Combustion Air Flow and Differential Pressure

There is nothing more frustrating than trying to start your boiler or burner; you hear the fan start, patiently wait for the sound of your ignition transformer to kick in and the roar of your burner light off, and nothing happens. The most likely cause of this problem is that your combustion air flow switch is not making.
The airflow switch is a burner/boiler safety device that proves (makes sure) that the combustion blower is running and providing the minimum amount of air pressure for safe light off, before we try to light off the burner. The airflow switch is wired in the preignition interlock circuit of a flame safeguard control. The sequence in which things happen is critical for safety in a combustion control system. The airflow switch being in the preignition circuit tells us that this switch must be made before the sequence can continue to ignition,which is why the combustion blower comes on and nothing else happens when the switch is not made. It could be very dangerous to open our main gas valves, allowing gas into the combustion chamber without having the necessary air flow that the combustion blower provides for proper and safe operation.
You can see “How to troubleshoot the combustion airflow switch” on Stromquist TV. Matt walks us through the steps of troubleshooting the airflow switch which include: 1) making sure the combustion blower is turning in the correct direction (you might laugh but it is one of the biggest causes of airflow switch problems our tech people get at Stromquist) 2) make sure your hook up tubes are the right size and not clogged 3) make sure your differential pressure is set properly. We like to use the Testo 510 ( available at Stromquist & Company) to set up our switches.
Differential pressure is an interesting subject in its own right. Not only is it used to prove things like air or water flow, but we also use it to measure flow. Put a know restriction in a pipe with flow.which creates a pressure drop, add an differential flow transmitter like the Honeywell STD900 or STD3000 and you have a very accurate method of measuring flow.
For more information on using DP ( differential pressure) to measure flow check out the following videos:

Meaning of DP transmitter high and low pressure ports

Back to Basics: DP Flow Measurement

If you are a Stromquist customer or are in GA or FL, call your Stromquist rep for help with your combustion and flow needs. All others, please refer to one of our affiliates at the Controls Group North America site to find a distributor in your area.


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