Building Systems Midterm

Thermal comfort

Body temperatures held within narrow ranges, skin moisture low, and the body's effort of temperature regulation minimized. Lecture wording: a condition of mind expressing satisfaction.

Which best describes body temperatures held in narrow ranges, low skin moisture, and minimized regulation effort?

Thermal comfort.

People experience thermal comfort when:

Heat loss EQUALS heat production. Not greater, not less. Being at 98.6 F is not the answer.

Thermal equilibrium

No difference in temperature or molecule activity from one area to another. The body loses heat as fast as metabolism produces it.

When there is no difference in temperature and molecule activity from one area to another:

Thermal equilibrium is reached. Not thermal resistance, capacity, or balance.

What does a heating system do to the body?

Slows the body's rate of heat loss. It does not add heat to the body.

Air conditioning (3 things at once)

Alters air properties, distributes air, cleans or removes air.

Heat loss or gain in buildings can occur through transmission, infiltration and ventilation. True or false?

TRUE. Those are the three routes.

Four ENVIRONMENTAL comfort variables

Air temperature, radiant temperature, relative humidity, air motion.

Two PERSONAL comfort variables

Metabolic rate and clothing insulation.

Dry-bulb temperature

Plain air temperature, unaffected by moisture or radiation. What people mean by the temperature.

Wet-bulb temperature

Expresses humidity. Measured with a wetted bulb rotated through the air.

Mean radiant temperature (MRT)

Weighted average temperature of all surfaces in direct line of sight of the body.

Operative temperature

The average of dry-bulb temperature and MRT.

New effective temperature (ET)

An index of discomfort or dissatisfaction. Determined experimentally, not read from a thermometer.

Kelvin is used for what in design?

Color temperature in lighting. Based on absolute zero, written with no degree sign.

Formula: F to C

C = (F - 32) / 1.8

Formula: C to F

F = 1.8 x C + 32

Formula: C to K

K = C + 273

Which way does heat always move?

Hot to cold, never the reverse. Bigger temperature difference means faster transfer.

The four modes of heat transfer

Radiation, conduction, convection, evaporation.

Radiation

Heat flowing in electromagnetic waves from hotter surfaces to DETACHED colder ones, through any medium including empty space. Requires line of sight.

For heat transfer by radiation the cooler object must touch the hotter object. True or false?

FALSE. Radiation needs no contact and no medium. It needs line of sight.

Conduction

Heat flows by direct contact, as when a hot pan touches skin. Responsible for only a small amount of body heat loss.

Convection

Heat leaves an object into a moving stream of fluid. Skin is warmed or cooled by air or water running over it.

Warm or cool air, or water running over your hand, making you feel hotter or cooler, describes:

Convection.

Natural vs forced convection

Natural occurs without wind or a fan and tends to create layers at different temperatures. Forced is circulated by wind, fans, or pumps.

Stratification of different temperatures indicates:

Natural convection. Fans and pumps would mean forced convection.

Evaporation

Liquid changing to gas, drawing heat from the surface. It absorbs energy. Condensation releases it.

Sensible vs latent heat

Sensible is the dry heat that changes temperature and a thermometer reads it. Latent is the wet heat of phase change and temperature stays flat.

A dark stone absorbs heat by radiation all day. At night it will:

Radiate the heat back out to cooler surroundings. Emittance is important for cooling in desert climates.

Body heat loss percentages (lecture)

Radiation 60-65, evaporation 20-25, convection 10-15, conduction 2-5.

What happens at 98.6 F ambient?

Radiation, convection and conduction all stop. Only evaporation still cools.

Relative humidity

Water vapor present as a percentage of saturation at that temperature. A ratio.

Does RH tell you how much moisture is in the air?

No. It depends on both moisture content and air temperature.

RH effect at 75 F

0 percent RH feels like 69 F. 100 percent RH feels like 80 F.

Humidity limits to know

ASHRAE at or below 65 percent RH. EPA 30 to 60 percent to limit mold.

ASHRAE 55-2017 winter operative range

68.5 to 75 F.

ASHRAE 55-2017 summer operative range

75 to 80.5 F.

Why do the winter and summer ranges differ?

Clothing selection.

What do the ASHRAE 55 ranges assume?

Air movement under 40 fpm and 50 percent indoor RH.

met

Metabolic energy per unit body surface area. 58.2 W per square meter, an average person seated at rest.

Air stratification

Layers of air at different temperatures staying intact. Caused by natural convection. Fixed by controlling airflow.

Measurement heights, seated

4, 24 and 43 inches. Operative temperature at 24 inches.

Measurement heights, standing

4, 43 and 67 inches. Operative temperature at 43 inches.

ASHRAE 55 air velocity targets

Under 50 fpm cooling, under 30 fpm heating.

Draft perception rule

Every 15 fpm above 30 fpm feels like a 1 F drop.

Most draft-sensitive body parts

Neck, upper back, ankles. Noticed when incoming air is 3 F or more below room temperature.

Stack effect

Density difference from indoor-outdoor temperature difference. Warm air out the top, cool air in the bottom. Stack pressure decreases with height.

Wind-driven ventilation

Positive pressure on the windward side pushes air in. Suction on the leeward side pulls air out.

Thermal capacity

A material's ability to store heat. High capacity materials heat up slowly and release heat over a longer time, like a cast iron pan.

A brick has:

High thermal capacity. Dense and heavy, so it stores a lot of heat.

Thermal mass requires which three properties?

High specific heat capacity, high density, moderate thermal conductivity.

Thermal lag

Delay before stored heat is released. Cavity brick 7-8 hours, brick veneer 5-6 hours, lightweight minimal.

Thermal capacity vs thermal resistance

Materials with high thermal capacity have low thermal resistance. They are inverse.

Best thermal resistor in buildings, and why insulation works

Air. Mineral wool holds air still. Disturb that air and resistance drops to about a quarter.

High mass or low mass?

High mass when outdoor temperature swings above and below your target. Low mass when it stays consistently above or below.

Thermal feel: what do we sense?

How fast a material pulls heat from us, not its temperature. Steel feels colder than wood at the same temperature.

Two continuous layers the envelope needs

A continuous insulating layer and a separate continuous airtight layer, because insulation is not airtight.

Thermal bridge

Heat bridging around the insulation through a conductive path. Can cut R-value by 50 to 80 percent.

Five passive building principles

Continuous insulation with no bridging, airtight envelope, high-performance windows and doors, balanced heat-recovery ventilation, minimal space conditioning.

HRV

Balanced ventilation that uses outgoing stale air to warm incoming fresh air. Two fans and a heat-exchange core where the streams never mix.

Which organization provides performance standards, test methods and recommended practices for HVAC and refrigeration?

ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Which best describes the Americans with Disabilities Act?

A comprehensive civil rights law with four sections, covering new buildings with public accommodations and commercial facilities.

Heating and cooling loads are the amount of energy required to make up for heat loss and heat gain. True or false?

TRUE. Heating load is the hourly rate of net heat loss, cooling load the rate of heat gain. Both in BTUH.