VENTILATION AND HEAT RECOVERY

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1 VENTILATION AND HEAT RECOVERY In last month s article we looked at the energy cost of ventilation. The cost of the heat lost when stale air is extracted is much greater than any energy used by fans. But by using a heat exchanger, most of the heat that would otherwise be lost can be saved. Heat exchangers are incorporated into: House ventilation systems. Room ventilators. House ventilation systems The Building Regulations Approved Document F, Means of Ventilation, details four systems of ventilation for dwellings. System 4 is continuous Mechanical Ventilation with Heat Recovery, MVHR. (This mouthful is gradually being superseded by Heat Recovery Ventilation, HRV.) In these systems, air is extracted from the wet rooms (kitchen, bathroom, etc) and sucked through ducts to a heat exchanger. This is usually located in the loft or a cupboard. (Easy access is desirable for periodic maintenance, in particular for washing or changing the filters.) In the heat exchanger, much of the heat in the warm, extracted air is transferred to the incoming fresh air to pre-warm it, without any mixing of the air streams. The tempered fresh air is driven by a second fan to the dry rooms (living rooms, bedrooms). The system should be designed so that the airflows to these different outlets are appropriately balanced. There are three types of heat exchanger: Cross flow The two air streams flow at right angles to each other. In general, this sort of heat exchanger is about 70% efficient. Rotating discs This method is not uncommon in very large installations, and it has now been incorporated into some domestic heat exchangers. Metal discs rotate slowly through both air streams thereby transferring heat from one air stream to the other. In the outgoing air stream, water vapour tends to condense on the cold discs. The water droplets are carried on the discs into the fresh air stream where they evaporate. So these heat exchangers don t need a condensate drain, unlike other types. More significantly, the humidity in the fresh air stream is increased. But is this desirable? Manufacturers of the other types of heat exchanger claim that the low relative humidity of their incoming air stream is beneficial. (We ll have a look at the subject of humidity next month.) Disc exchangers are reckoned to be about 80% efficient. Counter flow The two air streams flow in opposite directions. This is the most efficient method, with efficiencies of about 90%. VENTILATION AND HEAT RECOVERY 1 MARCH 2007.

2 When warm, moist air is cooled in any of these heat exchangers, water may condense out, giving up its latent heat. Much of this heat gets transferred to the incoming air, increasing energy efficiency. In very cold weather, the outgoing air might be cooled to below zero, which could cause a frosting problem. More sophisticated units have a defrosting mechanism. Most of the exchangers have three speed settings for the fans, for example using 40, 80 or 160 Watts. Normally, the middle setting is used, with an occasional boost as required either manually or automatically by a sensor in a wet room. Note that this boost affects the whole system, not just the local difficulty. To find very energy efficient systems look at the Appendix Q website for making SAP assessments. (See Further Info). Although this site is aimed at professionals, it gives useful information to anyone intending to purchase a house ventilation system. Manufacturers of the most efficient systems will want their systems to be listed here. On the site, you can check out the efficiencies of their systems, as derived from an impartial test. And you can download a checklist with which you can later check the installation of your system. In an airtight house (q50 lower than 3 m 3 /hr per m 2 ), a house ventilation system is likely to save more energy than it consumes if: Efficiency of heat exchanger is 85%, or better. Fans are very efficient The specific fan power is less than 1 Watt per litre/sec, say. To meet the specification of the PassivHaus standard (see Further Info), house ventilation with at least 80% heat recovery is required. The PassivHaus specification also requires an extremely high standard of airtightness a q50 value lower than 1 m 3 /hr per m 2. Furthermore, the specification requires the incoming air to be pulled through underground pipework to extract heat from the ground. A more powerful fan (consuming more energy) is required to pull the air through, but the Coefficient of Performance (heat gained/energy used) is about 8, so it is worthwhile. (Fellow columnist, David Olivier, laid the pipework drain pipes in the foundation trenches of his selfbuild.) Heat exchange plus a heat pump It is possible to incorporate a small heat pump into a heat exchanger. This can extract extra heat from the exhaust air by cooling it below the temperature of the incoming air. The extra heat is used to further warm the incoming air, to above the temperature of the outgoing air. So the efficiency of this unit can be more than 100%, and it can help to warm the house. As a concept, this is a very good use of heat pumps. Temperature rises are small (cf, a heat pump for central heating) and for small temperature rises heat pumps have their highest COP (Coefficient of Performance). Such units have been available for a decade or two in Europe, but they are hardly available in the UK. Perhaps their extra expense and complexity has so far made them unviable. By the way, under development at Nottingham University is a novel apparatus for the domestic market that combines the heat pump, heat recovery and fan functions VENTILATION AND HEAT RECOVERY 2 MARCH 2007.

3 into one. If development is successful, MVHR could move out of its niche market into the mainstream. Kitchen ventilation Some installers of house ventilation systems recommend that a separate extractor fan is fitted in the kitchen, over the hob, for fast extraction when required. Perhaps more importantly, extracting kitchen air through a heat exchanger tends to result in grease clogging up the surfaces of the heat exchanger. To counteract this, some heat exchangers are protected by a filter in the extract duct. Ductwork For the maximum air flow with the minimum fan power the ductwork should be as short as possible, should have has few bends as possible, and the surface should be smooth rather than corrugated. Ductwork in the loft should be insulated. It may be appropriate here to mention that in Sweden where MVHR is more widespread a research project has led to some concern about contamination of the incoming air by dirt in the ductwork. There seems to be a need for further research. Room ventilators As we saw last month, extract fans throw away heat eg, on a freezing day a 30 litre/sec fan is removing heat at the rate of 730 watts. This waste can be more than halved by using a room ventilator with heat recovery instead of an extract fan. A room ventilator contains a mini heat exchanger so it can suck out stale air from one side of the room and blow in tempered air to the other. Efficiencies vary from 50% to 85%. Whether or not a room ventilator is worth the extra expense will depend on how much ventilation is required in practice. (To comply with the regulations I had to fit an extract fan in my utility room. The fan has never been used the days of top-loading washing machines and mangles have long gone, and I suspect that most utility rooms, nowadays, don t get humid.) Unfortunately, Approved Document F requires a room ventilator to be run continuously if it is used in place of an extract fan in a wet room. Surely this unnecessary discouragement to their installation ought to be changed? NO HEAT RECOVERY At present, most of the ventilations systems installed in new dwellings don t incorporate heat recovery, so it s worth mentioning a few points about the simple extraction methods. Extract fans Basically there are two types of fan in use: the axial fan (which is rather like a propeller) and the centrifugal fan (which works by throwing the air out centrifugally and is a more boxy apparatus). The centrifugal fan creates more air pressure, so this fan should be used with ductwork over a metre long, or in windy locations. Some fans are powered by DC motors (Direct Current). These are more energy efficient than AC motors and last longer. VENTILATION AND HEAT RECOVERY 3 MARCH 2007.

4 On the ground storey, it may be preferable to fit an in-line fan in the ceiling void above, with a duct from a ceiling grill and a duct to a grill on the outside wall if the joists allow it. Or, on an upper storey, fit the in-line fan in the loft, with a duct to a soffit grill. Light tubes Sometimes a room such as an ensuite is lit by a light tube, rather than a window. For such a situation, light tubes are available which incorporate an extract fan. There is even one model with the fan powered by a battery charged by a small PV panel. Other light tubes incorporate PSV (Passive Stack Ventilation) even simpler. While we are on the subject of light tubes, Sun Pipe in the USA might be worth a mention. They don t incorporate ventilation into their tubes, but they do claim their tubes are the best in the world they are silvered for high light reflectance. Moreover, they will despatch from their factory in the USA to the UK, absorbing most or all of the delivery cost themselves and their prices are substantially lower than those for light tubes marketed in the UK. (The subject of purchasing materials from abroad, either at a lower price and/or a higher quality, was covered by Lynne Oakerby in the August 2004 issue of SelfBuild & Design.) Central extraction With the increasing number of ensuites in modern homes, an increasing number of extractor fans need to be fitted. The overall capital cost might be lower if, instead of fitting numerous extractor fans, one central fan unit is fitted, in the loft say, with ductwork to the wet rooms. (This is System 3 in Approved Document F continuous Mechanical Extract Ventilation, MEV.) Some trickle vents are required, but less than for a system with intermittent extract fans. These systems are supposed to run continuously, so the running cost of such a system will be higher than that of a system based on intermittent extract fans. Typically, the fan consumes 30 Watts normally, and 100 Watts in boost mode. Trickle vents are not required in the wet rooms, and if you don t build very airtight they may not be required anywhere. FURTHER INFO: SAP2005 Appendix Q This website contains info for incorporating new technologies into SAP ratings. At present the only new technology covered is Ventilation. The house ventilation systems listed at the time of writing are: Greenwood Fusion HRV1 and MVHR 90 Vent Axia HRE 350 Xpelair Xcell 270 (91% efficient) PassivHaus UK Energy Saving Trust Good Practice Guides: Energy efficient ventilation in dwellings a guide for specifiers. (GPG268.) VENTILATION AND HEAT RECOVERY 4 MARCH 2007.

5 Improving airtightness in dwellings. (GPG224) Free downloads from their website: Office of Energy Efficiency This is a department of the Canadian Government, and their sub-website gives a lot of info about Heat Recovery Ventilators (HRV) and other energy efficient forms of heating. (On the site you can also find a DIY method of measuring incoming airflow at ventilator grills.) Sun Pipe Company Inc Monodraught Manufacturers of Sola-Vent, a light tube combined with PV powered extraction. (No connection with Sun Pipe, above.) Words: Copyright article by Robert Matthews in SelfBuild & Design magazine, March, VENTILATION AND HEAT RECOVERY 5 MARCH 2007.