Unless situated in very sheltered climate, an high performing HRV system needs some kind of frost protection for the intake air. The problem is related to the condensate from the extract air, which gets cooled down in the heat exchanger. With frost outside, the condensate can actually freeze in the heat exchanger and block its tubes. The higher the efficiency of the HRV unit, the more is the system affected by frost. Our HRV units need frost protection below minus 2 to 3 degrees outside air temperature. Less efficient units can cope with minus 8 to minus 10 degrees.
There are various ways of frost protection:
1) Frost protection controls: Our HRV units have an in-built frost protection, that protects the heat exhanger from frost by disbalancing the air flow rates and thus making the system less efficient. Often the supply air rate is thereby be reduced in various steps; If the temperatures are low enough, the supply air will be completely shut off. This solution without any other method is only viable when the climate is very sheltered and frost below minus 2 degrees occurs very seldom.
Unfortunately it is common practice to install HRV systems with frost protection mode only, irregardless of the climate. In prolonged periods of frost, the air supply for the dwelling will not be sufficient. Alternative means of ventilation (window opening) have to be sought. We do not advocate this option, as it is leading to an influx of cold air during periods of frost, exactly then, when you need warmth the most.
2) Electrical pre-heater (defroster) in the duct between intake and HRV unit. This is the most common and least expensive approach for high quality HRV systems. ISO defroster are controlled in such way, that they adjust their power consumption on the temperature levels. The defroster can be operated in safe mode (operates below zero) and in eco mode (operates below minus 3 degree Celsius). With a defroster in place, the HRV unit will not need to operate in the frost-protection mode, as previously explained.
The defroster is protected from dust and insects by either an integrated filter or a filter box between intake and heater unit. Some HRV systems have got an in-built pre-heater. The Paul novus rage will have such in-built defroster from mid 2013.
A defroster operated with warm water is not recommended as in case the warm water is not flowing, the unit could freeze and be damaged.
3) Ground heat exchanger for the intake (GHX): This is a 30-50m long intake duct of 200 to 250mm diameter buried 1.5m deep in the ground. The duct should be anti-bacterial lined and allow for drainage of condensate; its terminal should have a F7 filter to prevent the pipe from soiling. The GHX takes the frost out of the intake air and provides gentle cooling in summer.
4) Brine loop with heat exchanger in the duct between intake and HRV unit.
5) Frost protection flap. This unusual form of frost protection is used in some HRV units. At low outside temperatures periodically a flap opens to take air from the surrounding area into the intake channel. If the place is cold, where the HRV unit is installed, e.g. a cold loft, the system does not work properly. It also draws in air from the installation room into the dwelling, which can cause smells to enter in. Out of these reasons, our systems are not equipped with such devices.
The higher the performance of a MVHR system, the more it is vulnerable to frost in the intake air. With systems, that are of more than 90% efficiency, the threshold is about minus 2 to minus 3 degrees Celsius.
Example: If the outside temperatures are minus 3 deg.C, the inside temperature is 20 deg.C, we have a temperature difference of 23K. A 90% efficient system will raise the intake temperature from -3 deg.C by 20.7K (=90% of 23K) to 17.7 deg.C. The extract air will likewise be reduced by 20.7K from 20 deg.C to -0.7 deg.C. As the extract air during the cooling down process looses its capacity to retain moisture, condensate will fall out.
At 20 deg.C 1m3 can retain 17.3g/m3 of moisture at 100% saturation. At zero deg.C it can only retain 5g/m3 at 100% saturation. Given the fact that the relative indoor humidity is 50%, it carries 8.6g/m3 moisture. This means that 3.6g of condensate will fall out per cubic meter of extract air. This condensate will freeze over when the outside temperatures drop below minus 3 deg.C.
If the real heat recovery rate of the system is lower than 90%, then the system can cope with more frost. Also if a high-performance latent heat exchanger is used, the fallout of condensate can in most cases be eliminated and therefore the need for a defrost pre-heater.