Comfort, humidity & air quality

MERV ratings, comfort ranges, dew points, recommended humidity, CO exposure.

MERV filter ratings

MERV	Particle size	Catches	Typical use
1 to 4	over 10 µm	Pollen, dust mites, sanding dust, textile fibres	Window A/C, basic residential
5 to 8	3 to 10 µm	Mould spores, hair spray, cement dust	Better residential, commercial
9 to 12	1 to 3 µm	Lead dust, flour, vehicle exhaust, welding fumes	Superior residential, hospital labs
13 to 16	0.3 to 1 µm	All bacteria, most tobacco smoke, copier toner	Hospitals, surgery, superior commercial

MERV 12 to 13 is a good target for air quality. A HEPA filter removes 99.97% of 0.3 µm particles. Filters do not remove gases or odours: use activated charcoal or an oxidizing media.

Comfort ranges

Season	Relative humidity	Comfortable temperature
Summer	30%	76 to 82 °F
Summer	60%	74 to 78 °F
Winter	30%	69 to 78 °F
Winter	60%	68 to 75 °F

General comfort zone: 30 to 70% RH at 70 to 85 °F. For air quality aim for 40 to 60% RH and 72 to 76 °F. Indoor to outdoor summer difference of 10 to 15 °F is comfortable.

Recommended indoor humidity by outdoor temperature (indoor 72 °F)

Outdoor temp	Indoor RH
−10 °F	20%
0 °F	25%
10 °F	30%
20 °F	35%
30 °F	40%

Go slightly higher if the house is cooler than 72 °F and lower if warmer. Mould grows above 50% and thrives at 70%. Windows fogging means the humidity is too high for the outdoor temperature.

Surface temperature where condensation starts

Room RH	Room at 70 °F	Room at 80 °F
100%	70 °F	80 °F
90%	67 °F	77 °F
80%	64 °F	73 °F
70%	60 °F	69 °F
60%	56 °F	65 °F
50%	51 °F	60 °F
40%	45 °F	54 °F
30%	37 °F	46 °F
20%	28 °F	35 °F

Use it to answer "will this window or duct sweat?". Anything colder than the number in the table will collect condensation.

Carbon monoxide and other exposure

Level	Effect
0 ppm	Target indoors
200 to 400 ppm	Slight headache
1000 to 2000 ppm	Palpitations within 30 minutes; staggering within 1.5 hours; confusion, nausea and headache after 2 hours
2000 to 2500 ppm	Unconscious within 30 minutes
2500 ppm and over	Collapse and death before warning symptoms
CO₂ up to about 700 ppm	Good ventilation. A higher number marks poor ventilation
Ozone 0.1 ppm	Maximum for 8 hours. Continuous occupancy: 0.01 ppm

Air quality problems and fixes

Problem	Fix
VOCs and odours	HRV or ERV, air purifier, remove the source
Particles and bio-aerosols	MERV 13 or better filter, UV light at the coil
Humidity	Keep 25 to 55%: humidifier or dehumidifier
Temperature	Better thermostat and airflow, 68 to 81 °F
CO₂	HRV or ERV, fresh air, new filters
CO	Repair or vent the appliance, no running vehicle in an attached garage, CO alarm

About 20% or more of occupants with symptoms that clear when they leave is a sign of a sick building. Use an HRV in cold climates and an ERV where moisture control matters. Some ionizers make ozone: do not use ozone-generating cleaners.

Other numbers

A water heater set at 140 °F or higher helps control Legionella in a single-family home.
UV germicidal lamps are 254 nm. Clean quarterly, replace yearly. An access door on a UV lamp should have a shut-off interlock. Never look at a lit lamp.
Electronic air cleaner cell: about 9,000 V DC. Clean cells and prefilter every 2 to 3 months, dry completely before refitting.
Degree days = 65 °F minus the average of the daily high and low (cooling degree days are the opposite).

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Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.