Wednesday, November 01, 2006

Comfort levels and "temperature"

Most thermostats for heating and cooling systems measure air temperature. Mostly, they do it that way because its easy and cheap. It has it's limitations however. That's because humans perceive temperature by three different mechanisms, one of which is air temperature.

This is a worthwhile topic because superinsulated house design with intelligent choice of heating systems addresses all three mechanisms. Folks who build conventional houses are generally ignorant of this phenomenon, and their finished product may turn out pretty good by accident, or not so good.

Intuitively, many people are aware of this on some level. Perhaps you have experienced a house where the thermostat/thermometer said you should be comfortable, yet you feel cold. Another common example is a cool, sunny, still day (outside), yet you feel comfortable walking around in short sleeves.

Humans perceive temperature in three ways:

1. Air temperature
2. The temperature of the objects that they are touching (eg sitting on a nice cold toilet seat in January)
3. The temperature of the objects in the room that they are NOT touching.

So let's unpack this a little bit. These three mechanisms of perceiving temperature are really just another way of describing the physics of heat transfer. If we state the three mechanisms like the physicists do, we get this:

1. Convection
2. Conduction
3. Radiation

1. Convection is heat transfer from object 1 (like your furnace) to object 2 (that would be you) by means of an intermediate heat transfer fluid (in this case, air). The furnace heats the air, and the air heats you. You don't have to physically touch the furnace to feel its effects.

2. Conduction is direct transfer of thermal energy from object 1 to object 2 and requires physical contact, like your cold toilet seat in January.

3. Radiation heat transfer is where a hot object radiates heat in all directions as infrared radiation. Common examples would be standing in front of a campfire or woodstove. You can feel the heat even though you are not touching the fire (hopefully) and it feels great despite the fact that there may be cold air all around you. The air is not heated as an intermediary, the heat is transferred very efficiently and directly to you. It is just in the nature of the universe that heat radiates from the hotter object to the colder object.

To flesh this out a bit, we'll do a thought experiment.

Imagine yourself in a room that has an air temperature of 70F. That's considered "warm" or comfortable, all things being equal. But all things are never equal. Now imagine that it's 0F outside and this room has a lot of old single pane inefficient windows along one wall. Even though it is not windy, and the room is not drafy, the room feels cold. You (the hot object) are radiating heat like mad, and those single pane windows act like an infinite heat sink. You will never warm the windows up, they will just keep sucking the heat right out of you.

Now imagine this room, and imagine that it's blustery and howling windy outside. In addition to the massive radiant heat loss, you also have a bad draft. Even though the measured air temperature is still our "toasty" 70F, this room will be uncomfortably cold, and it will be virtually impossible to sit on a couch for any length of time without a blanket. To attain comfort in this room may require air temps in the 80's, if it can be attained at all. If you have your feet in a tub of ice cold water (very high conductive heat losses), you can't make the air temperature hot enough to compensate. As you can readily see, air temperature is only part of the story.

Finally, imagine that our room now has a "normal" (ie modest) window instead of a whole bank of windows. The window is double or triple glazed (two or three layers) and is argon filled (to lower convective heat loss) and a Low-E coating (to reduce emissivity, which reduces radiant heat losses, sometimes known as a heat mirror). Also, let us imagine that the exterior wall is insulated to a true r-30, rather than an effective r-8 in our first room. Further, let us imagine that we have radiant heating in the floor rather than conventional forced air heating.

This does a number of things. The reduced area of glass dramatically reduces radiant heat loss, even if we had the old inefficient single pane window. Keep in mind that even a "good" window still has only r-5 to r-7 at the most. Lots of glass is a bad idea. We try to minimize the losses through the glass that we do have with modern window construction. This does reduce radiant loss and convective losses.

The airtight house construction (which is an integral part of superinsulation techniques) eliminates air infiltration from outdoors, regardless if it is windy or still. The warm radiant floor heats the objects in the room including you and the furniture and walls, by direct radiant heat transfer. Since the objects in the room are no longer cold, your radiant heat loss goes effectively to zero. In fact, you are now the recipient of radiant heat gain. And since your feet are probably on the floor, you also benefit from conductive heat gain, rather than heat loss to a cold floor. Lot's of folks have the idea that heat rises. That's not really true. Hot air rises. Pure heat (in the form of infrared radiation) radiates in all directions, eliminating thermal stratification (hot ceiling/cold floor).

So now that the floor feels warm, and the windows don't feel cold, and the furniture isn't cold, and you don't have a cold draft, you may be totally comfortable and warm with an air temperature of 63F or less. Purveyors of radiant heating systems often claim 20-40% better efficiency than forced air heating. Radiant heating is a more efficient way of giving you good perceived comfort without resorting to high air temperatures.

Congratulations, you now know more about perceived temperature and comfort issues than >95% of all the contractors out there.

Finest regards,

troy

1 comment:

Jeanette said...

Thank YOU! I don't understand why contractors don't know this (rather they do, but value economy over comfort!).

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