Ventilation Question & Answers

Want to know more about our ventilation services and systems? – please read our most frequently asked questions.

Positive Input Ventilation (PIV) FAQs

Do I have mould due to condensation, or have I got another form of damp?
Condensation tends to occur in rooms where lots of moisture is produced and on cold surfaces in other rooms. Examples are on windows, which may stream with water, or external walls on solid walled properties. Condensation does not form well-defined patches like other forms of damp and will tend to be worse in cold weather. Black mould on walls or green mould on furniture are often found as a result of condensation dampness. Damp caused by plumbing or drainage problems, on the other hand, usually forms well defined areas, localised near to the source of the problem. Rising damp is usually typified by a tide mark rising some 600-1000mm above floor level.
Will using a dehumidifier to cure my condensation solve my mould problem?
You could use a dehumidifier to remove moisture from the air. However, this is not tackling the root causes of condensation in the way that adequate ventilation will. Dehumidifiers are noisy in operation, require frequent emptying and are expensive to run. For example, a typical 250W unit will cost around 3p per hour when it is running. By comparison, most PIVs will cost around 1p per day to run, are virtually silent in operation and will require no maintenance other than a quick filter clean or change every three to five years dependent upon make.
Is my property suitable for a PIV?
Virtually any home with an accessible loft is suitable for the installation of a PIV. The units are mounted within the loft and ducted to a diffuser mounted in the ceiling below. Some of our PIVs are suspended from a cord into the heights of the roof apex, whilst others are secured directly to the rafters. The electrical connection is made via a fused spur, generally to the upstairs lighting circuit. In common with all electrical installation work, it is important that the property’s electrical circuit to be used, as well as house earthing and bonding is up to the required standards (As an NiCEIC approved electrical company, SWES is able to assess this and quote for any remedial works if necessary).
It is also important that your loft is adequately ventilated, normally achieved by vents incorporated into the soffits when the roof is built. If you have had loft insulation fitted it may be necessary to pull this back slightly from the soffit area to ensure ventilation is not impeded. Any water tanks within the loft should be covered.
If you do not have a loft in your property, there are other forms of ventilation systems that SWES offers as a solution.
Which type of PIV do I need?
All PIV systems cure condensation problems in exactly the same way and perform identically in this respect. Nine times out of ten, a standard unheated PIV will be sufficient as it offers the best value and lowest running costs. There are PIVs that can be beneficial when areas of a property are particularly hard to heat or stay colder than surrounding areas for other reasons. A temperature sensor mounted in the cold area will trigger a boost in fan speed when the unit detects warmer air in the loft than in the property. This provides a useful source of subtle ‘free heating’. The heated PIVs generally have built in 400-500W electric heaters which can be set to warm the incoming air (tampering) when its temperature drops below a certain level. This can be useful if the diffuser outlet is mounted in a living area or area of frequent use, such as the hallway in a bungalow, as it can help prevent cold spots. However, if a property already has central heating, this will invariably be cheaper to run and we recommend that this be used in preference to heated PIVs.
How much does a PIV cost to run?
All of the PIV systems that SWES specifies use extremely energy-efficient DC motor technology. As such, running costs are only around 1p per day as standard. However, PIVs with Heat contain a 400/500W electric heater which can be triggered to come on when the incoming air drops below a certain temperature. Electricity costs per hour when the heater is running at full load/range continuously would cost more, however as stated by the manufacturers; up to 10p per day as an average as the unit tempers the pocket of air around the diffuser, models set at mid-range and taking into account warmer periods of weather in a year (dependent upon energy suppliers, unit settings, size of property and local weather conditions, the quoted running costs may vary).
Will I be blowing cold air into my property?
PIVs take in fresh air from the loft and filters it, before introducing it into the property at a low level rate. This air, while typically being a few degrees warmer than outside, will often be a little cooler than the air in the property. This is counteracted by some degree of ‘heat recovery’ – the air in the loft is often warmed slightly from heat escaping from the rooms below (even if the loft is well insulated). This warmed air is reintroduced into the property by the PIV. The incoming air also mixes with the ‘unused’ warm air that gathers at ceiling level, recirculating it around the home. Whilst systems are set up to minimise draughts, a cold spot may sometimes be evident in the area around the diffuser. Since this is usually in a hallway or on a landing, away from main living areas, the effects do not generally cause a problem and will be further reduced with a heated PIV system.
Will my heating bills be effected?
There is no conclusive evidence to suggest heating bills will be either increased or decreased when using a PIV system. Despite cooler air being introduced into the property, the result will create a drier and fresher environment, this means your heating system needs to work less (dry air is easier to heat than moist air). Also, dry air feels warmer to us than damp air, so it is possible that your heating thermostat can be reduced slightly without effecting comfort levels! The inbuilt temperature sensing and heat recovery ability of the general PIV design also means that when loft temperatures reach (typically) 18 degrees, due to warming from the sun (solar gain), the airflow is boosted to provide an additional source of heating. Studies from the Nuaire manufacturer for example have shown that these factors could potentially contribute to provide over 500kWh of ‘free’ heating over a year.
Is there anything I could do to help minimise the effects of condensation?

As we’ve already seen, efficient ventilation is key to eradicating condensation and mould growth problems in the home. However, here are examples of a number of small steps that will help at minimal or zero cost:

  • Cook with saucepan lids in place (cooker hood extraction on) – this not only prevents moisture escaping but reduces cooking time, saving energy
  • Make sure tumble driers are vented to outside
  • Make sure heating is turned on in all rooms (condensation migrates towards the coldest parts of the property)
  • Try not to dry clothes on radiators indoors
  • Ensure wet room extraction is in place and used
  • Insulate your home
  • Ensure brick vents and window trickle vents are clear of blockage and open
  • Do not over fill rooms, and keep walls clear of boxes and furniture that have no airflow or minimal use
  • Ensure all internal doors have a 10mm gap at threshold to allow for room to room airflow movement and balancing

System 1 – Intermittent Extractor Fans and Background Ventilators FAQ’s

What are intermittent extractor fans?
Mechanical ventilators normally fitted to walls and ceilings and used in bathrooms, washrooms or kitchens and they do not run all of the time, usually only used when pollutants and vapour are required to be removed (e.g. during cooking or bathing). The intermittent operation may be automatically or manually controlled.
What are the benefits of intermittent extraction fans?
Just simply that there is some extraction rather than having none is the main benefit. There are more powerful and quitter intermittent extractor fans now available and with the right product planning and installation will be relatively effective to cope with most demands. Another benefit is the cost to purchase and install is lower than any other type of system, and that some rooms will only accommodate this type of system.
What are the negatives of intermittent extraction fans?
Whilst any extraction is better than none, cheaper and more convenient installations may not be the best choice for some builds. The energy consumption is normally much more than a continuous extractor fan for instance, and dependent upon manufacturer; most intermittent fans are generally noisy, and also lose their stated extract rates due to poor installation of ducting runs and distances. Finally, if the room in question has high humidity then intermittent fans just may not run for long enough to be effective enough.
What are background ventilators?
These are small ventilation openings designed to provide controllable whole building ventilation (e.g. window trickle vents).

System 2 – Passive Stack Ventilation (PSV) FAQs

How does a PSV work?
A ventilation system which uses ducts from terminals in the ceiling of rooms (typically situated in wet rooms or kitchens) that extract air passively (without the use of fans or mechanical means). Along with the effect of wind creating pressure to extract air, the natural buoyancy of the relatively warm humid air means the stale air runs up the ductwork to the colder outside air where it will disperse and give up its internal energy.
Is PSV better than mechanical extract ventilation?
PSV is an extremely simple form of continuous ventilation which has no direct running costs and no energy consumption however, it could be said that the PSV method is oversimplified; a change in wind direction may result in there being no extraction of stale air and instead, fresh air being brought into the property resulting in the potential spreading of moisture and creating drafts. This compared to a continuous Mechanical Extract Ventilation system (MEV) are really worlds apart. The only negative would be the running cost of a mechanical system compared to free. However, the PSV could cost more in terms of labour for install especially as the stack is to be vented inline with or above the roof ridge line so will require a roof tile stack pipe.
Is the design and install easier than other types of ventilation systems that require ducting?
Certainly one of the cheaper ducting options of install compared to a Mechanical Ventilation with Heat Recovery system (MVHR), but still requires precise planning for ducting routes and very limited to bends/sweep that can be used which could create pressure which may fail the PSV system due to the precise and fairly effective natural ventilation strategy it uses; combined cross ventilation, buoyancy (warm air rising) and the venturi (wind passing over the terminals causing suction) effect. All this means that a simple design could end up being ineffective, very disruptive to build and quite frankly end up working against the intended reason for its installation.
Will a PSV system solve condensation and mould issues?
A PSV helps in the natural reduction of moisture taken out of the home, however quite possibly not at the required rate of the dwelling. This is not a recognised ventilation system specifically aimed at solving the issues of condensation and mould issues.

System 3 – Continuous Mechanical Extract Ventilation (MEV) FAQs

What is continuous Mechanical Extract Ventilation (MEV)?
A continuous MEV system is a low energy, mechanical extract ventilation system designed with multiple extract points to simultaneously draw stale and moisture-laden air at a low rate out and constantly of wet rooms, whilst minimising the migration of humidity to other rooms.
What are the benefits of having continuous extract fans over intermittent extract fans?
The systems use less energy than traditional intermittent extract fans and are therefore a much more cost effective and low carbon option. They also run more quieter than intermittent, can be boosted or humidity sensor controlled ensuring all moisture is driven out due its continuous operating/running set up.
What are the negatives of having continuous extract fans over intermittent?
Not much, as both only draw polluted/moist air out of a property resulting in small heat loss. However, with the continuous centralised system the multiple points will require ducting which obviously effects installation cost and disruption if not a new build.
If installing a centralized system, at what stage in the building process should design and installation take place?
Much like MVHR, this needs to be as early as possible after planning permission has been granted so that definitive ducting runs can be designed and potentially installed during or after joist completion and/or build water tightness.
What maintenance do I need to carry out?
Dependent upon system installed, then in most cases outside of keeping air vents clear and clean the required maintenance would be none.
Is an MVHR more suitable for my house than a continuous centralised MEV?
Both MEV and MVHR systems are suitable for most properties however an MVHR is most suited to a new build as the system requires a good airtight property where virtually all of the airflow can pass through the heat exchanger, if they are to perform efficiently. The obvious is that heat exchange is more beneficial and that an MVHR system is ultimately the best system, but with the financial cost for the system, installation and building disruption. In saying that, in older dwellings with poor house airtightness an MEV would possibly be a more suited choice.
What’s a typical running cost of a continuous MEV system?
Typically, a standalone continuous decentralised wall or inline extractor fan will cost around £1.30-£2.00 per year as per manufacturers feedback. However, dependent upon the number of units in a centralised network design, the typical cost could raise up to 10p per week.

System 4 – Continuous Balanced Mechanical Ventilation with Heat Recovery (MVHR) FAQs

What is an MVHR system?
An MVHR supplies fresh air to all habitable rooms whilst also removing moist stale air, and reusing the heat in extracted air to pre-heat the fresh air entering the building.
At what stage in the building process should the MVHR system be designed for my building?
As early as possible! Ideally just after the project has received planning permission as it is really hard to add an MVHR system into a low energy or passive house building design later on in the process. SWES will work closely with customers, builders and architects to overcome any possible issues at the design stage.
At what stage in the building process should the MVHR system be installed?
At joist erection if a new build as most of our systems use either spiral wound rigid or glue and screw ducting which is usually designed to pass along timber joist runs or through metal web ‘pozi’ joists. In any case it is essential that the ductwork passing through the ‘pozi’ joists are installed as they are erected, otherwise it may be necessary to cut the duct into small segments, drop it between the joists then reconnect it, which is very labour intensive, can effect performance and requires extra materials. The electrical and MVHR unit are reasonably straight forward to install and can be planned in once building is watertight.
For existing dwellings, either plasterboard or floor boards will require lifting in order to install ductwork.
Is MVHR suitable for all types of property?
Although MVHR could be installed in all types of building, we would only recommend it for buildings with good airtightness levels. As a rule of thumb, this would be where the air permeability of the thermal envelope is at or below 3 m3/m2.h @ 50 Pa. If this level of airtightness is not achieved, then the heat recovery and consequent energy saving benefits of MVHR would be lost.
Can I open the windows?
Yes, you can and the MVHR system will continue to operate in the background. However, the efficiency of the MVHR will usually be slightly reduced, affecting its ability to keep the building warm in winter and cool in summer. The exception to this is when windows are opened for summer purging (see below).
Does the system run 24/7 for 365 days of the year?
It is set for automatic use all year round so you do not have to adjust any settings. However, you may want to over-ride the automatic system on some occasions (see below).
How much electricity does it use?
The MVHR systems typically used have energy efficient motors which push the air through your system with a heat exchange. Most MVHRs use around 22 Watts. This is similar to having a low energy light bulb on all day in your home, and costing around 10p per day.
What will happen if the electricity supply is cut off?
You may start to feel that the air is becoming a little stale, but do not worry you will not suffocate! In the unlikely event that it happens whilst you are asleep, then your bedroom might seem a bit stuffy in the morning. All of our MVHR units are set up to automatically restart when the power is restored.
Are MVHR systems noisy?
The MVHR systems specified all have minimal noise impacts and they should run almost silently. It is important not to block any of the air vents as this will put the whole system out of balance.
What maintenance do I need to carry out?
Apart from keeping all the air vents clear and clean, the only maintenance is periodic filter changes. There are several filters which need to be changed to keep the air flowing properly. How frequently depends on where you live and how clean the air is. The filters keep your air clean, but are also needed to keep the system working properly. The kitchen extract filter may need changing/ cleaning more often than the others, normally every 3-6 months, and ventilation unit filters vary from 6 months up to 3 years as recommended by manufacturers.
Do I need to clean inside the MVHR ducting?
We would advise that if an MVHR system is present in the property, a recirculating cooker hood is used rather than an extractor. This cooker hood will act as the primary filter, taking the air in through a charcoal filter removing smells and grease. The MVHR extract has a fleece filter to protect and leftover airborne grease from entering the ducting system. Providing that you keep on top of the filter changes including the kitchen extract filter then there should be no need for cleaning of the ductwork system. After a few years, it may be necessary to unscrew the extract valves in bathrooms and have a wipe in the duct behind them with a cloth, as wet dust and towel lint can collect here. As all of the ducts are within the thermal envelope the air does not condense until it hits the heat exchanger, by this time it has gone through the filter which will remove any airborne debris.
Will an MVHR system heat my house?
MVHR will re-use the heat which already exists in your home produced by occupants and electrical equipment. Some systems can be designed to have an additional in line heater, to heat supply air, but this can only provide a small amount of heating.
Can I dry clothes?
Yes, clothes dry very well indoors and create some moisture which is beneficial. You also might like to grow plants.
What is summer purging?
If you experience unusually high outdoor temperatures and your home is too hot, it is recommended that you ‘summer purge’. (This is standard practice in hot climates). Open windows during the night so that the cool air can enter and reduce indoor temperatures. During the hot day close your windows and increase the fan speed on your control panel to increase ventilation.
How do I get rid of cooking smells in a Passive house?
An MVHR system, with a recirculating cooker hood, performs as well as a standard extractor in a non-Passive house. If all of the filters are maintained in the MVHR system AND the recirculating cooker hood, the smells will be minimised. A well-designed MVHR system with a high extract rate in the kitchen creates a negative pressure in the kitchen, so air is being sucked under the doors into the kitchen and not the other way around, therefore minimising the spread of smells.
Can I boost the fan whilst cooking?
Yes. Just go to your wired/wireless control panel and touch the boost button. It will stay on for a pre-determined time (usually around 30 minutes), and your system will then revert to normal operation. You may have an additional boost switch in your kitchen or bathroom which will have the same function.
Is it possible to have a woodburning stove with a MVHR?
It is possible to have a log burner as well as a MVHR. However, there are a couple of things to consider:
The air supply to the log burner should be ducted directly to it, in order to avoid draughts and leakage in the fabric of the build.
If you have built a low-energy house, the room in which your log burner is situated may get too hot. A lot of people think that MVHR redistributes heat around a house, but it doesn’t, so any hotspots remain hot and cold spots cold.
What do I do when I go on holiday?
You can reduce the ventilation while you are away. Simply go to your control panel and reduce the fan speed to the minimum. When you return home just return to the standard setting and your system with return to normal operation.
What should I do if there are more people in the house or if there is a party?
You can increase the ventilation for ‘high occupancy’. Simply go to your control panel and increase the fan speed. When people have left, return to the normal ventilation setting, which will keep the air feeling fresh. If you forget then systems can be boosted after the period of high occupancy.
What happens to water vapour that is produced within an MVHR system?
When water vapour is extracted from a kitchen or bathroom it is carried along the duct to the MVHR unit where it condenses and is drained away via a small 20-25mm pipe into nearest the nearest wastepipe run. This is why it is important that all of the ducting is contained within the thermal envelope of the building. If the duct was cooler, the air would cool upon entering the duct and it would lose its capacity to hold as much moisture, resulting in the air condensing (warm air holds more moisture than cold air) within the ductwork as appose to the heat exchange point.

Single Room Heat Recovery Ventilation (SRHRV) FAQs

What is a SRHRV system?
A SRHRV is effectively a decentralised version of the MVHR system. These systems are installed in wet rooms, and although a smaller independent version of a MVHR, most have the heat exchanger (which prevents unwanted heat loss in the dwelling) contained within the duct that passes through the wall.
What are the benefits of the SRHRV system?
A good alternative to a MVHR where maybe cost is the issue or not feasible. The system is localized to one room and therefore may require additional background ventilation. Unlike Intermittent extraction fans and possibly some continuous extraction fans the heat loss from the localized room is negligible.
What are the negatives of a SRHRV system?
The system will most definitely need other ventilation systems throughout the build that need extraction, and will most probably also need background ventilation where building permeability is low. The system is quite large and may be viewed as unsightly on the wall within the wet room.
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