EXHAUST AIR HEAT PUMPS Mini heat pumps

EXHAUST AIR HEAT PUMPS Mini heat pumps Last month, we looked at heat pumps which extract their heat from the ambient air outside the house Air Source Heat Pumps. Another source of heat may possibly be exhaust air being expelled from a house by a mechanical ventilation system. Heat can be extracted from this by an Exhaust Air Heat Pump (EAHP). The amount of heat available is small, limited by the amount of stale air that is to be expelled. However, it may be more than sufficient for heating the Domestic Hot Water. It may even be enough to contribute significantly to the space heating of an ultra insulated and airtight home. At present, some readers may be unfamiliar with Exhaust Air Heat Pumps. In Sweden, the country which has been leading the way in the development of heat pumps, 90% of new houses have a heat pump and the most popular type of pump is said to be the EAHP. (Sweden gets most of its electricity from hydro sources, so heat pumps are a very carbon-light way of heating in that country.) With the coming requirements for our new houses to be ultra energy efficient, EAHP s will surely become more popular in the UK. But how much heat is available? In my article about ventilation in the February 2007 issue of SelfBuild & Design, I showed that if 8 litres of fresh air is required for one person every second (nearly 30 m 3 per hour), and if the temperature of the fresh air is to be raised from 0ºC to 20ºC, then the power required is nearly 200 watts. Conversely, for the same ventilation rate, if an EAHP is reducing the temperature of the exhaust air from 20ºC to 0ºC, then heat is released at the rate of 200 watts (ignoring any heat released by condensation). So in a family home with five people, heat can potentially be recovered at the rate of about 1 kw not a lot, but useful all the same. Continental manufacturers have been very ingenious in finding ways of utilising the heat made available by Exhaust Air Heat Pumps. Some describe their appliances as mini heat pumps, and this seems an appropriate description in view of the small output. Anyway, as outlined below, there is a wide, even bewildering, variety of applications. It will probably take a few years before one or other of them becomes the mainstream favourite. Until then, selfbuilders who are ready to be eco-pioneers will have to find their own way. MEV and HRV Approved Document F of the Building Regulations suggests two forms of centralised mechanical ventilation: Mechanical Extract Ventilation MEV Stale air from wet rooms is ducted to a single extract fan and expelled from the house. Heat Recovery Ventilation HRV Also known as Mechanical Ventilation with Heat Recovery (MVHR). As above, stale air is extracted centrally via ducts from wet rooms. Then the exhaust air passes through a heat exchanger where it gives up heat to a stream of incoming fresh air. The pre-warmed, fresh air is then ducted to the dry rooms. EXHAUST AIR HEAT PUMPS 1 MARCH 2010.

(See my articles in SelfBuild & Design, January to July, 2007 for more about ventilation systems.) There are appliances of both types (MEV and HRV) which incorporate heat pumps. MEV with Exhaust Air Heat Pump The exhaust air is at room temperature, typically 20ºC advantageously warmer than the sources of other heat pumps. Moreover, the air is likely to be fairly humid, so when the air is cooled water vapour condenses, releasing its latent heat. Because the amount of heat available is small, many appliances combining MEV with an EAHP limit themselves to heating DHW only. An example is the Vanvex ( 3K, or less), by the Danish company, Genvex. Combined in one unit is the extract fan, the heat pump, and the DHW cylinder. The heat pump extracts heat from the exhaust air and delivers it into the DHW cylinder. (The heat pump also uses outside air as a source of heat when required; and, in fact, most of the heat supplied may come from the outside air.) The heat output claimed is modest, about 1.3 kw, and the Coefficient of Performance (COP) is about 3.2 for a water temperature of 55ºC. The big Swedish manufacturer of heat pumps, Nibe, produce the Fighter 360P (about 4K) with provision for space heating as well as DHW. Instead of the single DHW cylinder of the Vanvex, this appliance has a double cylinder, the inner one for DHW and the outer one for space heating. Unlike the Vanvex, the 360P doesn t use outdoor air as an extra heat source. Rather, it has an in-built electric immersion heater. Because the amount of heat available from exhaust air is so limited, I suspect that much of the heat for space heating in midwinter will be derived from the immersion heater. Anyway, an annual COP of 2.6 is claimed. MEV with EAHP/ASHP and a gas boiler The Dutch company, Daalderop, offer the CombinAir, a unique product which integrates an MEV system, an air-to-water heat pump, and a gas boiler. You can have the reassurance of a familiar and flexible heating technology (a 24kW condensing gas boiler) combined with extract ventilation and a heat pump. The Dutch were the pioneers of condensing boilers, and Daalderop boast that this boiler is 107% efficient. (If you are wondering how a boiler can be more than 100% efficient, see my article about the efficiency of wood fuels last July.) Overall, the combination is 40% more efficient than a standard Sedbuck A rated condensing boiler. Price about 2,700. Like the Vanvex, the heat pump of the CombinAir takes its heat from both exhaust air (about 20%) and outdoor air (about 80%), and the combined COP is about 3.2. A side benefit of the combination is that radiators are viable, as the gas boiler can supply the water at the high temperature (70ºC 80ºC) required for radiators. The company suggest UFH on the ground floor and radiators on the upper floor, a combination popular with selfbuilders. Less popular might be their suggestion of installing the CombinAir in the loft, though this is a common location for gas boilers in Holland. EXHAUST AIR HEAT PUMPS 2 MARCH 2010.

MEV with EAHP interacting with a GSHP And now for something else completely different. Nibe s exhaust air module, the FLM30, is designed to integrate with their Ground Source Heat Pumps. Essentially the module is an MEV unit for extracting stale air, combined with a heat pump which takes heat from the extracted air. In wintertime, the heat is used to warm the brine flowing from the ground loop to the GSHP. Thereby, the temperature lift for the GSHP is reduced, making the pump more efficient. How effective is this combination? If the FLM30 is extracting air (temperature 20ºC) at the rate of 200 cubic metres per hour, and transferring the heat to brine at 5ºC, then Nibe claim that heat is transferred at the rate of 1 kw a useful bonus. In summertime, the ground loop might be used for passive cooling while the heat pump itself is not in use. ( Passive cooling will be covered in a future article.) In this case, the heat from the EAHP can be transferred to the brine flowing to the ground loop. The warmed brine helps to raise the temperature of the ground to supercharge the ground ready for winter. HRV with Exhaust Air Heat Pump Heat Recovery Ventilation will become increasingly common as new housing is built to ever higher energy standards. Indeed, HRV appliances are standard in Passive Houses. However, notwithstanding the superior insulation and airtightness of Passive Houses, in midwinter they may well need some supplementary heating. One way to do this is to post-heat the incoming fresh air. The heat exchanger of the HRV unit pre-warms the fresh air nearly to room temperature, and the fresh air is then post-warmed to, say, 35ºC 'post' because the warming takes place down-stream of the HRV unit. This warm air is distributed around the house by the HRV system a simple and effective method of central heating, albeit limited in power. (For a different reason, air may also be 'pre-warmed'.) One way of post-heating is to utilise an Exhaust Air Heat Pump. The EAHP can either supplement or supplant the heat exchanger of the HRV system. An example where the EAHP supplements the HRV heat exchanger is Genvex s Premium ventilation unit ( 3K+): As is usual in an HRV unit, the heat exchanger reduces the temperature of the exhaust air nearly to the temperature of the ambient air outside. The heat pump then extracts further heat from the exhaust air, so the temperature of the air expelled is about 5ºC. (And as described above, the heat extracted by the heat pump is used to post-heat the supply air.) Some appliances offer more: the heat pump not only post-heats the supply air, it also heats the DHW in an integral cylinder. Example: Genvex s Combi 185. (The heat pump heats the hot water only to 45ºC.) An example where the EAHP supplants the HRV heat exchanger is Nibe s Fighter 410P. Like the Combi 185, this post-heats supply air and heats DHW. But as in the Fighter 360P (mentioned previously), all the transfer of heat is achieved by the heat pump. The 410P is basically a 360P with the extra facility of a supply of warmed fresh air, so transforming an MEV system into an HRV system. But there is no heat exchanger as found in most HRV units. Instead, the warming of the supply air is EXHAUST AIR HEAT PUMPS 3 MARCH 2010.

achieved by blowing the fresh air past a hot water coil the hot water for the coil coming from the space heating water in the outer cylinder. My scepticism mentioned earlier about the performance of the heat pump of the Fighter 360P applies here, too. The abilities claimed for the 410P seem to be too good to be true. Merely from the expelled air, cooled down to 0ºC, it is said that the supply air is post-heated, DHW is heated, and there is hot water for UFH or radiators, too! In fact, the unit has an integral 6 kw immersion heater which switches on only when required. My suspicion is that in midwinter it will be required frequently! Nonetheless, provided the limitation of this versatile appliance are recognised, it may still be very appropriate for an ultra low energy home. As an aside, I ll mention that many heat pumps, of all types, have integral electric heaters for back-up. So it is not always clear how much of the heat output is from resistance heating (inefficient) and how much from the heat pump (efficient). In short There is a bewildering variety of ways in which a mini heat pump can be used to extract heat from exhaust air. Because the amount of heat available is small, an Exhaust Air Heat Pump usually has to be used in conjunction with another heat source. Next month: Water Source Heat Pumps. FURTHER INFO: Drexel und Weiss Aerosmart Compact Units. English pages of: www.drexel-weiss.de. Genvex Vanvex, Premium and Combi 185. www.genvex.co.uk. ICS Heat Pumps Suppliers of delonghi Thermacell EAHP for DHW. www.icsheatpumps.co.uk. Isoenergy Suppliers of the Danish made, Nilan VP18 all-in-one unit, which can be integrated with a GSHP or ASHP. www.isoenergy.co.uk. Nibe Fighter 200P, Fighter 360P, Fighter 410P, Module FLM30. www.nibe.co.uk. Stiebel Eltron Module LWM 250. www.stiebel-eltron.co.uk. Words: 1880. EXHAUST AIR HEAT PUMPS 4 MARCH 2010.