Is Excessive Back Pressure an issue with your Heating and Cooling System?

Is your heating and cooling system operating under too much internal static pressure? IS there too much air resistance through the system. It might be impacting your comfort right now. It might also be damaging your system and causing issues which will mean premature component failure.

We better find out what is happening and if any of the related issues are impacting you.

Resistance in a ducting system is measured in inches of water column – in. WC This is a bit esoteric and quaint to the profession, but we can live with this. The static drop over a conventional air distribution system is .4 to .5 inches.

There is a necessary pressure drop from the fan motor blowing out into the supply ducting to the return air duct at the air handler. This static pressure across the system is the accumulated effect of the duct design… Professional design here would follow from Manual D as formulated by ASHRAE – American Society of Heating, Refrigerating and Air-Conditioning Engineers and accepted by the ACCA- The Air Conditioning Contractors of America.

A well air distribution system designed to such standards with the static pressure in the range of .4 o .5 in. WC would perform well with components that are well matched and configured properly.

But a variety of things can upset this pressure balance.
First, we must understand that not all central air distribution systems have been designed with Manual D. Your system has a designed static pressure, more or less, depending on its history.

If your system is under the care of an HVAC service company, they have a sense of the proper pressures for your system according to the manufacturer’s specifications for your component system. You do have a maintenance agreement? Here are more reasons why?

Related: Duct caveat here… Oh and other parts of the ducting system can leak and reduce global resistance, causing additional problems, but not really solve the problem with air resistance. Duct leaks are quite harmful in themselves. Make sure you are are aware of these fundamental issues, but that is a separate theme and a separate story. More on the issues with duct leaks here LINK

When the components through which air must move, create too large a local or global air resistance, this global pressure drop impacts the function of the entire system. Air resistance impacting the proper pressure drop can leave you in discomfort. Greater resistance and your components can be damaged causing premature failure and unnecessary expense. We will talk about what can happen after we look at the effects which can upset the proper static pressure with excesslive added air resistance.

Let’s look at filters first.
There are a couple critical issues which can arise around your filtration system.

The system was designed with a filter system in place. It might have been a simple fiberglass filter intended to protect the internal components from contamination and airborne debris. So you or someone else just might decide that little crappy filter is not filtering well enough and find a better filter that traps finer particles. This is a big deal. We talk about selecting filters here in more detail LINK, but here we are just discussing the pressure aspects.

There are also issues with getting and installing proper filters. Everyone wants good clean indoor air. So it would make intuitive sense to put denser, higher MERV rated filters into the filter tray of your home. We talk about filter types and their ability to filter here. LINK.
But the wrong filter in the wrong filter tray of a susceptible heating and cooling system, can upset the pressure balance and create these same issues with inadequate pressure drop across the system and the emergence of issues related to poor air flow. We talk about finding the right filters to suit your needs here. LINK

If a filter with a pressure drop of 0.25 is used in an air handler, the filter alone can use half of the system’s available static pressure. If a high MERV filter is installed without designing for it and the static pressure is increased to an amount much higher than the optimum, several negative consequences can result.

The second critical issue with filters is the ignored filter. It gradually fills up with crud and the air resistance can go up enough to upset the balance of airflow. It first affects your comfort and then impacts your components. When is the last time you saw your furnace filter? LINK

Restricted air flow, i.e. abnormally high static pressure, can have a devastating effect on your heating and cooling components.

To understand the possible impacts we must also understand how the types of motors in our HVAC system will react to an increase in static pressure.

Do you know what kinds of motors you have? The traditional motor is an analog PSC – permanent split capacitor motor. It will come on and run up to a set speed. You might also have ECM motors in our system and they are more flexible on speed and torque and are driven by controllers to deliver a set volume of air. We go into great detail about comfort and saving money and the relationship of these issues with motor choice here. LINK

Here we are only interested in how each of these motors reacts to increased back pressure in the duct system.

The effects depend on the motor. An ECM is programmed to deliver a quality of air per minute. If the static pressure goes up, the ECM motor ups its speed and torque trying to drive the proper amount of air through the restricted air passages. Pressure in the system might increased to 1 in. WC – double the pressure.

The ECM will strain to its limits which would be heat sensors cutting off current to the fan. Continuing to push the upper limits of power used in the ECM will limit it life, and the right amount of air will not reach the system when it is needed. THe ECM will use a lot of power trying to push air where it cannot.

The PSC motor acts completely different. A PSC blower motor will work up to about 0.5 in. WC, then it will “fall off” i.e., the motor will stop pushing, although the fan will continue to turn.

The ECM, while most efficient at 0.3 inches of static pressure, will continue to work up to about 1 in WC, and this just might be enough to collapse your overfilled filter causing it to collapse and quite filtering at all. This can allow gunk into your system which can gum up your evaporator coil and prevent proper exchange of energy across it. This is where the cool is generated. PSC motors can also collapse a dirty filter also, but less so than an ECM. This can also gum up your blower motor and create more problems.

This conversation about abnormally high pressure in your heating and air conditioning system segues right into what high static pressure can do to your system.

As the air slows down in the ducts, during a heating season, the slow moving air could actually cool, reach dew point and create mold in the duct or mold at the register. This is also exacerbated by problems with poorly insulated ducts, slow moving air in warm ducts while traversing a cold unconditioned environment like an unheated attic or crawlspace could be a key to a mold growth and exposure to allergens generated right in the heating and cooling system.

Static pressures can go on up from here if the situation is not mitigated.
Abnormally high static pressure can choke off air flow so severely that your evaporator coil can freeze up and create frost out the refrigerant lines and freeze up and destroy compressors in the outdoor unit. We discuss this particular issue in detail here. LINK

Do you have a gas furnace? or a furnace powered by any other fossil fuel? THe heat from a furnace funnels through a heat exchanger on the way to the flue. A lack of proper airflow across a furnace heat exchanger can can cause the heat exchanger to crack prematurely. More on heat exchangers here. LINK

Other issues can impact your static pressure.
It is not a good idea to close of the vent in that unused bedroom. You are increasing the static pressure and setting any number of critical parameters out of balance. Lots of good reasons not to close off ducting. More on that subject here. LINK

Flexible ducts need to be stretched out and put in tension to operate properly. Has someone been in the attic and smashed a duct? Things happen. It is best to know what is happening in the attic. LINK

And then object placed in from of supply duct registers can restrict flow and create excessive back pressure. And cold air returns are such a big factor here also, and in so many ways that can create a problem. Make sure you check out the variety of issues around possible duct defects as part of your analysis to understrand what is happening within your air distribution system. LINK

In conclusion:
Abnormally high static pressure in your HVAC system is quite serious. Make sure the filters you are using in your system are not bollixing up your efficiency or worse. The dollars you save are yours to keep, or spend frivolously, elsewhere.

And don’t let pressure issues actually muck up you level of comfort.

Clogged filters are a big deal make sure you have a plan to change them on a schedule. LINK
Proper maintenance and close communication with an HVAC service company you trust is essential. You will save money, and never know how many times you headed off a disaster with loss of conditioned air. That is the trick of it. You cannot add up money you save, you can only complain later about the money you have to spend. in an emergency.

Any questions? Any comment? Feel free to add them in below. I you feel this info is useful, please share it. You might save someone from unnecessary expenses.

Stu Langley

meet the author at

Leave a Reply

Your email address will not be published. Required fields are marked *


This blog is kept spam free by WP-SpamFree.