SUNDRY ASPECTS OF HEAT RECOVERY VENTILATION In my earlier articles I have described the basics of a Heat Recovery Ventilation system: The HRV unit, where the exhaust air warms the fresh supply air (or possibly cools it in summertime). And: The ductwork and air valves through which the supply and exhaust air streams are conveyed. To wrap up the series up, here are some diverse observations. Maintenance An HRV system needs some regular maintenance, in particular, filters of the HRV unit need to be replaced or cleaned (washed or vacuumed), say two or three times a year likewise, any filters at extract air valves. The grease filter and the charcoal filter of a recirculating kitchen hood will probably need more frequent attention depending on your cooking habits. Every few years it may be prudent to clean the ducting. There are commercial companies that offer this service. For a DIY approach: Blow a light thread through the duct. Use the thread to pull a strong cord through. Use the cord to pull a 'pig' through. 'Pig' is part of traditional pipework terminology. In the present context, the pig may simply be a furry ball, either home-made or purchased. Some semi-rigid ducting has an anti-bacterial inner surface, and cleaning of this is said to be unnecessary. Summer bypass In the UK, night-time temperatures are usually below 20 C even in mid-summer. (Following the Passive House approach, it is desirable to keep internal temperatures within the range 20 C 25 C.) So on summer nights, the ventilation system can be used to help cool the house, if that is required. Heat recovery is not wanted, and the heat exchanger is bypassed. Nowadays, the operation of the bypass is under automatic control, dependent on the temperatures of the fresh and stale air streams, and on a target temperature (say 22 C) set by the householder. There are two situations in which the heat exchanger is bypassed: For cooling (in summertime): If: The interior temperature, as given by the temperature of the extract air, is warmer than 22 C, AND The temperature of the intake air is below 22 C, SUNDRY ASPECTS OF HRV 1 (JANUARY 2018.)
Then bypass occurs and the fresh air cools the interior. For warming (in spring or autumn): If: The interior temperature, as given by the temperature of the extract air, is cooler than 22 C, AND The temperature of the intake air is above 22 C, Then bypass occurs and the fresh air warms the interior. But note that in sustained hot weather, the night-time cooling supplied by the HRV system is unlikely to be enough to reduce the interior temperature of the house sufficiently. In which case, it could be advantageous to have some windows open during the night or early morning this can greatly increase the amount of cooling of the fabric in readiness for a hot day ahead. Be wary: The design of some HRV units results in the summer bypass also bypassing the filter. The bypass is most likely to come into use in the summer, when the pollen count is high and the filter is most needed! Summer Mode Some HRV units can be switched to operate in a 'summer mode'. The supply fan is switched off; the extract system still operates. Supply air is drawn in through open windows and/or trickle vents. If you intend to use this method, then fit windows with trickle vents. Power losses from the use of electric fans are halved. (In wintertime such power losses might contribute to space heating, but in summertime this is unwanted.) Purge ventilation HRV does not satisfy all ventilation needs. The Building Regulations also require a means of purge ventilation. What that means in practice is the facility to open a window in almost every room or perhaps two windows in a large room. Purge ventilation might be wanted after re-decorating, toast burning, flooding, etc, or for cooling in summertime. On an upper floor, some opening windows are also required as a means of escape in case of fire. Cat flaps Can a house with a cat flap be airtight? 'Cat doors' and 'dog doors' that use double flaps and magnets are made in the USA and available on the web. (See 'Further Info'.) Wood stoves with HRV If a small wood stove is to be fitted in the house, it should not use room air for combustion. It should be 'room sealed' of a type that allows the combustion air to be piped in from outside. (Many people who fit a small wood stove into a Passive SUNDRY ASPECTS OF HRV 2 (JANUARY 2018.)
House find that they never use the stove in practice.) The cooking hood conundrum In the early days of domestic HRV, the extract in the kitchen was often via a cooker hood. Generally this is now considered to be bad practice, not least because the ductwork can become greasy. (But see the next section.) The NHBC recommend that the extract air valve should be at least a couple of metres from the cooker (measured on plan). Over the kitchen sink is often a good position. The usual recommendation nowadays is to fit a recirculating cooker hood that has a filter to capture grease and charcoal to reduce smells or should it be 'aromas' in a kitchen? The charcoal filter needs to be replaced every few months. (Unfortunately, a recirculating hood does not remove steam.) A multitude of designs are possible. 'Hood' no longer seems an apt description some look more like fancy light fittings. Cooker hood! ('Vintage' pendant hood. Source: Best Hoods.) But some householders don't find a recirculating cooker hood to be effective enough. So to remove cooking smells, steam, etc faster they might fit an extract cooker hood ducted directly to outside. Unfortunately, such a cooker hood tends to interfere with the HRV system. The solution according to one source (bpc ventilation) is to use the cooker hood as the HRV extract outlet for the kitchen despite the comment above about this being bad practice. The hood must have a filter to clean the air, and a Boost switch linked to the HRV unit. Steamy bathrooms Some householders find that the HRV extract rates in wet rooms are insufficient. Eg, after a shower the bathroom mirror remains steamed up for too long. A simple extract fan would have been more powerful and cleared the air faster. (Approved Document F requires the minimum extract rate for a local intermittent fan to be nearly twice as large as that for HRV see the table.) SUNDRY ASPECTS OF HRV 3 (JANUARY 2018.)
Continuous extract on Boost Litres/sec Intermittent extract Litres/sec Kitchen 13 30 over hob 60 elsewhere Utility room 8 30 Bathroom 8 15 WC 6 6 Minimum extract ventilation rates for continuous HRV and for Intermittent Extract fans (Approved Document F of Building Regulations) A simple way to avoid a steamed-up mirror might be to fit a heated mirror. More fundamentally, the HRV system could be set up to have a boosted extract rate that is well above the minimum required by Part F. It will be even better when the technology has improved and HRV systems become smarter as described last month. For example, when it is only the bathroom that requires boosted extraction, it would be only the bathroom that gets it, and not also all the other wet rooms. Focussed solely on the bathroom, the boosted extract rate for the bathroom could be set much higher. Cascading ventilation systems For completeness it may be worth mentioning cascading ventilation systems. In a typical case, each bedroom has its own supply valve, but the other habitable rooms do not. The supply air 'cascades' via the hallway and doorways from the bedrooms into the living room and other habitable rooms,. Less ducting is required, and the HRV unit can deliver a lower airflow. Such a stem is likely to be of more use in a small apartment than a house, and more useful for retrofits than new build. Single Room Heat Recovery Ventilators As an alternative to a centralised HRV system, installing a number of single room heat recovery ventilators may be worth considering. These individual ventilators give heat recovery ventilation without any ductwork (except for the short duct through an external wall). They can be used in place of a traditional extract fan; these ventilators have two fans, one for extract, one for supply. The heat recovery efficiency is 75% 90%. They are sold primarily for use in wet rooms (including the kitchen), but they may be useful for habitable rooms, too. They usually have two speeds, Boost and Trickle (ie, normal). HeatSava ventilator SUNDRY ASPECTS OF HRV 4 (JANUARY 2018.)
These ventilators are likely to be of more use in a renovation (where installing ductwork is difficult) than in a new build. Two HRV technologies little used in the UK There are a couple of technologies which have been promoted in the past by the Passive House Institute but which have never become popular in the UK, with its relatively mild climate: Earth tube heat exchangers. Compact units (or 'Exhaust Air Heat Pumps'). Intrepid selfbuilders might like to investigate further, so here are brief descriptions of the two technologies: 1. Earth tube heat exchangers The temperature underground is much more stable than that of the air above. A metre and a half underground, the temperature is likely to stay at about 10 C all year. A Passive House technique has been to draw in the supply air through an underground pipe some tens of metres in length it is sometimes called a sub-soil heat exchanger. In wintertime, the supply air will be pre-warmed, and in summertime it will be cooled. The theory sounds good. Underground pipework for supply air (Source: Rehau.) But there are two possible drawbacks (beside the extra installation cost): Drawing air through a long length of pipework requires energy. The fan has to work harder. In some circumstances, condensation may occur within the underground pipe, and this could lead to microbial growths. That might theoretically lead to biological contamination of the supply air, though studies of actual systems have not shown this. (It is possible to obtain special pipes that have a bactericide coating of silver ions to deter fungal/mould growths.) SUNDRY ASPECTS OF HRV 5 (JANUARY 2018.)
There is another way to use the steady temperatures underground, which may be of interest to technophiles. Glycol is pumped by a small pump through a ground loop to extract warmth from the ground (or in summer, to extract 'coolth'). The pipework is connected to a special pre-heater where the intake air is pre-warmed (or in summer, pre-cooled); the intake air is then ducted into the HRV unit. 2. Compact units Exhaust Air Heat Pumps A 'compact unit' to use Passive House terminology incorporates both an HRV unit and a heat pump. The HRV unit may be very energy efficient, say 85% 90% efficient, but that still leaves some 10% 15% of the heat in the exhaust air going to waste. A compact unit extracts extra heat from the exhaust air before it is blown away outside (possibly even at a lower temperature than the outside air). The recovered heat can be used for heating Domestic Hot Water or for space heating (as in the diagram). Exhaust air heat pump (In the summertime, the heat pump can be used to cool the supply air.) (Source: Genvex.) The theory sounds good, but in the UK it is probably a case of too much technology chasing too little heat. FOOTNOTE 1 (for techies): Energy Recovery Ventilation 'Enthalpy' is a measure of the total energy in a gas both the 'sensible' heat energy (due to its temperature) and the considerable 'latent' heat (available if the gas eg, water vapour condenses). In a conventional heat exchanger, only the sensible heat is exchanged between the intake air and the exhaust air. In an enthalpy exchanger, both heat and moisture are exchanged, leading to ERV Energy (or Enthalpy) Recovery Ventilation. But this technology is of little use in the British climate. FOOTNOTE 2: Air changes per hour, ach Question: If a room has a ventilation rate of 1 ach, how long will it take to change all the air in the room? Answer: For ever. SUNDRY ASPECTS OF HRV 6 (JANUARY 2018.)
That answer might surprise readers, but it is mathematically correct if the fresh air mixes in the room with the stale air (as is normally the case). So extracted air becomes a mixture of stale air and recently introduced fresh air. Only if the fresh air replaces the stale air (so extracted air consists only of the original stale air) would all the air in the room be changed in one hour. If the fresh air and stale air mix as is likely in practice it can be shown mathematically that after one hour only 62% of the air in the room will be fresh air (introduced during the past hour). 'Air changes per hour' are a convenient unit for measuring air flows, but don't be mislead by the terminology into making false inferences. FURTHER INFO: The PH+ Guide to Heat Recovery Ventilation Excellent article in Passive House+ magazine, Issue 23. Ebuild Forum A forum for selfbuilders. Unfortunately, in 2016 it closed as an active forum as it had become too much work for its philanthropic proprietor. Currently, past posts are still available. View it while you can! There are no less than 109 MVHR topics. www.ebuild.co.uk. Buildhub Forum A successor to Ebuild created by former users of Ebuild. Currently there are 95 topics in the MVHR forum and many other forums of interest. www.buildhub.org.uk. Freedom Pet Pass Energy efficient cat and dog doors from the USA. www.energyefficientdogdoors.com. BEST Hoods Cooker hoods. www.besthoods.co.uk. The cooker hood conundrum Web page of Bpc Ventilation. www.bpcventilation.com. Fresh-r Single room HRV. www.fresh-r.eu. Genvex / Total Home Environment Ltd HRV units with integral heat pumps for space heating or water heating. www.genvex.co.uk. 2311 Copyright article by Robert Matthews, January, 2018. SUNDRY ASPECTS OF HRV 7 (JANUARY 2018.)