First, this is what the proper calculations would say about Risinger's installation. OR THIS Second, short circuiting a hood, air curtains, and generally any makeup air introduced too close to the range will produce counterproductive turbulence that will reduce the effectiveness of the hood to the point that he may as well have just put in a 400 CFM fan without makeup air. Third, the floor under the range is required to be sealed from drafts on most ranges. But, you can't usually shove ALL that air through the HVAC system either

Hello with an update: So, I contacted many HVAC companies and it was a hassle to even get them to understand what I wanted. Most have never even heard about Make up air... The company that serves our HVAC unit sent someone and gave us an estimate of $1500.00 to do the make up air through the HVAC system (but they were upfront they had never done this before and actually took a while to do their research into Fairfax code, etc..). I had another HVAC contractor give me the same quote. I kept looking (with no luck) and then I had the idea of calling Bray and Scarf (sp?) to ask them for a recommendation, since I am pretty sure they sell this kind of hood. They told me to call R.A. Dobson (heating, ac and plumbing) and they came over. They wanted to do something simpler, without involving the HVAC unit. George (their guy) said he didn't think the HVAC could support and work efficiently with the addition of the make up air damper. He gave me a quote of $1400.00. At this point, I have decided to just have my contractor do the simple version of it, without involving the HVAC unit and he is charging me $1200.00. I went with him because his price includes everything, including fixing the walls, the roof part, etc. The other companies don't fix dry wall or deal with the roof part. But yeah, if I had known it was so complicated and expensive would have gone down to a 300 CFM hood.

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.

20 year old duct system is pretty old, if your ducts are made of flex duct or duct board there is a good chance a duct cleaning will not reach the cleanliness of a new duct system. With that said if you have a 2 story structure, it's unlikely you will have the access needed to replace every piece of the duct system. So there are trade offs going from replacement to cleaning, not to mention truth in cleaning. How are you going to know how clean the duct system is after the cleaning? The ads of duct cleaning typically start around $59 a system. At this price point it would be impossible for duct cleaner to stay in business, but that is the con game of this kind of service. If the duct system isn't thoroughly cleaned, it won't be long until you'll need to do it again. the duct above is a 35 year old duct. This type are impossible to clean well as the inner part of the duct has a cloth like material. This supply plenum was replaced recently. I don't know how recently, but you can clearly see the stuff that was growing in the ducts is making it's way toward the cool dark ac supply box. Here's another picture of same plenum. Nothing is cleaner, healthier than a new duct system. I service the Katy, Texas area.