Heat Pumps - The Science and Benefits For Cooling/Heating

Heat Pumps – The Science and Benefits

While most people are familiar with the purpose of air conditioning, many people don’t know how central air conditioning or heat pumps work or what they actually do.

A heat pump is a type of HVAC (Heating, Ventilation, and Air Conditioning) equipment that can both heat and cool a home. A heat pump moves heat from one location to another. When a heat pump operates in cool mode, it moves heat from the inside of the home and releases it to the home’s exterior using the outside unit. In heat mode, the system moves heat from the exterior air into the home. A heat pump (just like a refrigerator) doesn’t create cool air, it instead moves heat. Removing heat from the air, thereby makes that air cooler.

A heat pump outside unit unit looks essentially the same as a standard A/C system. The main difference is that a heat pump outside unit has a reversing valve to allow it to heat and cool the home. This reversing valve is what changes the direction of the refrigerant depending upon what mode (heat or cool) the user chooses. The manufacturer’s tag would most often also list whether the unit is an air conditioner or a heat pump.

How does it work?

Like a central air conditioning (A/C) system, an air-source heat pump system (ASHP) consists of 5 main components: a compressor, a condenser coil, a refrigerant (R-22 Freon® or R-410A Puron®), an evaporator coil, and a line set. Unlike a central A/C system, a heat pump also has a reversing valve located in the outside unit. When running in cool mode, the indoor coil is the evaporator coil and the outside coil is the condenser coil. There are also two types of heat pump systems: air-source and ground source (aka ‘geothermal’). Generally, the cooling capacities (typically measured in tons which is 12,000 BTUs) of the indoor and outdoor coils are matched for proper system operation although sometimes an indoor coil with a slightly higher rating (half a ton or 6,000 BTUs) is used to help improve system efficiency. A BTU is a British Thermal Unit and is defined as “the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit”. Data shows that you get approx. $3 of heat out of a heat pump system (in regular “heat” mode) for every $1 of energy consumed. This is why heat pumps are so popular in many climates.

Warning: Science Content!

The cooling process (whether for a heat pump, central air conditioner, or even a refrigerator) is as follows: in cool mode, the compressor receives a cool gas (refrigerant in the vapor state) from the indoor evaporator coil through the large insulated suction line. The compressor then compresses the cool refrigerant gas to a superheated gas at high pressure. The superheated gas is needed because, to remove heat from the refrigerant to the outside air, the hot refrigerant needs to be hotter than the outside air temperature. Remember, heat moves to cool. The superheated refrigerant then expels its heat to the outside air via the condensing coil with the help of the condenser’s fan blowing exterior air across the coil. The hot refrigerant gas condenses back into a liquid at high pressure when enough heat has been removed from it. More efficient systems have more or larger coils which further assist in heat transfer.

Next, the refrigerant passes through the small liquid line and through an expansion valve (a metering device such as a TXV) where the refrigerant quickly expands and the refrigerant’s pressure drops becoming a cool liquid at low pressure. If you feel this small liquid line after it leaves the outdoor unit, it will be slightly warmer than the outside ambient air. Then, the liquid refrigerant passes into the evaporator coil (in the air handler) where the home’s warm return air is blown across the evaporator coil. This warm return air causes the liquid refrigerant to absorb heat (thereby cooling the home’s air) and the refrigerant boils to a vapor (gas). Modern refrigerants boil at about -15° F at normal air pressure. We can control this boiling temperature by controlling the pressure otherwise the coils would freeze. The home’s air handler’s blower sends this cool air into the home through ductwork, unless the system is a ductless mini-split. The refrigerant then returns to the outside compressor where the cycle starts all over again.

Keep in mind that when air is cooled, its capacity to hold moisture decreases, so the cooling process dehumidifies the air as well. This humidity condenses on the evaporator coil (think of how the outside of a glass of ice water gets wet in humid summer air) and drips into a pan below it, and then needs to be removed either into a floor drain or condensate pump.

In heat mode, the refrigeration process reverses direction and the evaporator and condenser coil functions are swapped. Where, in cool mode, heat was released from the refrigerant to the air at the condenser, now in heat mode, heat is absorbed at the outside unit (now, the evaporator coil) and is transferred inside the home (now, at the condenser coil). At this indoor coil in heat mode, the heated liquid then gives up its heat via condensation. Again, notice that the roles of the inside and outside coils are reversed when switching between heat and cool modes. The device that allows the refrigerant to flow in either direction, depending upon heat or cool mode, is the reversing valve. The pressure changes caused by the compressor and the expansion valve allow the refrigerant gas to evaporate at a low temperature outside and condense at a higher indoor temperature.

Heat pumps have a high operating performance when running as a ‘true’ heat pump, meaning in “heat” mode. There are limitations, however. Remember, in heat mode, heat from the outside air is absorbed at the evaporator coil and transferred inside through a refrigeration process. When the outside air temperature drops below around 25~30° F, the amount of heat in the outside air (called latent heat) is much lower and some help may be needed to adequately heat the home.

Air-source heat pumps will most often have electric strip heat built (aka electric resistance coils) into the air handler. The strip heat is a series of electric coils (similar to what is inside a hair dryer) and can be used to supplement or replace the operation of the heat pump. This strip heat is also called “EMHeat” or “Auxiliary Heat”.

One issue with standard air-source heat pumps (ASHPs) is that they generally start to lose operating efficiency in heat mode once the exterior temperature drops below approximately 50° F. Below that, there is less latent heat in the exterior air so the amount of heating capacity to be able to heat the home on a cold day drops. The common output temperature (at a supply register near the air hander) of a heat pump is often in the 90~110° F range, but that output temperature will often be closer to 90° F as the exterior temperature approaches the 20~40° F range. While 90° F supply air will still certainly heat your home, 90° F air blowing on your skin will almost feel ‘cool’ to some people, especially those used to gas-fired furnaces which have output supply register temperatures higher than 110° F. In this case, most air-source heat pumps will activate their auxiliary heat (the electric strip heat mentioned above) to supplement the lower output heat of the heat pump. The electric strip heat (see photo below) is all electric and more expensive to operate, therefore the less often that the electric strip heating needs to run, the better for your bank account.

This photo shows what electric resistance coils inside a heat pump air handler look like. They are essentially like a toaster or hair dryer inside your system.

Heat pump technology has greatly improved in the past decade especially as many homeowners are moving to electricity-based heating and cooling systems. Inverter-type air-source heat pumps now exist which can maintain an output (supply register) temperature closer to 110° F even when the exterior temperature is 10° F. Inverter-type heat pumps will become the norm as we move further into the future.

Air-source heat pumps will normally also have an EMHeat setting on their thermostat. Changing from Heat mode to EMHeat mode effectively turns the heat pump off and turns on only the electric strip (auxiliary) heat. This is much more expensive as the heat is all electric-generated. In most situations, EMHeat mode should only be used if the regular Heat mode can’t properly function, such as due to a failed outside unit. Normally, homeowners should otherwise never manually turn on the EMHeat mode. Some thermostats call the back-up heat “EMHeat” and some call it “AUXHeat” but it is the same thing.

In Heat mode, if the desired indoor temperature setting is more than 2~3° F above the actual room temperature, the auxiliary heat should automatically turn on to supplement the regular “‘Heat” mode. Normally, heat pumps will try to heat the home only in the most efficient (regular “Heat”) mode for cost efficiency reasons. Some systems have an exterior thermometer which can be set to turn the back-up heat on automatically when the exterior reaches a certain temperature. Ideally, this setting should be no higher than about 30° F. Any higher, then the back-up heat will come on more often raising your electric bill unnecessarily. In effect, the electric back-up heat should run as rarely as possible in order to keep the home warm on cold days and nights.

When working as a heat pump in either cool or heat mode, the system’s operating performance is quite high and operating cost is relatively low. The heat that exists in nature’s air is free. When the system needs to run in Auxiliary or Emergency Heat (‘EmHeat’) mode, the source of heat is artificially generated. Remember, a good amount of electricity is used to heat the coils in this mode. This is why I mentioned above that in true heat mode, you get approx. $3 of heat out of the heat pump for every $1 of energy consumed. This is not true when running in Auxiliary or Emergency Heat mode.

Air-source heat pumps will also run in defrost mode if, due to low outside temperature, frost or ice develops on the outside unit. The electronics in the outside unit will detect this low temperature and turn on the defrost cycle. In this cycle, the “cool” (air conditioning) mode actually runs allowing some of the home’s heat to warm the outside unit up and melt the frost or ice. The auxiliary (electric strip) heating coils (in the air handler) will also run in order to prevent cooling the home at this same time. You will often see a small amount of steam (looking like ‘smoke’) coming from your outside unit when running in defrost mode; this is normal. Don’t think that your outside unit is on fire.

What is SEER?

The Seasonal Energy Operating performance Ratio (or SEER) is used to rate operating performances of air conditioners and heat pumps (in cool mode). One can think of it similar to miles per gallon (mpg) for a car. A higher SEER unit consumes less energy than a lower SEER unit (for a given BTU rating) to provide the same cooling effect. Per Federal government regulations, manufacturers of cooling equipment can now only manufacture systems with a 13 SEER rating or higher. Also, ‘operating performance’ and ‘efficiency’ are not technically the same thing. If you compare a 10 SEER 2.5 ton system with a 13 SEER 2.5 ton system, both will provide the same cooling effect, however 13 SEER unit will consume less energy than the 10 SEER unit.

To achieve maximum efficiency for any central heating and cooling system, a manual J load calculation should be performed by the HVAC professional to determine what size of heating and cooling systems should be installed in a particular home. A manual J load calculation takes into account various factors of the home such as the total finished square feet, amount of insulation, type of windows, etc. What may work best in one house may not be best in another home. Oversizing or undersizing an HVAC system often results in wasted energy and less interior comfort. Also, proper system duct sizing is critical in achieving good comfort and efficiency with these systems. Also, heat pumps and air conditioner systems need to have the proper refrigerant charge to work properly. Over or under-charging these systems can greatly affect their efficiency.

What about Heat Seasonal Performance Factor (or HSPF)?

Since SEER only measures performance during the cooling cycle, you need to look at a different performance factor to determine how a heat pump’s heating portion will perform. HSPF is calculated using the total amount of heat provided during the heating season compared to the amount of electricity used by the heat pump during the same time frame, taking varying outdoor temperature conditions into account. In northern states which have a longer heating season and cooling season, HSPF can be more important than SEER when choosing a new heat pump. High HSPF systems do not always also have a high SEER rating and vice versa. The US Department of Energy has established 8.2 HSPF as the minimum efficiency for residential, air-source, heat pumps.

What about geothermal heat pumps?

Geothermal (also sometimes called ‘ground-source’) heat pumps work on the principal that the ground temperature (20+ feet below the surface) stays relatively stable throughout the year. While air temperatures in this area generally range from 10° F to 90° F through the year, the same seasonal temperature range in the ground down 20’+ will likely only vary between approx. 50° F to 65° F. In the winter, if the ground is still 50° F at 20′ deep, this provides a nice source of free heat. The same idea happens in the summer if the ground temperature at 20′ deep is 60° F. While considerably more expensive to install (often $20k+) compared to an air-source heat pump, a geothermal heat pump can be less costly to operate over its life, even compared to an air source unit. These systems also produce very little noise since there is no exterior compressor unit. Federal and/or state tax credits may help offset the cost of a geothermal system installation in some areas.

An air-source heat pump is the type more commonly found which includes an outside unit normally installed next to the home and an interior air-handler. Geothermal heat pumps have an interior air handler although its exterior coils are actually buried underground out of view either in a large coil of plastic tubing and refrigerant buried maybe 20~40′ below the surface or may be a single loop installed down a deep well. The compressor in geothermal systems is actually built into the indoor unit. Either format (a deep well or coils closer to the surface) can be used based upon the amount of available land and/or excavating or drilling costs… the important factor is contact area between the earth and the tubing filled with refrigerant. The refrigerant used with most geothermal heat pumps is a glycol mixture of water and anti-freeze. Calculating the efficiency of a geo-thermal (aka ‘ground source’) heat pump is done a little differently than for air-source heat pumps.

Alternates to electric strip heat

Some homes using a heat pump also have propane (LP) or natural gas service available. Instead of using electric resistive (strip) heating for emergency heat, these heat pumps can be modified with an LP or natural gas furnace, to minimize electricity costs on those colder days and nights. LP or natural gas systems can be very efficient (exceeding 95%) and lower in cost to operate (compared to the electric strip heat), so you can have the best of both worlds. These would be called heat pump/furnace hybrid systems.

A fuel oil-fired furnace could theoretically also be used as the heat pump’s auxiliary heat source (if natural gas is not an option), but the cost of fuel oil may not be reasonable or cost-efficient compared to using the electric strip heating since oil-fired furnaces can only achieve about 82% efficiency. Electric strip heat is 100% efficient. A qualified HVAC professional can determine what is most cost-effective.

There are also some new heat pump systems now on the market that are called ‘all climate’ models and can effectively work as a ‘real’ heat pump system below 0° F meaning they can adequately heat a home in Heat mode instead of needing an auxiliary heat source. These systems can be rather expensive currently, but we may see these becoming more popular and lower cost in another 10 years or more. Also, geothermal heat pumps normally don’t need a back-up (auxiliary) heating system.

Mini-Split Heat Pumps

Another type of heat pump becoming more popular for existing homes that don’t have ductwork (such as the home’s primary heat source being electric baseboard or a boiler) is the mini-split heat pump. These are units have an exterior unit and the interior unit is often mounted up on a wall. Like regular heat pumps and A/C units, a refrigerant line-set runs between the inside and outside units. Most modern mini-split systems can serve 4 to 8 interior wall-mounted unit with a single outside unit. This can allow you to add air conditioning in many rooms in the home without having separate outside units for each interior room. Mini-splits allow adding air conditioning to existing homes without the extra cost of running ductwork for a conventional heat pump. Of course, the mini-split will also heat these same rooms.

Refrigerants

The manufacture or import of Freon® (R-22) is now banned in the USA as of 1 January 2020. Most A/C and heat pump systems manufactured after 2003 use Puron® (R-410A) refrigerant. If you have an older A/C or heat pump system (that uses Freon®) and it needs a refrigerant recharge, this will likely me quite expensive (upwards of $100 per pound of Freon®) since only recycled Freon® is now generally available. A/C and heat pump systems manufactured after 2006 were permitted to use Freon® but were shipped from the factory dry (no Freon®) and the refrigerant was added onsite upon installation. Consideration should be given to replacing older (Freon® based) systems due to the maintenance cost of refrigerant recharging plus they likely also have lower efficiencies than new units and are older equipment so getting parts may be limited. Most A/C and heat pump systems have a typical life expectancy of about 15 years assuming annual professional servicing. Also, A/C and heat pump systems that use Freon® (R-22) can not be converted to Puron® (R-410A) since these units are engineered differently based upon the type of refrigerant used.

As you likely have heard by now, even R-410A is also on its way out after less than 2 decades in use. Starting within the next year or so, you will begin to see some new refrigerants on the market used in new residential A/C and heat pump equipment. They are R-454-B [also known as “Opteon XL41” or “Puron Advance”] and R-32. They have no chlorine in them and are safer for the environment. R-410A systems can not be converted to use these new refrigerants. It appears that R-454-B will become the dominant ‘new’ refrigerant as we look ahead a decade or so. Before these new refrigerants (considered mildly flammable) can be used in residential applications, the building code needs to change to allow their use in homes. Carrier®, Trane®, and a few other major manufacturers have announced that they will be transitioning newly designed equipment for use with R-454-B.

HVAC companies will still be able to get R-410A to service brand new (as of 2023) systems. R-22 stockpiles should be available, at least, through 2030 and R-410A well beyond that. As stockpiles decrease, costs for replacement R-22 or R-410A will increase. Using R-22 or R-410A equipment is still legal. I’ve heard some HVAC companies try to tell their customers that it’s illegal to use these older refrigerants in an effort to try to get their customers to buy new systems. That is simply a sales ploy.

You can learn more about the phase out of Freon® that started in January 2020 by reading my article “The End Of Freon®”

Operation of A/C and Heat Pumps

For regular air conditioning (A/C) systems, they should not be run when the outside temperature is less than ~60° F (equipment manufacturers vary slightly on this temperature). Doing so can cause major damage to the compressor (liquid may collect in the compressor in cold weather). If the outside temperature is less than that, the inspector should not run the A/C system and should note in their report why the system was not operated.

Similarly, heat pumps should not be run in cool mode when the outside temperature is lower than 60° F. They should not be run in regular heat mode when the exterior temperature is higher than 60° F. During a home inspection, the inspector will typically only run the heat pump in one mode (regular heat OR cool) and should report this and explain in the home inspection report why the other mode was not tested. The Emergency (backup) Heat mode can be run at any time to test the system since its operation is not dependent on exterior temperature and doesn’t use the outside unit. A home inspector should always test the backup heat mode (whether running the heat pump in Heat or Cool mode) to confirm that one is installed and will function.

Central PA is generally about as far north as you will generally see regular heat pumps due to the length of our cooling and heating seasons. Remember, heat pumps can be more expensive to run when it’s very cold outside since a backup heat source may be needed. The further north you go, the cooling season is shorter and the heating season is longer. In Minnesota, a heat pump would be extremely rare since most of its service would be in heat mode and more expensive to operate due to exterior temperatures easily reaching below 0° F during much of the winter. As you go south, heat pumps are more common due to the longer cooling season, plus since the winter months don’t get very cold along the Gulf Coast, a backup heat source may not even be needed. This idea may change over time, however, as all-weather heat pumps become more common.

How does a home inspector inspect a heat pump?

As mentioned above, since ~60° F is the general rule of thumb cutoff for running a heat pump in cool or heat modes, the inspector will typically operate an air-source heat pump system in either heat or cool mode, but not both during a home inspection. Per the American Society of Home Inspectors (ASHI) Standard Of Practice, the home inspector operates the unit using normal operating controls. This means ON/OFF using the thermostat. The inspector does not disassemble the unit to clean and service it, however, the filter should be accessed. If the air handler’s (indoor unit) front panel can easily be removed, the evaporator coil can be viewed. The inspector does not inspect duct interiors, remove vent covers, or calculate HVAC efficiency. Also, the backup heat mode should be tested to confirm its presence and functional. Testing this is as easy as setting the thermostat in Heat mode up about 4 or 5° F warmer than the interior temperature. One could also manually switch the system to EMHeat as well. For geothermal units, both heat and cool mode can both be run during an inspection as long as the unit is cycled off for a few minutes before changing modes.

The type of system installed should be noted in the inspection report and most inspectors will report the brand, the year of manufacture, and its approximate cooling rating (based upon the unit’s manufacturer tag or model number) as well as inspecting the exterior of the visually accessible duct system and condition of the outside unit. Two similar homes may have different cooling and heating needs, based upon several key factors, such as finished square footage, type of windows, amount of insulation, etc. The inspector, however, does not calculate the home’s cooling or heating needs. This can be done by a qualified HVAC professional by performing a Manual J load calculation. Most home inspectors will also report on whether service records are present indicating whether the system has been professionally serviced within the past 12 months.

Since A/C and heat pump systems generate condensate (water) due to the cooling process, this water needs to be drained somewhere away from the indoor unit. Modern high efficiency gas furnaces also generate condensate due to their operation. This condensate can be drained through the basement floor, into a sump pit, to the exterior, or into the sewer line, for example. Many A/C and heat pump installations utilize a condensate pump to discharge this water to a specific controlled location. If the air handler is located above (such as in an attic) or adjacent to living space, a secondary drain system should also be installed in case the primary drain fails, water damage could occur. In lieu of a secondary drain, a pan with a float switch can be installed to detect a non-functioning primary drain. The float switch will detect water filling the pan if the drain line gets clogged. Condensate drains should not connect directly to plumbing vent stacks since sewer gases can enter the home through the A/C system. If the dual-drain set up is used, each drain should be run individually and they should not connect together downstream of the air handler. In other words, the 2 drain pipes should should not connect downstream since a clog beyond this point, would affect both drains.

Confirming operation of the condensate pump during a home inspection can be difficult unless the pump happens to run while the inspector is in the area of the HVAC equipment and the cool mode has been running for some time before the inspector arrived. It takes time for sufficient condensate to be generated in order to cause the pump to run. Without taking the plumbing into the unit apart, extra water can’t easily be added to manually test the pump in many cases.

A condensate pump commonly installed next to a furnace, A/C system, or heat pump’s air handler.

Some other basic things that the inspector will look for:

at least 24″ clearance around the exterior unit (no vegetation, stored items, etc.) to ensure needed air flow;
the exterior refrigerant line’s insulation is intact;
normal operating sound when the compressor is running;
outside unit is level;
the outside unit’s fins aren’t considerably damaged by corrosion, impact, etc.

The inspector will normally also take temperature measurements to ensure proper heat rise or cooling drop (mode dependent). For cooling (A/C mode), a typical temperature drop (called “delta”) is 14° F to 25° F between a supply vent and an intake vent although these temperature ranges give basic confirmation of cooling. When the outside temperature is closer to 60° F or the humidity level is very high, the delta may be on the lower end of this range. In heating mode, the output temperature should be in the 90° F to 110° F range. Delta measurements are less important with modern Puron® (R-410A) A/C equipment and delta measurements may exceed 22° F with these newer systems. Taking refrigerant charge, air flow, and current draw measurements are each considered well outside the scope of a home inspection; these can be done by a qualified HVAC professional.

Also, keep in mind that some thermostats will work with heat pumps and some will not. Before replacing a thermostat, make sure it is designed to work with a heat pump. A thermostat only designed to work with furnaces, boilers, and air conditioners will not generally work with a heat pump since it will have no way to operate the EMheat (emergency electric strip heat) mode. The thermostat’s packaging will list the types of systems it will work with. A thermostat designed to work with a heat pump will normally have an EMHeat (or similar) setting.

If the measured supply temperatures in A/C mode appear to be low, the most common culprits are:

improper refrigerant charge;
possibly a frozen evaporator coil;
dirty or wet air filter, and/or;
inadequate air flow (such as partial blockage or inadequately sealed ductwork);
considerable vegetation or stored items blocking air flow around the outside unit.

Each issue will normally require service by a qualified HVAC technician, other than a homeowner should easily be able to change the air filter. If the system has not been professionally cleaned and serviced within the past 12 months (unless relatively new), this should also be done. As with just about every other system in the home, preventive maintenance is crucial and, if done regularly, will help minimize expensive repair bills down the road and help prolong the system’s serviceable life.

The Federal government is trying to encourage homeowners to install heat pumps (instead of furnaces or boilers) as they are very efficient. The issue is that our national electric grid is currently not up to the challenge, as of yet. The national electric grid needs to be greatly expanded in order to provide the extra electrical capacity needed to add millions of new heat pumps to American homes and businesses.

New heat pump and air conditioning standards as of 2023.

Read my Heating Energy Cost Comparisons article.

I also wrote Energy Saving Air Conditioning Tips and Energy Saving Heating Tips articles.

Matthew Steger is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at matthew@thehomeinspectorsnotebook.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.