Furnace Rating Plate Read Before Start-Up

Why this matters

Every number on a start-up sheet is judged against a number the furnace already told you. The rating plate inside the blower door is the furnace's own acceptance criteria: what it should burn, what it should deliver, how hot the air may get across the heat exchanger, how much duct it can push against, and what circuit it needs. A tech who fills in a start-up sheet without reading the plate first has a page of measurements and nothing to compare them to. Read the plate before the first reading, and write the plate values in the "target" column of the sheet so every measurement sits next to the number that judges it.

The plate

Below is a gas furnace rating plate laid out the way most makers print one. The values are typical of a single-stage, 80 percent, induced-draft residential furnace and are illustrative; the job reads the plate on the unit in front of you.

GAS-FIRED FORCED AIR FURNACE   CATEGORY I
Model ............. (model number)
Serial ............ (serial number)

Input ................ 80,000 BTU/h
Heating capacity ..... 64,000 BTU/h
Temperature rise ..... 35 - 65 F
Max ext. static ...... 0.50 in w.c.
                       (heating)

Gas ............ NATURAL
Manifold pressure .... 3.5 in w.c.
Inlet pressure min ... 4.5 in w.c.
Inlet pressure max ... 10.5 in w.c.

Electrical ..... 115 V  60 Hz  1 ph
Max total amps ....... 9.6
Max overcurrent ...... 15 A

Approved for altitudes to 2,000 ft;
above, see installation instructions
Clearances: see instructions / label

The plate is a list of limits and targets. The rest of this card takes each field to the start-up reading it governs, then shows a start-up where two plate limits disagreed with each other.

Field by field: what each number judges

Input, 80,000 BTU/h. The gas the burners should consume per hour at the plate manifold pressure on the gas the plate names. The start-up check is two readings: manifold pressure set to the plate (see Gas Pressure Set-Up Numbers Field Card for the values and taps) and, on natural gas, clocking the meter to confirm the burners are actually consuming the input, within the tolerance the maker states. Manifold right and clock wrong points at orifices, gas heating value or a conversion problem. See related: How to Clock a Gas Meter to Check Furnace Input.

Heating capacity (output), 64,000 BTU/h. What the furnace delivers into the airstream at plate input, under rating conditions. 64,000 / 80,000 = 0.80, which is the steady-state efficiency behind the 80 percent class. This number is the numerator in the sensible-heat airflow formula: plate output and a measured temperature rise give an airflow estimate. That formula, its constant and its altitude caveat belong to Temperature Rise and the Sensible Heat Airflow Formula; this card only needs you to know the plate is where the output comes from. If the clocked input came in low, the real output is low by the same ratio, and the airflow estimate has to use the corrected output.

Temperature rise, 35 to 65 F. Supply air temperature minus return air temperature, measured after the furnace reaches steady state on high fire. The range is a heat exchanger limit and a combustion design limit, not a comfort setting. Above the top, the heat exchanger runs hotter than designed and the high limit may start cycling the burners; below the bottom, flue gas cools enough that a Category I vent and the heat exchanger itself can condense. A rise inside the range is necessary and not sufficient, as the worked example shows. See related: How to Measure Temperature Rise on a Gas Furnace.

Maximum external static, 0.50 in w.c. (heating). The most duct resistance outside the furnace cabinet the furnace was rated against. Total external static is read across the furnace, return side to supply side, with the cooling coil and filter counted as external if they sit outside the cabinet as most do. Exceed it and the blower cannot move the air the rise range assumes, and the plate's capacity number is no longer what you are getting. Some plates list a separate cooling static, or none, leaving it to the fan table. See related: How to Measure Total External Static Pressure on a Furnace.

Gas type, manifold and inlet pressures. The gas the furnace is set up for as shipped and the pressures that go with it. A furnace on propane whose plate still says NATURAL has either been converted without the conversion label being applied, or not been converted. The conversion kit ships with a label that is applied next to the plate and records the new gas and pressures. If you cannot find one on an LP job, stop and find out which it is before you light it.

Electrical: 115 V, max total amps 9.6, max overcurrent 15 A. The circuit the furnace needs. Max total amps is the unit's full-load draw including the blower and inducer; the max overcurrent figure is the largest breaker or fuse allowed. The start-up check is that the existing circuit's breaker does not exceed the plate figure and that it is a dedicated circuit where the instructions or the local code require one. A new circuit, or a circuit that does not meet the plate, goes to the electrician. Measured running amps above the plate's total point at the blower motor or a capacitor, not the circuit.

Category I. The vent category the furnace is listed under, which decides what vent material and configuration it can use. Category I here means a negative-pressure, non-condensing vent, which is why low rise matters on this furnace: it is the condition that pushes it toward condensing in a vent not built for it. See related: Furnace Vent Categories and What Each Allows.

Altitude line. The plate tells you up to what elevation the as-shipped orifices and pressures apply. Above that line, the maker's high-altitude table governs, and a job at 4,500 feet left on the as-shipped orifices and manifold value is likely firing above the derated input the maker calls for; clock it to know.

Clearances. Most plates refer to a separate clearance label. Read it on any replacement, because the new furnace's clearances to combustibles may differ from the old one's and the closet may be tighter than it looked.

Worked example: rise passes, static does not

A replacement furnace, the plate above, in a basement with the original return duct. Gas inlet 7.0 in w.c. under load, manifold set to 3.5, meter clock 79,200 BTU/h, which is 79,200 / 80,000 = 0.99 of plate input, close enough to treat plate output as delivered.

First readings, blower on the factory heating tap (medium-low).

• Return air 66 F, supply air 134 F (in a straight run past the plenum, out of the heat exchanger's line of sight). Rise: 134 - 66 = 68 F.
• Total external static: return side -0.38, supply side +0.44, total 0.38 + 0.44 = 0.82 in w.c.

Judge each against its plate number. Rise 68 F against a 35 to 65 F range: over by 3 F. Static 0.82 against a 0.50 maximum: over by 0.32 in w.c., which is 64 percent over the plate value.

The tempting fix. Move the heating speed up a tap to bring the rise into range. The tech does it:

• Medium tap: supply 127 F, rise 127 - 66 = 61 F. Inside the range.
• Static rises with the extra airflow: -0.43 and +0.51, total 0.43 + 0.51 = 0.94 in w.c.

What the plate says now. The rise passes. The static is 0.94 against 0.50, now 88 percent over. Two plate limits, and the start-up sheet must record both: one in range and one out. A sheet that records only the rise reads as a clean start-up; it is not. The furnace is operating outside the conditions it was rated under, the blower is working harder than intended, and on a replacement that is the existing duct telling you something, not the new furnace.

Where the static went. The tech measures across the filter rack: 0.31 in w.c. on a 1 inch pleated filter in a 16 by 25 slot serving 1,200 CFM or so. That one component is a third of the total. The return is a single 14 inch round drop. See related: Static Pressure Budget Across a Residential System and Return Grille and Filter Sizing by Face Velocity.

The disposition. Leave the blower on the medium tap so the rise is inside range today, because that protects the heat exchanger. Record the 0.94 in w.c. total static as out of plate. Write up a return and filter correction as the recommended repair with the readings attached, and tell the customer in plain terms: the new furnace is installed and safe to run, and the duct feeding it is too small for what the plate allows. Do not write "start-up complete, all readings normal."

How the plate fields map onto the start-up sheet

Combustion readings are the one major start-up check with no plate value; the targets come from the installation instructions and the analyzer method in Gas Furnace Combustion Analysis and Tuning.

References

• Furnace rating plate, conversion label and installation instructions
• See related: Gas Pressure Set-Up Numbers Field Card; Temperature Rise and the Sensible Heat Airflow Formula
• See related: How to Measure Total External Static Pressure on a Furnace; Furnace Vent Categories and What Each Allows
• See related: The Gas Furnace Start-Up Sign-Off SOP