How to read the data plate on your own equipment

Gas furnace. Inside the cabinet, usually on the inner wall behind the front panel or on the back of the panel itself. You will normally have to take one door off to see it. Turn the appliance off at the service switch first, and if the panel does not come off easily, leave it — this is not worth a torn hand.

Outdoor unit. On the outside of the cabinet, usually on the side nearest the pipework, at or below waist height. Often faded by sun and weather to the point of being hard to read. Nothing needs to be opened.

Air handler or indoor coil. On the cabinet side, or behind the access door on the coil casing. On a system where the furnace does the air moving, the coil sitting on top of it has its own smaller plate.

Boiler. On the jacket, often on the side or behind a hinged front panel, and sometimes a second plate for the burner separate from the boiler itself. Older cast-iron boilers can have a plate that is genuinely illegible; the section castings usually carry marks too.

The single most useful string on the plate. It identifies the exact unit, and with it anybody can retrieve the installation instructions, the parts list and the specification sheet.

• It settles the altitude question. Whether this model needs a high-altitude kit or a pressure change at Greeley’s elevation is stated in its own installation instructions, and those are found by model number.
• It tells you whether a part quoted to you is the right part.
• Digits within it usually encode the capacity — but the encoding differs by manufacturer, so read the capacity off the plate rather than decoding it yourself.
• Photograph it. One photograph of the whole plate saves a return visit and lets a part be ordered on the first call.

This is where the real age lives, and it is the field that most often contradicts what somebody has been told.

• Most manufacturers encode the week and year of manufacture in the serial — commonly the first four characters, sometimes the last four, sometimes a letter code. The scheme is different for every manufacturer, so it is looked up rather than guessed.
• It dates manufacture, not installation. A unit can sit in a distributor’s warehouse for a year, and equipment fitted to a new build is often a year older than the house.
• Why it matters to you: “that’s twenty years old, it needs replacing” is checkable in one minute. So is a warranty claim, because registration and coverage both run from this number.
• If the plate is unreadable, the serial is often stamped on the heat exchanger or the compressor as well.

Two numbers that are frequently confused, and the difference between them is the whole efficiency argument.

• Input is the gas going in. Output — sometimes called capacity or bonnet capacity — is the heat reaching the house. Output divided by input is roughly the AFUE.
• A quote that replaces “100,000 BTU” with “100,000 BTU” may be swapping an old unit’s input for a new unit’s input and delivering considerably more heat, or comparing input with output and delivering less. Ask which number is being matched.
• The old number is not the target. Furnaces installed before about 2000 were routinely oversized by a large margin. A heat loss calculation, not the old plate, sets the new size.
• At this altitude the input on the plate is a sea-level figure. What the appliance actually delivers here is lower unless it has been set up for elevation.

How much cooling the equipment is rated to do. One ton is 12,000 BTU/h, so a three-ton unit is nominally 36,000.

• It is a nominal rating measured under standard test conditions. What the unit does on a 97°F afternoon with a dirty coil is a different figure.
• The outdoor unit and the indoor coil have to match. A three-ton condenser on a two-and-a-half-ton coil performs like neither. If only one was ever replaced, this pair of plates is where you find out.
• Bigger is not better for cooling. An oversized air conditioner cools the air fast, shuts off, and never runs long enough to remove moisture — which matters less in a dry climate than in a humid one, but still produces a house that feels cold and clammy in one room and hot in another.
• Model numbers often contain the capacity in thousands of BTU/h, which is why 024, 036 and 048 appear so often.

The smallest conductor the unit may legally be wired with. It is a wire-size instruction, not a measurement of what the unit draws.

• This is the field that decides whether a replacement is a simple swap. If the new unit’s MCA is higher than the circuit that is there, the circuit has to be upgraded — and that can mean a new run, or in an old house a panel that is already full.
• A quote that does not mention the electrical supply on a like-for-like replacement of a larger unit is a quote with a gap in it. Ask.
• It is calculated by the manufacturer from the running loads with an allowance on the largest motor, which is why it is not the same as the running current.
• Undersized wiring on an outdoor unit is common on installations where somebody upsized the equipment and left the circuit alone.

The largest breaker or fuse permitted to protect this unit. Usually printed right beside the MCA, and sometimes labelled “max fuse” or “max breaker”.

• It is a maximum, not a requirement. A smaller breaker is allowed as long as it is at least the MCA and does not nuisance-trip on start-up.
• A breaker larger than the MOCP is a genuine fault, not a preference. It is one of the most common things we find on equipment that has been replaced without the circuit being revisited, because the old unit’s breaker was simply left in place.
• You can check this yourself in thirty seconds: read the MOCP on the plate, then read the number on the breaker handle in the panel. If the breaker number is larger, that is worth raising.
• It also tells you what a fuse in an outdoor disconnect should be, which is where undersized and oversized fuses hide.

Which refrigerant is in the system and how much of it left the factory in the unit.

• R-410A is what most equipment of the last twenty years uses. R-22 means the unit is old — it has not been used in new equipment for many years and it is no longer produced or imported, so only reclaimed stock exists and it is expensive. Newer equipment uses lower-GWP refrigerants such as R-454B or R-32.
• Refrigerants are not interchangeable. A system is designed around one, and a “drop-in” substitute changes capacity, oil compatibility and often the warranty.
• The factory charge is the charge for a stated line length only — typically 15 or 25 feet. A longer line set needs additional refrigerant, and an installation where that was never added runs short for its whole life.
• A system that needs refrigerant every year has a leak. Refrigerant is not consumed. Being topped up annually is a subscription to a fault, and finding the leak is the actual job.

The rating that gets quoted in sales conversations, and the one that means the least on its own.

• AFUE is seasonal heating efficiency for fuel-burning appliances, as a percentage. SEER or SEER2 is seasonal cooling efficiency; HSPF or HSPF2 is the heating figure for a heat pump. The “2” ratings came from a test procedure change and are numerically lower for the same equipment, so a SEER2 figure cannot be compared with an older SEER figure.
• All of them are laboratory ratings. They assume correct sizing, correct charge, correct airflow and sealed ducts. A high-rated system on leaking ducts in a vented attic will not deliver its rating and never could.
• It is not on the plate of every unit, because efficiency is a property of a matched system rather than one box. It is often on the AHRI certificate for the combination instead.
• For a rated efficiency to mean anything the outdoor unit, indoor coil and air handler have to be the combination that was tested. Mixed equipment does not carry the rating.