Are the Mitsubishi Heat Pumps Meant to Continually Run Whwn it is below zero oitside

Many homeowners who want to install a heat pump wonder: At what temperature does a heat pump stop being effective (specifically in Heating mode)?

In cooling mode, ALL heat pumps are very efficient, and work from northern Canada all the way south to Central America.

Typically, heat pumps have cooling operating temperature range of 15°F to 115°F (-10°C to 46°C). This means that if you live in ANY part of USA, a heat pump will provide adequate cooling.

When it comes to heating, some heat pumps can be limited in their ability to provide sufficient heat in extra low ambient temperatures.

Overall, (air source) heat pumps are VERY effective in terms of energy efficiency and operating ranges. What homeowners need to know is that to get efficient heating with a heat pump, they need an "inverter" type unit, which can modulate output, and provide heat when outdoor temperatures drop below 5°F. These high efficiency "inverter" heat pumps can cost significantly more than a basic 1 or 2 stage heat pumps, which are essentially over glorified central ACs.

However, before we delve into figuring out at which temperatures heat pump systems stop being efficient and effective, let's make another differentiation:

There Are Two Types Of Air Source Heat Pumps

1) Ductless (mini-split) heat pump.
2) Central or whole-house heat pump

There are also so called PTAC or "Packaged Terminal Air Conditioners", but these are not typically used in private residences or even commercial buildings.

PTACs are usually found in hotels/motels, where they do excellent job, but that is a topic for another article, so here we will not review them.

Now let's see when and where heat pumps are and are not effective:

At What Temperature Is A DUCTLESS Heat Pump NOT effective?

There are all kinds of ductless units, which primarily offer better efficiency and quality, based on how much ductless systems cost. Four our purposes we will look at most efficient units from Fujitsu and Mitsubishi (Fujitsu 12RLS3YH & Mitsubishi MZ-FH12NAH) in 12K BTU capacity , which is enough for 400-500 sq. ft.

Both of these heat pumps come in Low Heat packages, which allow them to produce heating in -15°F (-26°C) for Fujitsu and -13°F (-25°C) for Mitsubishi. Low operating terms are pretty close.

Now the question is – are these units effective at such low temperatures?

Here is a review of my 9000 BTU (33-SEER) Fujitsu Mini Split working in Heating Mode:

As you can see, this Extra Low Heat model of has no problem heating my 600 sq. ft. loft during cold Massachusetts winter.

Best Heat Pump: Fujitsu vs Mitsubishi

Let's look closely at specifications:

Fuji 12RLS3YH has "Nominal Heating Capacity" of 16,000 BTUs and max heat capacity of 22,100 BTUs.
Mitsu MZ-FH12NAH has "Nominal Heating Capacity" of 13,600 BTUs and max heat capacity of 21,000 BTUs.

Nominal Heating Capacity usually means the rated capacity (typically at 17°F). However specs for these two units do not state "nominal" temperature. So we will use low temps:

Temp	Fujitsu	Mitsubishi
+5°F	22,080 BTUs	13,600 BTUs
-5°F	20,000 BTUs	11,690 BTUs
-15°F	15,680 BTUs	9,920 BTUs

Typical Cost To Install Ductless Mini-Split Average: $3,070 - $4,380

As you can see from the table above, the lower are the outside temperatures, the lower is the capacity or amount of heat, which these heat pumps can provide.

However, even at the coldest limit, Fujitsu is rated at 98% of its nominal capacity, whereas Mitsubishi would only provide 73% of its rated capacity, and 37% less heating that its Fujitsu counterpart.

If you have electric (resistive) heat, you should switch to a Heat Pump NOW!

ANY brand name heat pump (Fujitsu, Mitsubishi, Daikin, LG, and many other less known brands) will have 2.5-4 times higher efficiency (also known as COP or "Coefficient of Performance") than electric baseboards / space heaters. That means you will use 2.5 to 4 times less electricity to heat your house!

For example, if your electric bill in the winter is $600, and you get a 3 COP heat pump, you will then be paying only about $200/month. You can compare the cost and efficiency of a central air system (furnace) vs a ductless mini split (heat pump)

Now, lets answer the question whether heat pumps are EFFECTIVE in low temps. I would say YES, with some limitations.

If you regularly experience winter outdoor temps below -10°F (-23°C) then you should look at more common fossil fuel heating systems such as Gas/Oil Boiler or Furnace.

However, the majority of US households have average winter temps at or ABOVE -5°F, and thus we would recommend having a Ductless Mini-Split heat pump as a primary heating system for most US homes, with some conditions/limitations:

• You should have a backup source of heating, such as your old(er) boiler/furnace.
• If you have access to natural gas, a high efficiency condensing boiler will cost less to operate, compared to electric Heat Pumps.
• If you live in any New England state (Connecticut, Massachusetts, Maine, New Hampshire, Rhode Island, Vermont), where electricity is very expensive (above $0.21/kWh) we still recommend going with a Gas Boiler or furnace, in terms of fuel costs. Here we delve deeper into comparing the pros and cons, as well as costs of a boiler vs furnace.
• If you plan to have a large Solar PV array installed on your roof, then going with a Heat Pump is a no brainer!
• If you live in any area in the Climate Region 1 (see map below), you should have a backup heating system for when it gets TOO cold for heat pumps to operate, or if your power goes down.

Also keep in mind that as total system capacity of a ductless system goes up, its efficiency goes down. However, both Fujitsu and Mitsubishi have extra low heat models with 4 zone or more, connected to same outdoor compressor, and operate in temps as low as -13/-15°F.

Also, for larger systems (over 36K BTUs /3 Tons), Mitsubishi and also Daikin (not mentioned above) have more efficient systems than Fujitsu, and more options for 4+ zones. However Daikin does not have many models that heat in temps below -5°F. That's why we did not include them in the comparison above.

At What Temperature is a CENTRAL (whole-house) Heat Pump NOT effective?

Central heat pumps are a whole different market, and these HVAC systems have a different purpose. They are intended to replace Central AC (air conditioners) that are low efficiency, and to provide some heating in mild temperatures, as well as "smart fuel switching" in colder temps.

Basically, as you get close to freezing temps (+32°F or 0°C), most Central Heat Pumps, stop producing heat, and switch to either Gas furnace or resistance heating element installed in place where furnace would be.

Think of the latter as an oversized space heater – very expensive to operate.

Basically, Central Heat Pumps are only effective in warmer climates (Regions 3, 4 and 5 on the Climate Region Map above).

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Source: https://www.remodelingcalculator.org/heat-pump-efficiency/

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