that slow down air flow a bit across the pane (any additional convection across the panes will increase heat transfer, in part by diminishing boundary layer thickness) a little air flow can be good but too much is bad.

          And, we do get cold. In November, had two nights at -20 C (-7 F) with no problems maintaining the temperature at 21 C (70 F) inside. My installer commented that they have not had any issues with Mitsubishi Hyper Heat heat pumps in Montana.

  9. masincalgary | | #19

    I just learned what a mini split is! See what a novice I am. I'm a home owner, not a tradesperson. I will have a mini split, which means a unit inside (the air handler) and a unit outside (the compressor). I was told that the higher price is the way HVAC people make their money- the product, a % of the product, and then paying their men of course. I suppose different people might have different ways of business modelling.

  10. masincalgary | | #23

    Thank you Steve Grinwis and PBP1. I have no a/c needs by the way (though I understand I will have one when I get a heat pump). I still am not confident in my answer to question #4 above. It refers to specs on Mistubishi and Daikin, and I want to see their BTUs or COP at temperatures from -20 to -30. Steve, how did you size your Mitsubishi? Did you look at the BTUs you need and then the BTUs provided by the heat pump at your expected low temps?
    PBP1, my energy audit people worked out a heating design load of 34,500 BTU/h and two vendors here want to sell me at 42K. One tells me that the 36K will not provide enough heat at -20 C. This seems like the most important information, yet to hard to get! (I have asked, so it should be coming). Meanwhile my beloved oil space heater just gave out (I've three more space heaters, so not worried).

    1. PBP1 | | #32

      I was told to not oversize because of (a) the way a heat pump works and (b) the way multi-zone works. A heat pump is more efficient when operated at/closer to full capacity and when one zone is satisfied, the manifold switches to another zone, etc. For example, a 28.6 kBTU/h heat pump (just want to be sure that units are correct "per hour" as you have indicated) with a 28.6 kBTU/h heat load (say at 5 F = -15 C) can be paired with three air handlers 15 kBTU/h, 12 kBTU/h and 9 kBTU/h for three different zones. I understand that once the zone with the 15 kBTU/h is satisfied (brought to target temperature), the manifold can switch hot fluid from the heat pump to another zone/air handler and so on. You can see that 15+12+9 is greater than 28.6 but each zone is less than 28.6. There is downtime for the heat pump too, for example, downtime due to a defrost cycle. You can look on the Q&A for CollieGuy, he has a lot of good data - and some others too. Check out https://www.mitsubishipro.com/pdfs/m-series-catalog.pdf As to being that much over design load (+ 7.5 kBTU/h), I think an expert would be the best to comment. I am in Zone 6 and pretty much right at the design load. Also, please take note that the MXZ 36 kBTU/h from Mitsubishi is rated at 45,000 BTU/h (45 kBTU/h) max heating capacity at 5 F (-15 C) (see p. 40) and the MXZ 42 kBTU/h is at 48,000 BTU/h (48 kBTU/h) max at 5 F (-15 C) (also p. 40). Thus, you really need to read the specs because sometimes the numeric model number does not coincide with the actual rated max heating capacity.

      If it is a MXZ-5C42NAHZ*6, rated at 42 kBTU/h cooling - that has 48 kBTU/h heating at 5 F (-15C). That's way over (I'd consult an expert).

      "One tells me that the 36K will not provide enough heat at -20 C." The MXZ-4C36NAHZ*6 is at 36 kBTU/h cooling and at 45 kBTU/h heating (only 3 kBTU/h less than the 42NAHZ*6). So, if the 36K (model) is "not enough", then why would 3K more in heating capacity all of a sudden "be enough"?

  11. Expert Member
    Akos | | #27

    Usually most hyper heat units provide between 80% to 90% of their nameplate capacity at -20C. A 3ton unit will be very close, I would say close enough that even adding in small wall heater cover you for polar vortex days. For example the 3ton mitsubishi unit will deliver 34200BTU at -20C.

    You can take a loot at the engineering manual here for the PUZ series, it shows output down to -25C:

    https://nonul.mylinkdrive.com/files/P-Series_Engineering_Manual.pdf

    There doesn't seem to be that much efficiency difference in this case between the 42k and 36k units, so there is not much issue with upsizing a bit.

    1. PBP1 | | #33

      I noticed that the Mitsubishi MXZ-4C36NAHZ*6 max rated heating capacity is at 45,000 BTU/h at 5F (-15C), while my unit MXZ-3C30NAHHZ is at 28,600 BTU/h at 5F (-15C). That's a 16 kBTU/h difference for only a "6" difference in nameplate.

      Thus, the nameplate capacity (for heating) can be off the actual. The MXZ-4C36NAHZ*6 is at 36,000 kBTU/h for cooling. Above "30", the nameplates are aligned with cooling capacity and not heating capacity (42 = 48 kBTU/h heating, 48 = 54 kBTU/h heating).

      There are threads on GBA that warn of oversizing problems.

      1. Expert Member
        Akos | | #34

        Name plate and heating capacity is generally all over the place for hyper heat units. Fujitsu is the worst for this as some of their units deliver 60% more heat than their nameplate capacity at 5F.

        In the case of the OP, I was looking at Mitsubishi one-to-one system with a high static air handler. These are generally the easiest to install when replacing an existing furnace unit as it pretty much just slides in. For those at least the nameplate and cold temperature capacity seems to match.

        When it comes to selecting these units for cold climate, one needs to get the engineering data. Without that, it is all just a wild guess and little chance of getting it right.

        1. PBP1 | | #39

          Thanks! And need to read "original post" (OP?) more carefully, yes, a single high static air handler to replace a furnace unit as it fits in with existing ducting, makes sense. I had new construction with three low static SEZ (max CFM around 529 with 0.2 in WG).

    2. masincalgary | | #43

      Akos, see my latest post, Dec 6 re specs.

  12. masincalgary | | #36

    VERY MUCH not wanting to guess. Will look at all the data I've received thus far. It is certainly not easily available. Is one to one system = one heat pump and one air handler (correct me if I'm wrong). OP = Outdoor temperature performance (i.e. difference in performance at different outdoor temperatures?)
    Not in favour of wild guessing when it comes to the heart of my home!

    1. Charlie Sullivan | | #41

      Yes, one to one system = one heat pump and one air handler.

      No, OP is not what you thought--it's "original poster" (the person who asked the original question--that's you masincalgary) or "original post" (the question you first asked).

  13. Deleted | | #40

    Deleted

  14. PBP1 | | #42

    Dana, an expert, weighed in with expert advice on:

    Gas Furnace or Minisplit System
    JBrian2020 | Posted in General Questions on November 11, 2020 09:11am

    Maybe see that post . . . .

  15. masincalgary | | #44

    I think the attached chart is helping me a lot. Still one needs to be sure of one's interpretation. I now have the specs for what I take to be the Mitsubishi zuba central (PVA-A42AA7). It would be interesting to note if they are improved over the PVA A42AA4 model. However, those specs as provided by Akos don't go down past -10, and perhaps the improvement is more testing at lower temperatures (?).
    The A36AA7 looks more efficient than the A42 bc the A36 only looses 8K BTu/h when the outdoor temp drops from -15C to -25C whereas the A42 drops 10K. It makes me think the smaller size would be better, and then bite the bullet of more resistance back up heat on the -20 and below days.
    Running both systems at the same time (heat pump and back up resistance coil) might be too much of an electric load for my house. There is some modelling that prices out the difference in utility costs when using the different sizes and I'm excited to try it.
    Daikin tells me that their system doesn't run both back up and heat pump at the same time, and they assure me that I do not need to upgrade my amps to the house from the city.

  16. Steve Grinwis | | #45

    So, don't take the BTU output loss as being related to efficiency. Look at the hspf or COP at low temperatures to determine this.

    My design heat load is 18k BTU at -18C and we have a 30k, so we're definitely oversized a fair bit, but she holds the whole house right down to -30C without backup. -32C is the record low here.

    We do technically have Backup heat in the form of a hot water coil running off the tankless water heater, but it never comes on.

  17. masincalgary | | #46

    Thank you. Good to know. My design heating load is 34500. The 42K provides 36K at a-25 C, and 35.5K at -30C. I wouldn't have to run back up until below -30 (which is only a few days) and they are telling me I could do without the back up.

    The 36K provides just under my design heating load at -20, which means I'd be relying on backup heat for about 7 weeks of the year.

    So far, I can't get the Daikin engineering data below -19.4 C. It feels important to me to see it. Mitsubishi did give me data at -19.4, -22.2, -25, -30 C.

    1. toothman2020 | | #55

      Masincalgary,

      Daikin Aurora series is supposedly their cold weather equivalent to Mitsubishi's H2I/ Hyper Heat technology.

  18. David Barnes | | #47

    I’m curious about how accurate your Heat Load calculation is, since you’re really concerned with matching it closely. Most calculations have a little fudge factor to be safe, but more importantly the auditor can fudge the inputs going in as well. I also saw that you weren’t familiar with ACH50, so maybe you haven’t had a Blower Door test to accurately measure the air leakage rate of your house “envelope”?
    If you don’t have a Blower Door test number to input into the Heat load calculation, it can be really far off. If you don’t have a Blower Door number in CFM of leakage at 50 Pa, then they instead can choose an input like “tight” “average” “leaky”, which is pretty broad and might not correlate to your home. This input is especially important the colder it gets, as it takes the stack effect into account. Those “descriptions” can also oversize your heat load vs a good Blower Door number input after efficiency upgrades have been done, as you’ve seemingly done.

  19. masincalgary | | #48

    Thank you for asking. I didn't understand ACH50 bc I'm unfamiliar with a lot of the referents and terms. Now I've just looked at the document that details my blower door test (I had before and after done). It does read under Airtightness:
    Air leakage rate at 50 pascals - and in the next column, 2.08 air changes/hour.
    It is this Energy Audit report that calculated my design heating load of 8.96 kWh, and added 10% to bring it to 10kWh, or 34,500 BTU/h.
    I tried to work it out on my own, based on my historic data just to compare, but I'm just too unfamiliar with the meanings for things to rely on my following a formula.

  20. David Barnes | | #49

    2.08 ACH50 is a pretty good number for a retrofit/existing home. It’s usually tight enough for the HVAC to have good control of room by room and stratification.
    In your audit report, did they also show the existing total static pressure (tesp), or total system airflow, just to evaluate and put numbers to your existing duct system, like return static vs supply static? Most existing duct systems don’t have enough return duct as well as filter surface area (large or multiple return filters).
    On the electrical, furnaces use 110v, typical central type Heat Pumps, like Trane, Rheem, Carrier need 220volt in the attic or basement at the air handler plus 220volt at the condenser unit outside.
    Mini splits, even central ducted mini splits like Mitsubishi only use 220volt outside and have a cable from the outdoor condenser that supplies the indoor air handler.
    Where is your current furnace? What size?

  21. masincalgary | | #50

    Hi.
    I don't see airflow or pressure numbers, and I could ask them. I'd copy and paste your questions because I don't understand them. My furnace was way oversized (at 80K BTUS/h min - 115K max), and the inflow wasn't big enough (it was the second furnace). I relied on someone else's research which is in part why I want to have a good understanding here and not repeat the same mistake, and not have an oversized furnace. It was way too loud and not comfortable. It is in the basement.
    I won't be controlling the temp. in individual rooms (if that is what is meant by individual stratification) - due to budget and avoiding dry wall work.

  22. masincalgary | | #51

    I have some data and it lists "Zuba-Central MAXIMUM heating performance without auxillary heat." On another page, "Zuba Central MINIMUM heating performance without. . . "

    What does Max and Min mean? I am only paying attention to MAX since I want to know the limits of the unit at the cold temperatures.

  23. masincalgary | | #53

    Someone told me that min performance is for the shoulder seasons of spring and fall. I suppose that means that during times of less harsh weather variance, the unit doesn't need to operate as strongly. And so on the min. performance data, the COP is slightly better. I'm guessing at my understanding.

  24. Expert Member
    Dana Dorsett | | #54

    If you have a heating history on the place, measure the heat load with the gas burner logging fuel-use against heating degree-days, as outlined here:

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    Sizing the heat pump for 0.9x - 1.2x from that number is about right for 1-2 stage heat pumps with low turn-down ratios. Add auxiliary strip heat to cover the shortfall when it's colder. With a cold-climate heat pump at -30C the efficiency of the heat pump is slightly better than the auxiliary strip heat, but not by leaps and bounds, and oversizing it to be able to cover the colder temps without strip heat risks oversizing, making it less efficient the other 90-99% of the time. With the smaller heat pump even though the peak electric draw at the peak cold outdoor temps is somewhat higher, the seasonal average efficiency will be higher.

    With the simpler compressors of NON cold climate heat pumps the efficiency is usually less than auxiliary strip heat even at -25C , sometimes even at -20C.

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