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Heat recovery unit with or without a preheater?

Preheater for heat recovery unit

If we are not one of those lucky people living in a sunny place where the temperature does not drop below zero, our heat recovery ventilation system requires protection of the heat exchanger against freezing.

At very low temperatures, condensate extracted from exhaust air may freeze in the heat exchanger. Freezing of condensate may cause blockages of fins in the exchanger, and air exchange gets significantly reduced. If the icing period of the heat exchanger is extended, the device may get damaged.

To protect the heat exchanger against icing in the winter season, manufacturers of the heat recovery units use:

1. anti-freeze function (automation)
2. pre-heater (built-in or external)

Anti-freeze function in heat recovery unit

The anti-freeze function is an integrated MVHR unit automation function to protect the heat exchanger from freezing. Device automation reads temperatures, and if the ambient temperature drops significantly below zero, the system changes the fan operating speed. This way, the amount of cold air supplied by the heat exchanger to rooms is reduced.

In extreme cases, the unit will no longer ventilate and will only operate in air extraction mode, thus heating the heat exchanger. Modern units with EC fans allow for a smooth reduction of the supply fan speed and a smooth increase of the exhaust fan speed.

It is cost-free protection of the heat exchanger against freezing. It does not require any additional electricity consumption as in the case of a pre-heater. On the other hand, during the period of the anti-freeze function, the air streams are imbalanced because the system only exhausts used air without supplying cold air. Thus, the efficiency of the heat recovery ventilation system is reduced.

The anti-freeze function used as the only freeze protection, i.e. without a pre-heater or a ground exchanger as additional protection, is a good solution only in climates where freezing days with temperatures below minus 3 degrees occur very rarely.

Pre-heater protects the MVHR unit against freezing

The second popular method of protecting the heat exchanger against freezing during high frost is a pre-heater. There are HRV units available on the market that offer a built-in heater, most often in the form of a heating element or a heating coil as well as external duct heaters, which are additional accessories for the heat recovery unit.
Heat recovery unit with build-in heater

SlimAIR ceiling and wall-mounted heat recovery units have a build-in preheater as a standard.

Check technical data for SlimAIR MVHR units

As a rule, the heaters offer a power of approx. 1–2 kW, which may seem too small. Nevertheless, if we need an energy-efficient ventilation system, any additional energy-consuming source is undesirable. However, it is worth noticing that the heater only switches on in case of very low temperatures.

An external pre-heater is installed on the intake duct upstream the heat recovery unit. The heater only heats air supplied to the air handling unit. It does not affect the temperature in the room. The operation of the external pre-heater is controlled by an automatic control system of the air handling unit, so we do not have to switch it on manually.

The advantage of the pre-heater is that it is a relatively cheap solution. On the other hand, the heater uses electricity which is expensive.
Heat recovery ventilator with preheater

HRU-ERGO energy recovery unit with HDW water heater (left photo); dedicated preliminary electric heater
for PremAIR heat recovery units (right photo).

The most effective way of operation is to combine the defrost algorithms described above, i.e. to activate the heater as the second protection level, regulating the rotation speed of the fans for as long as the ambient temperature allows it.

Important: Remember to install an air filter upstream the pre-heater to protect it against contamination.

Water heater

A water air heater connected to the household heating system can also be used as a pre-heater, but it requires proper protection against freezing (automation system with thermostat, valve with actuator, additional pump and additional shut-off damper), which increases investment costs and is subject to failure.

The less popular way, but sometimes used by Scandinavian manufacturers, is to use a bypass during the anti-freeze cycle. Cold air bypasses the heat exchanger, but a secondary air heater is required.

Would you like to know more? Read other articles on heat recovery ventilation:

  1. How can You install heat recovery unit in the ready-built house?  - the article about decentralized heat recovery systems.
  2. What is an Electrostatic Precipitator and how does it work? - the article about highest filtration efficiency solutions.
  3. 14 Things You Need to Know about PremAIR - pretty self-explanatory title about the best HRU for clients looking for "premium" solutions. 
  4. Air distribution system for the MVHR - overview of FLX-REKU system: radial, semi-rigid ducting, plenum and distribution boxes, etc.