There is nothing that can be done to increase a furnaces capacity past what is listed on its own nameplate. All furnaces must be running at + or – 2% of their nameplate capacity. If the furnace is too small for the application, the furnace must be replaced with a properly sized furnace.
Check gas pressure at manifold. For optimum efficiency of a furnace, the burners should be operated at 100% of their rated input. Under-fired burners can cause the furnace to produce less than its rated heat, while over-fired burners can cause the furnace to overheat. A water or oil manometer is used to measure gas pressure in inches of Water Column Pressure or (WCP). A manometer is clear and shaped like the letter ”U”. Readings from spring type dial gauges are not acceptable. In most locations, natural gas entering the home is regulated between 6 and 7 inches WCP. LPG gas is regulated to 11 inches WCP. The appliance pressure regulator reduces the incoming pressure to the required amount, which is usually 3.5 inches WCP for natural gas installations and 10.5 for LPG installations. To measure gas pressure (clock the gas meter) follow the instructions below, make any major adjustments by changing the main burner orifices.
Clocking the gas meter
a. Obtain average yearly heat value from local gas supplier.
b. Check and verify orifice size in furnace, NEVER ASSUME ORFICE SIZE.
c. Turn off all other gas appliances and let the furnace run for 3 minutes.
d. Measure (in seconds) for gas meter to complete 1 revolution.
Step 1. Gas flow rate=
Cubic feet per revolution x 3600 (seconds per hour)Time of one revolution in secondOR
“Example” Gas flow rate=2.0 cubic feet x 3600 (seconds per hour)48 seconds = 150 cubic feet per hour
Step 2. Heat input rate=
cubic feet per hour x BTU/cubic feet = BTU/hour
“Example” Heat input rate=150 cubic feet per hour x 1050 BTU/cubic feet = 157,500 BTU/hour
Adjusting the gas input
a. Remove regulator adjustment seal cap.
b. Turn adjusting screw counterclockwise to decrease input. Turn adjusting screw clockwise to increase input. Do not set manifold pressure less than 3.2 inches WCP or more than 3.8 inches WCP for natural gas. Make any major adjustments by changing main burner orifices.
c. When correct input is obtained, replace regulator seal cap. Main burner flame should be clear blue, almost transparent.
The furnace limit switch is a safety switch that will shut off the main burners to protect the furnace form overheating. Some reasons a limit switch might be tripping; dirty filter; filters should be maintained on a regular basis, dirty indoor air filters will cause restricted airflow across heat exchanger. Duct restricted; a duct supply/return duct that is kinked, smashed, pinched, crushed, flattened or any anyway restricted will have the same negative effects on the airflow as dirty filters. Dampers partly closed; dampers either motorized or manual are designed to restrict the airflow to areas that do not need or have too much air volume. Dampers must be used in moderation. Check if a damper has come loose or is positioned incorrectly thus unknowingly restricting the airflow. If the blower has adjustable speed taps, verify the blower is at a heating speed. If it is set too low, use a faster speed tap. Verify the “off” setting (the temperature at which the indoor fan will turn off after a heating cycle) on the fan control is set properly. If it is set to low it will shut of the indoor fan before all heat has been dissipated out of the heat exchanger. Reset higher if necessary. Check heat anticipator setting on the thermostat, it may be shutting off the furnace before it has reached set point; readjust.
The existence of a pilot is necessary to light the main burners. Check that the standing pilot flame has the correct color and size to prove the flame and that it extends above the burner carryover ports so that it will properly ignite the main burners. If the pilot gas pressure is too great it may cause the pilot to become unstable and lift off the pilot assembly. If the pilot pressure is too small it will not light the burners in a timely manner and possibly cause a build up of gas around the burners causing delayed ignition. If the pilot is out: relight the pilot per the instructions on the door of the furnace. If pilot does not remain lit, adjust pilot gas pressure or troubleshoot the thermocouple.
Check for tight bends, kinks or restrictions in the pilot gas line that might be restricting the flow of gas. Also check for internal build-up or restrictions in the pilot orifice located inside the pilot assembly.
A properly adjusted pilot flame has a soft blue color with some yellow at the tip and has sufficient height to impinge on the thermocouple. The flame should not be too hard or lift from the pilot burner. If the size of the flame is too small or too large, adjusting the pilot pressure adjustment screw on the gas valve can normally change it. On some gas valves, the adjustment screw is recessed and covered by a cap that must be removed; on others, it is readily accessible on the surface. Turn the pilot adjustment screw clockwise to increase and counter clockwise to decrease the gas flow to the pilot flame. Note that some gas valves may not have a pilot gas regulator. These are preset at the factory for a specific pilot gas pressure and cannot be adjusted in the field.
Check to see if main gas valve is operating. Verify that all manually operated gas shut off devices external to the furnace are open. Look for loose or broken 24-volt wiring connections. If the gas valve has 24-volts but will not energize, replace gas valve.
The thermocouple senses the heat of the pilot flame and generates a small DC voltage up to 30 millivolts (mV) or higher. A gas valve normally requires 14 – 18 mV. The heat from the pilot flame causes a current flow in the thermocouple that energizes a solenoid in the gas valve. The energized solenoid holds the safety gas valve open, allowing gas to flow through the main gas valve to the pilot. If the pilot flame goes out, the thermocouple cools and the voltage generated drops to zero. The current will then stop flowing to the solenoid, causing it to de-energize and the safety valve to close. This will shut off the gas flow to the pilot and main gas valve. Should the pilot flame go out, the pilot must be relit manually and the flame allowed to heat the thermocouple long enough so that it generates the voltage needed to energize the gas valve again. Verify the top of the thermocouple (typically 3/8 to ½ inch) is located in the path of the pilot flame. If the thermocouple does not produce enough mV it can be cleaned with a fine sanding cloth. If it still does not produce enough mV it is a bad thermocouple and must be replaced.
Look for signs of a problem, poor venting, or rusted/damaged vents and vent connectors, excessive moisture in the house. Check burner flame, it should be blue, not yellow. Check for heat marks or soot on furnace or water heater or soot on vent or other debris. Check the air shutters on burners, they may be closed or restricted; adjust till there is a soft blue flame. Check for restriction in the heat exchanger furnace venting system. The furnace venting system carries the products of combustion from the furnace to the outdoors. The type and size of the vent system must be carefully matched to the furnace. Undersized or restricted vent can limit the flow of gases, resulting in poor furnace performance and possible unsafe conditions. Lack of combustion air; see installation instruction for required amount. Also don’t block air ducts or door openings (undercuts or louvers) in furnaces or water heater closets. An over fired furnace can cause premature failure of heat exchanger; reduce gas pressure to required amount. Check to verify main burner orifices are sized properly. The correct orifice size should be listed on the furnace’s nameplate. If the heat exchanger is cracked, replace it. If the furnace is located in a garage, make sure the return ducts are sealed to prevent vehicle exhaust fumes from being pulled into the furnace.
Check to make sure the thermostat is calling for heat or “closed”. The thermostat may look like it is calling for heat on the outside, but if the thermostats contacts either internal to the thermostat or on the sub-base are dirty the thermostat may still be open. Also check for bad transformer or broken/cut thermostat wires.