Good ventilation is essential for the health and well-being of both a building and its occupants. Efficient space heating requires the building fabric to be air-tight; controlled ventilation is required.
The conventional approach is to install Mechanical Ventilation with Heat Recovery (MVHR). Retrofitting MVHR is costly, complex and overlooks the principles of thermodynamics; could a responsive, Decentralised – Mechanical Ventilation with Heat Recovery (d-MVHR) be a quicker|cheaper|better whole-house solution?
In this study, decentralised MVHR is installed in the kitchen and bathroom of a four-bedroom, three-story house, and air quality is monitored in every room. The principles of thermodynamics are observed, internal doors are operated, assisting naturally occurring convection currents, and the resulting air quality is now good throughout the home.
On this page
- Air Quality
- Internal weather
- Door operating strategies
Good ventilation is essential for the health and well-being of both a building and its occupants. Efficient space heating requires a building to be airtight. Controlled ventilation is essential.
In a successfully draught-proof house, extraction becomes less effective. A conventional extractor fan can’t push air out of a building without air being pulled in to replace it. Regardless of how powerful the extractor is, it would, in effect, be fighting against a vacuum.
In many homes, the existing kitchen and bathroom extractors are either absent or ineffective; more often, it is the vent itself that is making things worse. Even if effective, purge extraction wastes energy from cooking or showering moisture which could be recuperated to heat the home – that’s a lot of wasted energy. Could MVHR be a better retrofit solution?
Mechanical Ventilation with Heat Recovery (MVHR) balances the air pressure by simultaneously introducing fresh air while extracting stale air out. By exchanging the air, side by side, the heat can be recovered from the outgoing air and used to warm the incoming air.
However, vernacular MVHR systems do not respond to the individual needs of each room and a centrally installed system with ducting to and from each room is complex and costly to retrofit; inappropriate and invasive for a heritage home.
The conventional approach is centralised Mechanical Ventilation with Heat Recovery (c-MVHR). The system of components has a central unit usually located within a service void or attic space, from where ventilation ducting can connect to each of the rooms and then via the heat exchanger to the outside.
The technology developed as a solution installed into new-build. When retrofitted into an existing building, the installation is inevitable more costly, complicated, invasive and likely to entail irreversible damage to the original heritage fabric.
A centralised system is not equipped with sensors to detect the needs of each room and therefore can not respond quickly to problems. The system is simply running in the background to maintain a constant air change, rather than adjusting to being more effective or efficient.
Single Room sr-MVHR
The principle of heat recovery has also been applied to small, stand-alone units and has been available for some time; single room Mechanical Ventilation with Heat Recovery (sr-MVHR).
The Envirovent – Single Room MVHR unit is an affordable entry-level product. It is much more effective than a conventional extractor, ideal for the basic needs of a bathroom. When in use, the room warms up quickly and stays warm for hours after use, and the fresh air allows the bathroom to become a laundry drying space over winter.
However, the single room unit is not very smart. Running unnecessarily fast when there is a minimal temperature gradient from inside to out. It can also be intrusively noisy, resolved only by turning it off but then forgotten to be turned back on. This is perhaps not much of an issue in the bathroom but less tolerable in other more social rooms in the house.
The Envirovent sr-MVHR only monitors humidity. It, therefore, does not respond to other air quality issues. It does have a manual boost mode, which is operated via a pull cord, for when other non-humidity air quality issues occur. It tackles humidity issues in the bathroom well but not the sensors required to pick up on other air quality issues beyond the single room in which it is located.
In recent years hybrid systems have become available. Comprising of more easily installed single room units that offer a whole-house solution as a multi-room but decentralised approach; decentralised Mechanical Ventilation with Heat Recovery (d-MVHR). Without ducting, d-MVHR can provide a more affordable and less disruptive retrofit solution.
Some systems have multiple units that communicate a coordinated approach. Others simply respond quickly and efficiently to the needs of each room. Most devices only react to humidity, but some also monitor and respond to other air-quality indicators; CO² and VOCs.
Carbon Dioxide (CO²) is exhaled by people but can also be produced from baking and brewing. Elevated CO² is a good indicator of poor ventilation and can be equated to an increased risk of COVID 19. A build-up of CO² indicates an absence of fresh air and can cause drowsiness and headaches.
Volatile Organic Compounds (VOCs) are emitted by household products and materials, such as solvents, cleaning products, paints, furnishings and other building materials. The build-up of VOCs is also an indicator of poor indoor air quality. Symptoms can include nausea and breathing difficulties. Health risks vary as much as the variety of air-born compounds. VOCs can be unpleasant harmless smells. Others can cause cancer.
Could a decentralised approach be better? By tackling problems as they occur and preventing air quality issues from circulating around the house?
Is it necessary to install a decentralised unit in every room? Could good quality air be successfully recirculated to other rooms and remove the build-up of Humidity, CO² and VOCs in rooms not fitted with decentralised-MVHR?
Could a Decentralised-MVHR be a quicker|cheaper|better whole-house solution?
Prana Recuperator ERP PRO d-MVHR
The Prana Recuperator ERP PRO is a stand-alone autonomous decentralised MVHR unit with Humidity, CO2 and VOC sensors suitable for the complex requirements of a domestic residential setting. Other Prana models with fewer or no sensors are more suited to integrating into a building management system. Or when a specific reoccurring issue is being tackled, more likely in a commercial, industrial or clinical setting.
The ERP PRO is available in two diameters (210mm and 160mm) and various lengths (350mm to 3500mm). The more additional width and length, the better the performance. Optimising the performance will reduce the number of units required. The limiting factor will be the wall thickness, which can be adjusted using wall insulation. Each unit is then custom manufactured to the specific length within the parameters of each model.
Each model has a minimal and maximum length; listed incrementally, 160 ERP PRO MINI – (min 350mm – max 440), 160 ERP PRO – (min 440 – max 650), 210 ERP PRO MINI (min 350mm – max 450), 210 ERP PRO G – (min 450mm – max 650), 210 ERP PRO C – (min 500mm – max 3500mm).
The Prana Recuperator is installed through the wall with a 2-3º incline (1:24, running downhill from inside to out, allowing any condensation produced in the heat exchanger to run out. The hole through the wall will ideally accommodate the Recuperator fitted within a sleeve, making the installation more easily demountable for maintenance.
The internal aperture size of the sleeve needs to be slightly larger than the chosen recuperator unit. (160 > 165mm, 210 > 225mm). Expanding foam airtightness tape will secure the unit into the sleeve and fill this gap.
The hole cut through the wall (with appropriate incline) will need to be sized to accommodate the thickness of the materials used for the sleeve. Lining may not appear necessary when a clean bored hole has been neatly cut. However, lining the opening with suitable material will help to prevent unnecessary air ingress-egress from cavities and gaps in the masonry. It will be easier to seal the gap between the hole and the sleeve with mortar or expanding foam filler than directly to the Recuperator unit.
Simply creating a sleeve from a section of suitably sized pipe has proven difficult. The available diameters are limited and more often sold in 6m lengths. The cost is prohibitive for a one-off installation but more acceptable as an aggregated cost.
Alternatively, a sleeve fashioned from flexible sheet materials such as closed-cell foam or corkboard works well. Cork has become the prefered option, as it is rigid and bonds well into a lime mortar.
The hole for the sleeve can be chiselled by hand or bored using a core drill. Modern cement-based materials are more easily core drilled, and older heritage fabric can be more problematic. Regular extractors usually exit through a small 100mm hole which is relatively easy to bore with a handheld heavy core drill. The larger hole size required for a Recuperator requires specialist core drilling equipment, which is; more often mounted onto the wall.
Good quality heritage brickwork is often soft, well bonded and can core-drills well. Problems can occur at the top of the wall where without weight bearing down, masonry can rattle loss. (For example – when boring a hole in an upstairs bathroom, high up where the wall meets the roof rafters). Alternatively, when heritage brick is soft, larger holes, such as those required, can be easier to work by hand.
Rubblestone can be more challenging still. Attempting to cut with either a core drill or cold chisel can be very unpredictable. Better to disassemble a hole and then reassemble around a sleeve acting as a former.
The Prana Recuperator operates using mains voltage, 240 volts. It can be hard-wired to a 2 amp fused spur or connected to the 5 amp lighting circuit via a double pole isolator. A three-pin plug is also a suitable connection, being both a fused spur and a double-pole isolator.
In a bathroom, being mains voltage, the connection must be to a 30ma RCD protected circuit. The unit has an ingress protection rating of IP24. It is preferable to fit outside zone 1 (above a bath or shower cubicle). A minimum rating of IPX5 is the default requirement within zone 1. However, if there is a low risk of exposure to pressurised water, a rating of IPX4 is sufficient. For example – when installing in zone 1, but above the tiled area where cleaning using the pressurised water may take place.
The Prana Recuperator has an illuminated digital display and can present a choice of information; humidity – VOC – CO², indoor/outdoor temperature, ingress/egress fan speed or simply the date or time. The display brightness can be dimmed or switched off.
The unit is operated using a handheld wireless controller or a smartphone application. The application provides energy-efficiency data and allows real-time observation and control remotely over the internet.
The unit essentially has three modes, Auto, Auto+ and manual, and 10 power/speed levels of response. Auto-mode responds simply to the room environment, levels 1-10. Auto+ is limited to levels 1-3, preventing, when necessary, audible operation. Two simple manual override buttons can either silence the unit to level 1 or maximise air change at level 10.
The internal heat exchanger is demountable for cleaning and inspection, recommended every two years. The need for cleaning will depend on local conditions.
Temperature, Humidity and Air quality
The indoor temperature was improved, the house felt warmer and was easier to heat than in previous winters. The need for heating was reduced and energy consumption was lower. Additionally, the perception of warmth was improved by a reduction in relative humidity – “from cold and damp to warm and dry”.
Humidity levels were dramatically improved throughout the house. The peaks detected in the kitchen and bathrooms did not migrate to other parts of the house.
Air quality was consistently excellent throughout the home. When an issue occurred, reduced air quality, ie the use of solvents or burning toast, good air quality returned quickly.
Eve Room sensors were used to measure and record temperature, humidity and air quality in each room in the house The Eve Room sensor monitors air quality as a measure of VOCs. Volatile Organic Compounds are considered to be a good indication of the presence of fresh air and an absence of stale air. However, further monitoring is required to ensure that Co²levels are not elevated, especially in bedrooms overnight. CO² monitoring was not available this heating season but will be added in preparation for the next.
Anecdotally – the house smelt fresh and aired never stale or stuffy which had been observed due to airtightness in previous winters.
In this trial, only the kitchen and bathroom had units installed, and air change for the rest of the house relied solely on working with thermodynamics.
Internal air circulation is actively understood and encouraged. During the heating season, when windows are shut, and curtains are drawn internal doors are closed to retain the warmth within each room. In the morning, the curtains are drawn and the internal doors are opened throughout the house. Any build-up of stale air is quickly purged.
Good indoor air quality is essential for the health and well-being of both the building and its occupants. Draught-proofing to prevent heat loss is necessary to achieve successful energy efficiency. Therefore, good air quality must be maintained, heat losses recuperated, and ingress-egress pressure balanced.
MVHR is a prerequisite for an energy-efficient home. Decentralised is a more affordable and less disruptive retrofit solution, and a localised response can be more effective and efficient. A humidistat-only controlled solution will not respond to a lack of fresh air, burnt toast or unpleasant odours; CO² and VOCs monitoring are also required.
It had been anticipated that a decentralised MVHR could perform well, even comparably with a centralised MVHR system. Perceived perhaps as an affordable poor man’s substitution but a compromise worth making.
Now installed, the Prana Recuperator has surpassed expectations. Monitoring has observed excellent air quality throughout the house. No longer is heat wasted extracting energy-intensive activity in the Kitchen or Bathroom. It is not a compromise, nor a poor man’s substitute, it is a game-changer for MVHR.
The decentralised system is less invasive, more affordable, highly efficient, and effective – a quicker|cheaper|better whole-house solution.
The Prana Recuperator is available from the UK distributor, Ecostream
Cork sleeve lining
Airtightness expanding foam tape
Eve Home – Temperature Humidity and VOS sensors
Installation contractors – Bristol Area