Any time the flow of air (or part of the air) has to change its velocity vector (direction and/or speed), even slightly, work is done on the air, and this requires either heating or force times distance. Pressure is force over area. Velocity change occurs not only when a duct bends, but when there is turbulence in the duct or at the duct walls, or when the air is forced through a filter, around baffles, etc. Thus, there are many ways that pressure drops occur along a flow path. If not, one could blow through a straw without having to build up pressure in one's mouth. I use a large (three-foot-square) vaned ceiling diffuser to introduce MUA into a hallway adjunct to the kitchen. The diffuser is about 18 feet from the edge of the hood. This configuration was the best option relative to where I could take the air from the roof area. It is my expectation that by the time the air gets to the vicinity of the hood it is flowing relatively smoothly and low in velocity due to the size and length of the hall. Certainly other approaches would work for other kitchens depending on where the air is taken from and how a house is architected. Test kitchens use a porous diffuser panel in an adjacent wall to the hood under test. I am sure that from a functional point of view, MUA flowing out from enough toe-kick area would also work, so long as it wasn't somehow directed at the stove. For example, if a cooktop were on an island, and the entire toe-kick area were grilled using a set of registers, the MUA would flow out and then up in a way that did not disturb the rising effluent from the pans. Even down flow behind a refrigerator can work if the path behind and underneath the refrigerator is relatively open (several times the area of the supplying duct.) Never forget that perfect is the enemy of good enough, and what we are trying to accomplish is good enough capture of grease, moisture, and odor. Good enough is also user dependent, so not everyone needs to put up with the compromises that a nearly perfect MUA system would impose on a house. So, my best advice is to find a way to introduce the air that doesn't blow at the cooking zone and disturb the natural rise of the cooking effluent. kas

So you have the Broan 6" MD6T and you have it unfiltered, unconditioned, and ouputting under the fridge for conditioning purposes? So you are relying on the exhaust heat of the fridge to cool the incoming air? How cold did it get this past winter for you? How cold was that make up air? Detectable at all? Here we had -20 real feel in NY with the wind chill at times. It was brutally cold and cold often. I think I might have to put in a heated MUA. So update on Broan. The lady I spoke to today was completely useless. To make a long story short she had absolutely no idea how to use the Make Up Air Specifier on her site. It was quite pathetic and my impressions of Broan's customer service = suck. I spoke to Dean at Electro Industries at length about all the factors I'm thinking about. We went over two options - one using the MA-10 (10kw all in one 10" heater with damper, blower, filter, and current detector to control blower speed) and the less expensive heater unit EM-WX1025R. The broan MD6T comes with a 24volt 20VA transformer so I could then use the one signal to turn on both the Broan damper and the heater unit. Both options unit can accomodate up to 637cfm in a 50 degree temperature rise. It's a 10kW unit though and would require a 50A or 60A two pole (240V) breaker and its own dedicated lines (of course). I can take care of the electrical so no problem there but man I'm guessing my heating bill (which was astronimical last year) will be more than I can tolerate this year. Temperature Rise Needed/CFM EM-WX1025R 10000 Watts 80/395 70/452 60/527 50/632 40/790 30/1054 MA-10 $1917 + freight EM-WX1025R $800 + freight. I think that should be sufficient - it's better to have 50 degree air coming in than 0 degree air. I'm thinking that maybe I can get away with non-heated air if I can get fresh air to come up that old supply register near the range. The opening is closer than the fireplace but it might work if I put a blower in-line as well.

This is an instance that you get what you pay for. The higher cost make up air system will be automated to engage WHEN your hood is running and not when not and will balance the air flow. It will also be designed to bring the make up air close to the exhaust so that as you suck out your heated or cooled home air you are bypassing most of THAT air and getting new air direct from outside sucked near your appliance so as your appliance sucks out air, it is getting make up air from very close by and is NOT sucking out conditioned home air for the most part. So there is dual ducting system to the appliance, one in, one out. (Note: for the purposes of this discussion "appliance" means the UNIT of range/cooktop AND its accompanying vent hood.) Often the in air is brought up under the appliance and vented from under the appliance up the front of the appliance and then sucked out the top of the appliance. The energy savings from this approach versus some generically located damper are significant. You will also bring less fluctuation in humidity levels into the home with the engineered system, also saving energy and wood surfaces in the home. In addition to the integrated parts (i.e. the automated damper), you are paying for the secondary ducting system for input air, and the design and parts to install it to an optimal location. The caveats here are that the worth of the engineered system are increased in harsher climates where you are paying to make home air either hotter or colder than outdoor air or where maintaining constant humidity levels in the home is important, and increased as the CFM the system is run at increases. And then there are issues of code. So you'd have to take into account these in your cost benefit analysis, especially in a mild climate and using a relatively low CFM air flow. This post was edited by beautybutdebtfree on Thu, Jun 26, 14 at 9:44

building2017 wrote: ... - What are the silencers I have read about and how do they work/would they work here? These are like giant truck mufflers, except designed for various diameter ducts. They have low pressure loss (forget the value but it is a lot less than baffles). Fantech is the source. - Any benefit re:sound to using two smaller liners instead of one big one and only using one at a time when using only one side of stove? Or is this magnifying the cost for no good reason other than sound? (Because I'm assuming I'd then have two runs up and out and two things on the roof because they wouldn't combine to avoid a turn and to ensure proper function? If only one side were run at a time, then MUA costs could be lower, but where code requires MUA, I am willing to bet a donut that the AHJ will demand an MUA system that can handle both at once. In theory, the sound reduction will be only 3 dB less. Also, when using the central burners, you will still need both systems for capture and containment. Well, I guess three things on the roof then with the MUA. While commercial systems mostly use roof MUA entry, and I use roof MUA entry, the options for residences are wide open and may include side of house entry, or entry into a basement that has an air path access to the kitchen.