Large Range Hoods - Are they worth it?

We often come across clients who are excited to be getting a new large range hood, but are unaware of the implications of managing them with a make-up air system. To manage the depressurization of a 600cfm range hood, one would have to install a make-up air (MUA) system and interlock it to the kitchen range.  When the range turns on, so does a make-up air fan to simultaneously re-pressurize the home to prevent any form of depressurization.  This could be challenge to wire since the kitchen may already be finished.

Source: HRAI - Residential Mechanical Ventilation

If it could be installed and wired it would be located in one of the basement mechanical rooms with suitable air transfer grills from the mechanical room to the basement area.  A MUA system would ‘likely’ cost $2800-$3600 and would use a lot of electricity when running in the winter.

To give you some more information of the energy used with an electric MAU heater in the winter.  Here is a simple temperature rise calculation that shows how much heat is being exhausted along with the 600cfm of air.  

 

BTU/h = cfm x 1.08 x DeltaT  

 

Where Btu/h is the amount of heat and the DeltaT is the temperature difference from outside to inside…..in this case it’s in Fahrenheit (-25C = -13F) which is Ottawa’s outdoor design temperature.  Which means when it’s -13F outside the 600cfm of exhaust air must be reheated to 72F (22C) inside….which equals a 85F temperature rise.

 

Example:  BTU/h = 600cfm x 1.08 x 85F = 55,080 Btu/h.  

 

To put this amount of heat into perspective, that is more than enough heat to heat the average house.  Granted the fan only runs 30 mins at a time…it’s still a lot of heat.  I know people love the idea of having large range hoods but the economics of their installation is hardly justifiable.  Sadly, the retailers do not divulge these facts and even more concerning is the manufacturers of these fans do not express the liability associated with possible asphyxiation when a wood stove or fire place is present and a make-up air system is not installed.   It takes literally seconds to bring smoke or even flames back into a house when a fire is burning.

In order to properly size a make up air system, we can perform an HRAI W-3 calculation to manage depressurization in your home.

 

 

June 11, 2015 by Matthew Pedersen
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