How to Set Gas Manifold Pressure on a Furnace

Why this matters

Manifold pressure is the one adjustment that decides how much gas a furnace actually burns, and it is the one most often left exactly where the factory set it. Factory setting is a guess about a house the factory never saw. A furnace a few tenths high over-fires, runs a hot temperature rise and cycles on its limit; a few tenths low under-fires and can condense in a vent that was never rated for condensate. Neither shows at the thermostat, so the only way to know is to put a manometer on the valve and set it. The pressure values themselves live in the shelf's field card (See related: Gas Pressure Set-Up Numbers Field Card); this article is the sequence that gets the valve to them without a mistake along the way.

Safety and start state

Safety first. If you smell gas at any point, stop. Everyone leaves the house, nobody operates a switch, a light or a phone inside, and the call to the gas utility is made from outside. Wear a personal CO monitor clipped near your face for the whole job, because you will spend several minutes with your head beside a firing appliance with its door off. Every pressure tap you open is closed and bubble-tested at operating pressure before you leave, and that test is a step below, not a courtesy.

What you need before you start:

• A digital manometer that resolves 0.1 in w.c. or better, zeroed with both ports open to the room before you connect it. A meter that drifts 0.2 in w.c. makes a 3.5 setting meaningless.
• The furnace rating plate values: inlet minimum and maximum, manifold high fire, and manifold low fire on a two-stage valve. Read them off the plate, not off memory (See related: Furnace Rating Plate Read Before Start-Up).
• The installation instructions, for three things the plate does not say: whether the regulator is referenced to the burner box, the setting tolerance, and how to force each firing stage.
• Barbed tap fittings for a 1/8 inch NPT plug, gas-rated thread sealant if the valve maker calls for one, and leak-detection solution.

Check before you touch the valve: the venting is complete and connected, the gas cock is the one feeding this appliance, and the furnace lights and proves flame normally. Setting pressure on a furnace with a combustion problem you have not found only gives that problem a better gas supply.

The valve and the hookup

        COMBINATION GAS VALVE, two-stage
 gas cock                                to manifold
 ------->+--------------------------------+------->
         | INLET     REGULATOR    OUTLET  |
         |  tap      CAP: HI  LO   tap    |
         |   o        [adj] [adj]   o     |
         +---|---------------------|------+
             |                     |
        inlet hose            outlet hose
        (steps 1-2)           (steps 3-6)
             |                     |
          +--+---------------------+--+
          | MANOMETER   + port = tap  |
          |             - port = room |
          | or burner box reference   |
          | tube where maker requires |
          +---------------------------+

One manometer is moved between the two taps, or a two-channel meter reads both at once. The regulator cap covers the adjusting screws; on a two-stage valve there is a high-fire screw and a low-fire screw, sometimes under one cap and sometimes under two.

The steps

1. Read inlet static. Shut the gas cock, remove the inlet tap plug (or back out the test-boss screw about a turn), fit the hose, open the cock. With every gas appliance in the house off, read the inlet. Why: this is the service regulator's setting and your baseline for the drop in step 2. Skip it and you cannot tell piping friction from a regulator problem.

2. Read inlet under load. Fire the furnace at high fire and have the other large gas loads running: the water heater with a hot tap open, range burners, a gas dryer. The inlet must stay at or above the plate minimum. Why: the valve's regulator can only reduce pressure, and it needs headroom above the outlet set point to hold steady. Skip it and you may set the manifold on a supply that sags below what the regulator can work with the moment the house is busy, which is exactly when the furnace is working hardest. If the inlet under load falls below the plate minimum, stop here: the problem is the supply, and pipe sizing and alteration belong to the licensed gas fitter or plumber where your state or locality licenses that work, with the meter regulator going to the utility.

3. Move to the outlet tap. Shut the gas cock, reinstall the inlet plug with the maker's sealant, move the hose to the outlet tap. On a furnace whose instructions say the regulator is referenced to the burner box, tee the manometer's negative port to the reference tube. Why: on those furnaces the regulator holds a pressure relative to the box, and a reading against the room is off by however much the inducer is pulling. Skip the reference and you will "correct" an accurate setting into a wrong one.

4. Fire on high and let it settle. Call for high fire by the maker's method: W1 and W2 from a two-stage thermostat, a dip switch, or a board test mode. A board running single-stage-thermostat logic may sit on low for several minutes before staging up, so force it rather than wait. Give the reading a minute after the stage change. Low-voltage jumpers go on the thermostat terminals only; never jumper a limit, rollout switch or pressure switch to get the furnace to stay lit.

5. Adjust high fire to the plate. Remove the cap and turn the high-fire screw: on most valves clockwise raises the outlet pressure and counterclockwise lowers it. Move in small turns and wait for the reading to settle between them. Replace the cap and re-read, because on some valves the reading shifts slightly once the regulator chamber is closed. Skip the re-read and the set point you walked away with is not the one you measured.

6. Drop to low fire and adjust low. Call low fire only (W1 alone, or the maker's low-fire hold). Adjust the low-fire screw to the plate's low value, cap it, re-read. Then call high again and confirm it has not moved; on some valves the two adjustments interact slightly. Why low matters: a two-stage furnace spends most of its hours on low, so a wrong low setting does more seasonal damage than a wrong high one. A low fire set too far down also risks unstable flame and condensing in the heat exchanger and vent.

7. Restore and leak test the taps. Shut the cock, remove the hose, reinstall the outlet plug with sealant, open the cock. Run the furnace on a call for heat and brush solution on both tap plugs and the cap. The outlet tap only sees pressure while the valve is open, so test it with the burners firing; a bubble test with the furnace off proves nothing about the outlet plug. Watch for at least a slow count of thirty; a slow bubble takes time to form. Skip it and a tap you loosened leaks into the vestibule every cycle for the life of the furnace.

8. Confirm what the pressure produced. The manifold number sets the gas; it does not prove the firing rate. Clock the meter (See related: How to Clock a Gas Meter to Check Furnace Input) and then check combustion (See related: Gas Furnace Combustion Analysis and Tuning). Read temperature rise last, at the stage you just set (See related: How to Measure Temperature Rise on a Gas Furnace).

Worked example: a two-stage furnace running hot on high

A two-stage natural gas furnace, rated input 80,000 BTU/h on high fire, installed two winters ago. Complaint: the furnace shuts off and the blower keeps running on very cold days, then relights. Plate values: inlet 4.5 minimum, 10.5 maximum; manifold 3.5 in w.c. high fire, 1.6 in w.c. low fire. The instructions say the regulator is referenced to the room on this model, and allow a tolerance of a few tenths on either setting.

Step 1, inlet static: 7.2 in w.c. with all gas loads off.

Step 2, inlet under load: furnace high, water heater firing, two range burners on: 6.6 in w.c. The drop is 7.2 - 6.6 = 0.6 in w.c., ordinary piping friction, and 6.6 is well above the 4.5 minimum. Supply is not the problem.

Step 4, high fire as found: 3.9 in w.c. against 3.5 on the plate, so 3.9 - 3.5 = 0.4 in w.c. high.

What 0.4 high means. Flow through a fixed orifice goes roughly with the square root of the pressure across it. The square root of 3.9 / 3.5 is the square root of 1.114, about 1.056, so the furnace is firing about 5.6 percent over rating on high: 80,000 x 1.056 is about 84,500 BTU/h. Checked the other way: 84,500 / 80,000 is 1.056, and 1.056 squared is 1.115, which matches the pressure ratio. On a long high-fire call on the coldest day, that extra heat plus a filter that has started to load is enough to open the limit, which matches the complaint: the limit trips, the blower runs on, the limit resets, the burner relights.

Step 5, high fire set: two small counterclockwise turns brought it to 3.5; cap on, re-read 3.5.

Step 6, low fire: read 1.6, on plate, left alone. Back to high: still 3.5.

Step 7: both taps bubble-tested, the outlet tap with the burners firing. No bubbles in a thirty-count.

Step 8: the clock and the analyzer both followed, and temperature rise on high came back inside the plate range. The ticket records inlet static and under-load, both manifold values as found and as left, and the bubble test.

The trap in this example. The furnace was set high from the factory. A tech who skipped the manometer and changed the filter would have made the symptom go away for a month and left the over-fire in place.

What changes the sequence

A modulating furnace. There is no high and low screw in the usual sense; the maker gives a procedure, usually through the control board, to force maximum and minimum rate and adjust each. Follow it exactly; a modulating valve set by two-stage habit lands wrong across most of its range.

Propane. Same sequence, different numbers and different parts. A propane manifold commonly sits near 10 in w.c. and needs the maker's conversion kit to get there; a natural gas valve turned all the way up is not a propane setting (See related: How to Convert a Furnace From Natural Gas to Propane).

Altitude. Above the elevation the maker's high-altitude table starts at, the plate value is replaced by the table's derated value, by orifice, by manifold pressure, or both. The table governs, not the sea-level plate number. The field card carries the general derating guidance from NFPA 54, the National Fuel Gas Code, as adopted by your jurisdiction, which applies only where the appliance listing does not say otherwise.

A reading that will not reach the plate value. If the inlet is good and the screw runs out of travel before the outlet reaches the plate, the regulator is at fault, not the setting. Replace the valve rather than accept a number off plate.

How to know you got it right

The job is right when all of these hold, and the ticket shows the numbers:

• Inlet under load at or above the plate minimum, recorded alongside inlet static.
• Manifold high and low each within the maker's tolerance of the plate value, read with the cap on, with the reference port connected where the instructions require it.
• Both taps and the regulator cap bubble-tested at operating pressure, the outlet tap with the burners firing.
• The meter clock agrees with the rated input within the maker's tolerance, and combustion and temperature rise are inside their ranges.

If any one of those is missing, the setting is a number on a ticket, not a verified firing rate.

References

• Furnace rating plate and installation instructions: manifold settings, tolerance, reference-port instructions, staging procedure, high-altitude table
• NFPA 54 National Fuel Gas Code, as adopted by your jurisdiction, current edition
• See related: Gas Pressure Set-Up Numbers Field Card; Furnace Rating Plate Read Before Start-Up
• See related: How to Clock a Gas Meter to Check Furnace Input; Gas Furnace Combustion Analysis and Tuning
• See related: How to Measure Temperature Rise on a Gas Furnace