How to Tell a Customer Their New Hood Needs Makeup Air

Why this matters

The customer picked the hood in a showroom. It is big, it is quiet on low, and it came with the cabinets. Nobody mentioned that it moves several hundred cubic feet of air out of the house every minute on high, that the air has to come back in from somewhere, and that one of the easiest places for it to come back in is down the water heater's vent. The installer who hangs it is the last person with a chance to say so before it runs.

Most installers know the rule. Where the conversation goes wrong is the telling: it lands as an upsell, the customer hears "more money for a fan," and the installer, not wanting to lose the job, lets it drop. This card is the conversation, step by step, with the words. The engineering behind it, including how to estimate the depressurization and size a makeup air system, belongs to Kitchen Hood Makeup Air; this card cites it rather than repeating it.

Steps

1. Get the hood's maximum rated airflow off the spec sheet, not the box. The number that matters is the highest cfm (cubic feet per minute) the hood is rated for, on its highest speed, published by the manufacturer. The code trigger is what the hood is capable of, not how the customer plans to use it. Skip it and you argue from a sales description ("powerful," "pro-style") that the customer can dispute and you cannot.

2. Find every combustion appliance and how it vents. Walk to the furnace, the water heater, the boiler, a fireplace or wood stove. The question for each: does it take its air from the room and vent by draft (a draft hood on a water heater, a metal vent into a masonry chimney), or is it sealed-combustion and direct-vent, piped to outdoors for both intake and exhaust? Draft-vented appliances are the ones a big hood can reverse. Skip it and you do not know whether you are having a code conversation or a carbon monoxide conversation.

3. Explain the house as a box, in plain words. The customer does not need pascals. They need the picture.

INSTALLER: This hood is rated 1,000 cubic feet a
  minute on high. Every one of those has to come
  back into the house from somewhere.
CUSTOMER: It comes in through the house, right?
  It's not airtight.
INSTALLER: Some of it does, through little gaps.
  But you had the attic sealed two years ago, so
  there are fewer gaps. The biggest hole left in
  the bottom of this house is the water heater's
  chimney.
CUSTOMER: So it pulls air down the chimney.
INSTALLER: That's the risk. The water heater
  sends its exhaust up that chimney on draft.
  Pull hard enough the other way and the exhaust
  comes out into the basement instead, and that
  exhaust carries carbon monoxide.
CUSTOMER: I'll just run it on low.
INSTALLER: The code goes by what the hood can
  do on high, not what you plan to run it at.
  And the day you sear a steak, it will be on
  high.

Why this way: the picture is true, it uses something the customer did (the air sealing), and it names the consequence once without dramatizing it. Skip it and the next step sounds like an upsell.

4. Name the code rule, as adopted. The IRC requires a kitchen exhaust hood capable of more than 400 cfm to have makeup air sized to its exhaust (earlier editions asked for about the full exhaust rate; the 2021 edition, in substance, the amount over 400 cfm), through a damper that opens automatically when the hood runs (M1503.6 in the 2021 edition; numbered differently in some earlier editions, and amended in some states and cities). Say it as a rule, not as your preference, and say that your jurisdiction's adopted version is the one that governs. If your local amendment changes the threshold, use the local number. This is not legal advice; it is what the inspector will look for on the mechanical permit.

5. Offer two real paths, not one. Path one: a smaller hood, at or below the code trigger, mounted to capture well. Capture depends more on geometry than on flow: at least as wide as the cooktop, deep enough to cover the front burners, at the height the manufacturer specifies. Path two: the hood the customer chose, plus a makeup air system sized to it, usually a powered supply with a motorized damper interlocked to the hood and, in a cold climate, heat on the incoming air. A third path, if the water heater or furnace is near the end of its life, is replacing it with a sealed-combustion or electric unit, which takes the backdraft risk off the table even though a hood over the trigger still needs makeup air under the code.

Present them as equals: "Either one is a good kitchen. A well-mounted 400 does a fine job over this cooktop. If you want the 1,000, we add the air it needs and it opens every time the hood runs." Skip it and the customer is choosing between your recommendation and nothing, which is how makeup air gets declined.

6. Route the safety test to the person who owns it. Whatever the customer picks, if a draft-vented appliance remains, a qualified gas or combustion-safety tech runs a worst-case spillage test after the hood is installed, with the hood on high and the other exhaust fans running. That test is deliberately trying to make the appliance spill and belongs to someone with a combustion analyzer and the authority to shut the appliance off; it is not part of hanging a hood. See related: How to Screen for Spillage After You Add an Exhaust Fan, for the simple look a non-gas trade can take and where it stops.

7. Write the choice down. On the work order or quote: the hood's rated cfm, the combustion appliances found and how they vent, which path the customer chose, who is doing the makeup air or the test, and the customer's signature or acknowledgment. What that gives the shop is a dated record that the customer was told and what they chose. Whether it limits liability is a question for your own attorney; that it answers "did anyone tell them" is a fact you can bank.

Worked example: the 1,000 cfm island hood

The house: two stories, about 2,600 square feet, attic air sealed two years ago, with the energy auditor's after-test on file at 2,200 CFM50 (the airflow that holds the house 50 pascals below outdoors). The furnace and water heater are both draft-vented and share a masonry chimney. The kitchen remodel includes an island hood rated 1,000 cfm on high.

What the arithmetic says, in outline. Kitchen Hood Makeup Air gives the estimate: pressure is about 50 times (exhaust cfm divided by CFM50) raised to 1 divided by a flow exponent of about 0.65, which is a power of about 1.54. A rated 1,000 cfm hood on a real duct run delivers less; call it 700 cfm, an estimate, not a measurement. 700 divided by 2,200 is 0.318; raised to 1.54 that is 0.172; times 50 is about 8.6 pascals of depressurization. Check from the other side: 8.6 divided by 50 is 0.172, and 0.172 raised to 0.65 is about 0.318, which times 2,200 is 700. The two directions agree.

A draft-vented appliance works on a draft of only a few pascals. Eight and a half pascals against it is not a close call.

The smaller-hood path, checked. A 400 cfm hood delivering an estimated 300 cfm: 300 divided by 2,200 is 0.136; raised to 1.54, about 0.047; times 50, about 2.3 pascals. That is legal under the code trigger and still not zero. It is why step 6 applies to both paths: the code threshold is a mechanical rule, and the spillage test is the safety check.

What the customer chose. The big hood, with makeup air. The installer wrote it up: hood rated 1,000 cfm; furnace and water heater draft-vented on a common chimney; customer chose path two; makeup air by the HVAC contractor, interlocked and heated; worst-case spillage test by the gas tech after install, before the hood is turned over; customer acknowledged on the quote.

What would change the conversation. If the house had no draft-vented appliance at all (a heat pump and a heat pump water heater), the hood still needs makeup air under the code, but the conversation drops the carbon monoxide half and becomes a straight code and comfort item: no backdraft risk, just a house that whistles at the door gaps and a hood that underperforms. If the house had never been air sealed and tested at, say, twice the leakage, the same hood pulls far less, but the measured number decides that, not a guess about how drafty it looks.

When the customer says no to both

It happens: the customer wants the big hood and will not pay for makeup air. Three things are true at once, and the installer has to hold all of them.

The code question is not yours to waive. Where M1503.6 is adopted and the job is under a permit, a hood over the trigger without makeup air fails the mechanical inspection. Installing it anyway, off-permit, puts your license and your shop's name on work you know does not comply. Most shops decline that job, and saying so plainly ("we can't install it that way") ends more arguments than explaining it again.

The safety question survives the code question. If the customer takes the smaller hood instead, the draft-vented appliances are still in the house, and the 2.3 pascal estimate in the worked example shows why the spillage test still runs. If they buy the big hood elsewhere and have someone else hang it, you cannot stop that, but you can put the warning in writing on your quote and recommend a CO alarm on every level and near the sleeping areas (the IRC, R315 as adopted, already requires one outside each sleeping area in many houses with fuel-burning appliances).

A cheap fix is not a fix. An open window "when you cook" works once and is forgotten by February. A passive hole in the wall with no damper and no fan cannot bring in several hundred cfm at a pressure the water heater can live with; Kitchen Hood Makeup Air shows the size that opening would have to be. Do not offer either as an alternative, because the customer will hear it as permission.

How to know you did it right

The work order carries the rated cfm, the appliance list, the chosen path, the named owner of the makeup air and of the spillage test, and an acknowledgment. The hood is not turned over to the customer until the spillage test is recorded with the hood on high, and the makeup air damper is confirmed opening every time the hood starts, at every speed.

References

• See related: Kitchen Hood Makeup Air; The Range Hood That Backdrafted the Water Heater; How a House Pulls Flue Gas Back Down a Vent
• See related: Counting Every Exhaust Device in the House
• IRC M1503.6, makeup air for kitchen exhaust over 400 cfm, and R315, carbon monoxide alarms, 2021 edition, as adopted and amended by your jurisdiction
• Hood and makeup air manufacturers' installation instructions (rated airflow, mounting height, interlock wiring)