Author: Dr Tomas Mikeska, Passive House Institute

The standard solution for the ventilation of residential as well as non-residential Passive Houses is a ventilation unit equipped with a passive heat recovery or energy recovery core (in the following: “passive system”). The minimum requirement for certification of such systems is a heat recovery efficiency of 75% at a maximum electricity demand of 0.45 Wh/m³. At 15 K temperature difference, the COP of such a system, i.e. the heat recovered divided by the electricity consumption, is greater than 10. The most efficient heat recovery units (efficiency approximately 90%, electricity demand 0.2 Wh/m³) achieve COPs above 20.Field measurements confirm these calculations. [Peper 2002] reports a ratio of recovered heat to electricity – equivalent to the heating seasonal performance factor (HSPF) – of 16.5 (in US units: 56 BTU/(Wh)) for a residential heat recovery ventilator. For the more demanding situation of a school, [Peper 2007] measured a seasonal efficiency of 8.9 (30 BTU/(Wh)).

Some manufacturers suggest to replace these passive heat recovery systems by heat pumps which use the extract air as a heat source to heat the supply air (in the following: “active system”). The goal of this article is to compare the energy consumption of passive and active systems for different climates.

The comparison is carried out for nine different climates across North America. By means of an hourly simulation we compare different passive systems and an active system, installed in a building according to the International Passive House standard (criteria). The floor area of the investigated building is 145 m². The occupancy is 35 m²/person and fresh air supply is 30 m³/(h*person).

As a reference, the buildings are equipped with a passive HRV system with an efficiency that is appropriate for the respective location (Table 1). Frost protection is achieved by direct electric preheating of the outdoor air. This reference system provides a certain supply air temperature and requires a certain energy demand for fans and frost protection.

Table 1: Investigated cities, their design temperatures and the efficiency of the passive systems

All other systems are assumed to provide the same heating contribution, i.e. the same temperature of the supply air. For the other passive systems, with lower frost protection set points, this means that the supply air needs a small amount of post-heating, which, pessimistically, is assumed to be direct electric. For the active system, only that fraction of its capacity is considered that provides the supply air temperature of the reference system. Both the passive and active system use fans with a total consumption of 0.45 Wh/m³. The resulting total electricity demands can now be compared.

Three passive systems and one active system are investigated:

1. Passive heat recovery with a membrane heat exchanger. Frost protection is activated when the outside temperature drops below 3 °C.
2. Passive energy recovery. Frost protection below -10 °C.
3. Passive heat recovery core using dampers that regularly switch the direction of the airflow in the heat exchanger so that condensation and frost reevaporate. Frost protection is needed only below 19 °C.
4. Active system with COP values according to Table 2. It is assumed that the heat pump will go regularly to defrost mode when the temperature of the air leaving the evaporator drops below 3 °C. In that case the COP is reduced by 20%.

Table 2: COP of active system taken from standard DIN 18599-6:2016

As can be seen from Figure 1 and Figure 2, a heat pump used for heat recovery purposes will consume up to 4 times as much energy as a passive heat recovery ventilator. There is only one case where active and passive heat recovery have approximately the same energy demand: the heat pump compared to a simple heat recovery with direct electric pre-heating for frost protection in the climate of Winnipeg (825 hours with outside temperatures below -20 °C, 2949 hours with outside temperatures below -3 °C). Note that it is generally not recommended to use simple heat recovery ventilators with direct electric pre-heating for frost protection in such cold climates. Even in Livingstone, Riverton or Burlington, at least an energy recovery ventilator is advisable, resulting in 70% lower energy consumption as compared to the active system.

Figure 1 also shows that the additional electricity demand of a heat pump solution for heat recovery is typically on the order of 10 kWh/(m² yr), a value that is much higher than the total electricity consumption for heating in a Passive House with a heat pump system.

Figure 1: Comparison of final energy use of different investigated systems (including frost protection and energy for fans)

Figure 2: Comparison of final energy use of different investigated systems shown in %

Calculating active heat recovery systems in the PHPP requires that the ventilation heat recovery rate is set to zero. This will result in the correct annual heating demand and peak heating load that need to be covered by the heating system. The properties of the heat pump in the active heat recovery system, as a function of the ambient temperature, are entered in the Compact worksheet.

How to insert an active system in PHPP version 9

If space heating is provided by the active heat recovery plus an additional heat pump, or if the active heat recovery system is operating in different modes (outdoor air / recirculation air), the average COP of the total system needs to be entered into the performance map in the Compact worksheet. This will require that the fraction provided by each system is estimated in an auxiliary calculation.

This article compares a passive heat recovery ventilation core with an 'active heat recovery' system using an extract-air-heat pump. It clearly shows that the direct passive-air-to-air heat recovery is highly advantageous. Due to the significantly lower performance, extract air heat pumps are therefore not suitable as a mere replacement for passive heat recovery systems in Passive House buildings in cold climates.

It is also important to mention that products with active 'heat recovery' systems are sometimes presented as combined units for ventilation, heating and cooling. For this application the designer must evaluate carefully if the units have sufficient capacity to cover both the ventilation and space heating load.

It should go without saying that the most appropriate mechanical system must always be identified for a specific building. There are cases (e.g. very mild climates) where direct-passive-heat recovery is not absolutely necessary and there are also cases in which extract air heat pumps can be integrated efficiently into the overall mechanical system. Energy modelling provides guidance on the impact on the heating/cooling demand, as well as the final energy needs. The combination of both a passive core and an exhaust air heat pump can be a highly attractive integrated solution to efficiently provide heating, cooling, dehumidification and DHW - see this article on compact heat pump units.