We no longer supply these systems, but they are available from Stiebel Eltron, Baxi, Vent-axia etc. Check the energy efficiency and electrical power and airflow to compare the systems.
The data for the old LWZ161 has been left on this web site for comparison.
Make sure that the energy efficiency is no less than 90%, and that the fans are low-power DC types.

Overview.

The heat loss from the fabric of a building can be minimised through good insulation. However, there is also a considerable heat loss due to the necessary ventilation of stale air. This heat loss can be reduced dramatically..

A passive heat recovery ventilation system takes stale air from kitchens, toilets and bathrooms, passes it through a simple heat-exchanger where incoming fresh air is heated by means of the energy in the exhaust air. These systems have few moving parts, are very quiet and use very little power. They are excellent, and an obvious addition to a new-build.
(note, this 'passive' system is not a heat pump)

The air quality is much improved in buildings using this system. The total heat requirements for the building will be reduced, resulting in a smaller (hence cheaper) heating or heat pump installation.

Most systems have a centralised unit which is connected to the rooms by means of ducting.

Applications include new build housing and busy rooms such as offices and class rooms where high ventilation rates are needed. The heat produced by the occupants (about 90 watts of heat each person) can be mostly recovered. This heat is normally lost due in the ventilation air.

Channel 4, 'Grand designs', The Underground House, Cumbria
http://www.theundergroundhouse.org.uk/

The German made Stiebel Eltron LWZ heat recovery unit is an extremely efficient, high quality device that recovers an amazing 90% of the heat that would be otherwise lost. This is achieved by the use of a super-efficient parallel/cross-flow heat exchanger and low power DC fans.
This unit has now been superceded by the LWZ 70/170 and 270. The new units have a night summer cooling option which is of great benefit in summer. The energy efficiency is very similar, so the data below has been kept for interest.

Type	LWZ161 (old version)
Electrical Connection Maximum	1/N/PE - 230V 50Hz
Maximum current (A)	1.54 A
Loadings Stage 1/2/3 (Watts) (to run the DC fans)	10-12 / 30-40 / 78-105W
Airflow volume Stage1/2/3 (m³/h)	80 / 160 / 225 m³/h.
Airflow volume Stage1/2/3 (lit/sec)	22 / 44 / 62.5 lit / sec.
Energy use per unit air flow (stages 1/2/3 (Watts per lit/sec.)	0.54 / 0.83 / 1.6 W / lit./sec.
Protection Class	IP31
Dimensions H/W/D (mm)	602 x 675 x 420mm
Weight	31 kg.
Air connections diameter	150mm dia. (4 off)
Cross-flow heat exchanger efficiency	90%
Noise level (at 1m distance) Stage 1/2/3 dB(A)	26.3 / 40.4 / 49.9

Further notes on installation details.

 

The LWZ unit is attractively finished, and can be installed in any convenient position, e.g. above a stairway, in a utility room or in a loft space etc. Two 150mm diameter (6 inch) ducts to outside (intake and exhaust) are required. These should be insulated to stop condensation.
The main ducts to and from the rooms start at 150mm dia. These would reduce in size as they branch off. In general, the fresh air inlet goes to living spaces and bedrooms, the extract duct will come from the kitchen, bathroom and toilets. The cooker hood should not be connected directly to the ductwork.

The maximum air flow of the LWZ161 is 225 m³/h. . Section F of the Building Regs states that when using Whole House Heat Recovery Ventilation (see Alternative Approaches - BRE Digest 398), the necessary ventilation rates will be satisfied if an overall air change rate for the dwelling of between 0.5 and 0.7 changes per hour. LWZ 161 will therefore service a house of volume 315 m³ at 0.7 air change per hour rate. Or a house of 450m³ at the 0.5 m³/hr rate.
The fans within the unit are extremely quiet, and can be programmed to give the required air flows to suit the size of house or office.

It is important to make the house as air tight as possible when using heat recovery ventilation. Care must be taken to ensure that sealed stoves (wood burners etc) get the required air supply. Houses with open fires could not use this system.