Imagine the appeal of a new build home or social housing project that does not rely on fossil fuels. This is no longer simply the domain of the bespoke eco home, but a reality that has been made possible by advances in Heat Recovery Ventilation (HRV) and the development of Heat Pump Ventilation (HPV).
HPV and HRV systems work by recovering ‘waste’ heat from internal air and reusing it. At their best, and when combined with effective insulation, these systems can recover 100% of indoor heat, allowing users to obtain 4kW of heat energy from 1kW of power.
It was an HPV system, which enabled the first social housing flats in the UK to be built to Passivhaus standards. “The objective here was to do away with the need for a conventional fossil fuel-based heating system” say Gales and Snowden Architects, who were commissioned for the project by Exeter Council. “This was achieved using a heat recovery ventilation appliance, supplemented by solar power. The new tenants now benefit from much lower than average heating bills”.
HRV is now the accepted form of ventilation required to meet building regulations and to help achieve in excess of Level 3 for the Code for Sustainable Homes. Michael Hunt is managing director of Total Home Environment, one of the leaders in this growing field. He says: “If carbon neutral targets and building regulations are to be met by home builders and developers, an effective and efficient heat recovery ventilation (HRV) system is now an essential component as new buildings become increasingly insulated and airtight.”
For the end user, HRV and HPV have significant attractions – not only in cost savings, but in terms of comfort and, importantly, health benefits. There’s no need to open windows, so there is less heat loss, greater security, less polluted air available with no flying pests or allergens. There is no need to open windows since the internal air is completely replaced, on average, every two hours. Condensation, mould growth and odours are eliminated as moisture is extracted. Potentially noisy trickle vents are also avoided, and HPV which operates 24/7, is quiet to use.
Air quality issues
Asthma and allergy sufferers also benefit thanks to the greatly improved air quality. The best systems will have filters, which can trap pollen and allergens down to 0.01microns.
With air quality a very pressing concern, HRV looks even more attractive. A recent report from the World Health Organisation warned of the potential dangers from chemicals and airborne dust from everyday products in homes and workplaces. A cocktail of chemicals is emitted from chipboard and other building materials and soft furnishings such as furniture, carpets, hard floors, curtains, paint and cleaning agents and is thought to be responsible for a wide range of illnesses. Phthalates, used as a plastic softening agent and present in everything from PVC flooring to children’s toys, were singled out as posing a particular risk. HRV significantly reduces the impact of airborne pollutants.
How HPV and HRV workIn a typical system, a ventilation appliance is usually located in the loft or utility area of a new or existing property, connected via ducting to ceiling terminals in the rooms. A fan system extracts the stale, warm (and sometimes damp) internal air to the ventilation appliance to be exhausted outdoors. Meanwhile, another fan draws fresh filtered air from outside and supplies it to indoor areas via ducting.
Using a heat exchanger within the appliance, HRV extracts the heat from the outgoing air as it passes over metal plates and uses this to warm incoming cold air. In warmer months, the heat recovery process can be bypassed to ensure that cooler, fresh air enters instead.
Using this simple cross-flow heat exchanger system, a heat recovery efficiency of 75 per cent can be achieved. In more sophisticated set-ups, which use a counter-current heat exchanger, even greater efficiency is achieved.
The optimum set-up is an HRV installation with counter-current heat exchanger plus an air-to-air heat pump, known as heat pump ventilation (HPV). The heat pump functions like a fridge but in reverse: while a fridge takes heat from food, the heat pump extracts heating energy from warm exhaust air using it to heat incoming cold air. The market leaders in HRV can, if required, heat incoming air at -1?C to 34?C.
When the incoming air needs cooling – in summer for example – the coolant circuit is reversed. The heat pump works like a fridge, taking heat energy from the warm fresh air and providing cool fresh air for the property.
The principle behind HRV is firmly grounded in the drive for sustainability. THE’s Michael Hunt explains the carbon-saving implications: “Obviously, the more heat we recover, the less we need to generate and the more we can reduce our reliance on fossil fuels.”The system can be used in new build or retro-fitted homes, but to optimise efficiency the building must be properly insulated and sealed. There are other considerations too, particularly for specifiers: ? Ductwork can be quite large in diameter so in a retro-fit the challenge is to find the space to route the duct to each room. ? Many companies specify flexible ducting for cost saving purposes but, where this is used, beware of ducting being squashed or damaged as this will greatly reduce efficiency of the system.? Ducting should be insulated correctly, otherwise heat will be lost.? Where heat pump ventilation is specified, a heat loss report should be carried out in the first instance in order to avoid over-reliance on the micro heat pump (some houses may need towel radiators and a wood burning stove to create a comfortable home environment, for instance.)? In a new build, the specifier needs to ensure that there are no gaps whatsoever in the insulation.
End users need to adopt certain practises to maximise the benefits – for example, to keep windows closed and to leave water for an hour after a bath in order that the system can extract the heat from it for re-use.
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