Heat pump water heaters
Electric heat pump water heaters; application guidelines
Electric heat pump water heaters are significantly more efficient than conventional electric water heaters. With that said, certain considerations should be made prior to installation in order to determine if application is both viable and cost effective.
Hot Water Loads and Heat Pump Water Heating
In order to maximize the potential energy savings, heat pump water heating is best suited in sites with moderate to high annual hot water loads. The systems' efficiency and the overall cost effectiveness is compromised in applications with low hot water demand and/or frequent vacancy.
In fact, the U.S Department of Energy has set a first hour rating of 50 gallons or higher in order for a residential heat pump water heater to meet the energy star performance criteria and qualify for program certification.
Adequate Ambient Temperatures and Air Circulation
With both add on and drop in electric heat pump water heaters, the ambient air temperature can have a significant impact on the overall energy efficiency of the system. Manufacturers often specify adequate inlet or air source temperatures or provide temperature ranges.
With recent industry advancements, some add on and drop in heat pump water heaters can operate efficiently with inlet air source temperatures as low as 35�.
An application that fails to adequately support the specified inlet air temperature range will compromise the systems' overall efficiency, resulting in the extended operation of the back up electric resistance heating elements, and/or resulting in a decreased hot water output, and/or resulting in a decreased temperature rise.
Add on and drop in heat pump water heaters also require adequate amounts of air circulation in order for the system to operate efficiently. Proper air circulation is required for both the systems' inlet air and the systems' exhaust air.
Air conditioning and dehumidification
The air source refrigerant vapor compression cycle of an electric heat pump water heater not only efficiently heats stored domestic water, this process also expels cool and dry air through the heat pumps' evaporator. So with add on and drop in units, cooler and dehumidified surrounding air is a by product of operation.
It's important to note that both the cooling and dehumidification of the ambient air is generally modest. With that said, it's equally important to note that application in a conditioned living space can have an adverse effect on space heating loads, albeit modest.
With the by product of modestly cooler and dryer air, the potential effect the add on or drop in unit may have on annual space heating and cooling loads must be considered by both consumers and professionals prior to determining application.
The systems' vapor compression cycle also produces moisture build up. This liquid build up is commonly referred to as condensate. Both add on and drop in electric heat pump water heaters come equipped with designated connections and fittings for condensate drainage.
Some applications may require slight modifications during the installation process in order to adequately address condensate drainage. The drainage connection, more times than not, can be fitted with a tube, and this tube can be directed to a floor drain, a collection pan or container, or directed outdoors.
The amount of condensate produced will vary from model to model as well as from application to application. With all models and applications, condensate drainage should be considered prior to purchase and/or during installation. When in doubt, consult with a local water heating professional prior to purchase and installation.
Heat Pump Water Heaters are Best Suited...
- in sites with moderate to high hot water loads, requiring a system with the capability of providing a moderate to high first hour rating.
- in applications that annually provide mild to warm inlet air temperatures. Most manufacturers and professionals recommend inlet air temperatures between 35� F and 100� F. An ideal location for unit installation can be an attic, crawlspace, garage, basement and/or furnace room.
- in applications with a large air source. Most manufacturers and industry professionals recommend installation in spaces with at least 1,000 cubic feet of accessible ambient air for proper system circulation.
- in existing homes and/or businesses with an existing conventional electric powered water heater. With the addition of an add on HPWH, the overall efficiency of the existing unit can be significantly improved.
- in existing homes and/or businesses that require an efficient replacement water heater that's electric powered. In such applications, a drop in HPWH is most ideal.
- in new constructions that lack access to inexpensive natural gas and/or new constructions that lack an adequate solar resource, eliminating the possibility of installing a solar hot water heater.
- in applications with unconditioned spaces that can benefit from the year round by product of modestly air conditioned and dehumidified air.
Heat pump water heating in cold climates
With the importance of warm intake air temperatures and with the by product of cooler exhaust air, most industry professionals generally agree that electric heat pump water heaters are better suited in mild climate regions.
In any climate, the overall efficiency of the system will be determined not just by annual ambient air temperatures, or by the effect (or lack of) that the cool exhaust air has on annual space heating loads, efficiency will be determined by the amount of annual heat pump operation as well.
After all the constant operation of the back up electric resistance heating elements will reduce the overall efficiency of the system significantly.
With that said, in many applications heat pump water heating can provide energy savings in even the coldest of climate regions within the United States, especially when compared to conventional electric storage tanks.
Air Source Ducting and Exhaust Air Venting
In some applications (especially new constructions) the cool exhaust air can be vented and redirected to an alternate location other than the indoor ambient air surrounding the unit. An alternate air source can be utilized as the units' intake air source well.
The cool and dry exhaust air can be vented to an unconditioned space such as a garage or attic, or even outdoors. In some applications, the exhaust air can be vented directly to the central space cooling systems' ductwork. This will provide modest assistance during the space cooling season.
The units' air source can be gathered from a warmer and unconditioned space, such as a crawlspace, utility room or even a furnace room, provided the space in question has an adequate amount of air source and the annual air temperature of the ambient air is adequate as well.
Both air source ducting and exhaust venting can be achieved with duct flanges, collars, dampers and other common ductwork connections.
In most applications (especially new constructions) air source ducting combined with exhaust venting can provide the additional flexibility of locating the electric heat pump water heater in either a conditioned or unconditioned space.
In most applications (especially in cold climate regions) the addition of ductwork and vents can significantly improve the efficiency of a heat pump water heater by eliminating the potential adverse effects the cool exhaust air may have on space heating loads, while utilizing an air source that annually is warmer than the air source of a conditioned space, while ensuring an adequate hot water output.
Ducting and venting, while especially ideal in cold climates, can boost the energy efficiency and the output of the system in any climate region. With some HPWH, relatively inexpensive duckwork kits are available.
Consult with a local water heating professional before determining if ducting and venting is viable.
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