Passive House Cooktop and Range Hood Exhaust

Gas cooktops: what they require

There is something undeniable about cooking on an open flame, and you can put a gas cooktop into a high performance home. It is not a cheap addition, and here is why.

Combustion from a gas cooktop produces significantly more particulate matter and pollutants, including carbon monoxide, nitrogen dioxide, carbon dioxide, acetaldehyde and formaldehyde. These must be exhausted directly to the outside by a range hood. Exhausting that much air changes the air pressure balance, so more air leaves the home than is supplied. The result is infiltration. These homes are airtight, but not perfectly airtight, so in winter cold outdoor air can be drawn into the walls and roof, where it meets warm interior surfaces and condenses. Water and building assemblies do not mix.

To counteract this, the Building Code requires make-up air, which supplies fresh air to offset the range hood exhaust. The parts list for the exhaust side is long:

1. Insulated exhaust duct connected to the range hood
2. Motorized damper activated electrically by the range hood
3. Tape for the interior envelope penetration
4. Tape for the exterior envelope penetration
5. Exterior wall cap
6. Metal flashing at the head and sill of the wall cap

The make-up air side adds more:

1. Bulkhead or dropped ceiling with a grille to conceal the system
2. Insulated intake duct
3. Motorized damper activated electrically by the range hood
4. Forced-flow electric resistance heater to warm the incoming air
5. Tape for the interior envelope penetration
6. Tape for the exterior envelope penetration
7. Exterior wall cap
8. Metal flashing at the head and sill of the wall cap

Many trades have to install and coordinate those parts. Every time the range hood runs, it also sends heated air, and the energy paid for it, outside, and the make-up air has to be heated as it enters. Both add to ongoing operating costs.

Gas cooktops are possible in high performance homes. We have designed them and may again, because many people enjoy cooking with gas. Given the pollutants and the construction and operating costs, though, we suggest considering an induction cooktop.

Induction cooktops

• Energy efficiency: more efficient than gas
• Pollutants: fewer airborne pollutants
• Cost: typically lower to install and to operate in a high performance home

Because induction produces fewer airborne pollutants, exhausting them can be much simpler. We still include a range hood, but it is a recirculating hood with a charcoal filter instead of a duct to the outside. The filter removes immediate pollutants, and the remaining air leaves through the home's ventilation system (HRV or ERV). An exhaust diffuser is located above, and at least 6 feet away from, the cooktop so fats and grease are not drawn into the system.

This approach leaves the home's air pressure balance unchanged. The extra ducts, dampers, heaters and wall caps that a gas system needs are not required. Every high performance house already has a cooktop, a range hood and a ventilation system, so the induction approach needs no additional parts.

While cooking, you press a boost switch. The kitchen exhaust diffuser increases from a continuous 27 CFM to 35 CFM (46 to 60 m³/h), and the whole-house ventilation rate also increases, which helps flush the home.

This approach is not as fast at removing pollutants as a ducted range hood, which is typically at least 150 to 200 CFM (254 to 340 m³/h). However, there are fewer pollutants to remove, and the home exhausts air continuously. It also does not depend on anyone remembering to turn on the hood. With a ducted hood, whatever the cooktop type, the person cooking has to turn it on, and many people do not, often because of the fan noise.

Summary

If you are designing a high performance home, we recommend an induction cooktop with a recirculating range hood and continuous ventilation, because of its lower cost and reduced indoor air pollutants. We understand that everyone has a preferred way to cook. Our job is to explain the advantages and disadvantages of each approach and to design the system correctly for the choice you make.

For more on ventilation and heat loss, see Six Enemies of Heat Loss in Homes, or the FAQ.

References

1. Energy Impacts of Effective Range Hood Use for all U.S. Residential Cooking. Jennifer M. Logue and Brett C. Singer. Environmental Energy Technologies Division, June 2014.
2. Ultrafine particles and nitrogen oxides generated by gas and electric cooking. M. Dennekamp, S. Howarth, C. Dick, J. Cherrie, K. Donaldson and A. Seaton. 2001.
3. BSI-070: First Deal with the Manure and Then Don't Suck. Joseph Lstiburek. 2014.
4. The most screwed up, badly designed, inappropriately used appliance in your home: the kitchen exhaust. Lloyd Alter. 2013.
5. Uncut and Hoodwinked: The unintended consequences of monster-size range hoods. Robert Bean. 2013.
6. How to Provide Makeup Air for Range Hoods. Martin Holladay, GreenBuildingAdvisor.com, 2015.