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.