Modular Building Institute
Modular Building Institute

Heat Pumps Drip in the Winter

The Comfort Zone by Maury Tiernan, Geary Pacific Corp.

But it isn't because they catch a cold and have a runny nose. Regular readers of The Comfort Zone may remember several articles on heat pumps, which have covered the need for auxiliary heat strips, the costs of heating with electricity vs. gas, and a variety of other topics.

Additionally, an air conditioning unit doesn't really "cool" the building in the summer; rather it removes the heat from the inside, and rejects it to the outside. That is why the air blowing off the outdoor coil is hot in the summer. The indoor coil of the air conditioner would feel cold to the touch. The cooling cycle collects the moisture from the air, while it absorbs the heat from the building. A pan under the indoor coil collects the moisture which then flows out the condensate drain line.

Again, the cooling mode of a heat pump works the same as an air conditioner, but the coils are "reversed" for the heating mode. A heat pump has a valve that reverses the refrigerant circuit enabling the unit to capture the heat (energy) in the outside air and reject it into the building. During the heating mode, the indoor coil would feel hot to the touch and the outdoor coil would feel cold, exactly the opposite effect of the air conditioner cooling circuit operation.

In the winter when the outdoor coil on a heat pump is "cold" to the touch, it will collect the moisture from the outdoor air. For several reasons, most HVAC manufacturers do not provide a drain pan under the outdoor coil of a heat pump to collect this condensed moisture. Heat pumps are used all over the world and in many climates with winter temperatures below freezing. In those colder climates, if the condensed moisture were collected in a drain pan, it could freeze and crack the coil. Also, the water may not run off the coil, but instead form a block of ice, and restrict the airflow across the coil. To prevent this problem, heat pumps include a defrost control circuit.

The defrost control prevents ice build up on the outdoor coil by checking the coil temperature on a period basis. To defrost the outdoor coil, the heat pump goes into the cooling mode (making the outdoor coil hot momentarily) until the outdoor coil thermostat is satisfied there is no ice on the coil. This may cause your occupant some discomfort because cold air will blow from the registers for a short time. The heat pump's auxiliary heat strip (if equipped) helps temper the cold air for the (short) defrost cycle. Putting a heat element in the pan to keep the water from turning to ice would reduce the efficiency rating of the unit, and raise unit and operational costs.

HVAC manufacturers design roof mount or wall mount heat pumps for use in any climate or application, so the drain pan on an outdoor coil would be considered a "field option" to be fabricated and installed by a qualified HVAC contractor.

When a heat pump is installed in a colder climate without a drain pan, you are left with an outdoor coil that drips water in the heating mode in the winter. If a wall mount unit is over grass or a flowerbed, the drips are not a problem. However, be aware that if the unit is over a sidewalk, driveway, or any surface that has pedestrian traffic, the water will collect on the ground. Some water drips, and some sprinkles (blows) off the coil (due to the velocity of air blowing through the coil). Roof mount units may drip on the roof then onto a pedestrian walkway below. Algae may form on some roofs and sidewalks where a lack of direct sun light prevents evaporation of the dripping water.

Here are a few suggestions for those of you with this situation. One, fabricate and install a drain pan under the bottom of the wall mount or roof mount unit. Drain the condensate from the outdoor coil to the same place as the indoor coil condensate. Two, if the air blowing across the outdoor coil sprinkles more water than is tolerable, it may be possible to reverse the airflow across the coil, from blow-thru to draw-thru. Consult your local HVAC contractor or supplier to verify if this is possible, and determine what effects such a change such as this will have on your application.

Another option may be to temporarily disconnect the compressor and use the heat strip as your primary source of heat. The auxiliary heat strip in the heat pump may not be large enough to cover the heat loss of the building, or for morning warm up. A larger heat strip, electrical service, and some rewiring of the low voltage circuit may be required. Further, consult your electrical contractor, HVAC contractor, or HVAC supplier before using this final option. HVAC units have legal limitations (and liabilities) when field modifications are being considered. Your HVAC unit may not be rated by UL, and/or your desired modification may not be in compliance with the NEC.

Finally, (to be "legal") it may be necessary to change-out the wall mount or roof mount heat pump to either an air conditioning unit with a larger heat strip or to a gas/electric HVAC unit. One wall mount manufacturer has recently introduced a heat pump with an outdoor coil drain pan included as a standard feature. As always, ask your HVAC contractor, mechanical engineer, or HVAC supplier for their input when these difficult situations arise, and weigh the pros and cons of each type unit/installation.

The temperature in California is 78 degrees this week, and another terrific day is forecast for the Rose Parade in Pasadena. That isn't as peculiar as the temperature in Minnesota this week, a balmy 50 degrees above zero. Growing up there as a kid, I don't remember any winters like that. I used to walk to school ten miles, up hill, both ways, in sub zero weather.

Take care of yourselves until the next time we meet in . . . The Comfort Zone.

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