A whole-home air purification system cleans the air as your heating system circulates it, so every room on the ducts gets filtered air without a machine in each corner. For a Daly City home the choice comes down to two questions: what you want out of the air (household dust, pet dander, wildfire smoke, odors), and how much extra resistance your blower can take. We measure the second before recommending anything for the first.
Match the equipment to what you want removed
| What you want removed | What removes it | Worth knowing |
|---|---|---|
| Dust and pollen | Pleated media, MERV 8–11 | A filter that fits and seals may be all you need |
| Pet dander | Pleated media, MERV 11–13 | A deep media cabinet keeps resistance low at these ratings |
| Wildfire smoke particles | MERV 13 or higher, or a HEPA bypass unit | Works only with windows closed and the fan running |
| Odors and VOCs | Activated carbon, in a real quantity | Particle filters do not stop gases; removing the source comes first |
| Viruses and bacteria | Filtration captures particles; UV-C treats surfaces | We tell you what a device does, not promise a health outcome |
The main whole-home options
Media filter cabinet. A 4- or 5-inch pleated filter in a cabinet on the return side of the furnace or air handler. Its large surface reaches MERV 11 to 13 with modest resistance and lasts months between changes. It uses no electricity and has nothing to wash, which is why it is the most common upgrade.
HEPA bypass unit. True HEPA media is too dense for most home furnaces to push all their air through, so a HEPA system mounts beside the ductwork with its own fan. It draws part of the air from the return, filters it and sends it back. It needs a mounting spot, power and a filter-change schedule.
Activated carbon. Gases and odors pass straight through particle filters. Carbon adsorbs them, but only if there is enough of it and the air spends enough time in it; a thin carbon-coated filter does little. Carbon also fills up and has to be replaced.
Electronic air cleaners. An electrostatic unit charges particles and collects them on metal plates. Resistance is low, but the collector cell has to be washed regularly, it captures less as it loads, and some models produce a small amount of ozone. Ask for the ozone rating before you choose one.
UV lights are often sold as air purifiers, but they do a different job; see UV light installation.
It only cleans air the blower moves
This is the point that matters most in a fog-belt home. If your house has a furnace and no air conditioning, as many in Daly City do, the blower may sit idle for days in summer and early fall, which is also when wildfire smoke tends to arrive. A whole-home purifier does nothing while the fan is off. The answer is a thermostat set to fan On or Circulate on those days (thermostat installation if yours lacks the setting), and a look at the blower motor: an older single-speed (PSC) motor uses noticeably more electricity running all day than a variable-speed (ECM) motor.
Planning for smoke days
- Make the filter upgrade before the season, not in the middle of a smoke event.
- Keep a spare filter on the shelf; heavy smoke loads a filter quickly.
- Close windows and doors, and set the fan to run.
- Close the obvious leaks: a missing filter-slot cover or a return duct open to the garage pulls in unfiltered air.
- Keep a portable HEPA unit for a bedroom as a backup. It cleans one room continuously, whatever the furnace is doing.
Installation
Most installs take 2 to 4 hours. A media cabinet goes in on the return side of the furnace, which can mean reworking a short section of duct. A HEPA bypass unit needs two duct connections, a mounting spot and power. When the work is done, we measure static pressure again to confirm the blower still moves enough air, and show you how to change the filter yourself.
Cost and what moves it
Systems run $500 to $3,000. A media filter cabinet on an accessible furnace sits at the low end. A HEPA bypass unit, a carbon stage, duct changes or added power move it up. You get a written estimate before any work begins, and we explain why we suggest one option over another. Not sure what is in your air yet? An air quality assessment measures it first, and the filter replacement page covers what a better filter alone can do.