Heating Season — Gas Pressure

In my last post, I said we needed to get out brain-set back to heating.  one of the items that needs to be looked at is gas pressure — both inlet and manifold pressure.  It is important to understand that these settings can make a big difference in how that heating section is going to work.  This applies to both residential furnaces and packed roof top units, single stage and multi-stage equipment.

Improper gas pressure, on the side of the gas train can cause the unit to over heat if it is too high.  To little gas pressure can cause nuisance lockouts on the flame sensor. Likewise, too much on the inlet side of the valve will cause gas valve failures or no heat because the valve cannot open against “high pressures”.  To little inlet pressure can cause sooting in the heat exchanger and contribute to nuisance lockouts.

So,  what is the guidelines for inlet and manifold pressures? What should the inlet operating range gas line pressures for the furnace to  operate safely. The gas line pressure MUST BE a minimum of:

inlet gas pressure

in order to obtain the BTU input specified on the rating plate and/or the nominal manifold pressure specified on the rating plate and to work safely.

Manifold pressure will now be able to help set the correct firing rate.  On single stage units, most manifold pressures are set to

manifold gas pressure

You should always check with the manufacturer as some do vary, but as a “rule of thumb” this is where most manufacturers set the high fire pressure. This is based on the actual burner orifices requiring this pressure to deliver the rated output.  On multi-stage units, there is also a setting for low fire and that also needs to be checked.

So, hopefully now you see why inlet and manifold gas pressure is important to proper operation of a gas heating system.  So how do we check this?

manometer hook up

As you can see from the picture above, you need a something that your can read inches of water column (IWC) to check for proper pressure.  This could be a simple “U-tube” manometer (as shown) or it could be a digital manometer, could be a magnahelic , could be a simple pressure gauge — anything that can read inches of water. You  can then set the unit up with the proper manifold pressure or can verify where the inlet pressure is coming into the valve depending on what side you hook up to.

One thing to remember, on 2-stage gas valves, there usually are 2 pressure regulators – one for HIGH FIRE and one for LOW FIRE (see example below).  Be sure you know the manufacturer’s specs for setting these.  Most high fires will be 3.5 IWC  but the low could be different depending on manufacturer.  Some use 50% low fire, some 60% low fire and some as high as 80% low fire so it is important to verify, either on the data tag on the unit or with the manufacturer where these should be set.

2-stage gas valve1

One good check is to always leave the manometer hooked up to the inlet side and then watch it as the unit operates.  This is especially important on “high pressure gas” supply systems and propane applications.  A lot of times, you can watch the inlet pressure drop, possibly due to a faulty regulator, when the main valve opens.  A lot of techs will hook up the meter, see they have good pressure and then remove the meter.  Then they see there is a problem on the manifold side and condemn the gas valve when there is nothing wrong with it.

Gas pressure, both inlet and outlet, are important to proper operation of any heating system.  In the next post, we’ll discuss temperature rise, how to check, and why that is an important diagnostic.

About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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7 Responses to Heating Season — Gas Pressure

  1. KKinnaman says:

    I just wanted to let you know that your website is GREAT!! I have spent time reading all your blogs all the way back to your beginning, and have found so much helpful information. I was taught from the beginning to always check incoming gas pressure both before and during operation of system. Your post is a good reminder to all. I too have spent many years working for contractors 31+ to be exact. I recently just started a new job as Technical Support/ service manager for a distributor who supplies Unitary Products (York Brand) to contractors. I have been telling their technicians to check out your site that is well worth the visit. Thank you for what you do and I look forward to your next post.

    • You are more than welcome. I worked a lot with one of your predecessors — John Price — and we always wanted to make sure dealers got the information they needed. John taught it in schools for licensing and I do it on line.
      Thank you for passing it on to your dealers — it is appreciated.

  2. Jim says:

    First let me say that I do enjoy reading your blogs…. there is always something to learn in this business. I also wanted to comment on your blog about taking gas pressure. Although I do believe it is always necessary to check gas pressure, without preforming a combustion analysis your only seeing part of the picture. Before I bought my CA I set the gas press. to manufacture specs. and timed the gas meter to make sure I was getting the correct output, now that I have a combustion analyzer I see that these furnaces were not even close to burning properly. Some of those furnaces I have even had to re-orifice to get proper combustion. There are many variables that effect the combustion (airflow, heat exchanger design,orifice size,gas pressure, ect.). It would be nice if the manufactures gave us some combustion guidelines as well.

    • Jim: I agree that combustion analysis is the best way to go. However, on service calls, most techs don’t have that. My point is that both inlet and manifold pressure are very good diagnostics for heating issues and most techs have manometers or magnahelics, or simple gas pressure gauges and can do some very good diagnostics with this information. Complete combustion analysis is ideal for proper set up, but for service, just getting techs to get gas pressures sometimes in a chore in itself.
      Thanks for your comments — they are appreciated.

  3. rex says:

    Great blog, It’s packed with good info!
    We are at 4500 foot altitude that can increase to 6500 in just a few miles.
    You might want to touch on the effect high altitude has on the gas / air mixture to run correctly.

  4. Brad says:

    I have just installed a York 80000 btu furnace but couldn’t find where the gas pressure setting for low fire is I looked online and it said anywhere from 50% to 80% I set it at 1.7 iwc want to make sure this is sufficient

    • on page 36 of the install instructions it states:
      Table 18: Nominal Manifold Pressure
      Natural Gas (High Fire) 3.5″ w.c. (0.87 kPa)
      Natural Gas (Low Fire) 1.6″ w.c. (0.40 kPa)

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