Best Method for Bringing in Makeup Air
So we have a 600 CFM vent hood going into a “pretty good” house, new construction climate 6b.
I’ve read the pros and cons of using a vent hood and due to our cooking habits we feel the vent hood is a necessary evil although I’ve been pushing for something in the 400 cfm range my wife found one to meet her style requirements that is 600 cfm and doesn’t come with a smaller exhaust.
The hood can be bought with an interlinked make up air damper which we were going to procure.
My HVAC guy wants to run the make-up air directly into the return line of the furnace.
My thoughts were to run it directly into my mechanical room, have it get partially pre-warmed in the mechanical room, then run up to the kitchen (directly above) through holes drilled in the subfloor then out through some grills in the cabinet toe kick.
I’m concerned about running directly into the furnace return line because whenever the blower kicks in there is going to be a negative pressure pulling cold air from outside past the damper. I know the seals on those motorized butterfly dampers are not great and it doesn’t seem like a smart idea to have them be pulled on all the time because I’m sure the seals will wear out quicker and they will surely leak more with the suction.
Using my method there is no suction on the damper until there is a net negative pressure in the home i.e. when the vent hood is turned on. In that case the damper will be open which is what I want.
I think I might need to make it an active system so when the vent hood turns on there a damper opens and a fan kicks on to pull in the required air flow.
Does anybody see any problems with my plan to bring the air into the mechanical room? How do people “normally” bring in makeup air to the house?
Steve
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Replies
I am facing the very same challenge, and also in Zone 6. Given a tight envelope, and other machinery pushing and pulling air independent of cooking, I distrust an automated makeup air damper for the vent hood. I think a direct link between the damper and the vent hood makes most sense, open-on-demand. We are looking at Broan for the damper.
https://www.broan.com/Accessories/Range-Hood-Accessories/Ducting-Dampers-Adapters-Caps/MD6T
I do worry about cold toes while sautéeing, however.
https://www.buildingscience.com/documents/insights/bsi-070-first-deal-with-the-manure
Sounds like you're between the high and low CFM suggests in the link above.
Normally people live in leaky homes so make up air isn't such a big deal or they just open a window.
Geir,
I'm not sure what an automated makeup air damper is. I was planning to use one like in your link that is electrically connected so that when the vent hood turns on, the damper opens. I think I'm going to have the make up air be active so that there is a fan that turns on in the makeup air line that flows 600 CFM into the house (Per John's link it should be a little more than 600 cfm). I'm not sure if this can be made variable to match the selected hood speed or not.
Instead of a vent at the kick plate, maybe I could run it up inside the wall and have a vent above my cupboards. So cold head instead of cold toes?
Steve
An automated makeup air damper basically opens an air pathway from outdoors into your house when you run your vent hood. Assuming you have your vent hood venting to outdoors (not a recirculating type), then the air it blows out that vent to the outdoors needs to be replaced inside the house by an equal amount of “makeup” (replacement) air from outdoors.
In a typical leaky house, this makeup air comes through all the little cracks and gaps all over the house. In a tight house that is well air sealed, there sometimes isn’t enough leakage to makeup the air the vent hood exhausts, so the vent hood pulls a slight negative pressure inside the house. This can cause problems, especially for gas appliances that use natural draft exhaust where the negative pressure can cause backdrafting.
The automated makeup air damper intentionally opens to the outdoors when needed for the vent hood to operate, but remains closed the rest of the time so that you don’t lose conditioned air.
Bill
I think he's questioning the prior answer, as I am too because that person said "I distrust an automated makeup air damper for the vent hood. I think a direct link between the damper and the vent hood makes most sense, open-on-demand. " I also don't understand the distinction between what he is calling an automated damper vs an "open-on-demand". Maybe he really means the method of opening, whether a pressure sensor, a current sensor, or a manual on/off switch?
We have a variable capacity range hood, up to 1100 cfm. We need it for the type of cooking we do. We also find that the kitchen heats up quite a bit when the range is going full-tilt. In those conditions, we go native: Just open the windows for makeup air. Anything less probably won't work anyhow. Opening the top sash a couple of inches is enough to comfortably mix the makeup air with the rest of the kitchen air. A well-designed hood captures most of the smoke on the first pass. These events are relatively short compared to the rest of the time that the place is closed up. I find that the ease of use (opening a window) is well worth the small energy penalty vs. a complicated and interlocked system with motors, fans, ductwork, etc.
I'm resurrecting this thread as nobody commented on my HVAC guy's plan. I talked to him again yesterday and he is determined the best way of doing the makeup air is with the furnace blower.
Like mentioned in the OP he wants a direct line from outside with a damper connected directly to the return air of the hvac system.
When the vent hood starts the makeup damper will open the furnace blower will start and the zone damper for the kitchen zone will open and makeup air would be supplied to the whole zone.
What are your thoughts on using the furnace air handler (it's variable speed) for a 600cfm makeup air?
Steve
Steve,
A 600CFM hood doesn't need 600CFM or makeup air. Depending on how tight the house is and number of baths, you probably only need around 200CFM.
Your idea would work much better, there is no need to for all the interlocking/zones/blowers for the makeup air.
Makeup air should be supplied to outside the kitchen area, this gives you the best capture efficiency plus keeps any cooking smells more contained.
I would even go one step further and install and adjustable barometric relief damper instead of a motorized one, no need to let in makeup air unless the house is actually getting depressurized.
Good Lord of Mercy... does anyone read codes anymore? Ain’t that a shame that an HVAC contractor doesn’t have a clue about codes?
M1503.6Makeup air required.
Where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within a dwelling unit’s air barrier, each exhaust system capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with not fewer than one damper complying with Section M1503.6.2.
Exception: Makeup air is not required for exhaust systems installed for the exclusive purpose of space cooling and intended to be operated only when windows or other air inlets are open.
M1503.6.1Location.
Kitchen exhaust makeup air shall be discharged into the same room in which the exhaust system is located or into rooms or duct systems that communicate through one or more permanent openings with the room in which such exhaust system is located. Such permanent openings shall have a net cross-sectional area not less than the required area of the makeup air supply openings.
M1503.6.2Makeup air dampers.
Where makeup air is required by Section M1503.6, makeup air dampers shall comply with this section. Each damper shall be a gravity damper or an electrically operated damper that automatically opens when the exhaust system operates. Dampers shall be located to allow access for inspection, service, repair and replacement without removing permanent construction or any other ducts not connected to the damper being inspected, serviced, repaired or replaced. Gravity or barometric dampers shall not be used in passive makeup air systems except where the dampers are rated to provide the design makeup airflow at a pressure differential of 0.01 in. w.c. (3 Pa) or less.
> exhaust makeup air shall be discharged into the same room in which the exhaust system is located or into rooms or duct systems ...
It's worth noting that the discharge location can have a huge effect on needed hvac capacity and/or comfort.
Yep, make up air required in IRC-2015, M1503.4 Makeup Air Required
The assumption being we are all under the same code.
This is one of the more confusing areas, but after reading through sections of my local building code, it almost feels deliberately vague in certain areas to give inspectors a great deal of latitude
"M1503.6Makeup air required.
Where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within a dwelling unit’s air barrier, each exhaust system capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with not fewer than one damper complying with Section M1503.6.2."
This can be interpreted as if you have an induction cooking appliance, direct vent gas fireplace and power vent furnace and hot water heater, there is no code requirement for make up air even if there is a practical one.
My local building code states make up air is required to ensure that operation of exhaust system, exhaust equipment and combustion equipment is not adversely affected. Seems like an opportunity for the inspector to enforce his/her interpretation.
Your understanding is correct: Using the specific language of the code, makeup air is not required if you have no combustion appliances or if they are all sealed combustion. However, your 600 cfm fan won't draw 600 cfm if your house is well sealed and there is no makeup air. The house will simply depressurize to the fan's static pressure, then no more air moves. Even with passive makeup air, you've got to depressurize the house before the makeup air does any good at all. Powered makeup air is probably the best way to go, and then you've got the issue of all of that unconditioned air going somewhere. If you expect your mechanical room to be hot, your approach could work. But if you're going to have HPWH in the mechanical room, chances it will already be cool and flooding the room with cold exterior air would not be a good idea.
"But if you're going to have HPWH in the mechanical room, chances it will already be cool and flooding the room with cold exterior air would not be a good idea." I have this exact scenario. Still, better to decrease HPWH efficiency for a little bit than introduce a dedicated resistance heater for the makeup air in my mind. I don't see blowing 400 cfm+ of below freezing air directly onto my feet during winter going too well.
"Where one or more gas, liquid or solid fuel-burning appliance"
I don't think this thread is exclusive to fuel burning appliances.
I for example have an induction cooktop and still want good venting for the toxins created by cooking food.
Very late to this conversation I know but I’ve been thinking of similar MU air delivery scenario. Perhaps outside air could be dumped (assuming a feasible route exists) behind a refrigerator near the bottom where it has the chance to mix with the warm exhaust air from the condenser that typically vents from back/bottom of many fridges. Thoughts?..
Certainly not a bad idea as far as the comfort impact goes. I chose to deliver make up air directly underneath my range to help with the comfort issue, and hopefully help with the energy gains/losses from introducing a large volume of outside air.
My reasoning is that the goal of a range hood is to exhaust particulates and some gases that are products of combustion. Ideally, if we can entrain all those undesirable components in air we want to get rid of anyway, we minimize the impact to air we want to keep. Therefore, make up air arriving close to the range has the best opportunity to entrain the contaminants we want to get rid of and hopefully only spend a short time inside the envelope.
My theory seems to work reasonably well on days that aren't excessively cold (30 F and above). On days colder, despite the air current from the hood, some of that cold air ends up riding the floor of the kitchen as it gets pulled in by the range hood.
See attached for pictures of make up air ducting in an uninsulated state. You can also see the damper which is activated by a current sensing switch in the range hood. (Oh, ignore the thing that looks like an electrical outlet on the side of the filter box; that's a joke from a friend.)
Deleted
J_Sommer,
You may find this article and the following comments useful: https://www.greenbuildingadvisor.com/article/sizing-a-kitchen-exhaust-fan
https://youtu.be/RJ-WF7gaiXA
https://youtu.be/u2uVaTMXd_8
This is what I plan to do if at all
possible.
I always heard it can be bad on a gas furnace to have very cold air being piped in from outdoors. They aren't designed for that large of a temp delta was the claim.
This has come up before:
https://www.greenbuildingadvisor.com/question/kitchen-exhaust-and-make-up-air-all-in-one-hood
This is one of those that feels like it should work but it doesn't.
It is very effective at exhausting the makeup air directly without capturing much of the fumes coming off the range.
You can read the study I liked to in the above thread to see what works the best for capture efficiency.
The install from Patrick in post #20 is simple and very effective.
I had an idea for my particular project where a 500 CFM max range hood is being installed. I also think bringing the make-up air in under the range or a combo of under the range and toe kick vents very near the range is best option.
My idea is this: I have a "good" crawlspace. It's tied to the rest of the home's air exchanges, with ground vapor retarder and exterior wall insulation. I want to terminate the air intake into the far side of that crawlspace, with a motorized damper of course. This particular crawlspace is also home to radiant floor heat pipes (attached to wood subfloor) and even the main manifold for that, so it is relatively warm in colder times and relatively cool in warmer times. (Front range, Colorado) Just dump the make-up air in that space. Then, at the other end of that crawl (maybe a few feet into the crawl so it picks up the air I'm intending) and nearer the kitchen range, I could have an open duct there that leads up to under the range. This way, the air bought in is tempered by the crawlspace environment first, then drawn the rest of the way to the range. The damper will open only when the range is activated.
What do you think?
Second question: How in the world are you guys or anyone terminating 8" and 10" ducts to the outside world? That's a big hole in a rim joist, which seems about all you could do if you're bringing the make-up air in under instead of over.
A photo in an above post appears to show one going through the concrete basement wall, but even then it would need to be above grade.
That's the main reason I stumbled on this discussion in the first place, I wanted to see how others solved that problem. Seems like that's a tougher question than everything else that's been discussed.
Mike,
I have the same situation. I have a 398 cfm exhaust hood and putting a vent in the floor under the cooktop cabinet. On the opposite side of the house I have a 8" duct with a blower that is the same cfm as the range hood which can be activated when the range hood is on. Fortunately my crawlspace is mostly above grade, so I had about 2 feet of vertical crawlspace to go through without having to go through the rim joist. This way, you have ~55F air coming up from the crawl instead of ~30F air from outside that would otherwise need to be heated. Note that in this situation a passive vent at the other side of the home would likely not pull in enough cfm, it likely needs an active blower if you want to come closer to neutralizing the pressure. I do not have a hard measurement of what the pressure is between the room and the crawlspace is when the range/makeup air is turned on. Connecting the makeup air to the floor vent would potentially have some benefits, like not leaking cold/dirty/radon air from the crawl when not in use, and likely reducing pressure differences. The advantage of this setup though is that the air is tempered and you don't have to run an 8" duct across the crawl.