Heating Season — Air Flow

In the last post we discussed temperature rise and how to check that.We discussed how this affects the proper operation of a furnace and keeps it from operating too hot and cycling on the limit control. We talked about adjusting fan speed  to make sure we could get the temperature rise  where it should be. Now, if making fan adjustments does not provide enough air, then we need to look at the duct work and the Static Pressure of the system.

To properly check air flow, a magnahelic or manometer is needed to measure the E.S.P. (external static pressure) of the system.  Two reading are necessary – one after the filter in the return and one before the coil in the supply plenum.

Once this value is known, the use of the blower performance charts in the installation instructions can help determine the amount of air flow through the system.  Most manufacturers size the CFM capacity of the blower based on one half (0.5 IWC) inches of water column.

Looking at the above chart, find the static reading you had on your magnahelic, find the model number of the furnace and you can see , based on what speed tap you are using, just how much CFM you are delivering.

Of course, there is a limit to how much static pressure a system can handle.  this is where we can get into trouble because the duct work won’t support the size furnace you have. As I have mentioned so many times, we always need to look at everything as a SYSTEM. The DUCT WORK IS PART OF THAT SYSTEM. If that isn’t right, then how can the actual equipment part of the system be expected to work properly?

One last thing to mention about External Static Pressure — DON’T OVERLOOK THE FILTER! Some filters are very “high static” filters which is why they can taut being “highly efficient”, but because of this, they also reduce air flow. Return static really should not exceed 0.2 IWC.  Someone once made the comment that “the dirtier a filter becomes, the more efficient it becomes”!  This really is true but what did that filter do to the AIR FLOW through the furnace?  Same holds true for some of those “high efficient” filters  sold directly to home owners.  They put them in and never consider the air flow restriction until they have problems and the unit is cycling on the limit control or they don’t have enough heat on the coldest of days.

As you can see, maintaining proper air flow in a heating system is very important to the proper operation of that system.  Don’t overlook this when diagnosing problems.  Air flow is critical to the proper operation of a heating system as well as ANY system.


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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2 Responses to Heating Season — Air Flow

  1. Larry Howard says:

    The Problem is high static pressure drop coils. And pleated filters. Also fans that can only overcome .5 inches of static pressure. Coil drops of .32 or more and filter drops of .2, leaves nothing to design your duct work. Manufacturers need to install fans that can handle more static pressure.

  2. Pete Singh says:

    Thank you , I always read your blog never learnt so much when I was going trade school . Please write a text book based on your own experience . You will be a great asset to Hvac industry. Keep up the good work. Thanks Pete.

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