Winter-Heat-Transfer

Real-world heat transfer

Heat Transfer

In the last three weeks we have briefly discussed the three mechanisms for heat transfer: Conduction, Convection, and Radiation. This week we’ll wrap up the topic of heat transfer by examining how the three types work together in the real world.

Hypothetical Home

I’ve modeled a hypothetical house that would have used typical construction techniques in the 1970’s and 1980’s. There’s probably a hundred thousand of these houses built along the front range of Colorado.

This hypothetical house is on the small side, only 1200 square feet on a single level without a basement. In those days’ houses, builders often used 2×4 stud walls, which only allows R-11 in the walls. In the ceiling, there would be more insulation, but only about R-19. Under the floor slab, typically there was no insulation. Window manufactures introduced double pane windows in the 1980’s. For this example, we’ll assume a typical first-generation double pane windows with a U-value of 0.85. The doors are solid wood with an R-Value of 4.

In the real world, the insulation in a 40-year-old house, is very likely degraded, perhaps considerably. A 40-year-old house is also likely to have many air leaks. So, when looking at the R-values given above, you should consider them to be absolute best case.

Our hypothetical house is shown below.

Hypothetical House

Cold Winter Night

If we assume an indoor temperature of 72°F and an outdoor temperature of 0°F. We’ll add a little wind outside, 15 mph, just for fun. It’s also night, so no solar heat gain, but there will be radiation heat loss.

Expensive

Crunching the numbers for this real-world example of heat transfer, I find that this little house is losing more than 31,000 BTUs per hour. If the house uses electric baseboard heaters, the residents would need to buy about 10-kWhr of electricity every hour. At 14.5 cents per kWhr, that $1.45 per hour, which can easily add up to more $25 or $30/day. Of course, heating with natural gas would be cheaper, with a 60% efficient furnace the cost of gas would be about half the cost of electric heat.

In my next blog post, I’ll get into some of the details as to why actual R-Values are so much lower than advertised.

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