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. For clarification, HVAC means Heating, Ventilation, and Air Conditioning. 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. In heat mode, it does the opposite. A heat pump doesn’t create cool air, it instead moves heat. Removing heat from the air, thereby makes the air cooler.
This is a heat pump compressor unit. It looks essentially the same as a standard A/C system. The main difference is that a heat pump has a reversing valve. The manufacturer's tag would most often also notate whether the unit is an A/C 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, a condenser coil, a refrigerant, and an evaporator coil. Unlike a central A/C system, a heat pump also has a reversing valve. When running in cool mode, the indoor coil is the evaporator coil and the outside coil is the condenser coil. There are also two basic types of heat pump systems, an air source heat pump and a ground source (aka 'geothermal') heat pump. Generally, the cooling capacities (typically measured in tons) of the indoor and outdoor units are matched for proper system operation. 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.
Warning: Science Content!
The cooling process (whether for a heat pump, central air conditioner, or even a refrigerator) is as follows: in cool mode, a compressor sends a hot refrigerant in the vapor 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. 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 now 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 drops down into a pan, and then needs to be removed either into a floor drain or sump pit, or by using a 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 called a 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. 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 to near freezing, the amount of heat in the outside air (called latent heat) is much lower and some help is needed to heat the home. This extra help is called Emergency Heat (aka 'EMHeat'). At this point, in most circumstances, electric resistive heating coils are most often used as the backup heat source. These electric heating coils are located in the inside air handler and transfer heat to the air being blown across them. This process works similar to a hair dryer.
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 Emergency Heat ('EmHeat'), the source of heat is generated by man. Remember, electricity is used to heat the coils in this mode and electricity is not cheap.
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 to give the same cooling effect. Manufacturers of cooling equipment can now only make 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 less energy will be used by the higher 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, some heat pump models 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, so you can have the best of both worlds. There are also some newer heat pump systems just coming on the market that are considered ‘all climate’ models and can effectively work as a ‘real’ heat pump system below 0° F. 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 60~62° F . Additionally, most heat pumps should 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.
Since Freon® (R-22) is being phased out by the year 2020, most A/C and heat pump systems manufactured after 2006 use Puron® (R-410A) refrigerant. If you have an older A/C or heat pump system and it needs a refrigerant recharge, this can easily cost $50~$200 since Freon® is now in short supply. 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 systems due to the maintenance cost of refrigerant recharging plus they likely also have lower efficiencies than modern units. Most A/C and heat pump systems have a typical life expectancy of about 15 years assuming annual professional servicing. A/C and heat pump systems that use Freon® (R-22) can not be converted to Puron® (R-410A).
Our area is generally about as far north as you will see heat pumps due to the length of cooling and heating seasons. Remember, heat pumps are more expensive to run when it’s very cold outside since a backup heat source is 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 quite expensively. 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.
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 stays relatively stable throughout the year down a few hundred feet. 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 may only vary from 50° F to 65° 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 65° F. While a bit more expensive to install, 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 less 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 rule of thumb cutoff for running a heat pump in cool or heat mode, the inspector will typically operate the system in either heat or cool mode, but not both during a home inspection. Within the standards of practice for the American Society of Home Inspectors (ASHI), 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 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.
The type of system should be noted by the inspector and most inspectors will report the brand, the year of manufacture, and its approximate cooling rating (based upon the unit’s manufacture tag or model number) as well as inspecting the exterior of the visually accessible duct system and condition of the compressor 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 if the heat pump system has been professionally serviced in 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 air handler. 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 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. Condensate drains should not connect directly to plumbing vent stacks since sewer gases can enter the home through the A/C system. Confirming operation of the condensate pump during a home inspection can be difficult unless the pump happens to run while the inspector is next to the HVAC equipment. Without taking the plumbing into the unit apart, extra water can't easily be added to manually test the pump.
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 compressor 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;
- compressor unit is level.
The inspector should also take temperature measurements to ensure proper heat rise or cooling drop. 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 74° F and above, although when the outside temperature is in the 60° F to 73° F range, 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 (R410a) A/C equipment. Taking refrigerant charge measurements are well outside the scope of a home inspection.
If the measured 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).
Each issue will normally require service by a qualified HVAC technician. 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.
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© 2014 Matthew Steger
Matthew Steger, owner/inspector of WIN Home Inspection, is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), an electrical engineer, and a US Dept. of Energy Home Energy Score Assessor. 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.