Surface Temperatures & Comfort

Heating and cooling consume more energy than any other residential application – 47% of all energy consumed by homes goes to heating and air conditioning as of 2009, according to the Energy Information Administration (EIA). EIA also reports that US household spend an average of $2,000 on utilities annually. The ultimate goal of this heating and cooling is to make homes comfortable for those who live in and visit them. Unfortunately, many homes, especially older existing homes, fall short in this regard, and many households expend large amounts of money installing larger energy-consuming appliances with the goal of improving comfort. However, an oversized furnace or air conditioner won’t solve comfort problems alone – it will only waste even more energy than the equipment it replaced.

Thermal comfort is dictated by two factors – heat and moisture. Both heat and moisture are governed by thermodynamic law:

1. Energy cannot be created or destroyed, but energy can move from place to place and change forms
2. Energy will always move from a high concentration to a low concentration

Heat moves from hot places to cold places via three mechanisms:

1. Conduction – when two solid surfaces touch, heat moves directly from the hot object to the cold object
2. Convection – Heat transfers through air. A cubic foot of hot air will in a 1000 cubic foot room of cold air will disperse evenly. Also, since warm air is less dense (lighter) than cold air, warm air usually (but not always!) rises. This is how a forced air heating system works – concentrated hot air comes out of a small source in a room and mixes with the temperate air in the room to make the entire space a little warmer.
3. Radiation – heat is emitted from an object and absorbed by surfaces within the heat source’s line of sight. If you’ve ever sat around a campfire and noticed the front of your body was warmer than the back, this is the result of radiation. And when you turn around to heat your backside, your frontside becomes a heat source and starts emitting heat it has absorbed from the fire into the colder space around it.

Image courtesy of Building Performance Engineering

The diagram below depicts how heat moves from a person’s body in a home. If you keep your home at 70 degrees in the winter, the people inside it are constantly transferring heat from their bodies to the inside of the home, which is transferring heat to the cold outdoors. 70 degrees is right in the sweet spot for human comfort, since we need to lose heat our bodies are constantly producing as a result of physical activity and bodily functions like breathing and digestion.

Image courtesy of Building Performance Engineering

1. Heat conducts directly from the person’s feet to the floor of the home
2. Heat radiates from the body and is absorbed by the walls, ceiling, floor, and air molecules in the home
3. Air heated by the person’s body convects throughout the room, mixing with colder air

Once the heat has left the hot person for the warm home, it will eventually reach the cold outdoors, along with heat from the warm home that was produced by the furnace.

The first call many homeowners make if they have rooms that are uncomfortable in their homes is to a heating service company, who will often prescribe a larger furnace to send more heat to the house. Unfortunately, while a larger furnace will produce more heat, a poorly insulated or un-insulated home will just lose heat faster as the temperature difference between inside and outside increases due to the additional heat being produced by the furnace. The result is that when the oversized furnace is blasting heat into the house, the house feels warm (perhaps even uncomfortably hot), but when the furnace clicks off, the occupants quickly feel cold again, as the warm air leaks out of the house and the cold ceilings, walls, and floors absorb the heat radiated by the 98 degree bodies and send that outside, too!

If the floor, ceiling, and walls of the home are well insulated, they will conduct and radiate heat out of the home orders of magnitude more slowly than if they are un-insulated, and when the furnace switches off, some heat will radiate back into the house from the warm interior surfaces of the floor, walls, and ceilings. It is not the ability to add more heat to the inside of a home that keeps us warm and comfortable, it is the ability to retain heat within the home that keeps us warm and comfortable.

Energy conservation and comfort are often thought of in opposing terms. If we want to save energy, we must put up with wearing sweaters and being cold all the time. This notion couldn’t be further from the truth. By inspecting the whole home and the interactions between the systems and components of the home, energy auditors and home performance contractors routinely discover glaring deficiencies in insulation and air leakage. Correcting these deficiencies allows many homeowners to turn down the thermostat further, saving even more energy, while enjoying a greater level of comfort in the home than ever before.

Sources

EIA heating and cooling percentage http://www.eia.gov/consumption/residential/

EIA utility costs http://www.eia.gov/consumption/residential/reports/2009/state_briefs/pdf/ca.pdf

Human Comfort Diagram: “Building Analyst Training”, Building Performance Engineering, Boone, NC

Copyright September 2014 by Advanced Home Energy

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