How to Measure Total External Static Pressure on a Furnace

Why this matters

Total external static pressure, TESP, is the one number that tells you whether the duct system lets the blower do the job the furnace was rated for. It is the input to the fan table, the first thing a maker's tech support line asks for, and the quickest way to separate "the equipment is wrong" from "the ductwork is choking it." It is also one of the easiest readings in the trade to take badly: a port in the wrong place, a tip turned sideways, or a filter counted on the wrong side of the line, and the number is off by a third without looking wrong.

This article is the measurement on a gas furnace with an add-on cooling coil. How to read the instrument itself, zeroing and reference ports included, belongs to How to Read a Manometer Honestly; how the total splits across filter, coil and duct belongs to the Static Pressure Budget Across a Residential System card.

What "external" means, and why the ports go where they go

A furnace's rating plate lists the maximum external static pressure the blower is rated to work against. "External" means everything outside the furnace cabinet as the maker rated it. On a typical gas furnace that includes:

• the filter, unless the maker states the rating includes a filter in the cabinet;
• the cooling coil, which sits outside the furnace and is a separate component;
• all supply and return ductwork, grilles and registers.

So the two ports bracket the furnace and nothing else. The return port goes between the filter and the blower, downstream of the filter, so the filter's resistance lands in your reading. The supply port goes between the furnace outlet and the coil, upstream of the coil, so the coil's resistance lands in your reading too. Measure supply above the coil instead and the coil vanishes from the number; measure the return ahead of the filter and the filter vanishes. Both mistakes read low, both make a choked system look healthy, and both are common.

        SUPPLY PLENUM AND TRUNK
     +-------------------------+
     |      CASED A-COIL       |
     |      (drain pan)        |
     +-------------------------+
 P2 ->  supply port: coil case, below
     |  the pan, above the furnace flange
     +-------------------------+
     |  HEAT EXCHANGER         |
     |  BURNERS                |  UPFLOW
     |-------------------------|  FURNACE
     |  BLOWER                 |
     +-------------------------+
 P1 ->  return port: return drop,
     |  downstream of the filter
     +-------------------------+
     |  FILTER RACK            |
     +-------------------------+
            RETURN DUCT

On an air handler with the coil inside the cabinet, the coil is part of the rated unit, and the supply port moves to the supply duct just past the cabinet. Read the air handler's rating plate and fan table to see what the maker included.

Start state

• A digital manometer zeroed in the orientation you will use it, with two hoses and two static pressure tips: the L-shaped probes with a closed nose and small holes around the side.
• A step drill or bit sized to your static tips and your test plugs, commonly 3/8 in, and a supply of proper test plugs. Tape is not a plug.
• The furnace rating plate read and the maximum external static noted (see related: Furnace Rating Plate Read Before Start-Up).
• A clean filter of the type the customer actually uses, or a decision to measure with the filter as found. Write down which; the number means different things.
• Know which mode you are testing. Cooling speed is usually the highest airflow and the worst case for static. A wet coil adds resistance, so for a cooling-mode number the system should have been running in cooling long enough to wet the coil, commonly 10 to 15 minutes.

Step 1: drill the ports in the right spots, checking behind first

Before drilling, look and feel behind the sheet metal. A bit through a coil case can hit the coil tubes or the drain pan; a bit through a return drop can hit a thermostat wire, a humidifier line or a control. Shine a light through an existing seam or the filter slot first. Never drill into the furnace cabinet over the heat exchanger or into the vent connector. Drilling sheet metal throws chips; wear eye protection. Drilling through lined duct or duct board releases fibers you can breathe, so wear a filtering facepiece respirator for that hole, under the employer's respiratory protection program per 29 CFR 1910.134 where one is required.

• P1, return: in the return drop or transition between the filter rack and the blower inlet, in the flattest straight section available, a few inches from the filter rack so the probe is not sitting in the turbulence right at the filter face.
• P2, supply: on the coil case just above the furnace flange and below the coil's drain pan. Some coil cases leave very little room there; if the coil sits directly on the furnace with no gap, measure the coil's location before drilling, and where the maker provides test ports, use them.

Why it matters: a port beside a sharp turn, a damper or the coil face reads a local pressure that does not represent the section, and two techs at two holes get two different numbers.

Step 2: insert the tips facing the airflow

Insert each static tip so its nose points upstream, into the oncoming air, with the body square to the duct wall. The side holes then sense static pressure only. Turned sideways or backwards, the tip picks up part of the velocity pressure or a wake, and the error is large on the supply side where velocity is highest.

Connect P2 (supply, positive) to the high port and P1 (return, negative) to the low port if you want TESP in one reading. Or read each against room air separately, which is what you want the first time on a system, because the split tells you which side is the problem.

Step 3: run at the speed under test and wait for it to settle

Start the equipment in the mode being tested, door closed, so the blower sees the real system. A blower door off changes the return pressure because the cabinet now draws room air. Let the reading settle; a reading that keeps drifting usually means the coil is still wetting or a supply damper is moving.

In heating mode, a firing furnace means a combustion appliance running in front of you. Wear a personal CO monitor while you work at it, and if it alarms, shut the furnace off and step out before deciding anything else.

Step 4: read both, then add them

Supply static reads positive and return static reads negative. TESP is the supply reading plus the return reading taken as a positive number. Read to the hundredth of an inch of water column.

Worked example: a choked 3-ton system read honestly

An upflow 80,000 BTU/h input furnace with a cased 3-ton coil, bottom return through a filter rack under the furnace, laid out as drawn above. The plate lists a maximum external static of 0.50 in w.c. in this example. Complaint: long run times and a whistle at the return grille.

Tips set and checked: P1 in the return drop downstream of a 1 in pleated filter; P2 on the coil case below the pan.

Readings, cooling speed, coil wet after 15 minutes:

• P1, return: -0.38 in w.c.
• P2, supply: +0.47 in w.c.
• TESP: 0.47 plus 0.38 = 0.85 in w.c.

Against the plate: 0.85 minus 0.50 is 0.35 in w.c. over, which is 70 percent above the rated maximum (0.35 divided by 0.50).

What the split says before any further holes: the return side alone is 0.38 in w.c., about 45 percent of the total (0.38 divided by 0.85), on a system where the filter sits inside that return number. A one-hole check across the filter was the obvious next reading, and it is where the budget card takes over.

What an untrained reading would have reported: an earlier service ticket on the same system showed "static 0.42, OK." That tech had read the supply in the plenum above the coil, and the return ahead of the filter. The coil drop and the filter drop never entered the number. The furnace had been working against roughly twice what the ticket said.

Plug the ports: both holes got test plugs pressed fully home, labeled with marker "SP" so the next tech uses the same holes. Reusing the same holes is what makes before-and-after readings comparable.

What changes the reading, and what to do about each

• Filter as found versus clean. A loaded filter can add several tenths of an inch. Record which you measured. A clean-filter reading tells you about the design; an as-found reading tells you what the customer has been living with.
• Coil wet versus dry. A wet coil adds resistance, so a cooling-mode reading on a dry coil understates the worst case. A heating-mode reading is dry by definition.
• Speed tap. Static rises with airflow. A reading on the heating tap does not tell you the cooling tap's static, so measure each mode you care about.
• Supply registers closed, zone dampers moving, a bypass humidifier damper open. Each moves the number. Note the position of anything adjustable at the time of the reading.
• A variable-speed ECM blower. A constant-airflow motor ramps up to hold its programmed airflow as static rises, so a high TESP on an ECM shows up as higher watts and noise rather than lower airflow. The reading is still valid; its consequence is different (see related: How to Estimate Blower Airflow From Static and the Fan Table).

How to verify the number before you act on it

• Repeat it. Pull and reinsert each tip and read again. Two readings within 0.02 in w.c. of each other mean the tip and port are behaving; a bigger spread means the port is in turbulence.
• Check the sign and the zero. Return negative, supply positive, instrument zeroed in place. A positive return reading means the hoses are swapped or the port is on the wrong side of the blower.
• Make it agree with something else. A TESP over the plate maximum should come with a symptom: low airflow on a PSC motor, high watts on an ECM, a temperature rise near the top of its range in heating, or a coil running cold in cooling. A high static with no symptom anywhere is worth one more look at the ports.

Close-out

Plug both ports, put the filter back the way the customer uses it, close the blower door, and restore the thermostat to the customer's setting. Record on the ticket: the mode, the speed tap or setting, filter condition, coil wet or dry, the two readings, the total and the plate maximum. A TESP with no record of the conditions cannot be compared with the next visit's reading, which is the reason for taking it.

References

• Furnace rating plate and installation instructions: maximum external static pressure and what the rating includes.
• 29 CFR 1910.134, Respiratory Protection, where the employer requires respirators for fiber exposure from lined duct.
• See related: How to Read a Manometer Honestly; Static Pressure Budget Across a Residential System; Furnace Rating Plate Read Before Start-Up; How to Estimate Blower Airflow From Static and the Fan Table.