12/19/2015

“Tis the Season for DeFrost

As we move into winter in North America, the discussion at Zehnder America, Inc turns to the need for frost prevention in HRVs and ERVs.

With very cold air moving in one direction, and warm air carrying humidity from showers, cooking etc in the other direction, it is inevitable that frost can form in the heat exchangers of HRVs and ERVs.

With this in mind, we have several options for managing the prevention of frost, which can cause damage to the heat exchanger, reduce the efficiency of heat recovery, and cause an imbalance of air flows which can potentially depressurize the home.

These options are:

  1. Switch air flows to an imbalance, which results in more warm exhaust air flowing than cold intake air. This will warm the outside edge of the heat exchanger, and defrost the unit.  
  2. Switch the HRV or ERV to recirculation mode, which stops the cold incoming air and routes the warm exhaust air back through the heat exchanger. This is normally done in a cycle, with recirculation occurring for short periods of time based on outside air temperature.
  3. Utilize an electric pre-heater, which will defrost the inside edge of the heat exchanger. This is usually done with a modulating and/or cycling electric pre-heater, which limits the amount of power needed to manage the defrost.
  4. Install a ground-source pre-heater/pre-cooler which is triggered to operate when incoming air temperatures are either cold or hot, to either pre-heat or pre-cool and dehumidify the incoming air.

A comparison of the costs to operate these options as well as the impact on the comfort of the incoming air influence the decision of which of these options to choose.

Recirculation is relatively cost-effective, but during defrost cycles there is no actual ventilation happening. Exhaust air is re-routed back to the bedrooms and living spaces. And with the unit in recirculation mode, efficiency is also negatively impacted.

An imbalanced operation is also cost-effective, but there is depressurization of the home during this mode, and efficiency also is reduced, impacting comfort. There is also a possible issue with fireplaces, stoves and/or combustion appliances back-drafting. This function can be disabled with Zehnder units to prevent this potential issue.

Electric pre-heaters, as long as they cycle infrequently, are cost-effective and do not impact continuous ventilation. An estimate of the energy used in Zehnder pre-heaters in most units is modest. In a range of climates, based on heating degree days, we can see that power consumption on a yearly basis is indeed modest:

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The ground-source pre-heater/pre-cooler-dehumidifier provides the optimum for both comfort and energy efficiency in cold-climate applications. By simply running glycol through a loop of HDPE in the ground around or near the home, and utilizing a coil with that ground temperature glycol, frost prevention and comfort in the winter time in cold climates is enhanced considerably. Tested to temperatures as low as -14F, this system produced over 90% recovery efficiency even at that extremely cold level. With the pump circulating the glycol running at as low as 10 watts, the impact on comfort and efficiency is dramatic. And this option is also very helpful in the summertime when pre-cooling and dehumidification are enhanced for that same very small energy draw. The cool coil also creates condensation, which reduces the humidity of the incoming air, which also helps to reduce run times for cooling systems.

When planning your ventilation system with heat recovery, it pays to take into consideration the need for frost protection and to determine the best option for your climate. Zehnder can help with this decision-making process by discussing these options, and providing comparisons for energy use and comfort in your project.