zero degree heating

Zero Degree Heating

  • October 14, 2015
  • By MR COOL

Everybody knows there’s no such thing as zero degree heating when it comes to a heat pump. An energy efficient heat pump might work for mild temperatures, but they rapidly lose efficiency as the temperature drops. Everybody knows you need a gas furnace to stay warm in severe winter weather.


I’ve got some news for you. What everybody ‘knows’ is about to change.

Zero Degree Heating is Real

The conventional wisdom that a heat pump won’t work for really cold winter days used to be 100% right. Previous generation heat pumps simply could not function in freezing conditions. As I’m sure you know, technology does not stand still. Right now it’s entirely possible for a modern heat pump to deliver zero degree heating. In fact, some heat pumps can continue heating at 100% efficiency past -10 degrees.

Okay, so that’s great. Heat pumps can heat better, terrific. What, however, does it mean for the average American homeowner?

Expanding Regional Utility

Not too very many years ago keeping your house heated with just a heat pump was not a great idea for most people. Heat pumps were unable to heat effectively at low temperatures, so only people in very mild winter climates could reasonably rely on one. If you didn’t live in South Florida or Hawaii, you were probably not going to use a heat pump for zero degree heating.

This is no longer the case.

Being able to heat effectively down to -10 degrees means using a heat pump for summer cooling and winter heating is a real option for many more homeowners than it used to be. If you live in Alaska and routinely experience temperatures around -40, you’re still going to need a furnace. On the other hand, if you live in a region, the American South for example, where zero degree days are not too common, you can now depend on an energy efficient heat pump for all your air comfort needs.

Like I said before – what everybody ‘knows’ just changed.

83 thoughts on “Zero Degree Heating”

  1. We live in north western Pennsylvania. We would like a unit for heating purposes. We need help deciding which one. The hyper heat looks like it might be it. How efficient is the unit at 0°. Or how low Can the temperature go and still supply heat.

    1. A Hyper Heat is definitely a better option for winter heating than other ductless heat pumps. The AHRI only rates down to 17 degrees, and the Hyper Heat performs at around 65-70% capacity. The lowest it will operate is around -5 degrees.

    2. North West Pennsylvanian here. We did purchase and install a Hyper Heat last year and flat out LOVE it. We do have a fireplace but only had to use it a total of 5 days last Winter when the temps went below -10. It was one cold Winter; with an average of 15 for Nov-Dec 2018 with a low of -17.

      That said has there been an update in the efficiency of the units better than -5?
      Also who is the manufacturer of these units?

  2. So we live in upstate New York we see temps as low as 10 below but not a normal average is between 10,15 we need a system to heat and cool 750 sg.ft which unit will be best and how efficient are they

    1. A 24k Hyper Heat unit would probably be the best fit. It is an 11.5 HSPF unit and will perform effectively in below freezing temperatures.

      1. In South Dakota. Looking for best Hyperheat mini-split DIY 3zone. What % are 0°? What unit? Wall mount diy? Thanks

        1. That may depend on what unit you purchase. You may want to take a look at our Documentation page to see if the specs you’re looking for are presented there.

  3. I live in the Boston area and would prefer to be able to install my own DIY Mr Cool system. Being that it will be my only heating system, will this work during the cold winter months?

    1. The DIY will heat in cold conditions. However, like all heat pumps, it does lose efficiency as the outside temperature drops lower and lower. Capacity is significantly impacted between 32 degrees. It will continue operating down to around 17 degrees (as rated by AHRI).

  4. What should the air temp be coming out of my DIY 24,000 mini be if inside temp is 80 outside is 110 and a setting of 70?
    Thanks in advance

    1. That depends. Air conditioners reduce indoor air temperature in 15-20 degree increments. For example, if you set your unit on 70 and the interior temperature is 100, the air coming out of the air conditioner will probably be 80-85 degrees. It takes time for any air conditioner, MRCOOL or otherwise, to reach equilibrium during hot weather.

    1. The exact components in a product package can vary from dealer to dealer. In our standard Olympus product, we include a condenser, a variable number of air handlers (depending on specifications), Smart Controller, remote control, and a wall sleeve.

        1. The MRCOOL DIY comes with a 25′ line set, and cannot increase or decrease length. For the Advantage and Olympus, choose the length appropriate for the amount of space between where the interior components must be installed and the outside components must be installed.

  5. What is the lowest temperature the 1200k btu Mr Cool advantage heat pump will continue to heat? Also when the HVAC contractor installed my system he talked about installing a drain pan heater for the indoor unit as well as some sort of heating device for the outdoor unit compressor. Can’t remember what he called it. He said there was also electrical hookups in the indoor unit for both of these. Is this correct? I live in central Indiana and we can get some pretty cold days during the winter months. Do you sell these products he was talking about and if not can you recommend any 3rd party ones that would work?

      1. How do I run it in the winter without a drain pan heater and crankcase heater? Are there any other manufactures that make compatible crankcase and drain pan heaters that can hook up to another power source?

  6. I live in Central Pennsylvania. What Mr. Cool mini split unit should I get for a 351 sq ft room for AC and heat? I am especially concerned about heating this room during occasional very cold winters. We sometimes get single digits. 12,000 btu or 18,000 btu? I’d prefer a DYI unit but would consider a Hyper Heat unit if I can get it installed for a reasonable price. Thanks.

  7. I live in Central Illinois–Springfield and have a 15 x 15 room with double pane windows on east, south, and west sides., with door opening to rest of house on the north. Looking for the right sized mini-split that will heat in our wonky Illinois winters. I assume the Hyper Heat is the way to go, but which size? Your wisdom please.

    1. Yes, the Hyper Heat would likely be the way to go. For a space that size, normally a 9k unit would suffice for cooling. However, you may want to go with the 12k to ensure you have sufficient heating BTUs.

  8. I’m looking at the DIY-12-HP-115B model and want to understand the ability to hest in low outdoor temps. A chart would be ideal, but short of that:
    1. what is the heat rating at 17 degrees?
    2. what is the lowest outdoor temp that it can be used? what it the rating at that temp?

    1. The DIY 12k is AHRI rated to produce 7200 BTUs at 17 degrees. It will not operate below 5 degrees. The heat rate at that temperature would be effectively zero.

  9. Will the new MR COOL DIY Multizone such as the DIY-MULTI4-36HP230 heat in even cooler temperatures compared to previous models?

  10. Is there a submittal sheet for the DIY units showing minimum modulation and capacity at 47/17 degrees etc? Thanks

    1. You can review our DIY units on AHRI for performance ratings. You may also contact us at 270-366-0457.

  11. I live here in Crestview FL and the coldest it gets here is 15* in winter. I built a 18’x20′ hydroponic greenhouse for growing tropical fruit trees year round and knowing tropical fruit trees are easily killed at temperatures below 40*. I want to use a heat pump to help protect my trees during our NW Florida winters. Will the Mr. Cool heat pump keep my greenhouse warm enough to keep my tropical fruit trees from freezing even when we experience hard freezes? Please let me know. Thanks

  12. I live in Colorado, where we will have prolonged days of freezing/sub freezing temps and will get to below 0 at times. Will the Hyper heat single zone work in these conditions?

  13. I am in NE Indiana, where winters range from mild to severe +20 to -20F . Propane and electric are my only Options. At what temperature do your units lose efficiency? This is an older farm house of 1500 sq. ft. with some older blown in insulation, but not perfect, and decent double pane but 25 year old windows. will I need a supplemental heat source for the really cold -10 to -20F days, or is your unit capable of keeping up? And if not, at what temp would it be more economical to run supplemental heat?

    1. The new Universal is our best cold-weather heat pump. We installed it in a home in Grand Forks, ND in December 2019. Even though we did not install a heat strip with the Universal, it kept the house nice and cozy even when temperatures dropped as low as -28F.

  14. I live In New York where temperatures in the winter tend to be in the teens. Rarely dropping below zero. Would I be able to use the DIY system in these temperatures as a primary heat source? Or would I need the hyper heat?

  15. I live in Grand Forks, ND Am looking at using DIY mini split for my garage heating and cooling. Will this unit be able to keep up during the cold winters. Looking to keep garage at 45 when not using and 65 when using. Thanks

    1. For heating in Grand Forks, we would recommend a unit like our MRCOOL Hyper Heat rather than the DIY. The DIY single-zone would not be an effective heater in very low temperatures.

  16. What is the lowest temperature that I can expect the diy unit to continue to efficiently heat a garage? I’m in Michigan.

      1. And what if it gets colder than that? Does the unit fail? Will it be damaged? Or will it automatically turn off? Is that any provision (dry contacts, etc) to signal some sort of alarm to avoid allowing the home to freeze (water pipe damage, etc)

        What options would there be to automatically have a secondary fuel-based heater activate at that point?

        Could an “outdoor” thermostat be connected that would disengage the MrCool unit at low outdoor temperatures?

  17. I was recommended the DIY version and I live in Northeast PA. Is this the right choice? It does get into the teens during winter sometimes and it can get to 0 once in a while.

    1. The DIY could likely heat and cool for most of the year, but as it is not able to heat below 5° Fahrenheit, you would likely need supplementary heating for extremely cold days.

  18. How does wind-chill play into the minimum temperature rating? I live in an area where the average min temperature is in the mid-teens but we can often get a win-chill down to 0. Would the DIY and its 5 degree rating be ok with this since the actual temperature is still above 5?

    1. Wind chill describes the lowering of body temperature, so it does not affect things that are not alive. As long as the actual temperature stays above 5° Fahrenheit, the DIY ductless mini-split can continue producing heat.

  19. Hi, I have an Olympus hyper heat 24k. In heating mode, I need to set the temperature to around 50F and to keep from freezing most of the time however, the unit only can be set to a minimum of 62F, which is unnecessarily heating this space and wasting energy. Is there any other way to reduce this temp or use the remote mini stat to achieve this? Seems like it would be a simple fix in the factory programming.

    1. You can use the FP setting to put the unit in Freeze Protection mode. It will allow the temperature to drop, but will keep it above freezing.

  20. I live in Central NY with temps in the winter getting down regularly to the 20’s at night, and teens during a cold wave. I installed a 4 zone DIY with 36K outdoor unit, and a second 4 zone on the way for the rest of my house. We have an older gas furnace with forced air, but would really like to heat with just my Mr Cools. Can you tell me at what outside temperature my Mr Cool DIYs will become less efficient than my forced air gas furnace?

    1. The DIY will begin to lose efficiency below freezing, and will not function below 5 degrees. We recommend the Olympus Hyper Heat for winter heating in cold regions.

      1. So that means above 32 it is 100% efficient, and then a little less below that – so that if by 100% efficient it is 3 times as efficient, can you say that at 25 degrees it could be just as efficient as a gas furnace?

  21. The installation guide for the DIY Multi-zone says that it will heat with outside temperatures as low as -13F. But it sounds as though you are saying it won’t work in less than 5F. Do I misunderstand you?

    Assuming they do work down to -13, I have the same questions as everybody else. Where can I find a chart of expected performance at various temperatures? Does it simply shut down at -13? Hit a ceiling on how many BTU it can put out? I live in Chicago. Very occasional dips below -20F. I’m trying to figure out what I need to do for backup heat. If performance gracefully degrades it’s easy to spin up some supplementary heat. If it stops working entirely, then I need a beefier backup.

      1. Thank you for your response. Unfortunately, there’s no information of this sort.

        If you look under the “Olympus Hyper Heat” section you see “Spec Sheets”. This is exactly what we’re asking for. It shows the energy input and BTU/hour output at various temperatures. Under the “DIY Multi-Zone” section there’s no such documentation.

        Was this an oversight? Is it something you can upload there?

          1. Not sure what you mean by an endless number of combinations. Are you referring to head configurations? I simply want to know what BTU output I can expect from the 36k heat pump at decreasing temperatures.

      2. any new info on 36? there is a big dif between -13 and that pdf which doesn’t say anything? am i missing something?

          1. no, you stated the multi zone cutoff temp was lower than single then you never clarified if it actually is. in fact you seemed to backtrack. I need to either hear verification from you here or see clear documentation that it is. that pdf does not state it. if you say it does then please explain.

          2. The lowest temperatures that each system can handle are listed in each manual. Since the original question above was answered, we actually updated our specs as a result of recent tests that discovered that the single-zone DIY system can heat in lower temperatures. The DIY 12k can heat in temperatures as low as -13° Fahrenheit, and the DIY 18k, 24k, and 36k can heat in temperatures as low as -4° Fahrenheit ( The lowest temperature both multi-zone condensers can heat is -13° Fahrenheit (

  22. Hello, i bought and installed the Olympus series in my home. I have a 5 zone system with 3 9k btu and 2 12k btu to heat and cool my whole house. So far it has worked splendidly! Were on our first real cold night and its supposed to be down to negative 1 tonight. Should i have another form of heating or will i be okay? Im in Northern PA and it doesn’t get too much colder than that too often. My other question, do i need to install any sort of drain pan heater or anything on my outdoor unit to ensure proper functioning throughout winter?

    1. We recommend having a secondary source of heat in case temperatures do get too low for the Olympus to provide appropriate heat. If one is necessary for your climate, a drain pan heater can be installed. Give us a call at 270-366-0457 for the best assistance regarding this.

  23. A manual J shows that the 12k DIY is just about right for a 600 square foot well insulated house in zone 3. Two heads would oversize the system. The cooling load at design temperature in the larger room is about 9k, with the bedroom at a little over 2k.

    With a single head, what are my best alternatives for keeping both rooms cool? Transfer grates, fan in a jumper duct, just leave the door open? How about the bathroom? Still in the design phase for a remodel, so a lot of options are possible. If only you guys had a diy ducted head!

    1. We do have multi-zone systems available, but unfortunately, they are not small enough to work well for your space. The closest might be an 18k condenser with two 9k air handlers. If you still want to go with a single-zone, we might simply recommend leaving doors open. We wouldn’t recommend installing the air handler in the bathroom, because it’s a high humidity location, which could negatively affect performance over time. Overall, whatever works best for your construction will work best for you!

  24. I live in south central wyoming where average Temps during winter are between 5 to -35. Looking at McCool. My current house is a 2 story with 750 sq feet to heat and cool. What unit should I look at?

    1. You may want to consider the Universal central heat pump split system. This system can heat down to -22º Fahrenheit at a 78% heating capacity. You would, however, need an auxiliary electric heat kit for these kinds of temperatures.

  25. Looking to add the 4th Gen DIY 24k Heat pump to my property. In the winters temperatures can go down to about 10 degrees. Will this model be able to perform in those temperatures?

  26. have had a mr cool 24k hyper heat for 6 years, in this last cyclone bomb storm we had -16 F plus wind chill, worked flawlessly to heat a 1200n sq ft room to 70 degrees F, installed myself but had help from a friend to charge but only because he had the equipment, anyone with some mechanical skills can install this unit, to charge just evacuate the system per normal evacuating and and charging procedures and open the valves as the refrigerant is in the condensing unit

  27. Trying to decide if I need the supplemental gas add on for the 4-5 ton universal rig?

    1750 sqft with full treated basement and GOOD insulation (built 2002)

    Live in mid lower Michigan where we might see 5-10 days around or under zero degrees

    Also on extreme cold days how many kW do the units draw WITHOUT supplemental heat source???


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