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How should I plan makeup air for a 1,200 CFM hood in a tight home?
I’m trying to coordinate kitchen ventilation with the mechanical design for a tightly sealed custom home. The proposed range hood is rated at 1,200 CFM and would use a 10-inch duct routed through the exterior wall. The current airtightness target is approximately 1.5 ACH50, with no final blower-door result yet.
The primary heating system will be a sealed-combustion heat pump, but I’m still checking whether the home will include a naturally vented fireplace, furnace, or water heater. I understand that negative pressure from the hood could affect combustion appliances, increase drafts, or make doors difficult to operate, but I’m unsure how much the actual risk depends on the hood’s tested airflow rather than its rated maximum.
For those who have coordinated a high-CFM hood with a tight building envelope:
- At what point did you require dedicated makeup air?
- Did you size it close to the hood’s full 1,200 CFM, or use a controlled system that responds to measured hood airflow?
- Was the makeup air tempered and introduced near the hood, or distributed through the HVAC system?
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5 replies
The 1,200 CFM rating shouldn’t automatically be treated as the actual makeup-air requirement. Have the designer verify the hood’s delivered airflow with the planned 10-inch duct, fittings, termination, filters, and any silencer installed. A hood’s real flow can differ substantially from its maximum rating once duct losses are included.
I’d specify the system in this order:
1. Obtain the hood’s installed airflow at its operating speeds.
2. Interlock the makeup-air system with the hood, preferably with airflow proving so the hood can’t run without the required ventilation.
3. Temper the incoming air rather than dumping a large volume of cold outdoor air directly into the kitchen. The HVAC designer should coordinate the supply temperature and distribution.
4. Test the finished house under worst-case conditions, including the hood, other exhaust fans, closed doors, and any combustion appliances operating.
A heat pump removes one combustion-related concern, but it doesn’t make an untested 1,200 CFM exhaust harmless in a tight house. If the final design includes a naturally vented fireplace, furnace, or water heater, the ventilation and makeup-air design needs to account for worst-case depressurization and spillage testing under the applicable code requirements.
A 1,200 CFM rating alone doesn’t establish that a separate 1,200 CFM makeup-air supply is required.
Check the applicable code threshold for the jurisdiction, then review the hood’s listing and controls. Some high-output systems use demand control, reduced exhaust at lower settings, or integrated makeup air, so the required supplemental volume may be less than the nameplate maximum. The installed airflow with that 10-inch duct still needs to be verified, but that’s different from automatically designing a full-capacity outdoor-air system.
If the hood has no compensation or flow-control strategy, a dedicated tempered system may be appropriate. Final depressurization and combustion-safety testing are especially important if any naturally vented appliance ends up in the house.
For an integrated makeup-air option, confirm exactly what the manufacturer includes and requires: the sensor, motorized damper, control module, and interlock may all be needed for the listed performance.
Get the commissioning procedure in writing too. It should verify that the damper opens, measure makeup-air volume at each hood setting, and test the finished house with the worst-case combination of exhaust equipment running. A feature that is only wired to the hood is not necessarily the same as a commissioned system that responds to actual airflow or pressure.
The lower equipment price isn’t necessarily the lower installed cost. A dedicated tempered makeup-air unit may carry a higher equipment quote, but its scope is relatively clear: outdoor-air ducting, heater or tempering section, controls, electrical supply, damper, and hood interlock.
A pressure-controlled HVAC supply can look cheaper if the air handler and ductwork already have capacity, but the estimate should also include the pressure sensor and control sequence, motorized dampers and any required outdoor-air controls, added ductwork and balancing, heating or cooling capacity for the introduced air, electrical and controls integration, and commissioning at the hood’s operating speeds.
For a 1,200 CFM hood, the deciding cost factor is often whether the existing HVAC system can move and condition the required outdoor air without creating comfort or control problems. If it can’t, the integrated option may require enough modifications that a dedicated unit is more economical and easier to commission. I’d have both alternatives priced to the same scope, including final airflow and depressurization testing, rather than comparing equipment line items alone.
The control sequence should cover cold-weather operation as well as hood interlock. For a 1,200 CFM hood, specify how makeup air tracks the hood’s measured or manufacturer-defined exhaust demand, then verify actual airflow at each operating speed during commissioning.
It should also state what happens when outdoor conditions are too cold for the makeup-air equipment to temper the incoming air as designed. The applicable low-temperature limit, lockout, or alarm response needs to come from the selected hood and makeup-air equipment and be coordinated by the mechanical designer. A damper that opens whenever the hood switch is on does not by itself show that airflow is tracking the hood or that the incoming air is being tempered appropriately.