Heat Pump Water Heaters — Part 2

July 9, 2010
A few years ago, while attending an out-of-state family reunion, we decided to stop by a roadside fireworks store to put some extra boom in our Fourth of July celebration. Although it was a mild day, a wave of heat greeted us as we entered the block ...

A few years ago, while attending an out-of-state family reunion, we decided to stop by a roadside fireworks store to put some extra boom in our Fourth of July celebration. Although it was a mild day, a wave of heat greeted us as we entered the block building. The sales guy was sitting at a folding card table smoking a cigarette! A window air conditioner was flowing cooled air over his seated spot. Since there was not a single window in the four walls the air conditioner was sitting on the table next to the cash register — blowing hot air directly onto the customers. Smoking in a bunker stuffed with fireworks? Crazy — but not as crazy as running a window unit indoors where the heat generated by the compressor was causing the space to become a bake-oven. What in the world does this have to do with heat pump water heaters? Read on.

How it works: Take a standard window-unit style air conditioner and move its outdoor coil to wrap around or be submerged in the water heater storage tank (see drawing). All air conditioners are heat pumps by virtue of pumping heat. In the HVAC industry, it’s generally accepted to refer to air conditioners as a one-way heat pump while the term heat pump typically refers to a system that can be reversed to swap heating/cooling from indoor to outdoor — cooling indoors in summer and heating indoors during winter.

Although heat pump water heaters are a one-way air conditioner, it wouldn’t make any sense to call them air conditioning water heaters! The "magic" of extracting heat from the air-flow across the evaporator coil works because the expansion device restricts flow and that creates a resistance against which the compressor builds pressure. High-temperature high-pressure gas exits the compressor and enters the condenser where it turns into a warm-temperature high-pressure liquid as it gives up energy to the stored water. A portion of the Freon flows through the expansion device where it suddenly finds itself in a low-pressure environment, which causes it to flash-over from liquid to gas. As it makes the change in state, heat-energy must be absorbed to “boil” from liquid to gas. The fan drives air across the evaporator coil to enhance heat transfer and the low-pressure warm-temperature gas is drawn into the compressor, re-compressed, and the cycle repeats.

There are two basic HP water heater types being presented to us by supply-house sales people: add-on 115-volt HP models designed to be married to any standard tank-style water heater, and direct drop-in 220-volt HP models with on-board standard electric direct-immersion heating elements.

Recovery rates: Recovery rates for incoming 40°F water raised to 120°F target-temperature get longer when using 115-volt HP models (approximately 9 GPH) while 220-volt direct drop-in HP models (approximately 16 GPH) designed to replace a standard electric water heater (approximately 28 GPH) have faster recovery rates and are engineered to operate on the pre-existing 30-amp circuit. Either one can work well if you size the storage tank appropriately to match or exceed the occupants’ peak hot-water usage.

Operating Modes: Energy Saver (heat pump only), hybrid (combined heat pump/electric element) or electric heater modes offer your customer the option to recover hot water slower/faster, but compromise energy savings if not operated in the heat-pump-only mode. Target storage water temperatures have a dramatic affect on the EF (Energy Factor): the hotter you set the storage temperature, the lower the EF becomes.

Peak usage calculation: After adjusting the water temperature for comfort, measure the GPM flow rate using a bucket with gallon markings and your watch. (I've found 2.5 GPM shower heads that flow in excess of 4 GPM when the home's water pressure exceeds 70 psi.) Measure the adjusted bathing temperature at the showerhead’s outlet or tub spout if the shower head injects air to feel more forceful at lower GPM rates. Well water systems will see a relatively constant 55°F year-round while municipal systems will dip to 40°F. We’re designing for design conditions, so always use the lowest cold-water temperature your customers will experience for sizing tank-style water heaters.

We need to know the percentage of hot water in the flow to calculate peak demand, the formula looks like this: (adjusted temp – coldest temp) ÷ (target storage temp – coldest temp) = %. Therefore, (106 – 40) ÷ (120 – 40) = 66 ÷ 80 = .825 (82.5% of the flow must be delivered from the water heater).

Change that to 55°F well water and the percentage is 78.5%. Ask about duration for uses and when they occur over each day’s 24-hour cycle. From last month’s column, our customer using well water was using a peak-demand load of 60 gallons, which means our water heater must be capable of delivering a minimum of 47 gallons of hot water. Asking about the usage habits reveals the parents shower back-to-back while junior is also showering, creating overlapping usage that often finishes within a 45-minute time-frame. Due to the hot/cold mixing action that reduces available storage volume during active use, you can expect to drawdown 80% of the stored volume before outgoing temperature falls below 110°F, or 40 gallons using a 50-gal. tank.

Calculating recovery rates: In heat-pump-only mode, the direct-replacement 220-volt heat pump water heater will recover at roughly half the rate of conventional electric water heaters. As a result, our example sets up a situation where a 50-gal. heat pump water heater may not be able to meet demand on well water and likely fall short when municipal water dips to 40°F. To further challenge recovery-rate calculations, you need to consider where the water heater heat pump is located.

Room to breathe: HP water heaters air condition the surrounding space and unlike our fireworks friend, the harvested heat is off-loaded in the tank. You can't stuff a HP water heater in a closet or confined space and expect them to perform well, although there is at least one model that can be used in this manner, and we'll cover those issues next month.

Dave Yates owns F.W. Behler, a contracting company in York, Pa. He can be reached by phone at 717/843-4920 or by e-mail at [email protected].

All Dave Yates material in print and on Contractor's Web site is protected by Copyright 2010. Any reuse of this material (print or electronic) must first have the expressed written permission of Dave Yates and Contractor magazine. Please contact via email at: [email protected].

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