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Nuaire Drimaster Positive Input Ventilation System PIV from Nuaire in the U.K. - http://www.nuaire.co.uk Indoor Humidity vs Positive Ventilation

Positive Input Ventilation Systems

PIV & indoor air quality & humidity control.

This article series discusses indoor air quality, ventilation, and indoor humidity control.

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Supply-only Vent System Operation: PIV System Humidity Level Variations

Heat Recovery Ventilation System Installation Requirements

For U.K. readers, as of 1 October 2010 revisions to Approved Document F-F1 the Means of Ventilation, applicable to people living in England and Wales requires that all Mechanical Ventilation & Heat Recovery System (MVHR) installations require that such systems be commissioned using a qualified, competent expert in compliance with 2010 ADF2010 regulations.

Among the requirements for these ventilation systems are conditions that will improve the installation and performance of any mechanical ventilation or heat recovery ventilation system wherever you live: [Paraphrasing from the Nuaire Best Practices Guide cited above and again in detail atReferences or Citations ]

Nuaire Drimaster Positive Input Ventilation System PIV from Nuaire in the U.K. - http://www.nuaire.co.ukA U.K. reader wrote that he had recently installed a Nuaire "positive input ventilation" PIV system sold in the U.K. The installer monitored the temperature and humidity in the most suspect corner a building room.

The PIV system generally maintained 55% RH but sometimes the humidity increased to 60-62%.

[Click to enlarge any image]

The operating premise of a Positive Input Ventilation (PIV) system is the continued introduction of filtered outdoor air into the building at a continuous rate, presumably putting the building at positive pressure with respect to the outdoors and thus causing indoor air to move outside through other building leaks or vents.

Shown at left, the Nuaire Drimaster Positive Input Ventilation System (PIV). In the U.K. & Ireland Nuaire is at +44 (0) 29 2085 8200, www. nuaire.co.uk Email: info@nuaire.co.uk or international@nuaire.co.uk

This product is sold in the U.K. specifically intended to address indoor dampness & condensation problems.

[Click to enlarge any image]

A further underlying premise is that the relative humidity of outdoor air is always below that of indoor air. This is usually true for buildings in which there are significant moisture sources (use of plumbing, cooking, occupants, or a wet basement or crawl area).

Really? Well often but not always: In some climates and under some weather conditions, if outdoor humidity is quite high, even though we are introducing outdoor air, it may be more humid than indoors.

A widely-discussed example of this "reverse humidity" problem (insofar as the Nuaire design intends to bring in outdoor air) is the movement of humid outdoor air into cool crawl spaces below buildings during summer weather. The result in these areas can be a significant increase in the crawl area moisture level as moisture from warm, moisture laden outdoor air condenses out into the cooler crawl space.

Relative temperatures between indoors and outside are also a factor. A commissioned installation by experts would include measurements of building air flow, air changes per hour ACH, and humidity levels.

In the U.K. Nuaire offers an HRV Best Practices Guide (cited below) that may be helpful. The company warns that installing any ventilation system without first studying building conditions is (in my words) a bit uncertain. Here is what Nuaire says about controlling condensation or moisture when installing a PIV system:

Control of condensation/moisture

In ducted ventilation systems, condensation will occur when warm, humid air extracted from bathroom, shower, kitchen or utility room hits the cooler surface of your ventilation duct. If the extracted air is significantly cooled, the moisture will condense back to water and become trapped in your ducting, potentially causing damage to the fan and property.

For this reason building regulations stipulate the following installation guidance in order to prevent issues with condensation and moisture:

Horizontal ducting including ducting in walls should be arranged to slope slightly downwards away from the fan to prevent backflow of any moisture into the fan unit

Ducting rising vertically carrying extract air to outside requires the use of a condensate trap, this is to prevent any moisture forming inside the ductwork, dropping back down and into the fan unit.

This should then be connected to a suitable soil vent pipe using 21.5mm pipe. - Nuaire Best Practice Installation Guide [for] Mechanical Heat Recovery Ventilation (MVHR), Nuaire Group, retrieved 1/1/2015, original source: http://www.nuaire.info/bpguides/BestPracticeInstallationGuide-MVHR.pdf - requires creating an account and signing up.

PIV Fan Boost Feature

The unit air volume can be manually boosted to maximum speed by wiring a simple one way switch (part number 771532) to the PCB (located under the top cover). By switching the ‘boost’ all other functions are over-ridden. - Nuaire, retrieved 1/1/2015, original source http://nuaire.info/IandM/671179.pdf

To understand why humidity might increase in a building where a PIV system is installed, start by looking at the periods of higher relative humidity levels, temperatures, and comparing indoors and outside air conditions. An expert, if one designed and installed your system would also have looked at building air flow rates or air change rates expressed as air changes per hour (ACH).

An improperly-sized, installed, or located unit can give unsatisfactory results including indoor moisture and moisture-related mould problems, poor indoor air quality or fan noise issues.

I would also look for both apparent and clandestine moisture sources in the building. The fact that you found most concern at a wardrobe (do you mean a closet or a piece of furniture?) may simply reflect temperature differences in that location.

In the U.S. we also refer to PIVs as Supply Only Ventilation Systems. The operation of supply ventilation systems or PIVs is discussed in more detail by our contributor and expert Steven Bliss at

VENTILATION, SUPPLY-ONLY - inspectapedia.com/BestPractices/Ventilation_Supply_Only.php

Heat Recovery Ventilation System Installation Requirements

For U.K. readers, as of 1 October 2010 revisions to Approved Document F-F1 the Means of Ventilation, applicable to people living in England and Wales requires that all Mechanical Ventilation & Heat Recovery System (MVHR) installations require that such systems be commissioned using a qualified, competent expert in compliance with 2010 ADF2010 regulations.

Among the requirements for these ventilation systems are conditions that will improve the installation and performance of any mechanical ventilation or heat recovery ventilation system wherever you live: [Paraphrasing from the Nuaire Best Practices Guide cited above and again in detail atReferences or Citations ]

You might do some simple, low -cost tests using toilet tissue or talcum powder or smoke or even a match to see which way air is flowing at your windows or doors.

Don't forget to look for condensate leaks in the PIV system ducting too.

About boosting the airflow rate of the PIV system, the company says:

The unit air volume can be manually boosted to maximum speed by wiring a simple one way switch (part number 771532) to the PCB (located under the top cover). By switching the ‘boost’ all other functions are over-ridden. - Nuaire, retrieved 1/1/2015, original source http://nuaire.info/IandM/671179.pdf

References for PIV Systems Installation & Operation

Update: 2018/01/08: Unfortunately NuAire has made retrieving and reading that company's best practices guide almost impossible, replacing it with a little video. Here are some other resources


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