All About Reusing Old Central Air Systems

All About Reusing Old Central Air Systems
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Introduction to A Hot Air Central Heating System

Electric central heating is still a popular form of heating, the electricity powering two quite different mechanisms.

The earliest one was a hot air type which circulated air heated by an electrical element and forced through ducts under the floor which blew the hot air into the various rooms of the house. It was a grossly expensive and inefficient means of heating the house, especially in conjunction with an electric immersion heater and radiant fire. It was also notoriously noisy in operation and would wake the whole house when it fired up.

It went out of fashion on the introduction of off-peak electric total control electric central heating. This type of heating had large heating panels in the rooms of the house. These panels contained bricks of high thermal mass which were heated by electric elements during the night on cheap power. This heat was then released during the day, being controlled by a series of flaps, which when opened, emitted the stored heat.

However the hot air system was basically sound, albeit noisy, but very soon became unpopular because of the unit price of electricity. However, with the coming of gas and oil, a lot of folk converted the heating unit to one of these fossil fuels.

So in this article, we shall investigate the reusing of an old central air system, which includes using geothermal energy from under the ground in your garden instead of fossil fuels.

We start with the upgrading of the ductwork, then describe the components of the upgraded hot air unit, finishing with a look at the geothermal heating input.

Upgrading the Supply and Return Ductwork

The Ductwork

The ductwork for the supply and return of the air was usually of circular spiral wound aluminium, or light rectangular galvanized sections manufactured from steel sheeting. Both types were prone to leaky joints and soon lost their surrounding insulation (if it had any in the first instance) by constant repairs to air leaks. Air leaks were caused by a combination of poor joint adhesive and bad duct support arrangement which allowed the ducting to sag.

The thinner rectangular air return ducts run vertically upwards from the underfloor return ducts through the wall sections. These draw the air from the various rooms through individual grills which are set at positions in the walls for optimum system efficiency, returning the air back into the air heating unit through the air filter.

Attention to Joints and Supports

We shall use the existing ducting if it is light and non-corrosive, but reseal all the joints with a two-part epoxy adhesive, overlapping them with duct-tape 100mm on each side of the joint (duct-tape is waterproof vinyl fabric backed very tacky tape which stays put forever). I have used this method of jointing ductwork in the accommodation modules on offshore oil and gas platforms and can guarantee its effectiveness.

Insulating the Ductwork

Modern insulation can be purchased in rolls of various lengths and thickness which can be cut into strips and tightly wound around the ductwork, again applying duct tape to secure the sections, with the insulation being applied to both supply and return ducting.

Ductwork Supports

The supports were normally of the hanging type which consisted of a steel circular loop attached to the duct and to the floor joists with a screwed rod. These proved to distort the ducts if fitted too tightly around the duct, but vibrated if too loose. This caused a lot of the noise associated with this type of heating system.

The best method of supporting both the supply and return ducts is to support it from underneath with square-shaped wooden supports made from rough sawmills, 2” x 2” sized timber. These sit between the ducts and the top surface of the solum with a piece of soft foam rubber between the support and the bottom of the duct, transferring any vibration to the sand/earth of the solum.

Read on to see the rest of the components involved in upgrading your old central heating system and learn how geothermal energy supplies the heat to the hot air unit………

The Components

  1. The New Hot Air Unit
  2. The Circulating Fan
  3. The Air Filter
  4. Air Humidifiers
  5. The Thermostat and Programmable Timer

Geothermal Powered Hot Air Central Heating Unit

1. The Heating Source

We shall be using a heat pump to heat the water which circulates through the coil. (Please see a previous article of mine on geothermal energy for the home for a full description of the use of this renewable energy source)

Briefly, solar energy is absorbed by the earth and this heat is transferred to closed loop coils which are laid horizontally or vertically under the ground in your front or back garden. This heat is then transferred to the water mix circulating through the coils. The heated water is fed into a heat pump which raises the temperature considerably, and this heat is transferred to the water running through the heating coil in the central air unit.

2. The Circulating Fan

The new fan will be a lot quieter running and more efficient than the original one.

The fan draws the air in from the rooms via the return ducting and forces it through the heating coil (heat supplied from the heat pump) and out through the supply ducting into the various rooms.

3.The Air Filter

This is essential, and if there are any of the family with dust allergies or asthma, a high performance air filter should be fitted to the returning hot air ducting where it enters the heating unit.

4. Air Humidifiers

The humidity of the air in your home should be between 30 and 50%, anything lower than this giving dry eye and skin, a higher level encouraging condensation with misting of the windows. The humidifier consists of a device which introduces water vapor into the hot air system, preventing the above symptoms.

5. The Thermostat and Programmable Timer

The thermostat is usually wall mounted and located in the room of the house that is most frequented. It is a simple device which can be set to the desired room temperature, and maintains this by stopping and starting the hot air circulating fan. The timer is beside the heat unit and programmes can be set to stop and start the system as required, controlling the heat pump and geothermal heat input system.


Reusing an old central air system can be accomplished by upgrading the ducting and exchanging the old electrical powered heating unit for a more modern type, running on renewable energy supplied by geothermal energy using a ground source heat pump.

The old electric heating unit will go for recycling, while this new, quieter, more efficient, environmentally friendly one, which costs a fraction of the original unit to run, replaces it.

The underfloor ducting is maintained having all the joints remade, insulation applied and duct supports fitted under the supply and return air ducting.

The hot water produced from geothermal energy is pumped through a set of coils, which transfers the heat to the air. This heated air is then circulated through ducts under the floor which blow the hot air into the rooms. This is carried back to the unit through a grill on the wall of the room, by quite narrow ductwork which runs down behind the dividing walls of the rooms. It is connected into the main return duct under the floor.

A humidifier is incorporated in the system to maintain the recommended air humidity and a thermostat on the living room wall controls the temperature. A programmer timer is used to set the start and stop times of the heating system.

Sketches of an Upgraded Heating Unit and Hot Air Flow Diagram

Air Heating Unit

Internet Sites Visited

Merriam Webster: Warm Air Systems

This post is part of the series: Energy Savings Tips for the Home

In this series, you will find some energy savings tips for the home that will also end up saving you money in the long run. You will learn about better insulation for your home’s roof and walls, draft-proof windows and doors, hot water tanks, and more efficient central heating systems.

  1. Energy Efficient Fixes for Your House Reduces Carbon Footprint
  2. High Efficiency Central Heating System Components - Condensing Boilers and Programmable Timers
  3. Improve Your Home Heating Using Energy Efficient Heating System Components
  4. Recycling and Reuse of Old Hot Air Central Heating Systems
  5. Save Energy for Your Home - Take the Survey