If you've never examined a heat pump water heater, it's likely you'll have that opportunity in the near future. While not entirely a new idea, the combination of recent advances in the technology, a real concern for energy conservation and tax incentives, no doubt, will increase their popularity, which means sooner or later you'll be inspecting one.
In this report, I'll explain the basics of how these appliances operate, examine the different types and offer a few inspection tips as well.
There are essentially two types of heat pump water heaters (HPWH): drop-in and add-on equipment.
The drop-in versions have a similar footprint and are installed much like standard electric storage tank water heaters.
Add-on units are coupled in some way to an existing water heater.
Both have a compressor, evaporator and a condenser.
To understand heat pump water heaters, first, you must understand basic refrigeration. Refrigeration is the removal and relocation of heat. With household refrigerators, heat is pulled from the food stored inside the appliance and sent outside of the cabinet. Heat is absorbed through an evaporator and is cast off by a condenser. Evaporators absorb heat as refrigerant passes through its tubes and changes from a liquid to a gas. A condenser rejects heat as refrigerant flows through its tubes and changes from a high-temperature gas to a liquid. A compressor pumps the refrigerant through both of these heat-exchanging devices.
The benefit of heat pump water heating technology is simple to understand. The amount of electrical energy needed to transfer heat from the surrounding space to the water in the tank is much less than what is needed to convert electrical energy into heat. In other words, the electrical components of the HPWH are cheaper to operate, under the right conditions, than energizing the electric element.
Photo: Drop-in HPWH Accelera 300 by Stiebel Eltron.
According to Stiebel Eltron, maker of the Accelera® 300, its unit's compressor and fan consume only 1kWh of electricity to generate the heat equivalent of 3 – 5kWh. The efficiency of the unit goes up as the ambient air temperature increases.
Drop-in heat pump water heaters
With drop-in heat pump water heaters, an evaporator is placed outside the unit, usually on top, and the heater's tank often is wrapped with the coils of a condenser. A compressor moves refrigerant through the evaporator, where it absorbs heat from the space and sends it to the condenser, where it passes the heat into the water heater tank. When the coils of the condenser are attached against the outer surface of the tank, the area between the coils and the appliance's jacket is heavily insulated, so the condenser's heat moves into the tank of water and is prevented from passing through the jacket back to the room.
Illustration: Like with many drop-in units, the compressor and evaporation rest on top of the Accelera 300.
As well as attaching the condenser to the outer wall of the tank, some makers submerge the condenser in the tank to transfer heat. Others pull water from the tank, pass it through a heat exchanger and then return the heated water back to the tank to be consumed.
Along with a similar footprint, drop-in heat pump water heaters have other things in common with the electric storage tank water heaters. Both are capable of producing scalding hot water. They're delivered with a drain valve installed near the base of the tank, and they need a full-bore shutoff valve installed near the inlet fitting to the tank. As with standard tank-type heaters, drop-in HPWHs must be braced, anchored or strapped per local code if seismic concerns exist.
For some drop-in appliances, the plumbing connections are on the side of the tank to avoid conflicting with the refrigeration components resting on the top. The inlet fitting or cold connection will be the lower of the plumbing connections along the side. The outlet or hot connection will be higher on the side of the tank.
If the HPWH is installed in a closed system because of backflow preventers or pressure-reducing valves or other devices installed in the service line, some areas also will require preventative measures to eliminate thermal expansion problems. Thermal expansion is known to cause TPR valves to drip.
The installation of an expansion tank might be required in some localities as this is the fix most often employed when thermal expansion is an issue. As a plumber, I install my expansion tanks in the cold water line near the water heater.
For the major brand drop-in units, one or two electric elements are installed in the tank to provide heat when using the heat pump alone is not practical or cost-efficient. Sometimes, the elements can be turned on manually if the refrigeration system is in need of repair.
For the most part, the drop-in HPWHs I've examined can be connected electrically to the existing electrical circuit, where a tank type electric water heater was once installed. This is not always the case, so be sure to check the unit's data plate and match it to the circuitry supplied.
Clearance requirements for electric water heaters have always been a selling feature. They can generally be pushed into a corner or up against a wall with little or no concern for operation. Drop-in heat pump water heaters, however, need a little breathing and servicing room. Unlike electric water heaters, where only a little access is needed to check the elements, a HPWH has major components that might require servicing.
The need for good airflow through the evaporator and for filter accessibility also limits the unit's proximity to walls, ceilings, shelves, ducts and other obstacles. Because regular filter cleanings are required by most HPWH makers, the filter should not be obstructed to the point of making removal difficult, as this can result in poor maintenance practices. A dirty filter can damage the equipment or result in inefficient operation. Most of the clearance information can be found on the heater or in the installation and maintenance packet.
Another space issue relates to the actual volume of the room where the heater is to be installed. When HPWHs operate, cool air is blown into the space. Should the space be too small, the water heater can actually cool it to a temperature where the heater's efficiency will suffer. One manufacturer of a drop-in unit, RHEEM, states the room should be no smaller than 1,000 cubic feet in volume, while G.E. recommends no smaller than 700 cubic feet. They note that, if the space is smaller, a louvered door or other similar form of ventilation is needed. Some HPWH makers prohibit installation in a closet or other confined space.
The other type of heat pump water heater is an add-on unit. As the name implies, components are coupled together or added to existing water heaters with this equipment. The heat pump unit is placed near the water heater or, in some cases, set on and anchored to the tank's top. Unlike most drop-in units, you don't have to replace the heat pump portion when the water heater tank leaks.
Most add-on units pull heat from the surrounding space and dump it directly into the tank through a stream of water pumped from and returned to the water heater tank. The Geyser 3000 -3.0™ Heat Pump Water Heater from North Road Technologies has been in production since the late 1990s, and, according to Robert G. Montenegro, a representative for the company, it has about 10,000 units installed.
Photo: The Geyser 6000-3.0 from North Road Technologies is an add-on unit.
The add-on unit connects to a fitting arrangement that is field-adapted to the water heater tank opening where the drain valve is installed. Other adaptations are possible. Once the connections are put in place, two washing machine hoses serve as the conduit between the heat pump and the water heater. The makers of this HPWH recommend a space of 1,500 cubic feet or greater, with ambient temperatures between 40° F and 120° F. Garages, utility rooms and basement spaces are specifically mentioned by the manufacturer as good locations for this add-on HPWH.
In simplest terms, this equipment employs a water-cooled condenser to transfer heat to the water heater tank. A small pump circulates water between the water heater tank and the water-cooled condenser. The hot gas loses its heat to the water as both pass through the condenser coil. The Geyser operates on 115-volt current and primarily is designed to take over the operation of an electric water heater's bottom element. The top element remains operational and will continue to provide heat to the top third of the tank.
As well as the Geyser, you might see the E-Tech add-on heat pump water heater in the field. Though similar in many ways to the Geyser, there are differences. The manufacturer suggests a minimum of 800 cubic feet for the installation space is possible, but recommends 1,000 cubic feet or greater with a temperature range between 45°F to 105°F.
Photo: E-Tech add-on HPWH.
This unit is wired to the existing water heater circuitry, making it 240 volts. The AirTap™ HPWH is different from the Geyser and the E-Tech. Instead of using a water-cooled condenser and circulating pump, a coiled section of condenser tubing is inserted in the water heater tank itself. When done well, the condenser tubing will coil inside the tank. As the hot gas passes through the condenser, the heat passes directly to the tank water.
Photo: Airtrap by Airgenerate. Visit www.airgenerate.com.
Illustration: The AirTap add-on HPWH sends heat to the tank through a condenser that is submerged in the tank water. Go to www.airgenerate.com to see an installation video.
The power to the water heater is disconnected completely on this add-on HPWH. Actually, this unit can be connected to a gas water heater as well. The gas and the vent are disconnected when used with the AirTap™. The water heater tank for gas or electric installations merely becomes an insulated storage vessel for the hot water. To see one of these units installed, go to www.airgenerate.com and watch the installation demo.
Drop-in and Add-on
Like standard electric storage tank water heaters, both drop-in and add-on HPWHs require that a temperature pressure relief valve be installed in their storage tanks. In most cases, the rules for the installation and venting of the safety valve for drop-in HPWHs are the same as for electric storage tank water heaters.
To read more about temperature pressure relief valves, see this December 2008 ASHI Reporter article, "The Temperature Pressure Relief Valve".
Warnings about storing products that produce flammable vapors near HPWHs seem to be more prevalent than what I've seen for the standard electric tank-type heaters I've been installing for years. Likely, this is due to the arc-producing components such as relays, motors, thermostats and others that are installed throughout the system. Also, this is likely because a fan pulls air from the space through these components during operation. Electrical arcs and flammable vapor together in the same space are, of course, a bad idea.
Do You Love a Good Checklist?
Finally, most of the home inspectors I know love a good checklist to guide them through a product or system inspection. After reviewing the maintenance recommendations and procedures for several different HPWHs, both drop-in and add-on types, I thought I'd wrap up this report with a checklist that might be helpful when you come across these units in the field.
• Is the appliance installed in an area that meets the volume and temperature parameters for efficient and safe operation?
The information I've noted in this article concerning room volume and temperature ranges can be a helpful guide, but consult the manufacturer's installation packet for a more precise definition of space and temperature restrictions.
• Is the water heater well supported, level in all areas and properly restrained in areas where seismic issues are a concern?
• Is the water heater set in a properly installed emergency pan if water leaking from the tank can cause damage to the surroundings?
• Are condensate drains installed and is the condensate being disposed of properly?
• Compare the electrical rating and actual circuitry.
Are they correct?
• Is there an insulation blanket?
If so, is it taped securely in place, not covering any vents or coils?
The access panels, TPR valve and warning labels should also be visible.
• Are all covers in place?
• Inspect the TPR valve and the discharge piping. Also, identify any TPR valve maintenance steps, such as flushing, that are spelled out on the safety valve's labeling. Never perform the actual procedure during the inspection.
Additional items to cover with your client:
• Identify the air filter and discuss filter maintenance with your client.
Point out that the air filter, the system's major components and the heater's airflow should not be obstructed.
• Some HPWH makers recommend a professional evaporator cleaning each year.
If this is the case with your client's unit, suggest having the service performed during the annual HVAC system maintenance.
• Point out to your client that most manufacturers recommend an occasional flushing of the tank to prevent harmful sediment buildup.
Typically, step-by-step instructions are found with the installation and maintenance packet and the procedure is considered a D-I-Y chore.
• With some add-on HPWH models, the water-cooled condenser requires occasional flushing.
If maintenance documents recommend this, inform your client and mention that it is a procedure best done by professionals.
• Recommend insulating water lines, especially between add-on units and water heater tanks.
• Most HPWHs can produce water temperatures well above 125° F. Inform your clients of the potential for scalding as you would with any hot water producing device.
Many warning labels, including scald warnings, can be found on the heater or, in some cases, in the owner's manual. Suggest that your client become familiar with all of the warnings.
• Check for the presence of dangerous products stored near the appliances and point out to your client the dangers of storing products that give off flammable vapors.
Share your experiences
One of the challenges of being a home inspector is being prepared to inspect a 100-year-old home and to inspect the newest building components and updated systems. If you are now better prepared to inspect a heat pump water heater and can explain how they operate to your client, I've accomplished my purpose in writing this report.
If you have had any experiences with HPWHs that might interest other home inspectors, let me know. I make presentations to home inspector and trade groups regularly and would love to pass on the stories and photos you might have. To contact me, e-mail firstname.lastname@example.org.
What are Desuperheaters?
Photo: The ENVIRO-PAK (a desuperheater) by Turbotec Products
Another device used in heating water via refrigeration is called a desuperheater. Although it aids in the transfer of heat to a water heater, it must be tied to the house's air conditioning or heat pump equipment.
Desuperheaters take the heat actually being relocated by an air conditioner, air-to-air heat pump or geothermal heat pump and divert much of it to a storage tank water heater. They are an especially popular add-on component for geothermal heat pumps because these units are capable of moving a lot of heat and tend to be located inside a home. In many cases, all the needed components for installation are in one space such as a basement or utility room.
When the desuperheater can be mounted close to the heat pump's compressor and the storage tank water heater, higher temperatures can be realized and the recovery process is enhanced.
With desuperheaters, the discharge line from the HVAC system's compressor is cut and diverted through the device before passing to the equipment's condenser, for air conditioners, or to the reversing valve on heat pumps. As the compressor runs, high-temperature gas is sent through a heat exchanger, where the heat is transferred to water being circulated from the water heater, through the heat exchanger and back to the water heater again. As long as the compressor is operating, heat can be recovered.
Your clients may have heard that the cooling effects of the heat pump water heater (HPWH) can actually cause heating bills to rise in winter. First, this should be a concern only if the HPWH is installed in the actual living space. What's more, if the space volume requirements are met for the equipment, any cooling effect should be only a minor issue. Manufacturers are aware of this concern and counter that during the warmer months, it can lower the cooling bills. At any rate, the savings in the cost of heating water should outweigh any loss that might take place during the heating season.
Kenny Hart is a second-generation Master Plumber and Mechanical Contractor with more than 35 years of experience in the mechanical fields. He is a contributing editor to the ASHI@HOME Training Program, the president of the Hampton Roads Chapter and past chair of the ASHI Technical Committee. He currently chairs the Trades Division of the Alpha College of Real Estate located in Chesapeake, Virginia. To read more of Hart's articles or if you need a presenter at your next chapter event, go to www.theplumbingandhvacguy.com.