Adjusting manifold pressure on the gas furnace

A demonstration of the procedure used to set the gas input when setting up a gas appliance.
This video is part of the heating and cooling series of training videos made to accompany my websites: and to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

27 Replies to “Adjusting manifold pressure on the gas furnace”

  1. Hi, I have a lennox mid efficiency single stage gas furnace. The manifold on rating plate is 3.5"wc. But I am only getting 1.7"wc. I adjusted to 3.5"wc according to the rating plate. But the furnace overheats and my temperature rise is out of specification. The furnace also intermittently short cycles. Can this be a bad gas valve? Thank you.

  2. Hi, my low stage reads 3.8"wc. I've tried turning the adjustment screw to bring it down to around 1.7"wc. But turning the adjustment screw has no effects. Is it a bad gas gave then? Thanks

  3. hi, i have this sealed combustion lennox furnace. When i open the brass screw my pressure reads 3.5"w.c. But strange when i put the brass screw back my pressure drops to 3.1"w.c. Is this normal? I've tried with another same lennox furnace and the same result. thanks.

  4. hi, for a sealed combustion ( with a hose connecting gas valve to burner box) do you have to take the cover off first before adjusting the manifold pressure? thank you sir.

  5. I had a boiler manifold pressure at 4.05 and I set it to 3.5wc. Is .5wc that much difference

  6. i've heard conflicting information on when to zero out the manometer, some say it should be zero'd with everything off, some say it should be zero'd with the inducer running before the furnace ignites, could you please comment on which method is correct grayfurnaceman? thanks!

  7. on the video you are using a no-name cheep chinese manometer for gas application.  there is huge confusion out there with sellers listing Digital Air Pressure Meter for unlabled models that all look alike and some tag on Differential Pressure Gauge HVAC Gas Pressure Tester if they want to sell for more money, but without a manufacturer, and model number, how do we know if these are even safe to use with LP gas?  Too many people with too little knowledge.  Please do a video on what kind of equipment is safe to use for gas vs. air.

  8. GFM – Any idea why a manufacturer's (Williams in this case) tech support would say the manifold pressure is set at the factory and shouldn't be adjusted? They told me if I'm getting below the specified 4 WCs of pressure I should replace the valve rather than adjust the existing valve. Thanks.

  9. I used a manometer to check my inlet pressure, stagnant pressure with furnace off was read at 6.61 wc and running pressure was read at 4.75 wc at my residential furnace, would you say that is to low?

  10. hello graythefurnace man! quick question, about what should be the inlet pressure on a residential furnace? manifold pressure should be 3.5 wc, wondering what it should be before, thanks for any info!

  11. Shouldn't you also check the gas flow at the meter by timing how long it takes for 2 cu. ft. (or some metered amount) of gas to be consumed, then calculating the btu's of the consumed gas to be sure that the consumption is close to the furnace's input rating?

  12. Thank you for posting this video, my home furnace wasn't working and thanks to your video I decreased gas flow. The heater only worked with air filter removed which led me to realize there was too much gas flowing, when I lowered flow and installed filter the unit now works Perfectly. Thanks again!

  13. Mr. GFM, What is a ECO device for and do I need to use it there are no conductors available for this device in my Furnace and the old valve did not have one? Also, I fired the pilot and put thread compound on the connection points at both ends of the Pilot tube to eliminate any possible leaks. The connection at the inlet to the Pilot tube showed a flame at the bottom of the brass connector/fitting is this because the pilot flame pressure is to high and needs to be turned down?

  14. Mr. GFM, I'll perform the Clocking procedure as instructed to ensure proper pressure levels. My other question is why are the gas valves shipped in the ON position is this to ensure that the technician know its never be used? Also the valve once I installed It I turned the knob to the OFF position is this a problem? It's not been lit as of yet.

  15. Mr. GFM I had a Gas Valve that was leaking under pilot at the burners. I smelled gas and called the Gas Company. Since they have the proper tools for testing the unit that is what they found. I have schooling in Residential HVAC but have never worked on a Gas Valve. This unit is at my home and I understand the video I have viewed. My question is that I reviewed the Honeywell Gas Valve installation instructions and looked at my Lennox Furnace information. My Manifold Pressure requirement is 3.5 in wc my Honeywell Gas Valve indicates that the OEM shipped the valve preset at 3.5 in wc do I still have to perform a Meter Clocking if the Gas Valve was preset by the OEM? I'm ready to open the cutoff and test for leaks and light the pilot; My unit's BTUH is 82000?

  16. Agree with Vincent Edwards, I was hoping to see how you connected the manometer to the gas valve.

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