With the launch of the new Tornado Silent PIV, we thought it would be a good idea to look at the differences between positive and negative ventilation in relation to one of their other best-selling models, the Silent dMEV.
With the Silent PIV already looking to become one of the most popular positive input ventilation models due to its performance and quality at an incredibly attractive price, we have had an uptake in enquiries about what is the best ventilation option for your home. In this article, we will look at the pros and cons of both PIV and dMEV platforms, including which suits different housing situations best, which combat damp and mould issues more effectively, where & when both PIV and dMEV can be installed together, and when they can’t.
Let’s start by explaining what both of these systems are and how they work.
dMEV
dMEV fans look very similar to your standard wall or ceiling-mounted bathroom extractor fans, the main difference being that dMEV fans are continuous-running units. This means they extract 24 hours a day at a low trickle rate, constantly helping to remove damp and stale air and promoting fresher indoor air quality. As expected, these units also boost to a higher extraction rate when required, either through manual triggering and/or, as in the case of the ST100DMEVHT model, automatically via rises in humidity. By extracting air from a property, dMEV fans create what is known as negative pressure, which means fresh air is drawn into the building through passive air vents, air bricks and through gaps in the fabric of the building, ensuring a healthier internal air quality.
PIV
In contrast, PIV units continuously supply fresh air into a property. Rather than extracting damp and stale air, PIV units displace the existing internal air by introducing clean filtered air into the property, creating positive pressure within the building. This over-pressure forces the damp and stale air to leave the building via passive grilles, air bricks and through gaps in the fabric of the building - essentially the exact opposite process of the dMEV, but generating the same outcome. The only real difference is that a dMEV unit is designed to service a single room, whereas a PIV unit like the STPIVLH is intended to service a whole house of up to five bedrooms in size.
This last point is of real interest. A single loft-mounted PIV, like the Silent PIV, can be easily retrofitted into the loft of most homes with minimal disruption. The big brother version of a dMEV, called an MEV, can extract from multiple rooms (usually up to 6 wetrooms) and again is loft-mounted. However, these units, such as the Vent Axia 443298 MVDC-MSH, require ducting to be run from each room you plan to extract from, which, unless you live in a bungalow or are undertaking a new-build project or major renovation, can be very disruptive when retrofitted across multiple floors.
So, PIV and dMEV are not directly interchangeable, even though they are designed to address the same issues. This is certainly a “horses for courses” situation, where the requirement will ultimately direct the system choice.
If this is the case, how do I know which system to install in my home? We will begin with dMEV options as these are usually born from more traditional requirements.
dMEV Damp and Mould control
If you have a bathroom, wetroom, utility room or kitchen that struggles with a damp and/or mould problem, dMEV units are absolutely the best option. In fact, dMEVs were primarily designed to help combat damp and mould issues, so they make a fantastic alternative in these situations. As 4”/100mm fans, most bathroom upgrades allow you to utilise existing ducting positions and wiring. But it is worth mentioning that dMEV fans require at least a constant live and neutral connection (Humidistat models only) or a constant live, switched live and neutral connection to operate. Where intermittent fans only run when a room is in use, or when humidity rises in the case of humidistat fans, for the rest of the time, they sit dormant. In rooms with persistent damp and/or mould, the dormant fan is not helping to combat any residual moisture in the room, allowing the issue to remain. By installing a dMEV, while the fan is in trickle mode, it will draw warm, dry air into the room from the rest of the house to replace the damp air it is expelling. So not only are you removing more moisture from the environment, but you are also promoting the drying out and reconditioning of the room by maintaining air movement and the introduction of warmth from the rest of the house. It is important to note that there are two other actions that you need to take (regardless of whatever style of fan you install) that will help to promote a drier and healthier room: heating and insulation.
The warmer you can keep a room suffering from damp and mould, especially during colder weather, the better chance you have of combating the damp and mould issue. Keeping windows in bathrooms closed, for instance, so that the negative pressure created by the extractor fan doesn’t pull cold air in from outside, is very important. Sensible heating in the room is also key. Maintaining a decent average temperature in a bathroom will help to keep the air, walls and ceiling surfaces at a higher temperature, thus slowing down the speed at which moisture condenses in or on them. You can read more about this three-pronged approach in our previous article, Awaab’s Law Part 2: The Three Most Important Factors for Eliminating Damp & Mould.
dMEV Fans in Kitchens
Let’s look at the reasons for requiring ventilation in our homes. To begin with, we have Building Regulations to adhere to. These are a set of criteria that must be met during construction and during habitation of buildings. Quoting directly from the current regulations, “Building Regulations protect the Health and Safety of people in and around buildings,...” - and there is an entire section (Part F) dedicated to Ventilation.
One of the main stipulations of Part F is the requirement for adequate ventilation in certain areas. These are Kitchens, Utility rooms, Bathrooms and Sanitary accommodation - the common factor here is primarily moisture, as well as requirements for removing other airborne contaminants to keep the internal air healthy for the occupants. Each of these areas requires some form of mechanical ventilation, extracting to a specific minimum extraction rate, and in most cases, traditional intermittent fans are implemented in these areas effectively.
However, there are times when traditional intermittent fans can fall short. For instance, the building regulations require the following extraction rates as a minimum to be reached for each of the following areas:
N.B.: sanitary accommodation refers to a room with a WC or urinal which requires extraction to remove moisture and pollutants
Taking a kitchen without a cooker hood or one that recycles the air without extracting it out of the building, an intermittent kitchen fan is required with a minimum extraction rate of 60l/s (216m3/h). Due to the high extraction rate required, this will require a larger 6”/150mm diameter intermittent fan to be installed to reach this minimum extraction rate.
But there is another set of regulations specifically for continuous-running fans, such as dMEVs, which have a different set of extraction rates:
Here, the minimum extraction rate is far lower because of the cumulative effect of the continuous extraction when the fan is in trickle mode, meaning the higher boost extraction rate can be dropped whilst still giving the same air quality improvements required as a whole.
This means that a 4”/100mm dMEV can now be installed in a kitchen, allowing for a more compact and discreet unit gracing your kitchen wall or ceiling, and giving the added benefit of continuous ventilation.
You will note that there is an additional column on Table 1.2 above with the heading “Continuous rate”. This relates to the minimum extraction rate required for the fan speed when in trickle mode; however, it is specifically related to whole-house systems such as MEV or MVHR, where multiple rooms are being serviced by a system and can be ignored in relation to single-room systems such as dMEV units.
However, it is worth covering this regulation in case you do choose to install an MEV unit to service multiple rooms. In these situations, the unit must be set to a specific rate of extraction in line with the size of the home, where this rate is divided across multiple extraction points. As a simplified example, let’s take an MEV system installed in a house with four bedrooms. As stated before, the minimum requirement for the boost extraction in the kitchen is 13l/s, but you also have extraction points in the utility and two bathrooms sharing the extraction from the central loft-mounted MEV. This means that the minimum rate of extraction required for the fan to run at when it is on boost must be set to at least 52l/s. This is because the extraction rate will be shared across all four extraction points, and 52 divided by 4 is 13, giving a minimum of 13l/s in all rooms where an extraction point is installed, including the kitchen. The regulations also stipulate that the trickle extraction rate for a home with four bedrooms is 37l/s, which will give 9.25l/s of extraction in each of the four rooms with extraction points installed. It is worth remembering that this is the minimum extraction rate - both for the trickle and boost modes - you will likely have the boost extraction set to a higher rate for a more effective extraction. In the case of the Tornado Silent MEV, the unit is capable of a maximum extraction rate of 83l/s, giving plenty of wiggle room for you to set an appropriate level for your own personal requirements.
I did say that this was a simplified example; in real-world scenarios, the ducting runs between each room and the MEV unit will all differ in length, which in turn will deliver more or less resistance to the fan unit. This will therefore affect the airflow rates from each room, creating an unequal share of the extraction rate. Professional commissioning will measure the airflow rate at the intake grille in each room to make sure they are meeting the minimum extraction rate requirement, and adjustments to the overall flow rate of the MEV or MVHR will be made if necessary.
dMEVs in Non-Wetroom Environments
dMEVs can also be utilised in non-wetroom situations to treat other problem areas. For instance, perhaps you have a spare room that struggles with black mould or a damp living room? Installing a dMEV in these spaces can help to reduce the moisture content. However, in these scenarios, we suggest a dMEV that does not incorporate the humidistat feature and is simply manually boosted when and if required. For the most part, as these spaces are not wetrooms with a large source of moisture, such as a shower, results can be achieved by allowing the fan to trickle quietly in the background. It is worth noting that most dMEV fans are virtually silent while operating in trickle mode and are therefore suitable for installation in living areas such as lounges and even bedrooms if required. Having a manual boost model, such as the ST100DMEVS, allows for continuous trickle without being disturbed by an automatic boost kicking in at inopportune moments, but does allow for the inclusion of a manual boost function for purge venting if required. Not only can this improve the moisture levels in these living spaces, but it also promotes healthier air quality for the occupants of the room.
Where PIV becomes the better option
In the previous example, we looked at utilising dMEVs for damp and mould management in problem rooms, be it a wet room or a living space. But what if you have multiple rooms suffering from damp-related issues? Well, technically, you could install dMEVs in each affected room in the home, but aside from the cost of purchasing multiple units and the cost of multiple installations, you will have to power the multiple units 24 hours a day, which will become more costly in terms of energy usage. Now, it is worth pointing out at this point that part of the criteria for most modern dMEV fans is to be as energy-efficient as possible to make them suitable for installation in low-income social housing projects. Taking the power consumption of the Silent dMEV as an example, the fan uses 8.3W on its highest boost setting and as little as 1W on its lowest trickle setting - this is less than some energy-efficient LED lightbulbs! However, running a dMEV in every room of the house will end up being more costly.
Running multiple dMEVs can also cause another issue. With multiple constant extract points throughout the home, you will be removing a higher percentage of heat during colder months. While heat loss is not a major issue with a handful of dMEVs installed in wet rooms only, if the majority of the rooms in the home are being continuously extracted from, this can become a more noticeable side effect, both in terms of overall temperature drop and heating costs. To compact the issue, that which is removed from the home, causing negative pressure, must be replaced from outside, leading to a greater volume of cold air being reintroduced to the home in colder weather.
This is where PIV systems become a more sensible and effective solution to whole-household damp and mould issues. By introducing clean filtered air from a central point in the home, a PIV unit can help to reduce moisture levels in the home across all areas. And because positive pressure is being produced in the building, where dMEV systems require a return of air from outside to be drawn into a building, PIV systems turn the airflow direction on its head, utilising the same passive air vents, airbricks and gaps in the fabric of the building which allowed the return of fresh air into the building when extract ventilation was implemented, as exit points for the damp and stale air. As the incoming air is now centralised, many PIV models, including the Tornado Silent PIV, include a heating element which is used to raise the temperature of the incoming air when the outside air temperature drops below a certain point. These heating elements are not designed to replace central heating, but they are included to temper the air, taking the edge off the incoming air during colder months. This supply of air is also filtered, removing any particulate matter such as pollen and other pollutants from the supply, something else that the return of air in a negative pressure system can contend with.
It is also worth mentioning that, regardless of damp and mould issues, installing a PIV in your home is an excellent way of improving your overall indoor air quality. Having a constant low-level supply of filtered air into your home can improve not only your respiratory health, but also minimise the amount of airborne dust and debris entering your home, keeping it cleaner for longer.
Installing PIV and dMEV together
We mentioned above that in line with the building regulations, all wet rooms require extract ventilation - kitchens, utility rooms, bathrooms and sanitary accommodation. This does not change if PIV is installed and extraction is still required in these spaces; however, it does affect your choices of extraction units. In general, it is best to install traditional intermittent fans in these spaces if paired with a PIV. This is so that the supplied air has a chance to circulate throughout the home rather than being drawn directly to the first continuously extracting fan in its path.
To illustrate this, the image below shows two versions of the same household layout. One has an intermittent fan installed in the bathroom, and the other has a dMEV. In both scenarios, when the bathroom fan is boosting, it will monopolise more of the airflow from the PIV. But once the intermittent fan turns off, as shown in the top scenario, it becomes a passive outlet whilst not running, allowing air to leave the bathroom at the rate set by the PIV - essentially doing the same job as a dMEV on trickle. However, if the bathroom has a dMEV trickling as shown in the second scenario, the fan will draw more of the PIV’s supply, diverting the flow of air around the rest of the home and extracting it before it has a chance to improve the indoor air quality of the whole house.
If multiple dMEV fans are installed in the home, these will also monopolise the airflow supply from the PIV, meaning that other areas of the house will not get the benefit of the PIV.
The general rule of thumb, therefore, is not to pair dMEV fans with PIV systems in the same building. However, there is one scenario where pairing a single dMEV with a PIV system can work.
If you have a room situated at a distance from the PIV unit, say a utility room in an extension, the airflow may not reach the room effectively enough to help condition the room. In these cases, a single dMEV installed in the utility room will help pull the fresh, filtered air from the PIV to this space, without monopolising the airflow from the other areas of the house. Instead, it will help draw the supplied air through the downstairs areas en route to the utility room, which in turn helps all the spaces it passes on the way.
PIV or dMEV?
If you are looking for a solution for improving the overall indoor air quality of your existing home, PIV is an excellent choice as a straightforward retrofit option. The bonus of not only supplying filtered fresh air but also removing excess moisture from the house and condensation from windows on cold mornings adds to the effectiveness as a whole-house ventilation system. For individual rooms suffering from condensation, damp or mould, dMEV fans are a god-send, actively working 24 hours a day to quietly remove excess moisture, reconditioning the room into a drier and safer environment. Hopefully, this article has helped to draw attention to both of these excellent ventilation platforms, as well as an understanding of where and when they can and can’t be installed together to maximise the benefit they can deliver to improve the health of your home.