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Community and Q&A

Cold-Climate Air Source Heat Pump

MDMD | Posted in Mechanicals on

We live in Ottawa, Ontario (zone 6) and want to stop directly burning fossil fuels. That means replacing our townhome’s natural gas furnace and hot water heater with electric.

Our plan has been to replace the furnace (two-stage 28,000/40,000 BTU) and existing AC (18,000 BTU) with a ducted ASHP. After researching and speaking to some contractors, we had settled on the Trane XV19 because it’s compact and quiet. The proposal we have is for the 24,000 BTU XV19 and the TAM Air Handler with 15 kW heat strips (models 4TWL9024A1 / TAM9A0B30V31DA).

However, we now realize that the Trane is not a cold climate ASHP and we are concerned about the operating costs and performance in coldest weather. That’s because it appears that if we are going all electric in our climate (heating design temp = -13 F), we have to trade off between three things:
1) Sufficient ASHP heat output
2) Cost of backup electric resistance heat
3) The need to avoid oversizing the ASHP so that the cooling does not “short cycle”

All insights welcome and we particularly seek answers to the two related questions in the title:

   1) Do we need a cold climate heat pump to help balance the heating and cooling performance in our climate?

   2) What BTU output should we get?

Many thanks in advance!

** For those who dive deeper with data and arithmetic, here are lots of further details:
– Home built to energy star standards in 2019 – we bought in 2020 so have lots of accumulated data
– 1,450 sq feet over 4 levels
– Footprint is 12′ W x 42′ deep (heated space is all of top 2 levels and half of the ground floor and basement)
– Long walls are shared on both sides with attached townhomes so very low exterior surface relative to heated space
– Lots of passive solar heat all year round – front is south-facing and 50% energy-efficient windows
– Design loads for the home
– 39,500 BTU for heating – furnace is two-stage 28,000/40,000 BTU
– 17,00 BTU for cooling – AC is 18,000 BTU
– Design temps for our location
– Heating = -13 F
– Cooling = 86 F
– HDD are 8,000 F
– Heat load calculation (using
– Balance point 65 F
– Meter reading data from Jan 5 – Mar 4 (our gas utility only reads every 2 months) minus known summertime gas consumption for hot water only
– 9,691 average BTU/hr
– Ecobee settings and data records
– Settings are 70 F from 7:00 am – 5:00 pm, 72 F from 5:00 – 11:00 pm, 61 F 11:00 pm – 7:00 am
– Furnace only ever works at first stage
– Other than the warming to higher set points, the furnace runs very rarely

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  1. Expert Member
    Akos | | #1

    You have to finish the fuel usage based calculation for load at design temp. For a row house that size, I highly doubt it will be anywhere near 39.5k BTU. Best guess is less than half that number on the high side.

    You do want a cold climate heat pump and the VX19 is definitely not it. Plus even the smallest one will probably be oversized for your shoulder season load but cut out and switch over to strip heat at low temp where you need it the most.

    I would took at either a Mitsubishi SUZ/SVZ :!/product/31992/7/25000///0

    or Carrier/Midea:!/product/33267/7/25000///0

    Would have to check the engineering manuals but I would guess either would carry your place without backup heat.

    1. MDMD | | #4

      Greatly appreciate the reply and the product recommendations. For Mitsubishi, do you have any thoughts on whether the smallest PUZ (which is 24K) worth considering as an alternative?

      Regarding the fuel usage calculations, I thought that I had completed them according to the method in this post: Not sure how I got confused but here is what I did:
      1) Took two consecutive gas meter readings in winter and then again for summer, to find the amount to subtract for hot water (our gas consumption is only heat and hot water): 304 m3 total - 26 m3 for hot water = 278 m3 for heat
      2) Got the specifications on our gas furnace: 96% efficiency
      3) Downloaded the HDD for base 65 F for our location for the same period as the two consecutive winter gas readings: 3,163
      4) Looked up the heating design temp for our location: -13 F

      My calculations then were
      - Heat delivered into ducts = 9.4M BTU = 278 m3 gas x 96% furnace efficiency x 35,300 BTU/m3
      - BTU per degree-hour at 65 balance point = 124 = 9.4M BTU/3,163 HDD/24 hrs/day
      - BTU/F-hr = 9,691 = 124 BTU per degree-hour x 78 degree difference (65 F balance point and -13 F design temp)

      Would really value any guidance on how to get this right. The result looked reasonable to me given what I have read about HVAC installs being as high as 4x the heat load and because our furnace seems to work so little to keep us comfortable. But it sounds like I fooled myself.

      1. paul_wiedefeld | | #8

        Akos was correct - you have a very low heat loss of 9,691btu/hr! You did the calculations correctly. That means you could use an even smaller unit.

        How well does the existing AC cool? If it's also cycling on the hottest day, the 12kbtu unit might be enough.

        1. MDMD | | #11

          Thanks for confirming the calculations.

          Good question about the AC on the hottest days - my impression is that it does cycle but it working a good majority of the time. Given that and the fact that I can't isolate its electricity consumption with any accuracy (too many other devices/appliances plus a recent EV purchase) I am a bit skittish about sizing below 18kbtu.

  2. paul_wiedefeld | | #2

    9,691 * (65- (-13))*.95/24 = 29920 heat loss at 95% efficient furnace, lower if the furnace is less efficient. That's probably why your furnace never leaves 1st stage. That Trane in particular runs out of capacity quickly. The 24kbtu Mitsubishi with 8kw backup would be my go to but only if you have the amperage - you might be pushing it if you're at 100 amps. There's another play where you keep the gas furnace, add a heat pump, and forgo the electric backup. That's a scenario where switching 95% of heating to electricity and fitting in an electric water heater is better than going 100% electric for heating and skipping the water heater.

    1. Expert Member
      Akos | | #3

      Something is not right. I've lived in similar places, there was barely any difference in gas consumption between summer and winter. Definitely not 30000BTU heat loss unless there is a huge hole somewhere in the house.

      1. MDMD | | #6

        Confused by the mention of the huge hole/heat loss. We had a blower test in May as part of the application for a government grant and there are no meaningful leaks.

        Regarding difference in gas consumption in this region between summer and winter, our experience is that we consume far more on winter. Our utility bills for this house and our two prior ones - one was similarly ducted but the other was hot water boiler with cast iron rads - testify to that.

    2. Deleted | | #5


    3. MDMD | | #7

      @paul_wiedefeld - Thanks very much for the helpful feedback.

      Regarding the Mitsubishi, AKOS TOTH's recommendation above is for an 18kbtu system. However, if their 24k system is a good match, that opens the choice to either the SUZ series or the PUZ series. Any experience or thoughts to share on choosing between the two?

      Great catch on the 100A electrical service. We started this process unaware of that issue and we do have 100A. But due to the needs of this project and the rooftop solar that is our next project, we are upgrading to 200A over the next few weeks.

      We had initially planned to hybridize an ASHP with natural gas furnace as backup. But there are two issues that changed our mind:
      1) The fixed cost charges of our gas connection are high enough that the cost per BTU for gas heat approaches the cost of electric
      2) The forecast for future energy costs in our area has gas rising more quickly than electricity, partly due to steadily increasing carbon taxes

      As a result, we think the all-electric is a reasonable long-term investment but would be happy to know if my logic seems flawed.

      1. paul_wiedefeld | | #9

        Above, your heat loss calculation shows that the 24kbtu is too big, so the PUZ vs. SUZ debate is unnecessary. For what it's worth, I have the 24kbtu SUZ and really like it.

        Your reasoning is sound - with the 200A service, you can ditch the furnace.

        1. MDMD | | #12

          Thanks again and on to getting quotes on the Mitsubishi and any other ccASHP options.

  3. Expert Member
    Akos | | #10


    I think it was confusing that you labeled the initial 9k loss as average. The math you did is mostly right, for newer construction re-run the HDD and use 60F base temperature. This will bump up your peak heat loss a bit.

    Since your heat loss is so low, you don't need resistance backup, so even your existing service is big enough. 100A service is typically good for 3.6kW solar, even more if the panel busbars are rated for 200A.

    Once you select a brand (make sure to talk to your installer as to which one they can support), look through the engineering data for the actual unit. This is for Mitsubishi:

    Select a unit that would cover your heat loss at your design temperature with a small margin.

    1. MDMD | | #13

      Apologies for creating the confusion and thanks for the additional response.

      Repeating the calculations for the 60F base yields 10,002 BTU/hr.

      REALLY interested to hear that we might not need resistance backup at all from the perspective of load. But is there a scenario where the system's heat pump fails and the resistance backup still works? It's a different type of backup but just wondering.

      For the rooftop solar, we have a proposal that would give us 7.6 kW but the installer reports that a recent electrical code change here in Ontario prevents installing more than 5 kW on 100A service. So, the 200A will be needed to achieve maximum output.

      Last question: Given the 10kbtu heat loss at 60F, do you think that we will be comfortable with a 12k Mitsubishi or does the recommendation remain as 18k? My response to Paul above provides my observations on the cooling activity of our 18kbtu AC unit.

      Thanks again.

      1. paul_wiedefeld | | #15

        "But is there a scenario where the system's heat pump fails and the resistance backup still works? It's a different type of backup but just wondering."

        Yes, the resistance would work if something happened to the compressor. If something happened to the blowers, then you'd be out of luck. Electric baseboard would work independent of the blower.

      2. Expert Member
        Akos | | #18

        If you want to skip backup strip heater, you would need the SVZ-KP18NA/
        SUZ-KA18NAHZ combination. The 12k unit would not provide enough heat at -13F.

        As for backup, what is your current backup for your furnace? I'm guessing nothing, don't see why one would be needed for an ASHP. In a townhouse with two neighbors heating your place, it will never get cold enough to matter. You can also pretty much heat your place with a pair of 1500W plug in heaters anyways.

        If your electrical panel is not too far, adding in the small strip heat "just in case" should not add a lot of cost. You only need the 3kW unit, which will already provide your full heat loss, no need for a 10kW unit as proposed.

        1. MDMD | | #19

          I think that the 12k unit would not only be insufficient for our heating needs, it also wouldn't provide enough cooling. Our current AC is 18k and it works most of the time in the hottest weather.

          Good point about the lack of backup for the current gas furnace! Humans always seem to hold new technologies to higher standards than the existing ones, though I haven't seen any data that compares failure rates/frequencies of gas furnaces vs ASHP (does HVAC use mean time between failure (MTBF) as a metric?).

          Even better point about getting heated by our neighbours in case of our system failing! I find that the effect of heated neighbours and passive solar seem to be the elements that most fool the "rules of thumb" for sizing.

          I checked the Zuba brochure and it appears that the smallest capacity heat strip they sell for their air handlers is 8kw, which translates to about 28kbtu. That's obviously more than we need but can't see a why to size it smaller.

          1. Expert Member
            Akos | | #24

            If you look at the accessories for the unit, there is 3, 5 and 8kW strip heater available.


            If I remember correctly, the heaters in these are on/off control, so you want it somewhat close to your design load. Plus a big heater needs a larger feed which adds to your demand calculation, not a big issue with 200A service though.

  4. walta100 | | #14

    From a green point of view, it is a poor choice to replace modern efficient equipment in operational condition on a whim. Reuse is always the best option only then does reduce and recycle make sense.

    From an economic point of view if you are on city gas and still operate any other gas appliance it is impossible to lower the monthly bills. If you eliminated all the gas you might break even but never recover your cost.


    1. paul_wiedefeld | | #16

      Do we know this for certain? How much energy does it take to build an AC/ furnace? I'm curious - it certainly must be significantly less than the cost of the system, so maybe a couple hundred $ worth?

    2. MDMD | | #20

      I think it's a bit unfair to categorize a choice to de-carbonize as a "whim".

      I completely agree that disposing of something that has plenty of useful life remaining is wasteful, both with respect to resource inputs and money. But I would suggest that the traditional ecological thinking needs updating in the face of the present climate emergency: I don't think we are going to adequately reduce carbon emissions by simply optimizing the efficiency of carbon-emitting devices/appliances/transportation. Instead, we are going to have to substitute non-emitting products for our current ones, often earlier than conventional thinking would dictate.

      And yes, I am aware that one's carbon emissions vary greatly depending on how the electricity is generated. In Ontario, we are 80% non-carbon emitting on the grid but we can improve on that by sourcing clean electricity and later adding rooftop solar (our next project). That dumb luck (because I can assure you that no political leaders were planning a low-carbon grid) makes the switch to an AHSP more sensible than it would be in many other locations.

      Regarding the hard economics, we believe our plan is also a sound investment. Because we have no gas consumption other than heat and hot water, we will convert both to electric immediately. That allows us to turn off the gas and save the fix costs of a connection. And the forecast for consumer pricing of electricity vs gas in Ontario will increasingly favour electricity as rising carbon taxes take hold. I admit that this too is the dumb luck of where we live.

  5. owen_p | | #17

    Hi MDMD,

    I'm also in Ottawa and considering a similar ccASHP transition to replace a failing/ailing furnace in our 1968 home in Blackburn Hamlet.

    I can't find a way to send you a direct message through this forum, but if you are interested to chat I can share more about the local contractors and designers I've talked with and perhaps we can learn something from each other.

    I too have been trying to apply the estimating technique in the replacing a furnace/boiler article. I was able to trick the EcoBee API into getting me more than just 12 months of data (and 1 month at a time). But I've found that the runtime * nameplate BTU is over estimating the actual meter readings even after removing igniter warmup and pre-purge cycle time (30s). Once heating season starts I'll have to time how long it takes the furnace alone to burn 1m3 of methane as I suspect mine is running at less than the 69K BTU it is rated for.

    If you want to reach out, my email is on the top row of tab #1 of this google sheets doc with all my estimated heat/cooling load data (still a work in progress - we just had a kiddo so I don't have much time these days)

    I've decided to hire a local HVAC engineer to help guide me through equipment selection


    1. MDMD | | #21

      Would love to chat. Sending you an email message to connect.


      1. owen_p | | #32

        Hi MDMD,

        Exciting that you are so far along. I'm envious, next year at the earliest for us.

        It would be great to chat by email or on the phone, please find my email in Tab 1, cell D1 of the Google Sheets document linked above.

        Also thanks for the heats up on the new circuit rules, thankfully we did a 200A and panel upgrade a year ago and have ample room and short runs to where the air handler would go.


        1. MDMD | | #33

          Hi Owen,

          I emailed you at that address on Sep 8, as promised above. It didn't bounce back so not sure what happened. Did you check your junkmail/spam folder?

          I have just emailed that address again in hopes that helps.


    2. paul_wiedefeld | | #22

      Owen great work! Would like to see how the final product compares to the BTU/HDD fuel usage method.

  6. rockies63 | | #23
  7. MDMD | | #25

    Despite Mitsubishi's website saying that they have three certified Zuba dealers in the Ottawa region, only one of them was interested in quoting (the other two are inexperienced).

    The quotes are as follows (all prices $CAN net after federal Greener Homes Grant):
    18k BTU S-series (SUZ-KA18NAHZ/SVZ-KP18NA) with 3 kW electric heat backup - $12,340
    24k BTU S-series (SUZ-KA24NAHZ/SVZ-KP24NA) - $13,270
    24k BTU P-series (PUZ-HA24NAH1/PVZ-A24AA7) - $14,099

    The contractor agrees that the 18k system is sufficient. However, he notes that its operating costs in colder weather are greater and might negate the relatively small initial costs savings of $930. The data he uses to support that are:
    - the 24kbtu S-series system provides ~30% more heating capacity at -15C (5F) and ~47% more capacity at -25C (-13F, which is my heating design temp) than the 18kbtu system
    - the 24kbtu P-series system provides ~12% more heating capacity at both -15C (5F) and -25C (-13F) than the 24kbtu S-series system
    - the CoPs of the sytems at lower temperatures are:
    18k BTU 1.8 at -15C (5F); 0.67 at -25C (-13F)
    24k BTU S-series 1.6 at -15C (5F); 1.30 at -25C (-13F)
    24k BTU P-series 2.0 at -15C (5F); 1.47 at -25C (-13F)
    This means that in very cold weather the 18k condenser shuts off due to its C.O.P dropping below 1.0, which is why we need the heat strips in that system.

    I know that the average number of hours per year that a location colder than its heating design temp is 88 (3.65 days, or 1% of the year).
    I also know that our electricity average cost is $0.11 per kWh (we have time of use and three levels).
    But figuring out the difference in operating costs over a range of colder temperatures requires some complex math built on the CoP curves of the systems.

    My two questions:
    1) Can anyone provide a lead for a tool that can compare the total cost of ownership (initial investment plus operating costs) of two or more systems?
    2) If a TCO analysis suggests that one of the 24k BTU systems is the best choice, any downside in being "oversized "?

    Thanks very much in advance!

    1. paul_wiedefeld | | #27

      Quick and dirty - I just counted all Ottawa days over the past 3 years with 55+ HDD_65. It's about 20 per year. Even if you were at design load for all 20 days *24 hours/day, you'd only need 4,800,000 Btu or 1400kwh total, about $150/year using 100% resistance heat. The COP 1.3 24kbtu unit would save about 325 kwh/year, or $35. Not worth it, and that's putting a thumb on the scale for the 24kbtu unit. Besides, the online specs have the 18kbtu unit as being more efficient at 5F.
      HDD Cutoff 55
      Hours/Day 24
      Heat Loss 10,000
      Days > HDD Cutoff 20
      kwh Output 1,406.80
      Total BTU needed 4,800,000
      Model 18k 24k
      COP at HDD cutoff 1.00 1.30
      kwh input 1,406.80 1,082.15
      $/kwh $0.11
      Savings $35.71

      1. MDMD | | #28

        Thanks for doing this! I was able to follow almost all the calculations and now see what you mean about the COP being higher for the 18k S-series vs the 24k. In fact the COP is higher for the 18k at 47F, 18F and 5F (see attached brochure, tables with yellow highlights) and we spend most of our winter here above 5F.

        As with your earlier comments above, and those from AKOS TOTH, it's hard to make a case for going above 18k.

    2. Expert Member
      Akos | | #30

      I would shop this around a bit more. I'm guessing your existing AC has a 20A feed for it, so there is even no extra wiring to be done to install this.

      The online price for the SUZ18/SVZ is around $3.5k US, even add in the markup for CDN, $18k for a furnace swap seems on the high side.

      My local HVAC folks had no issues supplying Mistubishi/Carrier/Midea/LG, no need to get too attached to a brand. If you can save some money by picking a different brand (all assuming properly sized and can supply heat at your design temps), even if slightly more costly to operate, you'll likely be further ahead.

      Paying $1k or more for slightly higher COP at your low heat loss means the ROI is probably never, plus there is a good chance that some of that savings you loose by having the unit cycle at lower loads where it will be running 75% of the time.

      1. MDMD | | #31

        Really appreciate the COP insights - they help confirm our plan to go with the 18k BTU Zuba.

        Regarding cost, not sure where you got the $18k figure - the quoted price from the contractor is $16k CDN (I wrote $12k because our net cost is reduced by a $4k government grant). That works out to $12,300 in USD.

        I'm sure that stills sounds too high but two issues face us:
        - Changes in electrical standards here require that the air handler must now be powered directly from the panel instead of from the condenser. In addition, heat strips are powered separately. So we in fact require three separate two pole circuits - condenser, air handler and heat strips - two more than we currently have.
        - The includes shutting off the gas connection and closing off venting.

        Most important, the cost for this proposal is many thousand dollars less than Trane, Bryant and Carrier quotes I got before turning to the Zuba option. It thus appears that our costs here are significantly higher across the industry i.e. this is not a brand-specific issue.

        Many thanks again - you have been a great help.

        1. Expert Member
          Akos | | #34

          "air handler must now be powered directly from the panel"

          I'm pretty sure that is not right. Not right, as in it will most likely let out the magic smoke out of the electronics. Mini splits are not like North American equipment and use one of the power lines as reference for the communication signal, there is no separate low voltage connection to them. The outdoor unit has the connection for and provides power to the indoor unit. By separating the power from this communication line, there is a good chance it will cause problems.

          Depending on where your panel is, code only requires a disconnect for the air handler, this can be a standard 3 pole one installed in-line on the 14-3 feed from the outdoor unit to the air handler.

          As for wiring, I know people that won't allow HVAC folks to even pull thermostat wires as they can be very sloppy and has the electrician run them instead.

          With a service panel upgrade and electrician on-site, I would have them run the new feed to the outdoor unit if needed, the power feed for the strip heaters and the 14-3 from the outdoor unit location to the air handler disconnect. The only thing your HVAC person should do is point to point connection to the equipment.

          1. MDMD | | #35

            I am grateful for your continued efforts to protect us from misinformation.

            Not sure what to say about possible differences between the US and Canada on electrical requirements other than the quote we got for a Trane system also mentioned the electrical specs were different for Canadian equipment than US equipment. If that's true, the fact that the Canadian market is 1/9 the size of the US might explain why equipment made for this country is more expensive i.e. small production runs for Canadian models leads to higher unit cost.

            A few other points:
            - Our system will be central ducted, not a mini-split
            - I never questioned the cost of proposal because it was lower than all the others we got so what I was told about electrical requirements was not a defensive response from them to justify the cost
            - I agree with concerns about HVAC contractors' electrical work but one of the contractors' two partners is a master electrician and their company happened to install a split-ductless LG system for us over 20 years ago in our former home, including doing the electrical work. They did a good job on that entire project and continue to be well-reviewed and under same ownership, so we don't have concerns about them.
            - My philosophy about such projects is that all other things being equal, it is always preferable to have one supplier do everything related to the installation so as to have a single accountability for the project. That means that we will limit the electrician to the electrical upgrade.

            Much appreciation again for taking the time to provide wise guidance.

          2. Expert Member
            Akos | | #36

            I use the term mini split to refer to any of the overseas systems. Many of these can be parried with different indoor units (wall, floor, ceiling, slim ducted, multi position air handler) and they are all wired the same, the Zuba is no different.

            It very well could be the indoor air handler power feed line item was left on the quote after they changed from the Trane system to Mitsubishi. In any case, the air handler on the Zuba should be powered from the outdoor unit. I would assume they know this but might not hurt to check.

            I'm in Toronto, the Mitsubishi equipment I had installed was exactly the same as the US ones. There is a lot of hand wavy arguments of why this is more expensive up here in the great white north, the reality is that they charge more because they can.

            I still find $16k for installing about $5k of equipment and a roughly 2 day job for two people on the high side. Maybe a bit of the unknown factor so a lot of times the price is what it is and getting quality work is worth the extra cost.

  8. Expert Member
    Dana Dorsett | | #26

    >"... the Trane is not a cold climate ASHP"

    Perhaps THAT Trane, but Trane also sells Mitsubishi cold climate & minisplit units under their own nameplate (at least in the US), sometimes for less than what the Mitsubishi contractors quote.

    >"If a TCO analysis suggests that one of the 24k BTU systems is the best choice, any downside in being "oversized "?"

    Look at the modulation range at average winter & springtime outdoor temps. Any time/temperature the minimum modulated output is greater than the load the compressor is cycling, and if it's cycling a LOT there is a significant efficiency (and often comfort) hit.

    1. MDMD | | #29

      Thanks for the reply and also for your 2016 article on calculating heat load (, which was my breakthrough learning in understanding how to calculate sizes. Great stuff!

      Regarding Trane and Mitsubishi: Despite their business partnership there are no dealers in my area that sell both product lines, so when it comes to cold climate equipment, it's the Mitsubishi brand here.

      Our present heating is a 2-stage natural gas furnace that is 28k/40k and even on the coldest days, it's on for only an hour or two. So the modulation range and maximums for the 18k are likely to be fine except below -13F, when we will need the heat strips.

      For cooling, we currently have a 18k unit that cycles a fair bit, but I expect that the cycling will go down with the Zuba 18k given that it's range is apparently 9.4-18k. Is that assumption correct?

      1. tkzz | | #37

        Hi just found your thread when searching for heat pumps.

        I'm in a different situation (Toronto, detached house) but am considering a hybrid Trane system with the XV19, a Carrier(Midea) hybrid system, or splurging on a Zuba.
        Gas rates are definitely up so it gets me thinking.

        I'm wondering what you ended up getting?

        1. MDMD | | #38

          Hi there,

          We ended up getting the 18k BTU Zuba because we calculated that in our climate, we would be requiring the costly backup electric heat too frequently with the Trane XV19. The system was installed 4 weeks ago and is performing very nicely. But I have been reserving comment until we get through the coldest weeks of the year, which are early January through mid-February.

          By the way, the cost for the Zuba was lower than the XV19. ($17k vs $22k). It also qualifies for $5k of federal grant whereas the XV19 only gets $4k due it not meeting "cold climate" criteria.

          1. tkzz | | #39

            Sounds like a great choice!

            After looking at specs, I concluded the same for XV19... not so good in a cold climate. Thanks for confirming what I thought.

            Interesting that your Zuba was cheaper, that sure makes it an easy choice! Curious to hear your feedback when it gets colder.

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