Domestic Hot Water and Space Heating

Domestic Hot Water Production

As in any dwelling, a Passive House requires a system that provides domestic hot water (DHW). As with space heating, it is important that the system is energy efficient, well controlled and has an adequate capacity to meet demand. Generally the DHW system in a Passive House is combined with a heat source such as a wood stove, solar thermal collector, heat pump, gas or oil boiler (if not direct eletricity) for space heating. Most Passive House examples encountered have utilised solar thermal collectors as they reduce the use of primary energy and CO2 emissions. It is important to note, however, that the Passivhaus Standard is achievable without solar based water heating. The introduction of Building Energy Rating system as an indication of the energy performance of dwellings in the UK is likely to increase the installation of solar technology as it influences the energy rating of a home, and in particular CO2 emissions.

Domestic Water Heating - Solar Input

It is reasonable to expect that an optimized solar thermal system will produce up to 60% of total annual hot water demand. Solar thermal systems also have a shorter pay back period (when taking into account available grants), than other renewable energy technologies such as wind turbines or photo voltaic panels. In Scotland, the amount of solar irradiation received each year is approximately 900–1,150 KWh/m² (see table below). This is the approximate equivalent of 100 litres of oil. In terms of specifying a solar thermal system, the following outline guidance should be considered:
• The optimal orientation is directly due south (for the Northern hemisphere) and deviation from this will reduce the contribution of the collectors to DHW production. In places where there is no south facing roof, a larger area of panels can be fitted to east or west facing roofs.
• The optimal tilt of the solar panels, to meet approximately 50% of the annual heating demand for DHW, is approximately 45 degrees (at a pitch greater than 45 degrees the potential annual output is compromised somewhat). Examples for the optimum angles are mentioned in the tables below. Sometimes, a rack with different settings for summer and winter is used, as the winter sun is lower than the sun in summer, and therefore, in winter the panels should be more upright.

Irradiation (Wh/m2/day) in Edinburgh and Aberdeen (at 38º inclination / at 40º inclination):

Based on these figures, the annual solar irradiation in Edinburgh is 1025 kWh/m2 and in Aberdeen is 1048 kWh/m2

• There are two typical types of solar collectors, flat plate panels and evacuated tubes. A comparison of the performance of these types, based on 5m2 collector area, along with consideration of orientation and angle of incidence is provided here. The calculation was developed for a prototype Passive House using the calculation methodology for solar water heating in Dwelling Energy Assessment Procedure DEAP 2005, version 2. Three different inclinations of solar panels (30º, 45º, 60º) and three different orientations were calculated, with the following specification: standard number of 3.6 occupants according to DEAP assumption; water storage tank 300l, with 150l dedicated to solar, and 50mm factory insulation; with thermostat control:

Solar input [kWh/yr]   (Solar input to demand ratio)

5 square meters of FLAT PLATE collectors (η0=0.75 and a1=6), no obstructions:

5 square meters of EVACUATED TUBE collectors (η0=0.6 and a1=3), no obstructions:

Domestic solar water heating - solar input (flat plate collectors and evacuated tube) for the prototype Passive House
(described in Section 3), calculated with the Dwelling Energy Assessment Procedure DEAP 2005 version 2.
Source: UCD Energy Research Group.
 

As a general rule of thumb for a one-family-home, the area of solar panels is roughly 1-2m2 of collector area per person. The system should be capable of providing up to 50 litres of DHW per person per day in season.

In terms of sizing a solar tank, a minimum of 60 and preferably 100 litres storage per m2 of collector should be provided. In a typical Scottish home this could mean installing a tank with a capacity of between 200 and 500 litres. To ensure that the best levels of energy conservation are attained, it is important to use a proper very well insulated solar water tank and ensure that the hot water pipes are well insulated. The 40% or more of DHW needs that are not provided by solar energy can be met by several means including biomass boilers or stoves, immersion heaters or natural gas. An outline of the first of these is provided below. It must be remembered that space heating in a Passive House is often provided by using hot water to heat the air passing through the ventilation system. In such cases, hot water production is essential in the heating season when solar panels on the roof will not be sufficient to meet the demand for heating the hot water. Accordingly, many Passive Houses will have a biomass stove burning either natural logs, wood chip or pellets. The advantage of the latter two of these is that they are more easily automated so that they fire up and switch off in the same way as a conventional gas or oil burner.

The following issues should be remembered when considering installing a wood stove or boiler:

• The equipment must be sized appropriately to the heat load of the house. This will be defined by the ‘Verification’ page in the PHPP software. In a 110m2 house with 3.6 occupants, a stove of 3kW output would be sufficient for all space heating and DHW needs. For a small Passive House, most models will be too large, i.e. the output will be too high. Therefore suitable models have to be chosen carefully.
• A combustion air supply must be provided to any stove or boiler in a Passive House bearing in mind the level of airtightness that has to be achieved. The provision of an air supply and flue for stoves or boilers will generally not adversely impact on the airtightness or balancing of ventilation flows due to the 'closed' nature of their construction. Air required for combustion is drawn in through a relatively small diameter duct and expelled through the flue.
• A stove or boiler that directs most of the heat output to the DHW tank is essential if the hot water is to be used to heat the ventilation air. A model that simply radiates all the heat into the space in which it is located cannot generally be used for whole house heating. Most Passive House models with a backboiler deliver 20-30% of the heat produced into the room and 70-80% to the water.
• Wood (whether logs, chipped or in pellets) is bulky and a considerable volume is required for storage especially if it is purchased in bulk to keep costs to a minimum.
• Most wood stoves are highly efficient (up to 90%) and when burning pellets there is very little ash remaining following combustion. A flue and a fresh-air supply (independent of the room air if placed inside the thermal envelope) will be required.
• Biomass boilers have a slow response time (i.e. they need a while to fire up and even longer to cool down). Therefore, the DHW tank or thermal store has to be certified for use with biomass boilers (these might e.g. be a vented system type). To catch the sluggish response time, and in order to make the boilers efficient, you will often find accumulator tanks, even up to 3000l or more! This is not a must, of course, and will have to be decided in each individual case.

Below, you will find an example of the hydraulics of a system with an MVHR, solar collectors and a wood stove with back boiler feeding into a thermal store (with an additional immersion heater backup). From there, the heat for the wet heating system (e.g. the device at the fresh air outlet of the MVHR) and DHW is taken out. This system has also the advantage that the hot water is always freshly produced, and does not sit in a DHW cylinder for any length of time. The defroster at the air intake shown in the diagram is generally not needed for houses in the UK (even if no ground heat exchanger is used), as the temperature rarely drops below -5ºC.

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