What Temperature is a Heat Pump Not Effective?

Heat pumps – do they work well in all temperature zones? Heat pumps are nice because they will both cool and heat your home. They are being installed in a lot of new homes instead of a central air conditioning system and a furnace due to their outstanding energy efficiency. But in some places, they have the reputation of not working well in colder climates. Is a heat pump right for your climate? For your home?

This brings up the question of what temperature is a heat pump not effective? Is there a “magic number” at which they cease to be a wise choice? And if so, why is that the case? To answer these questions, we will begin by understanding a little about heat pumps, what they do and how they do it. Let’s set the indoor thermostat on “Cool”.

If you enjoy the technology of systems like heat pumps, then we suggest reading the entire post. If you’re here for a quick answer, jump to the section titled “What Temperature Is a Heat Pump Not Effective?”

How a Heat Pump Cools Your Home

Heat Pump Works

Just like an air conditioner, a heat pump operates on the refrigeration method using a refrigerant such as R-410a. This stuff is an amazing combination of chemicals that has some very interesting properties – like having a boiling point of -55°F, for example. Without going into more technical details, just understand that the refrigerant circulates inside a closed loop through components inside and outside your home. During this process, it is changed back and forth from liquid to vapor, and from very cold to very hot, depending on what part of the loop it’s in.

In all this, it has the capacity to absorb heat in one location when it evaporates into a gas vapor and release the heat when it is condensed or squeezed back into a liquid. If you like technical information, you will enjoy researching refrigerant and how it works. Here’s a primer on the subject:

The heart of the system is a compressor, which compresses the refrigerant and makes it a very hot vapor at a high pressure. From there, it goes through a condenser, where a fan blows outside air over the hot condenser coils to transfer heat from the refrigerant into the outside air. It then goes through a special expansion valve where it is changed into a very cold liquid/vapor mix.

It now enters the evaporator coils inside your home, usually in or near the furnace or air handler, over which the warm room air is circulated by the blower fan through the air ducts. The air is cooled by this process and at the same time moisture in the air is removed. The refrigerant absorbs some of the heat from that room air and is then sent back to the compressor as a cool, low pressure vapor. At the compressor, this cool vapor is compressed into a very hot vapor again, and the process repeats.

How a Heat Pump Warms Your Home

To state this operation simply, when a heat pump warms your home it transfers heat from the outside environment into your home by reversing the refrigeration process. It’s pretty amazing to think you can heat your home with “outside heat” when the temperature outdoors is in the 50s, 40s or even upper 30s.

When the indoor thermostat is set on “Heat”, a special reversing valve in the heat pump does just that – it reverses the direction that the refrigerant circulates. Now, instead of the coils inside your home being cold in order to cool the room air, they will be hot so they can warm the room air.

Instead of the coils outside your home being very hot in order to expel heat from your house into the outside air, they will be very cold so they can attract and absorb heat from the outside air.

This leads to the natural question, “How can you heat a house by starting with cold, outside air?” It’s all because of the properties of the refrigerant inside the system and the very basic fact that warm always travels to cold. By this, we need to understand that it isn’t what we think is warm, but what is warm relative to something else. Here’s an idea: “Cold” isn’t a real thing. It is just the absence of heat.

We might think that when the outside temperature is around 45°F it’s getting cold outside. But to the outside coils in your heat pump that have -37°F refrigerant inside them, that outside air is relatively warm. That “warm” outside air is going to be absorbed by the very cold refrigerant, which will then be superheated by the compressor and sent to the coils inside your house. Room air circulating over those coils will be warmed and circulated throughout your home.

What Temperature Is a Heat Pump Not Effective?

Heat pumps can pump some heat into your home at almost any temperature. But it might not be enough to sufficiently warm your home.

Efficient vs Effective – noting a technical difference.

When we speak of efficient we are referring to the degree of productivity – how well something works. When we speak of effective we are speaking of whether something works or not. Heat pumps will heat a home more efficiently when the outside air is above about 40°F. Once the outside temperature drops below about 25°F, heat pumps become less and less efficient, and require electric strip heaters or some other form of supplemental heat to maintain a comfortable home.

Having said that, a heat pump can continue to extract heat from outside air down to around 0°F. At this temperature, however, a heat pump would have to be considered ineffective. Even working at maximum output 24/7, it would still not be able to adequately heat your home without a significant supplemental heat source. And, it would greatly increase your energy bill.

Better than Before – Improvements in Newer Heat Pumps

In recent years, several improvements have been made in heat pump technology, resulting in better efficiency in colder climates. Some of these improvements are:

  • Coil design
  • Variable-speed compressor
  • Thermostatic expansion valves enabling more precise refrigerant flow control to the indoor coil
  • Variable speed blowers on air handlers and condensers
  • Copper tubing with internal grooves to increase their surface area and speed the transfer of heat

If you are planning on building a new home in the future – or upgrading your HVAC system – stay tuned for even more improvements that might make a heat pump right for you.

Geothermal Heat Pumps – outside temperature is a non-issue

The bulk of this article has applied to what might be referred to as the “standard” heat pump, which is the air-source type. As stated above – and as the name implies - this type draws its heat from the outside air and transfers it to the inside of your house.

Geothermal Heat Pump

Most geothermal heat pumps draw their heat from large loops of tubing that are buried in the ground around 5 to 8 feet deep. At this depth, the temperature is always well above freezing, usually around 55°F, and even higher in hot climates. Water runs through these tubes, which is then used to temper the refrigerant, making this the temperature from which the heat pump collects heat to warm the house in winter. It is through these same tubes that it also cools the house in summer.

Some geothermal units draw their heat from similar tubes that are submerged in wells or a body of water that always stays well above the freezing point.

While geothermal heat pumps can laugh at the temperature outside, the initial cost is much higher than an air-source unit, and the cost of installing the ground or water loops is many times more. They are more often found in commercial buildings than in residential homes, but they certainly work very well in a single home application.

The Final Verdict - the Temperature a Heat Pump Won’t Work

Is the climate too cold for a heat pump where you live? Having one unit to do both jobs of cooling and heating your home sounds nice. If you live where the weather rarely gets below 40°F, it may be the right choice for you. If the outside temperature is likely to fall below that, however, you should be sure to consult your local HVAC provider for their professional opinion. They will have had experience in your area and will be able to advise you as to what has worked best for others. Another good option is to go with the trustiest heat source around – the gas furnace.

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