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Community and Q&A

Gas Furnace or Minisplit System

JBrian2020 | Posted in General Questions on

I am currently renovating my home and am replacing the gas furnace that is installed in my attic. The reason for replacement is I am insulating the attic with foam where the existing normal furnace is which right now gets its combustion air from the attic space. Once the attic is reinsulated the attic will be sealed and a different furnace (condensing) will be needed.

I have gotten prices for condensing furnaces and mini-splits. The condensing furnace (furnace + outside condenser) is $12K and mini-split heating system is $20K. I already have ductwork in the home.

So here is my question Mitsubishi and Fujitsu each make systems that can go down to -13/-15. I live in NH Zone 5, and we do have times where we get below these temperatures. Will the mini-splits work in the colder climate with no backup heating source (most of the contractors are reluctant to install a mini split system as the only source of heat? ( house built in 2003 with normal 2X6 walls and normal batt insulation; house is normal to below normal insulated).

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Replies

  1. Jon R | | #1

    > Will the mini-splits work

    I suggest that "work" isn't the best question. Where the manual clearly states that minimum temperature is -13F, is it a good idea to operate it lower than that? Or, for example, is there a reduction in life due to lubrication issues? Or potential warranty issues (operating outside of specs)?

    I know someone who didn't change their car's oil for 50,000 miles. It "worked".

  2. Expert Member
    Dana Dorsett | | #2

    First things first.

    Since you have a heating history on this place, run a fuel-use based load calculation to measure the 99% design load. This method is good enough:

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

    With the heat load established you can search for a ducted or mini-ducted system with meets or exceeds your design load at +5F by 10-25% on this site:

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

    Then take a look at the main trunk duct sizing and calculate the duct velocity at the highest blower speed of the heat pumps that meet capacity requirements. This nomograph is useful for people who are calculator-challenged:

    https://www.dropbox.com/s/pwgwl5m8vryqh5c/Duct%20Sizing%20Using%20FPM%20Targets%20and%20CFM%20MacFarland.pdf?dl=0

    From a system efficiency point of view ideally the duct velocity in a buried attic duct configuration would be <500 fpm, but over 400 fpm. Any slower than that could lead to higher heat/coolth loss from the ducts, any faster results in high air handler power use, higher static pressures, and higher air handler driven infiltration from any leakage.

    Most mini-ducted Fujitsus are fully characterized with a rated capacity down to -4F, which is probably a hair cooler than your 99% outside design temperature, but most single-zone ducted mini-splits are not equipped with pan heaters for automatically clearing defrost ice from the outdoor unit. Depending on your load numbers you may want to break it up into a couple of zones and use a cold-climate 2-3 zone multi-split that comes with a pan heater and associated controls.

    To guesstimate the local 99% outside design temp, find something close to you on this list:

    https://farm-energy.extension.org/wp-content/uploads/2019/04/7.-Outdoor_Design_Conditions_508.pdf

    Manchester's design temp is -3F, Keene's is -7F, Portsmouth's is +9F. Yours is likely somewhere in that range, but with a ZIP code we might be able to guess a bit more closely.

    The Mr. Cool Universal 2-3 ton heat pump is fully characterized down to -22F, and delivers it's full "rated" capacity down to -5F. I haven't seen it on the NEEP site (yet), but it should be able to handle most ~2000' - 3000' houses with 2x6 /R19 + low-E window type construction at your local low negative single digit design temps. Unlike most of the Japanese & Korean competitors, their single zone mini-ducted systems also come with pan heaters in the compressor unit too. Their 2-ton compact-ducted system might cut it for smaller houses as a single zone:

    https://ashp.neep.org/#!/product/30937

    As I understand it all of the Mr.Cool heat pumps are designed and manufactured by Midea, the largest first tier Chinese refrigeration, AC, & heat pump manufacturer, and the world's largest manufacturer of air conditioning equipment, much of it OEMed to other vendors. It's likely that the same MrCool systems can be found under not only Midea's label, but many are also distributed by Carrier, Senville, Pioneer, etc.

    There are (of course) others, but don't be afraid to specify a particular model and stick to it once you get comfortable with your load calculations. Most HVAC contractors in our area are pretty clueless about just how much the low-temperature performance of this equipment has improved over the past 15-20 years.

    And even with a first tier Japanese vendor a ducted single zone mini-split shouldn't cost anywhere near $20K, assuming there aren't major revisions being made to the duct system. In competitive bidding it should be no more than ~$5K/ton, and most houses would not need more than 3 tons. A 2-3 ton Midea/MrCool Universal would in most cases be comparable to your gas-burner quote.

    1. HoverDA | | #7

      Mr.Cool Universal may be manufactured by Haier. For 4-5 ton outdoor unit, it uses Zhuhai Landa (subsidiary of GREE) Compressor QXAU-F516zX440A. You could check at 2:13 in the YouTube video (https://www.youtube.com/watch?v=y8WVXlzYegQ). By searching the compressor, you could find the detailed info/data of this heap pump on Haier GE appliances website.

      Innovair also sells the product.

      NEEP has data for 2-3ton AUH2436ZGDA: https://ashp.neep.org/#!/product/33204

  3. Josh Durston | | #3

    If your truly concerned I would stick with Fujitsu since they keep running (at reduced capacities) with no low limit cutoff. My only concern at extreme low temps with the XLTH series would be whether the capacity is adequate.

    EDIT: Concern was the wrong word, more like caveat. The XLTH class Fujitsu's IMHO are the top ASHP choice for truly cold whether.

    1. Expert Member
      Dana Dorsett | | #4

      >"My only concern at extreme low temps with the XLTH series would be whether the capacity is adequate."

      There's a guy who used to post here under the handle "Jin Kazama" who successfully heats his place in Quebec with four -9RLS2 ductless units who reported that it sailed through -28F temps without losing ground. His location hits the -30C (-22F) range every winter at some point.

      The capacity goes down with lower temps, but it doesn't fall off a cliff until somewhere south of -40F or so. At temps in the -30s F it's likely to be running a COP less than 1, but at -15F it's likely to be 1.5 or a bit better.

      The (IECC zone 5) southern tier counties of NH aren't really much of a challenge for better class mini-splits- it's simply a matter of sizing them correctly. Fujitsu isn't the only one that keeps on going- MOST do. Mitsubishi is the only vendor that I'm aware of that automatically turns itself off (and auto re-starts) at some colder temp.

      1. Josh Durston | | #5

        Agree with Dana,
        To be clear have no concerns, just a caveat that the capacity goes down as temps drop (like all other ASHPs)
        I was trying to be positive about the XLTH, basically saying they are built for low temps, and I wouldn't be worried about beating up the hardware (oil return, etc).
        Just making sure I select equipment appropriate for the loads.

        1. Expert Member
          Dana Dorsett | | #6

          >"I wouldn't be worried about beating up the hardware"

          That's right, especially in this case. The number of hours per decade with temps run in negative double-digits is pretty low in southern NH.

          The bigger concern for IECC zone 5 use is managing the defrost ice, not beating up the equipment. There are many many mini-splits even in zone 6 VT that have run for well over a decade without a pan heater, but if you happen go without it's prudent to check on it occasionally during periods of weather that doesn't break the freezing mark for days/weeks on end. Even in somewhat cooler Keene the period of risk when it might not reach the freezing mark for more than a week is only at the cusp of January/February in typical years:

          https://weatherspark.com/m/25654/1/Average-Weather-in-January-in-Keene-New-Hampshire-United-States#Sections-Temperature

          In more temperate Manchester it's less common. Most weeks will see at least a mild thaw even in January:

          https://weatherspark.com/m/26350/1/Average-Weather-in-January-in-Manchester-New-Hampshire-United-States

          If you're the worrying type of person or would really HATE to manually thaw an ice plug in the pan drain once, maybe twice per season a pan heater is an absolute necessity. If you're willing to inspect and take action as-necessary during colder weather it isn't critical.

  4. Walter Ahlgrim | | #8

    I have to say the smart move would be to get the equipment and ductwork out of the attic and inside the conditioned space without making the conditioned space 50% larger.

    Maybe I missed it but why do you need a new furnace after you insulate? There are thousands of houses that use non condensing furnaces using indoor combustion air.

    Generally when people change heating fuels the local cost of fuel is the most important factor. If you are on city gas it is very unlikely that a fuel changes make financial sense.

    In dollar term I doubt you are likely to live in this house long enough for insulation and furnace to save enough to pay off.

    In green terms it is always a sad day when spray foam is your best option.

    Walta

  5. Joe Braun | | #9

    I will be getting a mini split based on a lot of different blogs with people stating that they do not have any problems in very cold weather. Here is one from here:

    https://www.greenbuildingadvisor.com/question/cold-weather-performance-reports-from-minisplit-owners

  6. masincalgary | | #10

    Hi.
    $20 K for an attic is a lot. I'm looking at less than that for a whole house in Canada, where we get below -30 for a few days. Why don't you try one winter with space heaters? I'm grateful for last yr since I upgraded my insulation to experience the house with no other changes. My gas furnace konked out, and for two weeks I've been OK!
    But . . here is where I'm asking similar questions: https://www.greenbuildingadvisor.com/question/evaluating-heat-pump-needs-for-cold-climate-4-qs#comment-192272?utm_medium=email&utm_source=notification&utm_campaign=comment_notification&utm_content=view (have a look).

    I'm looking at the Mitsubishi and Daikin, both of which have been around for decades. I'll post more about there.

    I've had to persist at looking at the engineering data (they don't make it easily accessible but I'm insisting on seeing performance between -25 and -30 C). I do have an energy audit which tells me the design heat load (amount of heat I need considering all factors). Then I look at the specs for max heating performance at low temperatures. This chart helps the perspective on the need for my furnace to perform at low temperatures: https://calgary.weatherstats.ca/charts/count_temp_m20-yearly.html

    One installer did suggest I go with a 42K Mitsubishi Zuba, and never mind the back up resistance heat. I might choose the smaller one with the back up resistance coil, so that I am not oversized, and so that it's more efficient most of the time (and then just accept higher bills when I'm relying on the back up). I aim to shut off the gas to house. There are some home owners in Calgary who have done that. My home is well insulated (2 airchanges/hr).

    Good for you for doing this work!

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