Furnace Temperature Rise

There is no normal figure, and that is the single most important thing to know about this measurement. The acceptable rise is printed on your own unit's rating plate as a range, and it is a property of that unit rather than of furnaces in general. The clearest evidence for that is where the manufacturers themselves send you: both manuals behind this tool route the answer to the specific unit. Carrier's service manual says the rise must be within the limits specified on the furnace rating plate, and Velocity's judges it against a table keyed to the model. So read the TEMP RISE line off your plate and judge against that, and treat any single typical range you are offered as a guess about your furnace rather than a fact about it.

Temperature rise is the supply air temperature minus the return air temperature, measured with the furnace running and settled. Take the supply reading in the plenum close to the furnace but where the probe cannot see the heat exchanger, and the return reading in the return duct. Then compare that difference against the range on the rating plate. The comparison is the whole measurement; the subtraction on its own tells you nothing.

Two things, and a temperature rise reading cannot tell them apart. In the manufacturer's own words, a rise above the plate range means the furnace is overfired or has insufficient airflow. Overfired is a gas-side problem, so check the input rate at the meter and the manifold pressure, and derate for altitude if the unit is high up. Insufficient airflow is an air-side problem: the heating blower speed is too low or the duct system is restricted. Check the gas input first, because it is the one that also explains the reading. A high rise damages equipment, and Carrier files a rise outside the plate limits under a furnace damage hazard naming two mechanisms: overheating the heat exchanger, which is the high-rise one, and condensing flue gases in heat exchanger areas not designed for condensate, which is the low-rise one. Being outside the band is the hazard, in either direction.

Again two causes and the reading cannot separate them: the manufacturer's wording is that the furnace is underfired or has too much airflow. Underfired means checking the gas input rate, the manifold pressure and the orifice; too much airflow means the heating blower speed tap is set too high. Check the gas input first. A low rise is not merely an efficiency problem, which is the intuition worth correcting. Carrier attaches the same furnace damage hazard to being outside the plate limits in either direction, and the low-rise mechanism it names is flue gas condensing inside heat exchanger metal that was never specified to handle condensate. That is why a rating plate prints a minimum rise as well as a maximum.

CFM = heat output divided by (1.08 times the temperature rise). The 1.08 is not a magic number: it is 60 minutes per hour times 0.075 lb/ft3 standard air density times 0.24 Btu per lb per degree F, which multiplies out to 1.08. So an 80,000 BTU/h output furnace running a 52 F rise is moving 1,425 CFM. Use the OUTPUT rating, not the input rating. If your plate only prints an input rating and an AFUE, input times AFUE is an estimate of output rather than a measurement of it, because AFUE is a rated seasonal descriptor fixed by a federal test procedure and carries its own installation assumptions.

Because several ordinary field conditions make the reading mean something other than what it looks like, and giving you a confident verdict on one of those would be worse than giving you nothing. A dirty or missing filter makes the reading a statement about the filter rather than about the furnace. A supply probe with line of sight to the heat exchanger absorbs radiant heat and reads high, which looks exactly like a starved blower. A reading taken before the temperatures stopped climbing is a reading of a system that has not reached steady state, and it will not match the one you get a few minutes later. A blower door left off changes the airflow, and the manufacturer calls the result an incorrect measurement. A bypass humidifier or zone bypass feeds supply air back into the return, which warms that reading, shrinks the rise, and can turn a starved furnace green. And without the rise range off your plate there is a number but no standard to judge it against. In each of those cases the tool says which one it is and what to do about it.

Yes, and this is an easy way to get a confident wrong answer. A two-stage plate can print a separate rise range for high fire and for low fire, and Carrier's instruction for its two-stage furnaces is to determine the rise for both and exceed neither. Compare a low-fire reading against the high-fire band and it can read low while the furnace is perfectly correct. This tool asks which stage you read it on for exactly that reason.