Air Source Heat Pumps — Water Heating

(This is an expanded version of an article published by Green Energy Times in their Dec. 2012 issue #17)

If you have not heard the term ‘heat pump’ before, you will be soon.  Most people who are familiar with heat pumps associate them with ground-source or ‘geothermal’ systems that move heat from underground into a building using pumped water.  This article is about a much less familiar, less expensive and rapidly growing family of technologies called air source heat pumps (ASHP’s).

Heat pumps are actually quite common and include refrigerators and air conditioners.  They collect heat in one space and deliver it to another through the use of special fluids, a compressor and usually a fan.  The fluids are designed to evaporate within tubes in the space to be cooled, and condense in tubes in the space to be heated, all made possible through certain laws of thermodynamics.

ASHP’s take advantage of this engineering phenomenon to heat domestic hot water. Over the last few decades they have become very efficient devices that deliver two to three+ times more energy in heat than they consume in electricity.

ASHP DHW cutaway copy

Stiebel Eltron Accelera 300 Hot Water Heater

There are energy companies in Northern New England now who are replacing oil-fired boilers with ASHP’s for space and water heating. The ASHP water heater units are up to seven feet tall and look like a large electric water tank (see image).  They draw heat from the space where they are located and deliver it to the water in the tank, using just over one third of the energy of a standard electric water heater.  The total energy savings is much greater than this, however, because this swap allows a homeowner to turn off their home’s boiler in the warm months, eliminating the tremendous standby energy losses that happen when an oil boiler runs just to heat a building’s domestic hot water.  It has been estimated that an average homeowner can save 150-200 gallons of heating oil a year by replacing their oil boiler with an ASHP water heater.  At $3.85/gal. for heating oil and a price tag of $1,200 for a cheaper ASHP water heater, the payback period for such a conversion is less than two years.

Another substantial advantage of the ASHP hot water heaters is that they dehumidify the space where they are located.  If this space is a damp basement in which the homeowner often runs a dehumidifier in the summer, the energy efficiency of the whole ASHP system improves from ‘great’ to ‘fantastic!’  As with all dehumidifiers, this will result in a water condensate that has to be dealt with.

There are a few disadvantages.  The ASHP water heaters can cost significantly more than conventional hot water systems.  ASHP’s for domestic hot water range from $1,200 to $2,500, but many utilities offer incentives up to $1,000 because of their amazing efficiency.  They are also said to be noisy, though the noise level, as well as the efficiency, varies substantially from unit to unit.  Another disadvantage is that an ASHP takes much longer to heat a tank of water than conventional water heaters do.  Because of this homeowners are often encouraged to buy the larger sized units, which cost more.  ASHP’s are also more complicated and have moving parts that can fail.  Different models also have different warrantees.

Air source heat pumps are not appropriate for every situation.  In the right setting, though, they have the potential to reduce energy usage dramatically.  These systems have been used in Asia and Europe for decades, and with energy prices consistently climbing in America we will be seeing a lot more of them here, too.  In fact, the federal government has all but mandated their use starting in 2015 by increasing the performance standards of water heaters.

In the future we expect these ASHP units to become even more efficient.  For example, warm outdoor air could be ducted to a water heater ASHP to increase its efficiency during warm months.  We can also envision an ASHP water heater being coupled with a refrigerator since they have opposite and complimentary thermal needs.  Likewise, waste ‘coolth’ from an ASHP water heater could be used in the summer to drop the temperature in a root cellar or for the cooling of living spaces.

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