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Mon, Oct. 21

Column: Pros and cons of heat-pump water heaters

Heat-pump water heaters have come of age. They are twice as efficient as electric-resistance water heaters, and support operating costs comparable to natural gas units.

A heat-pump isn't making heat as an electric resistance or natural gas unit does; it's transferring heat from the surrounding air to the water. As with all air-based heat pumps the higher the ambient temperature, the more heat is converted from the air to the water. This is done by circulating a "refrigerant" fluid that can be alternately evaporated into a gas and condensed into a liquid by changing its pressure. It's higher efficiency is because it transfers more energy from the air to the water than the input electricity used to run it.

Heat-pump water heaters are clearly more efficient than electric resistance or natural gas water heaters. A typical heat-pump efficiency factor (EF) or coefficient of performance (COP) ranges from between 2.0 and 2.5, depending on ambient temperature, while the EF of an electric-resistance water heater is always less than 1.0.

If we assume that the average heat-pump has a 200 percent EF, as opposed to the loss of the gas unit at 62 percent and the electric unit of 94 percent:

Energy required to heat water = 41124 BTU (from article 1)

Input Energy = 41124BTU/200 percent = 20562 BTU

Convert BTU to KWh x (cost per KWh)

(20562/3412) x $0.118 = $0.71 per day

Cost per year = $0.71 x 365 = $259.55 per annum (pa)

This is compared to $552 pa for the electric-resistance unit and $274 pa for the natural gas unit. Based on cost, the heat pump is the most economic to operate, and less complex to install and maintain than the gas unit.

Unfortunately, heat-pump water heaters can't heat water as quickly as electric resistance or natural gas units. While the electric-resistance unit can heat 20 gallons per hour, a heat pump can only manage about 8 gallons per hour. To make up for this deficiency, heat-pump water heaters are equipped with electric resistance elements that are energized whenever the heat pump can't keep up with the demand for hot water. This feature improves performance of the unit but introduces an energy penalty.

A disadvantage of heat-pump water heaters is that they rob heat from the room where they are located. The units are often installed in a basement or garage, but they should not be put in a space that can drop below about 40°F, as the EF will drop as the ambient temperature drops, and the cost to operate increases.

Many homes don't have a good place to put a heat-pump water heater, because manufacturers advise that the room should measure at least 750 or 1,000 cubic feet; heat pumps are also taller (63" to 82") than electric-resistance units. The location must also allow for the installation of a condensate drain and be far enough away from occupied areas to prevent noise complaints of about 60 decibels.

Finally, heat-pump water heaters are higher-priced. The average cost of a 50 gallon tanked heat pump is between $1,200 and $1,400, as compared to an electric resistance unit of between $300 and $500. However, their operating efficiency provides a payback period of three to four years, and with new Energy Star standards for electric water heaters (not gas) now in effect, only heat-pump models make the grade. Heat-pump water heaters are also an ideal solution when using photovoltaic solar panels, as they are electricity efficient and use power in off periods.

It is always necessary to compare savings with capital cost and payback time. It is also important to consider health and safety effects when using different fuels. To maximize water efficiency, a timer and switch-based recirculation system is recommended, as it prevents water loss while waiting for hot water to reach the use point from the tank.

Next time we will look at the pros and cons of tank-less water heaters.

For more information contact Paul Scrivens via www.greenhomeenergyadvisors.com.

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