myths about cfm's and mua
I was going to post this as a response to an active thread about how many CFMÃ¯Â¿Â½s can be added without makeup air. Since this comes up a lot on this forum, I thought it deserved its own thread so new posters can find it in a search.
For the OP and anyone else considering the tradeoff of wanting higher CFM but thinking they need costly makeup air (MUA), I would like to dispel some myths that get presented on this and other forums. I am currently going through this process and the city building department has given me quite the education about the process so I thought I would share.
We live in Minnesota where the temperature can vary more than 120F in the course July to January. People toss out these generalizations that if you live in a cold climate you have to have heated makeup air to get above certain CFM without MUA.
Generalizations about maximum CFM's that are allowed without MUA are just that, generalizations. Your house is unique to any other house when it comes to determining if you need MUA. Your contractor should work with your local building code department to perform some calculations that will determine if MU is needed. Some factors in the calculation include; number of external fans (bath for example) their CFM's, number and types of fire places, type of water heater and furnace, efficiency of furnace, if an air exchanger is installed, age and materials insulate and wrap the home. With this information they can determine how many CFM's you can pull out of the home without causing negative pressure that could back draft the home. As others have posted, if you back draft, you could literally suck the CO from the water heater, furnace, etc. right back into your home, or even pull embers from your fireplace out onto the floor.
The idea that you have to have heated makeup air (HMUA) is also a generalization. As I said, we live where it gets -20F BELOW zero in a 6 year old very tightly wrapped home with 2x6 framing and crazy high insulated R factor. . We do NOT have to have HMUA to add 600 CFM. All we need is cold MUA. This is a very simple solution if you live in a home with a basement or crawlspace that is not finished. All that is needed is a flexible 6" duct from the outside (where there is no other exhaust ports) that goes into the mechanical room and just hangs there open in the ceiling. The purpose of the MUA is that when you turn on your high CFM hood, if enough other exhaust is also running at the same time (bath fans, etc) and the home needs more air, it will draw from the MUA duct. If is kind of like leaving a window open when there is no wind. If the home needs it, it will draw through it.