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 piece of HVAC (Heating, Ventilation, and Air Conditioning) equipment that can perform heating and cooling functions. 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, it does the opposite (moves heat into the home from the exterior air). A heat pump doesn’t create cool air, it instead moves heat. Removing heat from the air, thereby makes the 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. 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, a heat pump system consists of four main components: an outside compressor unit, a condenser coil, a refrigerant (Freon® or Puron®), and an evaporator coil. 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) of the indoor and outdoor coils are matched for proper system operation although sometimes the indoor coil is half a ton (6,000 BTUs) larger to help system efficiency. 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 sends a hot refrigerant in the vapor (gas) state under high pressure to the condenser where the vapor condenses into a liquid and cools while still under high pressure. During condensation, heat is released to the outside air. Next, the refrigerant passes through an expansion valve where the refrigerant expands and the refrigerant's pressure drops and becomes a cool liquid at low pressure. 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. The warm return air blowing across the evaporator coil, in turn, causes the liquid refrigerant to absorb heat from the home’s air and the refrigerant boils to a vapor (gas). Heat has now been removed from the air, causing it to cool and the air handler’s blower sends this cool air back into the home through ductwork. 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 exterior of the evaporator coil (much like how an icy glass of 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, sump pit, 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 temperature indoors.
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. When the outside air temperature drops below approx. 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 into the air handler. The strip heat is a series of electric coils (similar to what is inside a hair dryer or toaster oven) and can be used to supplement or replace the operation of the heat pump.
Most heat pumps will keep running when the exterior temperature is in the 20’s and may satisfactorily continuing to heat the home. The heating capacity of the heat pump will drop at such low exterior temperatures so the air supply output temperature (at the supply registers) may be lower than normal. In this case, some units 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 is all electric and more expensive to operate, therefore the less often that the electric strip heating needs to run, the better.
Air-source heat pumps will normally also have an EMHeat setting on their thermostat. Changing from Heat mode to EMHeat mode turns the heat pump off and turns on only the electric strip heat. Again, this is much more expensive. In most situations, EMHeat mode should only be used if the regular Heat mode can’t properly function, such as due to a failed compressor unit.
In Heat mode, if the desired indoor temperature setting is more than 2~3° F above the actual room temperature, the auxiliary heat will automatically turn on to supplement the regular Heat mode. Normally, heat pumps will always try to heat the home only in the most efficient (regular Heat) mode for cost efficiency reasons.
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 generated by man. Remember, electricity is used to heat the coils in this mode and electricity is not cheap. 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.
Heat pumps will also run in defrost mode when outside temperatures are low due to frost or ice forming 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. 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.
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.
Some homes using a heat pump also have propane (LP) or natural gas service available. Instead of using electric resistive 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 90%) and lower in cost to operate (compared to the electric strip heat), so you can have the best of both worlds. These would be call 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 feasible money-wise compared to using the electric strip heating since oil-fired furnaces can only achieve about 82% efficiency. Electric strip heat is 100% efficient.
There are also some new heat pump systems now on the market that are considered ‘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 and auxiliary heat source. These systems are rather expensive currently, but we may see these becoming more popular and lower cost in another 10 years or more.
Most heat pumps (in cool mode) and air conditioners should not be run when the outside temperature is less than 60° F (some manufacturers say 55° F, 62° F, or 65° F ). Similarly, heat pumps should generally not be run in heat mode if the outside temperature is higher than 60~62° F. During a home inspection, the inspector will typically only run the heat pump in one mode (heat OR cool) and should report this and explain why in the home inspection report. The Emergency (backup) Heat mode can be run at any time to test the system since its operation is not dependent on exterior temperature. 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.
Freon® (R-22) is now effectively ‘obsolete’ 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 cost $500~$1,000 per pound of Freon® since only recycled Freon® is now generally available (as of 2020). Freon® can now no longer legally be manufactured or imported into the USA. A/C 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. 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.
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.
Another type of heat pump that we see occasionally is a ground source or geothermal heat pump. This type of system works on the principal that the ground temperature (40 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 40 feet will likely only vary between 50° F to 60° F. In the winter, if the ground is still 50° F at 40 feet deep, this provides a nice source of free heat. The same idea happens in the summer if the ground temperature at 40 feet deep is 60° F. While considerably more expensive to install (often $20k+), a ground source 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.
How does a home inspector inspect a heat pump?
As mentioned above, since 60~62° F is the general rule of thumb cutoff for running a heat pump in cool or heat modes, the inspector will typically operate the 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 Emergency (EMHeat) mode should be tested to confirm its presence and functional. For geothermal units, both heat and cool mode can be run during an inspection as long as the unit is cycled off for a few minutes before changing modes.
The type of system 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. The home inspector should also report on whether service records are present indicating whether the heat pump 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 noise when the compressor is running;
- outside unit is level;
- the outside unit's fins aren't considerably damaged by corrosion, impact, etc.
The inspector should 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 22° F between a supply vent and an intake vent. When the outside temperature is closer to 60° F, 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 (R-410A) A/C equipment and delta measurements may exceed 22° F with R-410A units. Taking refrigerant charge measurements are well outside the scope of a home inspection.
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 heat pumps. 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 type(s) of systems it will work with.
If the measured supply temperatures in A/C mode are below these ranges, the most common culprits are:
- improper refrigerant charge;
- possibly a frozen evaporator coil;
- dirty 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 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.
You can learn more about Freon® being phased out as of 1 January 2020 by reading this article: "The End Of Freon®"
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© 2020 Matthew Steger
Matthew Steger, owner/inspector of WIN Home Inspection, is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at: 717-361-9467 or firstname.lastname@example.org.
WIN Home Inspection has provided a wide array of home inspection services in the Lancaster, PA area since 2002. This article was authored by Matthew Steger, ACI - owner of WIN Home Inspection in Lancaster, PA. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.