HEAT RECOVERY VENTILATION For healthy, energy efficient ventilation Heat Recovery Ventilation (HRV) is an integral part of the Passive House approach to achieving ultra-low energy houses. In the UK, HRV is often known as Mechanical Ventilation with Heat Recovery (MVHR), but that is too much of a mouthful, and it won't be used again in this article. The common method of ventilating new houses is by natural ventilation (with trickle vents in windows) augmented by intermittent extract fans,. Like me, you might be attracted to what is 'natural', but leave this inferior method of ventilation to the mass house builders. Selfbuilders do better. 'Build Tight Ventilate Right' is a much quoted slogan. Many well informed people think that the best way to Ventilate Right is to install a Heat Recovery Ventilation system. The need for fresh air Most people appreciate that 'fresh air' is good for us. But indoors, there are many sources of pollution: Moisture from cooking and washing. Carbon monoxide and nitrogen dioxide from cooking with gas. Carbon dioxide from people, pets, and cooking with gas. Volatile Organic Compounds from aerosols, paints, and some furniture. Odours from cooking, people and pets. Allergens eg, from house dust mites. Tobacco smoke. Ventilation rates required to control pollutants If the ventilation rate is sufficient to control moisture build-up, the other pollutants are also controlled. (Source: 'Energy Efficient Ventilation in Dwellings', by the Energy Saving Trust.) Excessive moisture in the air can lead to condensation, mould growth on cold surfaces, and mould spores in the air. In a 2006 publication, the Energy Saving Trust HEAT RECOVERY VENTILATION 1 (JANUARY 2018.)
suggested that to control moisture (and thereby control other pollutants) a ventilation rate of 0.5 to 1.5 ach (air changes per hour) is required. (See Further Info.) But nowadays, in the quest to conserve energy, 0.5 ach is considered by many to be at the top end of the range for optimum ventilation. (A typical ventilation rate for a Passive House is about 0.4 ach.) By the way, you could eliminate one of the pollutants in the above chart by cooking with electricity, not gas. The advantages of HRV The traditional form of ventilation is 'natural' ventilation augmented by extract fans in the kitchen, bathroom, etc. Most of the time, the replacement of stale air by fresh air depends on the airtightness of the building envelope the latter depends on both the incidental 'leakiness' of the fabric and the deliberate usage of trickle vents and open windows. The ventilation rate depends also on the vagaries of the weather: On the wind Which creates a pressure differential between the windward and lee sides of the house. On the temperature difference between inside and out Which usually creates a stack pressure due to warm air rising. In short, natural ventilation is likely to be haphazard. (Moreover, in many households I suspect that trickle vents are closed through much of the winter.) In contrast, an HRV system allows ventilation to be controlled. Provided the house is airtight, the ventilation is independent of the weather. Used air (brown) is continuously being removed from the rooms with high levels of pollution and humidity. Fresh air (light green) is supplied to the living areas. There is airflow throughout the house. The system shown here uses rigid ducting more about ducting in a later article. Note: The diagram seems to show the fan unit installed in a heated attic. It is a poor idea to install the fan unit in an unheated, inaccessible loft. An HRV system HEAT RECOVERY VENTILATION 2 (JANUARY 2018.)
Besides control, an HRV system has another substantial advantage, one implied by its name: 'heat recovery'. With natural ventilation there is no heat recovery when the cold, fresh air replaces the warm, stale air, the heat of the stale air is lost. I explored this topic in an article, 'The Cost of Fresh Air' (January 2012 issue). Suffice to say that, for a house with a modest floor area of 140 m 2, on a cold day with a temperature difference of 20 C and a ventilation rate of 0.33 ach, heat is lost at the rate of 680 watts. That ventilation rate is considered acceptable by the Passive House Institute, but it is at the low end of what is generally recommended. As we saw earlier, the Energy Saving Trust has recommended 0.5 1.5 ach. At 1 ach, for example, the rate of loss of heat from this house due to natural ventilation on this cold day would be about 2 kw a very appreciable amount. The ventilation heat loss can be slashed by installing an HRV system. In its heat exchanger, most of the heat of the exhaust air is transferred to the incoming fresh air (without any mixing of the two air streams). Although the fan unit continuously uses electricity (at perhaps 50 watts), over the course of a year, the energy saved by heat recovery is greater. (See Footnote 1.) HRV is an energy efficient form of ventilation. Anyway, the electrical energy used by the fans is converted into heat, and, if the system is within the thermal envelope, that heat is useful in wintertime (albeit not in summer). A report 'Characteristics and performance of MVHR systems' shows that in money terms (with heating by gas) after the cost of filters has been taken into account the cost of running a MVHR system over a year roughly equals the value of the heat recovered. (The chart is probably over-pessimistic about the economics of MVHR. For a start, the analysis assumed that the filters cost 100 a year, which is now likely to be too high.) Ventilation costs with and without MVHR (Source: Characteristics and performance of MVHR systems.) An HRV system has another advantage: the fresh, incoming air is filtered. For a house in an urban setting, the filter might remove dirt due to traffic fumes, etc. For a house in the countryside, pollen might be removed, to the benefit of occupants who are allergic to it. Flying insects are kept out, too. HEAT RECOVERY VENTILATION 3 (JANUARY 2018.)
Two other, small advantages: Build Tight Windows don't need trickle vents. External noise doesn't enter wet rooms via the ducting of extract fans. A house that is to have an HRV ventilation system needs to be very airtight. Part L of the building regulations requires the airtightness of a house to be measured at the end of the build by a 'blower door' test. (See my May 2012 article, 'Testing Airtightness'. Also see Footnote 2, 'Measures of Airtightness'.) But for HRV ventilation to make sense, the airtightness should be much better than Part L's minimum standard. So a permeability, q50, of 1 cubic metre of air per hour per square metre of the envelope area would be good. If q50 is worse than 3 m 3 / h.m 2, installing HRV would be a waste of money. Part F and many diverse methods of ventilation In the Approved Document, 'Ventilation', for Part F of the building regulations, detailed requirements are given for four ways of ventilating a new dwelling: Background ventilators and intermittent extract fans System 1. The background ventilators are usually trickle vents in windows. This is the most widely used systems, especially by spec house builders, though in my view it is obsolescent. (Ventilation is weather dependent and wastes heat see above.) Passive Stack Ventilation System 2. For extract: Vents in wet rooms are ducted to terminals on the roof air is drawn up the vents by the stack effect and by wind. Intake of air: Through trickle ventilators, and the leakiness of the building fabric. Continuous Mechanical Extract Ventilation (MEV) System 3. A fan unit, often located in the loft, extracts air from wet rooms via ductwork. The fan operates at normal speed most of the time, with an optional 'Boost' when required, eg, when a shower is used. Intake of air is via trickle vents in habitable rooms and the leakiness of the fabric. Note that some HRV units have an optional 'Summer Mode' that gives MEV rather than HRV, for use in summertime the supply fan in the HRV unit is switched off. (If 'Summer Mode' might be used, windows require trickle vents.) Continuous Mechanical supply and extract with Heat Recovery (MVHR) System 4. This method is also known as Heat Recovery Ventilation (HRV). It is the method favoured by many selfbuilders to give the healthiest air quality and the least waste of heat. Other methods are allowed by the Approved Document eg, Positive Input Ventilation (PIV), or even Air Supply Windows but in such a case it has to be HEAT RECOVERY VENTILATION 4 (JANUARY 2018.)
demonstrated to building control on a more individual basis that the chosen method will be satisfactory. I wrote an article about PIV in the May 2007 issue. To summarise: Positive Input Ventilation (PIV) A fan unit, often located in the loft, blows air into the dwelling, usually into a landing or hallway. Air is exhausted via trickle vents and fabric leakiness. The fan operates at a low normal speed for most the time, with Boost when required. PIV is more useful for leaky, old houses than airtight new ones. Single room ventilators Recently yet another possible ventilation option has become available: Single Room Heat Recovery Unit (SRHR) A SRHR unit is like a mini HRV unit, with separate fans for extract and supply, and a heat exchanger. It is ducted straight through an outside wall. No trickle vents are required in the room. Such a unit may be fitted into a wet or a dry room. (These units are likely to be of more use in a renovation than a new build.) For more about the diverse ventilation systems above, see the Energy Savings Trust 'Energy efficient ventilation in dwellings'. (Details below). Next month: HRV units. FOOTNOTE 1: Electrical consumption of HRV units The power consumption of a typical HRV unit is about 0.3 W/m 3.hr. Let's consider a typical detached house with a total floor area of 150 m 2, and storey heights of 2.4 m. So its volume is 360 m 3. (360 = 2.4 x 150.) If the ventilation rate is 0.5 ach, this ventilation rate can also be expressed as 180 m 3 /hr. (180 = 0.5 x 360.) So power consumption for ventilation would be about 54 watts, continuously throughout the year. (54 = 0.3 x 180.) On the Boost setting (as used to clear a steamy bathroom or kitchen, etc) the consumption is higher, but generally Boost is only used for about 2 hours a day. FOOTNOTE 2: Measures of airtightness In the building regulations, airtightness is referred to as permeability, and is measured by the quantity, q50. The '50' refers to the pressure of 50 pascal used in the blower door test that measures air leakage. The measured air flow (m 3 /h) at this test pressure is divided by the envelope area to give the permeability, q50 (m 3 / h.m 2 ). (The envelope consists of walls, windows and external doors, top ceiling and bottom floor.) The Passive House Institute, however, uses a different measure of airtightness, n50. For this, the measured airflow is divided not by the area of the envelope, but by its volume. There is no simple factor for converting q50 to n50, as the conversion depends on the particular geometry of the house. As an example consider a box-like house with a HEAT RECOVERY VENTILATION 5 (JANUARY 2018.)
simple geometry: rectangular bottom floor and top ceiling areas of 10 x 8 metres, and a total storey height of 6 metres. (The loft is not habitable.) Envelope area = 2 x (10 x 8) + 2 x 6 x (10 + 8) = 296. Volume of house = 10 x 8 x 6 = 480. then: If measured airflow = 500 m 3 at 50 pascal (as shown by a blower door test), FURTHER INFO: q50 = 500 / 296 OR n50 = 500 / 480 q50 = 1.7 m 3 / h.m 2. n50 = 1.0 h --1. Home comforts Guidance on using ventilation, heating and renewable energy systems NF68. Published by NHBC Foundation, 2016. 60 pages. Free pdf download after registering. www.nhbcfoundation.org. Mechanical Ventilation with Heat Recovery in new homes Final report Published in 2013 by the NHBC Foundation and the late Zero Carbon Hub. According to the report: 'MVHR practice must improve.' 16 page free pdf. (Web address as above.) Energy efficient ventilation in dwellings A guide for specifiers. Originally published in 2006 by the Energy Saving Trust. (GPG268.) Free 20-page pdf (2010 version) now available to download from Beama: www.beama.org.uk. Designing and implementing a robust and effective ventilation system An Insight pdf published by Encraft, consultants for low carbon buildings. www.encraft.co.uk. Words 2110. Copyright article by Robert Matthews, January, 2018. HEAT RECOVERY VENTILATION 6 (JANUARY 2018.)