Lesson HVAC fundamentals · Gases and energy
Energy, work, and power
Energy does work. Work = force × distance (ft-lb). Power is how fast work happens (hp, watts). Conversions you will use: 1 hp = 746 W; 1 kW = 3413 Btu (heat output context). The compressor motor buys power to pump heat uphill in temperature.
Two energy forms in the trade
Most HVAC/R systems convert:
- Electrical energy (motors, controls, electric heat)
- Heat energy (gas, oil, heat from combustion or refrigeration transfer)
Fossil fuels store ancient solar energy. Electricity is often generated from fuel, nuclear, or renewables—you pay the utility for kWh (energy over time).
Work: moving something against a force
Work = Force × Distance → units ft-lb (pound-force × foot).
Example: lift 150 lb 100 ft:
Work = 150 × 100 = 15,000 ft-lb
No time limit yet—just total effort.
Refrigeration analogy: pumping heat from a cold box to a warm room is “uphill” work—requires continuous power input.
Power: work per time
Power = rate of doing work.
Horsepower (hp): classic definition—
1 hp = 33,000 ft-lb per minute
A motor doing 15,000 ft-lb in 1 minute uses 15,000 ÷ 33,000 ≈ 0.45 hp.
Average furnace blower might be ½ hp class—that is the scale of residential air movement.
Electrical power: the watt
Watt (W) = electrical power unit (what utilities bill in combination with time as kWh).
Key conversion:
1 hp = 746 W
So a 1 hp motor draws roughly 746 W if motor efficiency were 100%—real motors draw more because of losses.
Btu and kilowatts: comparing heat outputs
Electric heat comparison:
1 kW = 3413 Btu (often written 3413 Btu/h context for hourly capacity)
Examples:
- 20 kW electric heat ≈
20 × 3413 = 68,260 Btu/h - 100,000 Btu/h gas furnace output ≈
100,000 ÷ 3413 ≈ 29.3 kWequivalent
Useful when comparing fuel types or explaining why a 12 kW strip heater is not a 3 ton A/C in capacity numbers.
Fossil fuel units (awareness)
- Natural gas: often ~1000 Btu/ft³ (verify local utility data)
- Fuel oil #2: ~139,000 Btu/gallon
- Coal: varies by type
You buy volume or mass; you use Btu for heating capacity.
Efficiency: nameplate vs reality
Motors, compressors, and furnaces have efficiency losses. Conversions above are ideal heat/electrical equivalence—not accounting for COP, SEER, AFUE, or motor losses.
Fundamentals lesson: know the unit bridge. Application courses: apply efficiency factors.
Safety note
Devices that consume power—motors, gas valves, heat strips—can injure or kill. Only qualified personnel service energized or fuel-connected equipment.
How this ties to refrigeration
The refrigerator or A/C must purchase energy (electricity most often) to move heat from cold space to warmer surroundings. The ton rating (next lesson) describes heat moved; kW or hp describes motor work to move it—not the same number because efficiency ≠ 100%.
Field case
Situation. Homeowner wants to replace gas furnace with electric heat. Asks: “My neighbor’s 20 kW heat kit—how does that compare to my 100,000 Btu/h gas unit?”
How to think with this lesson.
- 20 kW × 3413 ≈ 68,260 Btu/h electric heat output (ideal)
- 100,000 Btu/h gas output is larger nameplate— but AFUE, climate, and cost per kWh vs therm decide economics, not Btu alone
Learning takeaway: convert units before comparing; then add efficiency and utility rates.
In the field
Symptom
Undersized electric heat; surprise utility bill; motor overload confusion
Where to look
Nameplate kW/hp; amp draw; Btu/h rating; service history
Likely causes
- Confusing heat **moved** (refrigeration ton) with heat **produced** (kW strip)
- ignoring 746 W/hp
What to measure
- Amps × volts for kW estimate
- compare to nameplate
- load calc Btu/h
What not to do
- Assume hp on blower equals compressor capacity in tons
Checklist
- I define work (ft-lb) and power (rate of work)
- I know 1 hp = 33,000 ft-lb/min and 1 hp = 746 W
- I convert kW ↔ Btu using 3413 Btu per kW
- I distinguish energy (kWh) from power (kW)
- I connect motor power to “pumping heat uphill” concept