Big kitchen range hoods cause a lot of problems for builders trying to get certificates of occupancy, for homeowners with appliances that might backdraft, and for me trying to energy-rate and size mechanical systems for homes. Even though they should, they’re not going away anytime soon. The rules for them have recently changed in NC and code enforcement is not uniform as a result. It pays to know the code and to understand the issues.
What’s the problem? I’ve written about these before. Large gas ranges often “require” a large, high CFM hood. Because the manufacturer’s specification for the range specifies this hood, builders are bound by best practice and essentially by code to follow the specification. It’s not uncommon to see hoods rated from 600 to 1200 CFM. This is a lot of air for a house, especially a new, tight one. Depending on the type of gas appliances present, there is a significant risk of back-drafting at these flow rates. It’s also quite possible (especially in a smaller, tighter home) to have problems with noise, doors opening, ashes flying out of fireplaces, and HVAC systems that can’t accommodate the additional heating and air conditioning required.
It’s very common for us to blower door test a home at 50 Pa pressure and get a result that’s less than 1200 CFM. That means that these hoods are de-pressurizing homes as much or more than our blower door test, which is designed to take place at a pressure that is much higher than normal operating conditions. When we run the blower door test, we turn gas appliances off for safety and make sure that there are no ashes that could be pulled out of the fireplace. The code is trying to solve a real problem here.
So what does the code say? Most people do not know that there was a recent change to the code in North Carolina. Some online sources don’t show it. Here is the link to the NCDOI web site with the current language. You have to scroll down to page 42 to see section 505.2 of the NC Mechanical code. The changes are underlined and became effective Jan. 1, 2015. I’m paraphrasing, but makeup air is required for hoods “capable of exhausting” more than 400 CFM. There is an exception for all-electric homes or homes with only direct-vent, power-vented, or un-vented gas appliances. For those homes makeup air isn’t required unless the hood is larger than 600 CFM. And in both cases, the makeup air has to be “approximately equal to” the amount of exhaust air in excess of the 400 or 600 CFM limit.
A lot of builders don’t know about the makeup air requirement at all, and a lot of code officials don’t know about the change. Prior to the change, makeup air was required for any hood over 400 CFM, and was supposed to be approximately equal to the full hood exhaust rate.
What should you do about this? The best choice is to install a hood with a flow rate less than 400 CFM. Especially if you are getting very tight blower door tests in your home, you should be avoiding over-sized hoods altogether. If that’s not compatible with a particular gas range, consider taking the money you’re spending on gas piping and the range itself, and buy a very nice high-end electric induction cooktop. I have one and find it to be even more controllable than gas. I haven’t met anyone who has actually cooked on one that doesn’t like it as much as cooking on gas. I’m going to go out on a limb and say that I think induction is a game-changer that makes gas ranges obsolete. In my opinion, builders aren’t doing nearly enough to talk their clients out of these cooktops and giant hoods.
Another option is to install a range hood that has its own built-in compensating outdoor air. These are fairly new in the residential market. The hood itself brings in some outdoor air that gets sucked right back up out of the house without needing to be conditioned or controlled separately.
Any other option is not ideal and should be avoided. Seriously, the advance planning to avoid having to do these things is worth it. The two other (semi)viable options that I know of are:
Option 1 – bring in an outdoor air duct to supply makeup air adjacent to the range hood in the high sidewall or ceiling. This would have a damper interlocked with the range hood’s operation, and in some code jurisdictions may need to have an inline heater (although to date, no one has been able to locate this requirement in the code for me). The goal here is to approximate a self-compensating hood as closely as possible. Here are my problems with this approach:
• It’s important not to hide this opening in a cabinet or other enclosed space where the unconditioned air could cause a hidden moisture problem.
• The incoming air needs to be as close as possible to the hood so that the unconditioned air will mostly go right back up out of the house, avoiding the need to condition it.
• It’s possible that someone may feel this unconditioned air and be uncomfortable. Personally, this doesn’t bother me, because it’s likely to be the person who made the stupid decision to buy this range and hood in the first place. But the inline heater would address this.
Option 2 – install an outside air duct on the return side of the air handling system for the house, with a damper interlocked to the range hood. Here are my problems with this approach:
• If you do this, you need to size the HVAC to accommodate this much air. In an efficient home, I’ve seen this nearly double the size of the HVAC system. No one wants to do that (or pay for it).
• I’m quite skeptical that this system will actually be designed and installed to bring in an “approximately equal” amount of makeup air, and no one is measuring it. Certainly the code official isn’t.
• Range hoods never exhaust as much air as the rating on the box. In fact, it’s quite common for a 1000 CFM rated hood to actually move 600. So if it were designed to balance the hood, it may cause weird pressure phenomena in the other direction, or just be very wasteful.
• Dampers fail. Fairly often. And even though people will tell you that the direction of failure is predictable, that is not my experience. I see dampers all the time that have failed or gotten stuck in a supposedly impossible position. If the damper fails open, the homeowner will have a (very) high bill or a “not meeting thermostat” complaint. I think it’s highly unlikely that the average service technician is going to accurately diagnose this as the problem.
Remember, a smaller range hood is easier, cheaper, and an overall better solution. It’s worth changing your range selection. Self-compensating probably isn’t cheap, but it’s also a reasonable solution. After that, you’ve got a choice between unappealing and difficult to coordinate options. And as usual, if you’re building a home, you should obtain specific advice from professionals in your area and your local code official. Outside of North Carolina, your state’s requirements may be slightly different.
Copyright 2015. Amy Musser