Upgrading the Supply and Return 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.
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.........