# HVAC and Solar Calculations

How much power does it take to replace all the warm air in a 2000 square foot house twice on a cold winter day?

Assume 70 F inside, 10 F outside, 9 foot walls; shv_air is 0.0013 joule/(cc deltaC):

```2000 square feet * 9 feet * shv_air * (70 - 10) deltaF * 2/day ? watt
= 510.902 watt
```
How much power is required for humidification to bring the air to 80% RH?

Look up: At 100% RH and 70 F air contains 19 cc of water/cubic meter, and pretty much none below freezing. So, how much water is in all the air in the house? 2000 square feet * 9 feet * 19 cc/m^3 * 80% ? gallons = 2.04667 gallons For humidification we must evaporate this much water twice per day, the power required is: 2 gal * hvv_water * 2/day ? watt = 389.699 watt What would this cost for a month if gas is \$19 for 41 hundred cubic feet and the heater is 80% efficient?

```19\$ / (41 hundred cubic feet) ; hcv_natgas * 80% ; (510 + 398) watt ? \$/month
= 13.1013 \$/month
```
Obviously, there are other mechanisms of energy loss in the home because my winter bill has never been below 40 \$
How long will 20 lbs of propane keep my camper warm?

Assume constant 30 deg F delta.
First Calculate the area of surface exposure of the camper: (20 ft long, 7 ft wide)

```2 * (20 ft * 8 ft + (20 * 7) square feet + (7 * 8) ft^2) ? square feet
= 712 square feet
```
Calculate heat loss assuming R11 insulation for this area of exposure:
```712 ft^2; 30 deltaF; 11 Rvalue ? btu/hour
= 1941.82  btu/hour
```
How long will 20 lb of propane last?
```(20 lb * hcm_propane_liq) / (1941.81  btu/hour) ? days
= 9.11907 days
```
I'd be lucky to get four days off in a row.

Calchemy Software Inc.
Fort Collins, CO Last Updated 6/21/2009