You have to in new construction too.

"HVAC guys" - I like that because it doesn't give them any credit for technical expertise which is generally the case.

First - most HVAC systems do not pull in outside air. That is a generalization and it is possible that they do in your area. The blower on an HVAC is relatively high CFM - like 1000 or 1500. No one needs to ventilate that much air. So a typical fresh air requirement is more like 100 CFM (house size dependent). So in no world that I can think of does it make sense to use a 1000 CFM blower for fresh air ventilation requirements.

Now - you can design an HVAC system to pull in outside air with the full blower power to do make up air. That requires a fairly large external filter - think 20x20 in with a damper. It maybe that this is customary in your area. In which case, on a new house, it would be no big deal. I find this hard to believe.

Make up air is a code requirement (not generally enforced). If you have combustion appliances in the home, you should follow the code. Since most people have a high powered hood because of a gas range, it follows that they should have make up air.

Clear as mud?

I suspect your HVAC folks are thinking that a typical fresh air intake is just fine and dandy for make up air. So that is what you need clarity on. Will you have an HRV or ERV?

Thanks for the reminder on the topic.

David Cary - Sorry for incorrectly addressing the HVAC guys as HVAC guys, they did not provide a business card with their official title, nor did they announce their title after meeting them. Henceforth, I will refer to them as HVAC masters.

I've been told by the HVAC masters that the inspectors here are a real stickler for certain things, one of them being makeup air. They quickly explained the system to me in great detail, and about 90% of it flew over my head. I had this idea in my head that the makeup air would be its own separate thing, piping, unit, intake, etc. Was just surprised that somehow with some magic that they are able to make the HVAC system do this job too.

I don't think I have a HRV or ERV. I did some research earlier in this process, and the architect and builder was telling me this was going to be a "tight" house, so I thought one of those would be helpful for the spring and fall when the HVAC does not run much, so the air does not get stale. But the builder said I did not need one of those.

Sounds like you didn't do enough research if you went with builder's recommendation of no outdoor air ventilation at all. Not even exhaust only?

Your MUA unit probably is separate but shares ductwork with heating and cooling system. Curious about size of vent hood wall outlet versus MUA intake.

In my opinion the HVAC guys are making up the story about the wire between your hood and HVAC system. What they are saying makes no sense.

How large is this house? Will the kitchen always have a door open to the rest of the house? How long a period of time do you envision running the hood?

I am not an expert, but I question whether make up air is necessary to run a 600 CFM exhaust. Will this house have a gas hot water heater? If yes, where will it be located in relationship to the range hood?

Anything over 400CFM requires MUA. It is required to operate simultaneously to your hood. As in either wired to the same switch, or powered by the pressure from the hood activating it.

House is 2600 sq ft, open floor plan so the kitchen is open to the rest of the house. Hood is 600 CFM. Induction cooktop. All other gas appliances are direct vent. Maybe we don't need 600 CFM with induction, but we do sear steaks and fry bacon alot.

You might need MUA more for the combustion appliances than your cooking. Even with the MUA, it's possible to backdraft your "sealed" direct vent combustion appliances. Combustion appliances are the reason for the MUA requirement.

Mike - you misunderstood me - I think "guys" is usually the correct term.

I think when people do induction, they usually do 400 CFM to be below the rather expensive/onerous requirement for MUA.

I would never ever ever trust a builder or an HVAC person to handle this correctly without getting an exact answer with details. But that is me.

Just look at the ridiculous oversizing of HVAC systems that is tolerated. The field is filled with technicians with very little physics background. It is a real shame.

Simple thing to do - ask where the external air intake is and what size it is. Your concern can be for aesthetics without questioning their competence.

Before Springtime says it.... If you are happy cooking without gas, why would you build using gas? Your structure will outlive the use of gas in homes - why waste money on it now? In 2018, in almost every market in the US, it is time to plan to use electricity only.

I just got done building. I needed a 600 cfm MUA unit for the range top hood vent( general rule of thumb is 100 cfm per burner). This is a separate monstrosity that is large and ugly and sits in the attic. It is dedicated for just the MUA and cant share itself for the purpose for the whole house ventilation. For that I needed to install an ERV (energy recovery system) which turns on every ten minutes per hour and is tied to the upstairs air handler.

The whole thing is ridiculous. I spent a fortune tightening and sealing and insulating, only to get an amazing blower door test result, but my reward is that I have to deal with 2 ventilations systems.

My rookie assessment is that we are approaching absurdity in requirements with the code, which actually just creates an unhealthy indoor air situation, even with the ventilation systems. Its all just too tight. It takes away from the efficiency when you need to use more electricity to run all these ventilation systems. And for bonus points I get to second guess the super scary idea of actually opening a window and throwing the whole house into a death spiral.

What is the code for ventilation for a gas range? Does it have to be vented to the outside? Why install a 600 CFM hood and all the issues it creates if it is not required?

No requirement for hood ventilation to outside with gas range, as long as vent hood is under 400 cfm, you are free to poison yourself and family all you want. The requirement is because people continue to put combustion appliances inside the home. When that goes away, so does need of MUA for life-safety.

Venting cooktops to outside is important for removing cooking pollutants, which goes for induction too.

It's not just tight construction. Backdrafting combustion exhaust happens in leaky houses all the time. Tight homes are more efficient and healthier with the proper outdoor air ventilation, which doesn't have to be complicated. It's not as complicated as combustion appliances; burning things indoors.

With current state of home industry, insulation is less important than a good blower door test. Insulation can't do it's job with air flowing through it or around it.

Leaky homes have no control over air movement compared to tight homes. Uncontrolled air movement causes a lot of problems other than bad efficiency.

Tight homes can better control the amount of outdoor air called for by building codes and indoor air quality research. Leaky homes without outdoor air provisions are usually only ventilated enough when the wind blows or windows are open.

400 is code required MUA. Physics happen despite code minimums and including MUA or using a vent hood <400 cfm is no guarantee for avoiding combustion exhaust backdraft. The best way to do that is not use combustion appliances.

My advice is to choose induction, stay under 400 cfm vent hood with more effective capture area and eliminate all combustion appliances. No need for MUA.

Mike, I must have been adding paragraph above as you posted that. Staying below 400 cfm does not necessarily stop combustion exhaust backdraft. It only helps improve the odds and satisfy minimum building codes. The easiest, least expensive and safest solution eliminates combustion appliances.

Forgot to include attached garages. Until cars are also all-electric, MUA is a good idea for homes with attached garages too.

Back drafting would be my concern. That is why I would avoid installing a powerful ventilation hood. If one is installed then run it at the lowest speed when cooking items that produce an odor during the actual cooking time.

Cooking with gas is much cheaper than cooking with electricity. I have tried cooking on an electric cook top. I find it very frustrating. Give me four gas burners and I am happy. On the other hand an electric convection wall oven is superior to gas in my opinion.

Mike, you should check out induction cooktops. They are safest, fastest, most efficient, even heating and easiest cleaning cooktop available. It seems there is a mass exodus away from gas to induction even among professional chefs who use cooktops everyday for a living. Search induction vs gas for all kinds of info.

Gas is nowhere as close to efficient as induction (different than traditional electric).

Yeah - Mike. Induction is probably an order of magnitude cheaper than gas. Honestly, gas is probably about even with conventional electric. Sure gas is cheaper for BTU created but most of those BTUs don't make it to the pan. It all doesn't matter because you must ventilate gas more to get rid of the pollutants. All that ventilation requires more space conditioning energy that cooking ever uses.

Now cooking with gas vs electric as far as feel is another issue altogether.

What was the topic again? ......

I'd start here: https://www.gardenweb.com/discussions/5161173/hood-faq

Where are you building (eg, what is the climate like?)? How tight of a house are you building?

Consider this, every bit of air that is exhausted out must be replaced with air coming in. Even if it's a 75 CFM bath fan. Where does this air come from? Many HVAC folks (I know a great system designer who's a woman) will put in an 8" passive duct. This is good for about 200 CFM of inflow but if crimped or lots of bends then much less.

Without a powered MUA system then the rest of the air must be sucked in through cracks. If it's humid out then this will create considerable moisture problems in wall and roof cavities where this air is being sucked in. Moisture hanging around means inefficient insulation and mold.

Ideally you really want to match MUA to total exhaust. For every 100 CFM you're exhausting you want to bring in 100 CFM - minus perhaps 200 CFM for the passive vent.

As to code, keep in mind that it used a house with an ACH50 of 7.0. That's extremely leaky compared to houses built today where in northern climates they are often under 1.0 and even elsewhere are often below 3.0.

Then there's the performance of your exhausting appliances. As mentioned above, backdrafting of gas appliances is the worst case scenario. Short of that there is also decreased performance. Cracks are inefficient and create extremely high static pressure. A cheap Panasonic bath fan having to suck air in through cracks and pull against 5"SP (inches water static pressure) will not perform to the 75 CFM that they advertise but at about 10 CFM. Your 600 CFM hood will actually only be doing about 150 CFM...

A properly designed MUA is a very good thing and worth the money.