Comparison of Home ventilation solutions part 1

We all know that effective ventilation is an integral part of maintaining a healthy home environment. Or at least we should know.

Ventilation removes moisture and airborne pollutants from the home and replaces this 'bad' air with drier, fresher air from outside.

The energy cost of ventilation

Ventilation is often the poor cousin of the critical triple act of Heating, Insulation and Ventilation. They're all equally important to maintaining a healthy indoor environment.  Unfortunately, due to our current fixation on reducing energy consumption and reducing our electricity bills, ventilation often gets forgotten or ignored.

Put simply, there is an energy cost to maintaining effective levels of ventilation. If you're expelling the damp, stale (and warm) air, and replacing it with fresher, drier (and cooler) air from outside, then you obviously need to heat the new incoming air.

But what's the alternative? Seal up your home to retain all that hard earned warmth and keep out the cold outside air? Considering an average family generates in excess of 12 litres of moisture per day (and that's conservative), it's not rocket science to see that the moisture content of the inside air will very quickly increase to unhealthy levels. This will lead to condensation, mould and mildew and unhealthy occupants.

Heat loss is an inevitable side effect of proper, effective ventilation. The key then is to achieve the correct rate of ventilation to control indoor humidity and pollution, whilst not losing more heat than is necessary.

It's worth noting that drier air takes less energy to heat than damp air, so if you get your ventilation rates correct, the cost to your heating bill can be quite minimal.

Factors in deciding on the right ventilation solution

So which ventilation solution is right for me, I hear you ask.  As you'd expect, this is not a straightforward answer. It depends on quite a few factors including:
- Building airtightness
- Location
- Local weather patterns and temperatures
- Building construction characteristics
- Occupant's behaviours and lifestyle
- Building design and layout
- Cost

Types of ventilation

Before we look at these factors in more detail, I'll introduce the main methods for ventilating a home, running roughly from cheapest to most expensive:

Passive ventilation

This is where the naturally occurring pressure differences around a building are harnessed to draw fresh air in and suck stale air out.

This is the way homes have been traditionally and successfully been ventilated for centuries and since the principles haven't changed, neither has the validity of this approach. There are two main principles that can be used:
1. Cross-flow - Where negative pressure on the sheltered side draws air out through openings and replacement air is pulled in through openings on the windward side
2. Stack effect - Where warmed air in the building rises and exits through openings in the top of the building and replacement fresh air is drawn in through openings lower down. This is also referred to as the chimney effect, since this is exactly how chimneys work.

In practice, passive ventilation can be achieved by opening windows, installing secure trickle vents, usually in the window system (www.easyair.co.nz), or by installing roof vents to take advantage of the stack effect, with trickle vents for the replacement air.

Passive ventilation is effectively the main method for meeting the building code requirements. These stipulate that openable windows, equivalent to not less than 5% of the floor area in a room must be installed. This of course makes an assumption that these windows will be opened from time to time, which in practice doesn't always happen, for reasons of security, weather, insects, noise and heat retention. This is where passive vents come in. They are usually installed securely into the window system. Some can be retro-fitted to existing windows. They provide a controllable level of ventilation, they are secure and they will keep out insects and weather. As they rely on natural pressure differences around the building, they are better suited to buildings which have reasonable exposure to the wind, although only the most sheltered of buildings would not benefit from passive ventilation.

Passive ventilation with intermittent extraction

This solution uses intermittent extractor fans in wet areas (i.e. Extractor fans in the bathrooms and rangehoods in the kitchen). These extractors must be ducted to the outside. They also need to be switched on long enough to remove the majority of moisture created when showering, cooking etc. It's recommended to have them either over-running on a timer, or to have them hooked up to a humidity switch so they automatically switch on when humidity gets above a set level.

To operate efficiently, extractor fans rely on passive vents to replace the air being extracted. They can only extract air if it can be replaced with fresh air. So passive vents are an integral part of this solution.

Passive ventilation with continuous mechanical extraction

This uses the same principles as above. Basically the house is under continual negative pressure, where an extractor fan mounted in the roof space draws air from one or more rooms through ducting and expels it from the house. Passive vents are positioned around the home to provide fresh replacement air. Again, without the passive 'inlet' vents, the system won't extract air at the rate required. It would rely on windows being left open which is inefficient and a security risk.

Positive input ventilation with passive outlets

This is essentially the opposite of the above system. A fan is housed in the roof space with ducting to one or more inlets through the ceiling. Air is blown from the roof space into the home, forcing the damp air out through openings, usually in the form of passive trickle vents. Although the vendors in NZ (e.g. www.HRV.co.nz) often simply rely on natural air leakage of the building, or on open windows. This is very variable depending on the house construction, and installing specific outlet vents would seem a much more reliable approach. Pushing air into the home rather than extracting it out also means the ventilation is less targeted, and path the air takes is more difficult to predict or control.

To counter these shortcomings, the companies promoting these systems emphasise that the roof space air is warmer and drier than outside, so there is some heating benefit from doing it this way.  This strikes a chord with homeowners in the current climate where reducing heating costs is a hot topic.

However the heating benefits have been questioned by a number of studies both here and overseas, and it appears the ability of these systems to provide free heat at the time of day, and year, when you actually want it is marginal at best. In addition, the roof space is often too hot during the summer, requiring a summer bypass to be installed, usually at extra cost.

One of the draw backs to a ventilation system claiming to also provide heating is that, when the roof space temperature doesn't suit (i.e. it's either too hot or too cold) the ventilation fan either slows right down or turns off. This means that the system is now not even providing ventilation, which is it's primary function.  Since ventilation is about maintain moisture levels in the air, any automatic controller should be based on humidity, not temperature.

Mechanical Extraction with Heat recovery

This system is a balanced pressure system, usually incorporating an input fan, an output fan and a heat exchanger. Fresh air is drawn in through ducted vents (usually under the eaves). This passes through the heat exchanger which transfers some heat (but not moisture) from the outgoing air to the fresh incoming air, essentially pre-warming it.  The warm air from the home is extracted through ducting, passes through the heat exchanger where its latent heat is transferred to the incoming air before being expelled to the outside.

The obvious key benefit of these systems is that they use a percentage of the residual heat from the extracted air to pre-warm the incoming air, thereby reducing the cost of bringing this fresh air up to temperature.  In the summer time, they can also work in reverse, as long as you have a way of keeping the inside of your home cooler than the outside (i.e. with an air conditioning unit). During the summer, the cooled inside air is passed through the heat exchanger to cool the hot incoming air.

Balanced pressure systems such as this (e.g. www.loss-nay.co.nz)are the only system where passive inlet or outlet vents are not recommended. These systems rely on the building being as airtight as possible so that all air coming and going is passed through the heat exchanger. Any additional gaps will reduce the efficiency of the system.

In the next section we'll compare each option with the determining factors - Read on >>