"All that said, if I had an HE furnace within the habitable space in our house, I would definitely make sure it got its combustion air directly from outside the envelope. Apart from anything else, I would not want to draw unconditioned air into the house to provide combustion air for the furnace". Couple of points/questions still on this topic, as I draw closer to actual demo date. First, our local community permit office is now pushing enforcement of mua damper such as above (where as before, they knew, but weren't enforcing). Broan8T or other universal 8" mua damper: for better or worse, my plan is exterior unconditioned air, with critter cage and down line filter; the exterior duct will be 10' away from exterior vent pipes, chimney, and at 1' above grade. How to ensure the newly made 8" exterior hole through our brick is not a magnet for wetness/other (foam insulation) Is a big concern for me. The 8" duct will run a straight line of about 15' through an open crawl space (but not draw from it), then turn to run in the small furnace room to the return of the 80% efficient furnace. Not wild about this solution but it's feasible and reasonably priced (except for me picking up it's installation/brick cut/return duct cutting/ mua analysis, low voltage interface with hood etc). Low voltage unit in hood will communicate with basement duct/furnace to kick on when hood does. Regarding our house envelope: I've worked hard to update chimney rain caps/proper pointing and installed a new, properly size chimney liner, as well as new properly sized ducts for both our gas furnace/gas water heater. I also installed for the first time four window well covers which surround exterior crawl space vents. These slightly angled well covers are on raised clips which allow air flow in for mua. I'm watching this situation closely for sufficient mua, as prior to this the exterior crawl space vents were usually open for free access to unconditioned exterior mua. I'd say the laws pertaining to mandatory make up air (for largely =/ >400 cfm vent blowers) are ahead of reality for many GCs and homeowners and should be given more time and known code meeting types of mua. I understand the need for mua to avoid backdrafting and carbon monoxide poisoning. The industry is just not there yet with feasible solutions. It's the make up air engineers and specialists who benefit the most, as they get hired to check your leaky envelope and help with the universal damper installation plan, if they even agree to come to your house. This is an important and big investment for most homeowners. Do due diligence and bring your check book. Plan early, no your code and invest yourself. Good luck. edited for spelling etc. This post was edited by SparklingWater on Wed, Apr 3, 13 at 16:11

without any information about local weather extremes it is hard to make any comments. What works in the middle of the country can be a complete disaster in the north (cold winter) or south (hot summers).

If the cooktop or range is against a wall, then the amount that the hood aperture extends past the burners front-to-back should be commensurate with the amount the hood overlaps the burners side-to-side. This is because the rising effluent expands equally in all directions as it rises. (A powerful hood will cause some distortion, but nominally and for the most part practically, the hood air flow does not "reach out" and suck in air that already has a velocity that dominates the air velocity of the hood intake air away from the actual aperture.) The best insertion of make-up air (MUA) is any method that doesn't cause any cross-drafts at the hood. In test kitchens, this is done by introducing MUA through porous walls. We can generally exclude this approach for residential kitchens. Floor, wall, or ceiling registers (diffusers are what you want for air flowing out, but are not really suitable for floors) are all potentially suitable options. Mine is a ceiling diffuser located down a hall that leads to the kitchen. This provides some flow straightening and area expansion as a means to minimize cross drafts. Cost of heating MUA is not high, but the assembly of parts needed may be high in cost. Unless you operate your hood 24/7 boiling off maple sap, you likely will only be running the hood at partial to full power for short durations. I have a heat exchanger hooked to my hydronic heating furnace that can utilize all of its (forget the exact number) probably 180 kBTU/hr output if needed for MUA heating. I think the furnace at this rate uses 1 point something gallons of oil per hour, so this high cost is around a few dollars an hour for usually less than an hour when the exterior temperature requires the maximum temperature correction. In any case, I believe that an electric heater in the MUA path will be an easier thing to integrate. Cost per hour may be higher in proportion to your electric rate's higher cost (per BTU) over oil. You may have to run significant power from breaker box to heater if you are going to make-up very high flow rates through the hood at -20F. (Fireplaces can have high flow rates that have to be made up also, but I don't know anything relevant about the flow rates of gas fired fire places, so that should be something you can find out for planning purposes.) If you are rugged enough, the MUA heating doesn't need to be perfect in temperature control, just good enough to keep the introduced air tolerable during the cooking. Your code requirement doesn't provide for different conditions needed for safety, it just mandates MUA. The safety requirement that should be driving a more sophisticated MUA requirement, applicable where combustion appliances are used without their own separate MUA sources (ducts to outside, operation in a closed room ventilated to outside, or whatever), is to keep the internal house air pressure high enough to not back-draft the appliances, and thus avoid pulling carbon monoxide into the house. For unblown gas appliances, the house negative pressure limit is 0.03 inches of water pressure, and requires very low pressure loss in the MUA system with a powerful hood. I think this is necessary for fireplaces also, as they depend on the draft for flow, and the draft depends on the pressure difference between hearth and top of chimney, given hot flames to provide upward momentum. All this means is that you can either provide a code-compliant MUA system that is rated for a given flow rate, and for safety do whatever is really needed to safety proof your combustion sources, or provide an MUA that is sophisticated enough to keep the house pressure up over the range of ventilation flow rates possible with the hood, fireplace use (potentially high flow rates themselves), bathroom fans, etc. While I am working toward a closed-loop pressure controlling MUA system for myself, I am certain that for most needing MUA, isolating appliances is the better cost approach, allowing the MUA to aid hood blower performance, and not be so critical for safety. My oil burning furnace does have its own MUA, but I wouldn't operate my fireplace with my kitchen ventilation running at full power, even with windows open (due to pressure loss in the screening), without a powered MUA path. Can't help with an educated HVAC company, but FanTech has a line of powered MUA systems (don't recall if they are heated). If not, a suitable heater can be found and inserted into the duct path. Someone on this forum back in the 2008 to 2010 period, possibly clinresga, provided an image of his basement ducting with coil electric heater in the path. My [not yet completed] system is in my attic and draws air from above the roof. A blower will push this air through a filter, through the heat exchanger (think car radiator), and into the diffuser. HVAC companies dealing with restaurants will understand what you have to do, but often their setups are not compensating -- flowing a fixed MUA flow rate against fixed vent hood flow rates. Large building MUA is usually compensated, but the schemes used are better suited (as far as I've been able to learn any details) for continuous pressurizing MUA systems that have to compensate for outside door openings. They can do this with passive air pressure control of dampers that recirculate MUA around the blower. The blower motor runs continuously. The relatively recent of introduction of residential MUA requirements has provided some impetus to development of residential MUA systems, so availability of systems and components should be expected to increase. Keep reading; be sure to check out the Greenheck paper available at their website: KVSApplDesign_catalog.pdf, as well as CKV_Design_Guide_2_031504.pdf, available somewhere on the Internet. These are commercially oriented, but should provide useful insight. kas

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