Did the builder/homeowner decide to put a 1200 cfm exhaust hood in the kitchen? Is the inspector now requiring conditioned make-up air? Don't worry, we get these calls often and have a solution.

The 2015 MRC prescribes in M1503.4 that any kitchen exhaust hood system capable of exhausting more than 400 cfm must be provided with make-up air at a rate approximately equal to the rate of exhaust. This air must be heated at a minimum as we know dumping 400 cfm or more of 0* air into a living space will cause discomfort, to say the least.

Can we dump it into the return of the furnace and condition it that way? Not likely. The minimum return air temperature of most furnace manufacturers is 55-60 degrees. If we have an 80k furnace with 1600 cfm and a return temp from the house of 70*, this allows us a maximum of 228 cfm before we dip below a combined return temp of 60*. Is your furnace oversized enough to handle this added 17,077 btus to condition that 228 cfm? If it is, then it is way too large for the space when we are not heating the make-up air. 

This is where we see the need and value of a dedicated make-up air system. We can achieve a heating-only system with a packaged electric make-up air unit from Electro Industries as you will see described below. To add cooling we would be looking at a commercial packaged make-up air unit or a Fujitsu outdoor air system; both of which start around $15,000... So lets focus on the heating-only option here.

Like all equipment selections, we need to determine the amount of energy needed which will be determined by the airflow, incoming air temperature, and desired air temperature. Electro also has some selection charts on their spec sheets below to simplify this process, but following is how we get there long-hand. The airflow is usually given by the builder, homeowner, or hood/fan provider, and we usually shoot for a temp rise of 70 degrees in West Michigan. Using our airflow formula of: BTUH=1.07xCFMxTD, where TD is temp rise, we can determine the BTUH needed. Take this value and divide by 3415 and you will get the kW of heater needed.

This unit will have a fan, electric heat coil, incoming and discharge air sensors, mesh filter, current sensor for the exhaust fan, and a control to tie all of this together; all packaged and assembled from the factory. Installation is relatively easy, leaving the most difficult part as the electrician getting the large amount of power to the unit. 

If you have any questions on this topic or would like a quote on a unit, we encourage you to submit a design ticket: https://supporthub-jswmi.happyfox.com/