In this 2nd of three: “Heat Pump Water Heaters”, it’s the author’s hope to explain in simple wordage, what a heat pump water heater is, and how it accomplishes what it does for you. However, first…
“Location, Location, Location”
Pete doesn’t want to pop anyone’s bubble of being able to employ a mass-produced, residential,
heat pump water heater. So, he needs to alert the reader ‘right out’ that these stand-alone* mechanical wonders only operate efficiently in year-round, stable air temperatures. Lower cost models may require between 40-90 degrees F. (4.4-32.2 C.). More expensive units have wider ranges like 37-145 F. AND they require a minimum cubic feet of air space around them. (This figure wavers between individual manufacturers but 1,000 cu. ft. was an accepted standard.) This could make them a non-choice for some. However, such things as louvered and trimmed (bottoms) doors can ameliorate some concerns for indoor installations. And, engineers design heat pumps providing not only space heat, AC, but hot water also with units remotely installed. But Pete hopes his article on heat pump water heaters will still prove useful to many readers.
How It Works
With a heat pump (in general) heat is moved from a cold space to a warm space. The author’s plumber friend and mentor Larry Weingarten will tell you: “Think of heat pumps and refrigerators as apparatus that concentrate and move heat from one place to another. A refrigerator extracts heat from its cold interior, making the inside colder yet, and moving that heat into the kitchen.“
As the author researched: a heat pump water heater moves heat from ambient air (inside or outside) into the water in its holding tank. Refrigerators depend on compressors and metal tubing to send gases to various places to keep food fresh. Heat pump water heaters also require compressors and tubing to move heat from the ambient air, into the water in the holding tank.
In a refrigerated system ambient warm air is ‘pulled’/sent (fan) across refrigerant-filled evaporator coils. These gaseous filled coils ‘pick up’ the heat from the passing air. The slightly heated refrigerant is then ‘sent’ (pumped) via compressor (which raises further the temperature of the refrigerant). This now really hot refrigerant is piped to the outside of the appliance (usually on side or back). The piping here is often ‘finned’ to greatly increase the exposed area. As the refrigerant moves through this exposed tubing, its heat is released back into the ‘air (kitchen).
The now much cooler refrigerant, losing more heat, condenses (liquid) and now at a very cold temperature is pumped through the cooling coils surrounding the ‘fridges’ interior, to maintain a cold and/or freeze temperature for the food it houses. As my friend Larry likes to put it: “A refrigerator extracts heat from its cold interior, making the inside colder yet.
As long as these mechanical components behave and stay on the job, our modern-day life ‘leaves a fine wake.” When the machine (either water heater or refrigerator) stops working for some reason, some of us may actually suffer some levels of agitation. Yours truly, personally, would prefer to lose (for a protracted time) the benefits of his refrigerator than his hot water supply.
Unlike the refrigerator, the heat pump water heater is not sending a chilled liquid refrigerant to maintain a ‘cold box’. The heat pump water heater (with its compressor and tubing) moves the heat from surrounding air and infuses it (heat exchange via submerged tubing coils) into the water in its holding tank. Engineers have a term Energy Efficiency. A simplified way of thinking of this is: A process of reducing the amount of energy required to provide products and services*. The heat pump water heater is providing a service.
*In our case it is the producing of convenient hot water. The efficiency of how this is done is referred to as “a co-efficient of performance.” (COP). When selecting a heat pump water heater its particular COP is listed. The higher the listed number, the more efficient that appliance is.
There are four possible heat source variables a heat pump can operate on. One of them is: air. Most stand-alone residential heat pump water heaters operate on free air. Many homes space-heated and air-conditioned via air heat pumps may also use heat in the ground (geothermal) to accomplish their tasks. Other enablers are water and exhaust gases from various apparatus. These larger units also can add ‘capacity’ in their design to include the serving you of your hot water demands. As you might imagine, these units are complex. In this article the author is solely concentrating on the residential, air served, stand-alone, electrically powered, holding tank heat pump water heater. Illus.1.
The Energy Department’s Northwest National Laboratory, teaming up with: A. The Bonneville Power Administration, Portland, Ore.; B. General Electric Corp.; and C. 10 other Northwest Utilities, designed and administered a multi-year test comparing the electrical current draw between standard electrical resistance water heaters and heat pump water heaters. This test was performed at a load (demand power) rate of 90%.
In comparing the two types of water heaters the engineers discovered that ninety percent of the Evening Peak Load power could be reduced by eliminating standard electrical resistance water heaters and replacing them with heat pump water heaters. The study included ten weeks of data collected on 250 water heaters. Of these 145 were heat pump water heaters and 86 were standard electrical resistance water heaters. One of the surprising facts that came to light was heat pump water heaters could, on average, operate their water heaters at 60% of what it cost owners of standard electrical resistance water heaters. This is a remarkable cost savings. For the majority of U.S. households heating water is their second most expensive energy expense. Only top loading automatic washing machines use more hot water.
Pete the Plumber surmises that the hefty purchase and installation costs of heat pump water heaters are the major cause for the slow growth in popularity (more than the “atmospheric restrictions aforementioned). But it’s not just the upfront costs that are deterrents. Surrounding Space concerns; Size (height of heater); and Noise (of fan/compressor) and Power (electricity supply installed?) are the big factors that must be taken into consideration when weighing the choice of a stand-alone air enabled heat pump water heater.
Let’s Open the Box
In Photo 1 we see a Rheem air heat pump water heater Pete’s contractor friend Eddie M. recently installed on a major remodel. The owners not only wanted major space/architectural modifications but also wanted to ‘get off’ heating water with Natural Gas. So, the heat pump water heater was incorporated into their plan.
If you think the water heater in this picture looks a bit tall compared to the average standard electrical resistance (or gas) it’s not because of choice of camera lens. The heater is 4 to 5 inches taller than the average standard electrical resistance water heater of equivalent holding capacity. (Where an owner can have installed an air heat pump water heater may require some head scratching.) Luckily those remodeling or creating a new structure encounter far fewer challenges to do so.
The air heat pump water heater Eddie installed was wired for 30 Amperes on 240 Volts. Some makes/models can function on 120 Volts, but recovery times are much longer. Basically, if an owner chose a water heater with a holding tank large enough to provide approximately 16 gallons of desired temp hot water for each inhabitant, in a shower ‘chain’, a longer recovery time might not be as big of an issue.
One factor that does need consideration is the noise associated with operating air heat pump water heaters. (To install one on one side of a not well sound-insulated bedroom wall would not be recommended.) Builders of additions and new structures can purposely design and create quiet space to moot this concern. A second ‘must remember’ is keeping the air filter (for compressor cooling) clean. (Pet hair to cobwebs to sawdust). (Put up another ‘fridge’ magnet with a reminder?)
In Photo 2 the reader will notice hanging down in the center of the tank is the anode rod. We were introduced to anode rods upstream, in Conundrum. The reader may also recall in Conundrum, Pete the Plumber comparing water heater warranties. One extra anode added 5 years to the warranty. These are important components. (In Larry Weingarten’s book The Water Heater Workbook he shows the reader how to locate, replace and add anode rods to extend the life of installed holding tank water heaters.) Now look at Photo 2 again. Where is the (single?) anode on this heat pump water heater? Directly under the compressor! (Pete would not covet the task of replacing that one.)
One worry that owners of standard electrical resistance water heaters and fuel gas water heaters do not have however, is the bother with a condensate drain line.* Heat pump water heaters (via compressor) produce condensation and need to always ‘pipe’ it away using a condensate line (drain line) (most always of easily assembled Schedule 40 PVC pipe and fittings.) The trick involved with condensate lines is: Always Be Going Down Slope (to a legal final destination). If the plumber cannot arrange this by gravity flow, then a sump and electric pump must be included in a heat pump water heater installation. Don’t panic. There are manufactured such ‘plug-in’ collector/pump combo’s (just for this purpose) available at plumber and or builder suppliers.
*This issue of the condensate drain line is really no more fuss than what a conscientious plumber does when installing a gas or standard electrical resistance water heater inside a building: Install heater in a properly sized drain pan with properly sized drain line, Code sloped to a final 90-degree bend, pointed down, 6 inches above grade (ground).
A Special Case
As mentioned upstream there are atmospheric limitations for air supplied heat pumps in general. It’s no different for heat pump water heaters. More than one manufacturer of air heat pump water heater has ‘the answer’ for the marginally sited: Hybrid air Heat pump water heater. These up-scale models are able to automatically switch back to using the standard electrical resistance heating elements when atmospheric conditions are not co-operating. This may be a game saver for those readers living in Northern latitudes.
The majority of we Yanks live from paycheck to paycheck. If/when our ‘silent, obedient, hot water servant’ suddenly gives up the ghost, are we going to spend maybe three or more times the money to get our hot water back? (Even if you’re a Tree Hugger?) For most of us the answer will be: “No, just don’t have the bucks for a heat pump water heater” (right now). Yours truly believes it will require generous rebates from both manufacturers and Government before we witness significant saturation of this energy saving water heater technology. The author’s upstream stated belief of new construction mandated acceptance will be tested.
There’s a hefty haul of information for those who would like to pursue this topic further, on their own. Pete has assembled the following list of inform-able sources on the subject of residential air Heat Pump Water Heaters: Have fun. Dream.
Further Reading Links:
How it Works – Heat Pump Water Heaters:
The Pros and Cons of Heat Pump Water Heaters:
Living With a Heat Pump Water Heater:
Heat Pump Water Heater Ultimate Guide:
Heat Pump Water Heaters as Clean-Energy Batteries:
Pete will be back sooner than later with the third and final installment in this series. In the meantime, have many readers ‘picked-up’ (media) on major metropolises green lighting the addition of treated sewage water into municipal fresh water supply systems? That practice might make for some future comments in The Pipe.
Peter Hemp is a San Francisco East Bay residential plumber and plumbing author and former R & D steam vehicle plumber. His hobbies are ocean kayaking and touring the Left Coast by bicycle.
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