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European – Air-source heat pump examples

European – Air-source heat pump

The purpose of the project was to build a low energy and low cost prefabricated house without compromising the comfort. Due to the overall reduced energy consumption in low energy houses the proportion of the heat required for DHW increases from 30% to 40%. This sets new requirements for a heating system. In this residence there are two air-source heat pumps: one for heating (from October to April) and the other for DHW (DHW; all year round).

MINERGIE-single-family house in Schötz (NTH4).

The former uses outdoor air and the latter indoor exhaust air as its heat source. The SPF of the 4.6 kW heat pump for space heating is 3.0. If auxiliaries such as fans for heat recovery and standby losses of the water heater are considered as well the SPF drops to 2.0.
This case study is based on information from (NTH4). For ordering information see below.

Building and design values

Building type:Single-family house
Year of construction:1998
Number of storeys:2
Heated floor area (m2):155
% of total floor area (%):No data (basement unheated)
Design outdoor temperature (ºC)Heating:-9Cooling:
Design indoor temperature (ºC)Heating:20Cooling:
Degree daysHeating:3 294Cooling:
Base temperature for degree days (ºC)Heating:12/20Cooling:

Heating and cooling (NTH4)

Application:Space heating, DHW
Heat pump type(s):Air-to-water (space heating), separate air-to-water heat pump water heater (DHW)
Heat pump installed capacity (kW)Heating:4.6
Refrigerant:R290 and R134a (DHW)
Heat sourceOutdoor air (space heating), exhaust air (DHW) from two bathrooms, toilet, kitchen, laundry room
Details:Circulation pump 35 W (heat distribution)
Distribution system(s):Floor heating elements/water without thermostatic valves
Supply and return temperature (ºC)Heating:35/302Cooling:
Auxiliary system:None
Heat pump design:
  • Heating demand of the house (20ºC, air change rate=0.3/h) according to a SIA3 model was calculated as 181 MJ/m2a. The actual amount used was higher at 242 MJ/m2a (23ºC) because of higher effective averag e room temperature, higher air change rate etc.
  • DHW (57 l/person/day, 50ºC) 12 months a year, space heating in winter period.
  • The return temperature control is dependent on outdoor temperature. Utility restrictions do not allow heat pump operation at 2-4, 10-12 and 17-19 o’clock.
Supplementary system:None
Heat pump system completion date:1998

1 A-7/W35: Air -7ºC, water 35ºC. etc.
2 With a utility cutoff period of 6 hours; continuous operation would allow 30/25
3 Huber Energietechnik, Ingenieur- und Planungbüro SIA; model no. 380/1

Performance (NTH4)

Energy for heating, DHW and ventilationHeat pumpAux. heating systemAuxiliaries 4
Energy input (kWh/year):4 859548
Energy output (kWh/year)1:13 8012745
Energy cost (CHF/year)2:535.66
Cost tariff (CHF/kWh)3:0.16/0.10
1 Measured data
2 Calculated from values during low (38%) and high (62%) tariff period and basic annual (CHF 57.60) cost. Costs include auxiliaries!
3 High/low
4 Auxiliaries include circulation pump for heat distribution and fan during off-time of heat pump water heater
5 Assumption: 50% of auxiliaries energy input can be used for space and hot water heating

Coefficient of performance (COP)
Heating:4.0 and 2.5 (space heating)
Test conditions:A7/W35 and A-7/W35
Test conditions:

Heat pump cost breakdown
Heat pump only (CHF):9 850 (space heating) and 4 660 (DHW)
Installation (CHF):4 000
Capital cost (excluding heat pump) (CHF):17 175
Maintenance (CHF/year):150

Comparison of annual costs
Figure 3: Comparison of annual cost of various systems for heating, ventilation and DHW. Options 1-6:

  1. Oil-fired boiler (16 kW), electric backup for DHW; balanced ventilation with heat recovery

  2. Air-to-water heat pump (4.6 kWth) and exhaust-air heat pump water heater

  3. Gas-fired boiler (13 kW), electric backup for DHW; balanced ventilation with heat recovery

  4. Wood-fired burner, electric boiler for DHW, balanced ventilation with heat recovery

  5. Integrated heat pump system for space heating, DHW, ventilation and heat recovery, back-up electric boiler (for application in “passive house”).

  6. Ground-source heat pump (4.9 kWth), electric backup for DHW, balanced ventilation with heat recovery

Operational experience and other comments
Schematic of the heat pump system (NTH4). This project has demonstrated that a low energy, low cost element (prefabricated) house can be built without compromising the comfort. Similar 2-storey houses exist on this housing estate, which is about 508 m above the sea level.
In low energy houses the overall reduced energy consumption significantly increases the proportion of the heat required for DHW, from 30% to 40%. This sets new requirements for a heating system.
Some heat pump-related improvement ideas were raised after completing the monitoring programme:

  • The heat pump capacity should be lower than required rather than higher than required.

  • Low temperature distribution systems (Tsup>25°C) are self-regulating

  • Mixing or thermostatic valves are not necessary.

  • Overheating in fall and spring may be a problem. Shading devices are essential.

  • The equipment should be in heated space.

It is estimated that the heat pump can be used for 15 years and the pipes etc. for 30 years.

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