Suspended Gas Fired Warm Air Heating

A modern suspended gas fired heater is designed around the heat exchanger. A section of pipe is formed into a coil shape and several formed tubes together create the heat exchanger. Each pipe becomes an individual pass of the same length. The strange shapes make the pipe length longer and increase the surface area of the heat exchanger. The discharge ends of the pipes join together into a welded box with a single exit fed into a combustion fan. The discharge side of the combustion fan is fed directly out to the flue. These pipes are held together inside a frame and form the main part of the heater.

Early versions of the tubular based heat exchanger suffered from various problems such as splitting and burning tubes. Various design and material changes have now made the heat exchangers last for many years.

The fuel (gas) is fed to the appliance and arrives at the Gas Valve. From the gas valve it follows down a pipe which is placed close to the input of the heat exchanger tubes. Pointing from the gas manifold are the actual jets which inject the gas directly into the tubes.

This primary side of the heater is contained inside sealed box and the heat exchanger tubes terminate to this primary box. Generally combustion air is fed into this primary box through a round supply air hole in the casing. This can also be ducted from the outside to supply fresh / safe air for the combustion process when used in some hazardous areas.

Because air is drawn into the unit, it always remains at negative pressure to prevent leakage back into the room.

Mounted on the rear of the heater is an axial fan with a finger guard. This will blow room air over the heat exchanger tubes and provide heat for the room.

A Safety Gas Control Box operates the sequence of the gas heater in the following way.

The combustion fan will start up and suck plain air through the heat exchanger tubes for a few minutes. This is a safety purge procedure to remove any gas which may remain in the heat exchanger from a shut down or failed start.

A dual purpose probe (ignition / ionisation) probe is mounted near the lower gas injector and mounted close to a good ground. The control box will turn on the ignition transformer and a spark will be formed inside the heater.

The control box now applies power to the main gas valve which starts to open up and allows gas to flow from the injectors. The gas mixes with the air being sucked into the heat exchanger by the combustion fan and is ignited by the ignition spark. The other gas mixtures combust from each other and form multiple flames one burning into each tube.

The ignition is generally held on by the control box for a few seconds and then is switched off.

Immediately the control box tests for a small current which will be generated by the ionisation probe if it is inside a flame. If the flame is detected the heater continues to operate and warm up.

If the probe detects no flame the heater immediately shuts down and enters “lock out”. The heater cannot be restarted for a period of time. Continued lock out will require investigation by a Gas Safe engineer.

The combustion fan pulls air and gas into the tubes where combustion occurs and pulls the hot waste flue gases around the tubes which continue to heat up and eventually cause the cabinet to heat up.

A thermostat mounted inside the cabinet detects that the heater is operating and turns on the fan at the back of the heater. Cool air is drawn from the room, blown over the heat exchanger and leaves the front of the heater and is blown into the work space.

The heater continues to operate and warm the workspace.

An additional thermostat called the High Limit turns the gas valve off if the heater becomes too warm. A failure of the fan motor would cause the High Limit to activate. This safety feature prevents the heater from catching fire.

Generally the High Limit can be re-set from the central control which is mounted near ground level for user convenience.

When the workplace controls signal to the heater to turn off, the control box closes the gas valve which extinguishes the flame and then turns the combustion fan off.

The main fan continues to draw cool air in to blow over the heat exchanger to cool it down. This cool down process is most important and prolonged failure to allow the cool down to complete will cause the heat exchanger to become damaged.

When the cabinet is sufficiently cooled the main fan motor is turned off and the heater has shut down. When turned on again the process is repeated.

It is important to have the heater regularly inspected and serviced and the integrity of the heat exchanger professionally tested. In the event of a failure to the heat exchanger components Carbon Monoxide could easily be transferred into the building warm air which could be fatal to occupants.

Qualified Gas Safe engineers hold the necessary training and testing equipment to ensure that continued use of an appliance is permissible.

(C)2011 Philip Hamer. Warmairheaters.com
www.magma-group.co.uk