Here's what to consider before you buy a ventilation system.

Capacity

To meet building code minimum requirements for ventilation, the system needs sufficient fan capacity to move the whole volume of air in your house every three hours. All makers listed in our table offer units that can meet this requirement, offering a range of models to match house sizes. If you have higher than usual ceilings or an unusually complicated house design, you may need a larger unit, and/or more outlets.

Extra outlets

Forced-air systems can be installed with one centrally located outlet, designed to push the damp air outwards, and out via windows and doors - or they can be installed with outlets in several rooms, especially those with severe condensation problems. Larger houses will ventilate better with several outlets rather than one large one.

Heat-exchanger systems operate across the whole house, and usually have at least two outlets and two inlets inside the house. The inlets are typically located in warmer areas, and the outlets in cooler areas. Be guided by your installer on the location and number you need.

Controls

When set up correctly for your house, the controls should need little adjustment. That's the ideal arrangement - set and forget. With forced air systems, it's handy to be able to do some basic things from a control panel in the house, like switch the system off in summer. But, otherwise, it's easier to let the system do the work.

Some of the more expensive forced-air models let you check temperatures in the roof and the house, and make your own adjustments to temperature cut-out settings or adjust the air flow. Others will automatically change airflow as required, and even switch to summer cooling or switch on heat transfer systems at pre-set temperatures.

Regardless of this, a basic system can provide ventilation just as effectively as one with complicated control options.

Extra features

You'll need to consider these if you're buying a forced-air system - although they're often standard on higher specification (more expensive) models. Heat-exchanger systems make some features like heaters and heat transfer unnecessary.

Most of the optional inline heaters for forced-air systems are small, just designed to take the chill off the air, But some makers offer larger units that can substitute for some of your normal heating. If you're thinking of a forced-air system and you want to maintain good ventilation during the colder months, we think additional heating is essential - but this will increase your running costs.

A heat-transfer feature allows the fan to take warm air from one part of the house (near a wood burner, for example) and move it to a colder part of the house.

Whether you need summer cooling depends on the design of your house, and on the local climate. In forced-air systems, extra ducts are added so that fresh air is drawn from outside (rather than warm air from the roof space). Look for a model with an automatic or remotely controlled changeover feature - or you may have to send someone up into the roof space to alter the settings.

If effective summer cooling is really important to you, air conditioning is better than a ventilation system - although heat-exchanger systems work well in conjunction with air conditioning.

A shower boost is offered only on heat exchanger systems, and uses sensors on the hot water pipes to detect when the shower is running and switch the fan to high speed to extract the extra steamy air.

Filters

Forced-air systems need good filters, as they draw on roof space air that can contain fibreglass from batts, dust, debris from bird nests, and other pollutants. Most of these systems come with EU4 or G4 grade filters, which can remove 80 to 95 percent of particles down to 10 microns. Some makers of forced-air system offer finer filters as standard, others offer this as an option.

If you know your roof space is very dusty or dirty, finer filters will prevent more of that dust from getting into your house. Heat-exchanger systems draw in outside air. So they don't usually need filters, unless there's pollen or severe air pollution.

If you want to keep costs down by changing or cleaning your own filter, check how easy this is. Some makers won't sell you a filter; they only provide a filter-replacement service. Some filters are washable - but wash them carefully by hand, and replace them if they get torn or damaged. Driveaire says its filters can be washed up to six times. Electrostatic filters require regular washing in special solutions.

Warranties

If there's some doubt whether a forced-air system will work in your house, make sure you get a money-back warranty. Look for a "three winter months" warranty, (not offered on most DIY systems) so you can evaluate the system's performance in more extreme conditions. Warranties are in addition to your usual protections (repair, refund, or replacement) under the Consumer Guarantees Act.

DIY

This is possible for both forced-air and heat-exchanger systems - provided you're reasonably handy. (Heat-exchanger systems are more complex, but they come with comprehensive instructions.) You'll still have to use an electrician to make the connections to the mains or to fit a 3-pin outlet, which adds to the cost.

DIY's also the cheapest option - Bunning's offer the Comfortmaster kit for as little as $600. It's a basic one-outlet forced-air system. Weiss and Smartvent are available through hardware stores like Mitre 10 and Placemakers, but you may have to ask your store to order one.

Some of the other makers of forced air units will supply systems for DIY, but they don't usually offer a winter performance warranty if you install it yourself. And, if you have to get an electrician to wire the unit up, the cost savings are minimal.

DIY kits are also available for Solar Venti solar systems, as well as Cleanaire and Vent Axia heat-exchanger systems. Usually you have to supply your own ducts and vents for DIY heat exchanger models.