Warm air-ducted heating is a product of the 1970s and still has its champions, but it’s fair to say it has fallen out of favour in recent years. But, the way we build houses has changed and warm air heating may have a place in the modern house.
Warm air heating’s big selling point was that it heated the house very quickly (by warming the air in the house rather than the house itself), but this meant it cooled as quickly as it was warmed. Modern methods of insulation – and particularly airtightness – mean this is no longer such an issue as the heat stays in the house.
These levels of insulation and airtightness also mean that mechanical ventilation systems are increasingly becoming the norm — which obviously lends itself directly to warm air heating. Mechanical ventilation with heat recovery is, effectively, warm air heating.
When to Use Warm Air
Warm air systems are capable of producing up to 30kW. However, the higher the heat load, the longer the system will need to run to keep the house warm. A well-insulated, thermally efficient single or two storey house, where mechanical ventilation is being installed, is the ideal situation for a warm air system.
Most gas and oil-fired boilers have a SEDBUK (Season Efficiency of Domestic Boilers in the UK) rating — which used to be expressed as a rating of A to G, but has recently changed to a percentage. It’s used to compare the efficiency of boilers. SEDBUK ratings are based on water temperatures.
Warm air systems don’t use water so they can’t be tested or rated. However, warm air boiler manufacturers provide the efficiency of their boilers in percentage terms too, but this can be misleading as it is a measure of different factors.
Types of Warm Air System
In all cases, air is pushed around a duct network using low-power fans; the difference lies in the way the air is warmed.
This traditional warm air system simply draws cold air across a gas flame and warms it — typically in a heat exchanger. It is a simple, robust and effective way to heat air.
Air-to-air Heat Pumps
Essentially the same as an air conditioning unit as they tend to be reversible to heat or cool the air. These can be standalone units or used in conjunction with a ventilation system and are typically quite inefficient.
This could be an air-to-air heat pump, a wet heat exchanger run from a boiler, or an electric-resistance heater, located in the ventilation system. It is usually a very small heat source – in the range of 1kW to 3kW capacity – only suitable for a house with a very low heat load — being at, or close to, Passivhaus standard. People like Genvex use exhaust air or loft space heat pumps in this sort of application.
The Rehau ground-air heat system (supplied in the UK by ADM System) exploits the fact that at depths over 1.5m, the ground temperature is a relatively constant 8-12°C. Air is drawn through a network of pipes and raised (or lowered) to the temperature of the ground.
Pre-heated air is used in an air-source heat pump connected to the ventilation system, or cooled air is used directly to provide cooling in summer. This removes the need for air conditioning as well as improving the overall heat pump efficiency.
Know the Facts
Should you believe the claims of warm air heating system suppliers, installers and manufacturers?
- Quiet in operation – This depends on the quality of the ducting and the quality of installation. Fans are now virtually silent, but flexible or poorly installed ducting can result in noise.
- Fast warm-up times – True, and as airtightness and insulation levels improve, the consequent fast cool-down time is less of an issue.
- Highly controllable – True. Switch it on and heat almost immediately enters the room; switch it off and heat stops entering the room. A radiator system will take 30 to 40 minutes to achieve the same thing and underfloor heating could take two hours.
- Cheap to install – True in single storey buildings, but a little less so in two storey houses. With three storeys, or more, a detailed cost comparison will be needed.
- Fuel efficient – It is a comparative thing and compared to conventional wet systems, warm air is less efficient. The best warm air system will reach around 80% efficiency while an average wet system boiler will be over 85% — not much difference, but moving air inherently cools it (fanning yourself on a hot day works) so there is a heat loss as the warm air moves along the ducting.
- Coughs and colds – Claims that warm air heating spreads coughs and colds, spreads dust and dries the air to an uncomfortable degree, are unfounded. The air is filtered, as it is in any ventilation system, and if your child brings a cold home from school then you’re likely to catch it whatever heating system you have.