Make-up Air Required for Range Hoods

Today’s Tech: Make-up Air Required for Range Hoods

Problem: I am installing a large range hood for a customer, and I need a way to open a make-up air damper whenever the hood is on. Is there a RIB® product that can help me?

This is a question I encounter periodically while providing technical support for Functional Devices, Inc. The answer is “Yes,” and we have several products that can help when installing such a system.

A little background...

Homes are becoming more airtight, which saves energy when heating and cooling, but this can lead to problems when you begin exhausting large volumes of air from the home. When air is exhausted from a home, an equal amount of air will want to enter the home. If there is no make-up air path created, a negative pressure exists and the air can find its way in by traveling backwards down appliance flues or a chimney. This is known as backdrafting, and it can be dangerous if carbon monoxide is being pulled back into the home. To help address this issue, the International Residential Code has a requirement to provide make-up air for any range hood moving over 400 cfm.

Solution: The customer will need a Current Sensor and a Dry Contact RIB® Relay.

The current sensor will go on one leg of power going to the hood fan, and its output will wire to the dry contact input of the relay. The final piece is to wire the damper circuit through the normally open contacts of the relay. When the hood fan draws current, the output of the sensor will close the input of the relay. The relay will close its contacts, the damper will receive power, and it will open. Now a make-up air path has been created.

What are some alternate solutions?

I have also recommended two different tweaks to this control topology for special cases. One customer was having issues with a similar setup, but they also were powering a make-up air fan in-line with the damper. Their problem was that the fan was strong enough that it held the damper closed, even when it was trying to open. To overcome this, I recommended using independent RIBs® for both the damper and the fan. A standard dry contact RIB® to control the damper, allowing power to it as soon as the range hood began drawing current, and a dry contact time delay RIB® to control the fan to delay its turn-on until the damper had sufficient time to open.

The other special case was a retrofit installation where the installer did not want to deal with the hassle of running wire, and their customer did not want the extra cost associated with the same. Because Functional Devices’ wireless EnOcean RIBs® also have dry contact inputs, they can control this system as well. One EnOcean enabled RIB®, as a transmitter, at the hood fan with the current sensor wired to its dry contact input, and the other, as a receiver, at the make-up air location with damper power wired through its normally open contacts. When the input of the transmitter closes, it sends the signal to the receiver to close its relay output.

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About the Author

Henry Smith is a 34-year-old engineer at Functional Devices, Inc. He has a BSEET obtained from Purdue University in 2014 and a lifelong interest in electronics. As an engineer at Functional Devices, he gets to provide Technical Support to our customers, from distributors to specifying engineers and installers.

Henry enjoys providing tech support, as it allows our company to assist at every level of our product’s lifecycle and exposes us to interesting and unique applications. While not every technical question is unique, even answering a simple question or providing someone with the information in a timely manner can go a long way to helping him or her meet a deadline.