Heat loss through an open door

[James Allsopp]

All the info I could obtain talks about heat loss through windows and doors. I am trying to get
several store owners in my town to close there doors in the middle of winter and summer.
The average door is 28 sq ft. I am assuming the inside temperature is 68 degrees and the outside temperate is 34 degrees. How much energy are they wasting? This is not a homework question. I am 65 years old. Too old to remember any of my physics.

 

[Andy Resnick]

Interesting application- here's a back of the envelope estimation:

Assume all the heat loss is through air loss: heated air leaving the store. The specific heat of air is about 0.001 J/(cm^3*K) (from wikipedia). What's the amount of air leaving through the doorway? 28 ft^2 is about 27000 cm^2, estimate a 'wind velocity' of 1 m/s with the door open gives about 3*10^6 cm^3/s air leaving the store. The air is heated 30 degrees, this gives 9*10^4 J/s (9000 Watt) energy loss.

That seems like a lot, so double-check my calculations- I did them in my head.

 

[Dale]

That seems reasonable to me. At 10 cents/kWh that is only about $1/hour, so I could see how a business owner might think that an open and inviting door is worth that cost.

 

[GT1]

Interesting application- here's a back of the envelope estimation:

Assume all the heat loss is through air loss: heated air leaving the store. The specific heat of air is about 0.001 J/(cm^3*K) (from wikipedia). What's the amount of air leaving through the doorway? 28 ft^2 is about 27000 cm^2, estimate a 'wind velocity' of 1 m/s with the door open gives about 3*10^6 cm^3/s air leaving the store. The air is heated 30 degrees, this gives 9*10^4 J/s (9000 Watt) energy loss.

That seems like a lot, so double-check my calculations- I did them in my head.

1 m/s sounds quite high velocity-it's 36 KM/Hr, I don't think I feel such strong wind when I'm standing at the door.It sounds more like a free convection problem then a forced convection. But there are some other solutions,in some malls I saw a blower installed above the door, the air flow from the blower creates an "air screen" which reduces the heat losses, on that way they can keep their door open and still reduce the energy wasting.