______________________________________________________________________ A friend has decided, due to warming temperatures, to reduce the gas flow to 80% of normal by partially closing the main shutoff valve. He has a natural draft furnace and says the flame is about 3/4 the height when the main gas valve is fully opened. The furnace now works perfectly with the gas flow reduction, other than a longer cycle time, which was his goal. What dangers has he overlooked? Replies: Brandx30 “Re(1):Dangers Of Underfiring A Gas Furnace” , posted Sat 5 Feb 20:28 ______________________________________________________________________ I don’t get it. If the system runs longer with a reduced gas flow where’s the savings? I liken that to something like putting the pedal to the metal if you’re gonna run out of gas hoping you will coast farther. 🙂 hvac_dude “Re(1):Dangers Of Underfiring A Gas Furnace” , posted Sat 5 Feb 15:31 ______________________________________________________________________ I have also closed down the gas valve on oversized units to get a better operation… but just slightly… You can also take the orifice off the last burner and install a plug ( if the unit is very oversized)… but you will have to move the flame sensor to the ‘new’ last burner to get the furnace to operate…. Mastertech “Re(1):Dangers Of Underfiring A Gas Furnace” , posted Sat 5 Feb 12:48 ______________________________________________________________________ I do that some times when someone has an oversized unit. But not more than 30%. It might cause the unit to rust more becuase the heat exchanger is cooler. I worry more about the unit not liting all chambers and getting a build up which we all know is a bad thing. Nothing like burning some hairs off to get your attention lol. Things i look for is will it light evertime with out a problem. and am i getting 40 degree rise. Freon “Re(2):Dangers Of Underfiring A Gas Furnace” , posted Sat 5 Feb 16:20 ______________________________________________________________________ The ultimate goal is to make a multi-stage furnace from a single stage. Here’s how it will work: Main gas line after the main shutoff valve goes into a T connection. Main gas valve will be full open. From one side of the T (branch A) runs a gas line that continues through another gas valve that’s set to 80% of full open. From the other side of the T (branch B) runs a gas line through a solenoid valve. Both of these parallel gas lines (branch A and B) each connect to the sides of another single T and then, through one pipe, on to the furnace’s gas valve. The solenoid valve, when opened by the Aux control of the thermostat, allows for full gas flow to the furnace through both branches, A and B. When no Aux call, the solenoid valve is shut forcing the gas flow through branch B that has the 80% open gas valve. sascuderi “Re(3):Dangers Of Underfiring A Gas Furnace” , posted Sat 5 Feb 18:19 ______________________________________________________________________ Whats the advantage of doing this? Freon “Re(4):Dangers Of Underfiring A Gas Furnace” , posted Sat 5 Feb 21:00 ______________________________________________________________________ The advantage is to lessen the number of short cycles. Heating and AC systems are the most efficient when they run for long times at stable conditions. That’s why the high efficiency heating systems are variable speed/load now. In winter a system will run most of the time at full BTU capacity. But in fall or spring, when the heating demands drop due to warming outdoor temps, the systems will run shorter cycles during the day. By underfiring, or lowering their heat delivery capability, the furnace will run longer delivering fewer BTU’s per hour, and therefore the system should be more efficient overall.