Don’t do ducts

In selecting home heating, a common choice is a ducted system.  Generally I think ducted heating is a bad idea for homes.  Here’s my take on the problems with ducted systems.

By Richard Keech

2018-10-20  based on earlier work from 2013

Last updated 2023-02-01

IMG_2224 - lo-res
Ducting has all sorts of problems

[This is an adapted and extended from the BZE Buildings Plan, page 82]

The use of ducted heating and/or cooling, although widespread in some states, is highly problematic for a number of reasons:

Ducting losses. A typical ducting arrangement in a home’s ceiling space has a surface area of many tens of square meters. In summertime the unconditioned space through which these ducts pass would often have temperatures exceeding 50°C, and approach freezing on a cold winter evening. The conductive loss of heating or cooling energy through these ducts is significant. It would not be unusual for 20 to 40 per cent of the cooling energy to be lost in the ducts alone. Even premium ducting only has R1.5 compared with R4 or more for many ceilings. Duct losses mean that high-efficiency ‘inverter‘ air conditioning units may actually reduce rather than improve overall efficiency because air is flowing more of the time when the unit is on.

Leaks. Ducting is typically held together with adhesive tape which tends to weaken over time thereby allowing leaks. To the home owner, it is generally not obvious if there are small to moderate leaks, accordingly a great deal of energy can be lost through leaks before it becomes obvious.

Air infiltration. A side effect of duct air leakage is that there can be a significant infiltration of outside air due to pressure imbalances. In other words, every litre of air that leaks out of ducting into the roof space needs to be balanced by a litre of unconditioned air drawn into the conditioned part of the house – usually through cracks and vents (discussed also here). Leaks in the system of ducts is not only an issue when the system is on, but lead to draughts via the ducting all year round.

Poor air mixing in winter. Warm air entering a room from a ceiling vent is generally too buoyant to mix properly because of the tendency of hot air to rise leading to stratification (see Figure below). Poor mixing can be especially problematic where a conditioned space adjoins a stairwell and/or if the return air point is in the ceiling. If air velocity is increased to give better mixing, then draughts and noise levels can become annoying. As a result of poor mixing the level of comfort can be significantly reduced. If the house has air leaks under the doors the problem is further compounded because the cold air pools at floor level and further impedes air mixing.

Poor air mixing leads to stratification

Poor fan-only operation. In summer there are many hours when a good fan alone gives sufficient comfort. Unlike split systems, ducted systems generally cannot provide well-controlled air movement to create useful fan-only cooling. One key reason is that horizontal air movement from a split system will generally give better cooling comfort than any air movement from ceiling-mounted or floor-mounted vents of a ducted system.

Whole-of-house circulation. Air in a ducted system typically travels around a closed circuit with a single return-air vent. This requires a clear air path from every conditioned space back to the return-air point, creating undesirable draughts in winter since individual rooms cannot be closed off. Secondly, lots of fan energy is required simply to move the air around. Split systems, on the other hand, recirculate air within individual rooms, requiring less fan energy and allowing for closing off rooms where appropriate. Thirdly, whole-of-house circulation means that areas such as hallways end up being conditioned spaces. Thus a side effect of whole-of-house aircon design is to condition a greater volume than is really required.

Zoning compromises. Even if a ducted system is divided into zones, allowing selective heating and cooling in difference spaces, then it’s unlikely that the zoning will allow control down to the level of individual rooms. Typical installation of ducted systems divides homes into zones comprising more than one room each, whereas split systems are typically handled on a per-room basis. So for example, heating a single (occupied) bedroom may lead to a second (unoccupied) bedroom in the same zone being heated unnecessarily. 

Zone temperature control. Each zone in a ducted system will have one temperature sensor even if multiple rooms are served.  Variations in heating/cooling load between rooms, and variations in the individual preferences at the room level are not well served with ducting.

Air quality. Ducted systems only rarely include a specific fresh air inlet in the air flow pathway. Without this a ducted system cannot introduce fresh air.  Ducts can also harbour dust and micro-organisms which can have health impacts. Such ducts, when dirty, can be difficult to clean and lessen indoor air quality;

Lack of advanced control. Ducted systems typically lack the advanced in-room control features now common on split systems. Advanced control includes, for example, the ability to use occupancy sensors to avoid heating/cooling unoccupied rooms.  

Compromised insulation. Ducted systems within roofs and/or under floors make it much more difficult to achieve good insulation coverage.  The vent registers penetrate the ceiling which, in most houses, is the main thermal-envelope boundary.  These penetrations are almost always poorly insulated, i.e., large and unnecessary gaps are commonly left in the insulation in the vicinity of the vent registers.

Insulation missing where vent register is installed

Noise coupling. Unwanted sound can be easily carried between rooms via ducting.