By-Pass Sizing Calculator

Often with zoning systems a By-Pass is needed to relieve excess air when only a small zone or number of small zones cannot deliver the required air flow thru the HVAC System. A by-pass is often ducted back into the return air or into non-critical, common conditioned temperature areas such as entry ways, hallways, basements, etc.

Select your Maximum Total System Cubic Feet per Minute (CFM), then enter the CFM of your Smallest Zone to calculate the type of By-Pass Damper you require.

Note: If the By-Pass CFM amount is 30% or less of the Total System CFM, No By-Pass is required.



The By-Pass Damper you are looking for is SPRD08.

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The By-Pass Damper you are looking for is SPRD09.

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The By-Pass Damper you are looking for is SPRD10.

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The By-Pass Damper you are looking for is SPRD12.

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The By-Pass Damper you are looking for is SPRD14.

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The By-Pass Damper you are looking for is SPRD16.

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The By-Pass Damper you are looking for is RDM18 w/SPC.

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The By-Pass Damper you are looking for is RDM20 w/SPC.

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The By-Pass Damper you are looking for is ZDSM2020 w/SPC.

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The By-Pass Damper you are looking for is ZDSM2420 w/SPC.

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The By-Pass Damper you are looking for is ZDSM3024 w/SPC.

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About ZoneFirst


For over half a century, ZONEFIRST has been the industry leader in zoning.

Starting with the development of the first zoning dampers and registers in the 1950’s to today, with exclusive “Plug & Play” dampers – Zonefirst has led the way in excellence and continued innovation.

ZONEFIRST is the first company that successfully developed and marketed HVAC Zoning Systems back in the 1950’s and is today a leading manufacturer of HVAC zone dampers and zoning systems. Several other major zoning companies are in business due to ZONEFIRST.

It was in the mid to late 1950’s when Richard Foster developed the first automatic motorized dampers, registers and difusers that were thermostatically controlled. He marketed these products under the trade name Zone-A-Trol with another company when in 1964 he founded Trolex Corporation and the start of the well known trade name Trol-A-Temp. In 1999 the Trol-A-Temp brand was acquired by Honeywell.

Since that time, Trolex has continued to manufacture zone dampers for a number of OEM’s. In 2004 Trolex re-entered the HVAC market as ZONE-A-TROL and later the ZONEFIRST tradename. This new name signifies the many firsts the company has developed in the HVAC industry. We were the first company to develop automatic dampers, registers, and diffusers for zoning, centralized zone controllers, the first zoning system to feature normally open dampers and the first to zone heat pumps in the early 1970s.



ZONEFIRST is now the first company to offer the new EZ-Wire controls and dampers. These New Color-Coded Push-In wiring terminals make wiring the thermostats, transformer, sensors and HVAC equipment fast and easy. The New Plug-In Dampers simplify damper wiring and make connecting multiple dampers literally a snap. The new DC motors consume 80% less energy than 24 volt motors and the wire is included with each damper.

ZONEFIRST is also promoting the concept of designing a HVAC Zoning System first, and incorporating that design into every new HVAC System. When you think about hydronic and radiant heating, zoning is always included. Even today’s cars include dual zone climate control. Imagine that. People are buying zone control in a space smaller than the smallest room in their home. It’s because Zoning makes sense!

What’s New In Our 50th Year