Efficiency and Air Displacement

By Richard Keech

2018-08-12

Some systems used for heating and cooling depend on air displacement.  Understanding what this means is valuable, because systems like this are usually inefficient.

So , what do 1) evaporative air conditioners, 2) fireplaces; and 3) solar air heating systems have in common?  Answer – their normal operation usually requires air displacement.

Evaporative aircon
Evaporative cooling and wood fires both involve air displacement

What is displacement?

The concept of displacement of a fluid goes back to ancient Greece and the apocryphal tale of Archimedes in the bath. Displacement is the pushing aside of something with something else. Displacement of water explains why ships float. Displacement can also relate to air in heating and cooling systems.

Draughts. Note that I’m not talking about draughts which may be common with these types of systems when they’re off.  The displacement I’m referring to is associated with the normal use of the systems when they’re operating.

Evaporative air conditioners

Evaporative air conditioners are very common in much of Australia and most people are familiar with them. Every litre of air delivered through the vents of an evaporative air conditioner is balanced by a corresponding litre of inside air lost to outside.  So the air conditioner delivers cool air which displaces the inside air and sends it outside through windows, doors, gaps, cracks and chimneys.

To put this in context, systems that move air around can either operate on the basis of displacement or recirculation.

In this type of system, the displaced conditioned inside air represents considerable wasted energy, relative to a system that can keep all the cooled air within the home.

Fireplaces

Less obviously, your basic fireplace is also a displacement system.  Fires need a source of air for combustion.  That air is normally drawn from within the room.  However, in winter, every litre of room air drawn into the fire has to be balanced by a litre of cold outside air drawn into the home, usually through gaps and cracks.

I think most people understand the way that fireplaces deliver heat into a home. But perhaps very few of those people would take into account the offsetting effect of cold air drawn from outside.  So fireplaces necessarily induce draughts unless they have an external source of air.  I think of fireplaces as draught amplifiers.

Fireplace - slow combustion
Wood fires are draught amplifiers

Solar air heaters

A less-well-known type of heating system is a solar air heater. Some have solar collector panels that fit on the roof.  Others effectively use the roof itself as a solar collector – taking advantage of the normal warming of the air within the roof cavity.

These types of systems often, but not always, operate as displacement systems.  So, much like evaporative coolers, every litre of air delivered means a litre of inside air lost to outside.

See more on solar air heaters here.

Displacement as a design feature

Displacement-based systems are sometimes specifically preferred in larger commercial situations if air quality is paramount.  Balancing the need for both good energy efficiency and high air quality is tricky.

Heat-recovery ventilators

An example of a displacement system that has good excellent efficiency is a heat-recovery ventilation system (HRV). HRV systems provide ventilation in air-tight homes and use a heat exchanger to extract heat (in winter) from stale air and transfer that heat to incoming fresh air.  These systems shouldn’t be confused with HRV-branded systems which don’t have a heat exchanger.

HRV systems are not heating or cooling systems by themselves. However, they are mostly used in high-efficiency designs where very little active heating or cooling is needed.

What makes HRV fundamentally different from the other three types of displacement systems in this article is the use of a heat exchanger to recover heat (or cool) energy. So HRV is the right way to balance the efficiency and ventilation requirements. However they don’t come cheap.

Analysis

What these three types of systems share is unintended consequences of heating or cooling methods that involve the exchange of air with outside.  In each case the heating or cooling benefit of the system is diminished or offset as a consequence of internal air being lost to outside.

The lost heating or cooling benefit in each case amounts to a significant loss of system efficiency, relative to systems that can retain all the heating or cooling benefit within the building.

Implications

If you are choosing a heating or cooling system then keep the displacement effect in mind.  Displacement systems are usually a bad choice.

New systems

Aircon. Evaporative aircon systems are nearly always a bad choice.  Displacement effects are just one of a number of reasons why evaporative aircon systems are problematic.  For more information see the BZE Buildings Plan, Part 3, Section 3.

Wood fires. Heating with wood has many problems, discussed elsewhere. However, if you must use a wood heater the I recommend:

  • using a slow-combustion unit (not an open fire), that;
  • has an external air inlet, that sources combustion air from outside.

Solar air heating. I’m not a fan of solar air heating (I’ll do a separate article on that later). If you must use one for heating, then choose one that recirculates rather than drawing in outside air.  If it draws air to heat from within the home then, ideally, it should draw air from floor level where air is coolest.

Existing systems

Wood fires.  Operation of wood fires needs a source of air. One way to achieve this and reduce the impact of induced draughts is to fit a closable floor vent in the floor near the fireplace. An example, from ecoMaster, is shown below.

closable floor vent
An ecoMaster closable floor vent

Evaporative systems. There really is no way to mitigate the displacement problems of existing evaporative air conditioning because it’s so intrinsic to how they operate.

 

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