Heating Systems Flashcards
7 cards from real HVAC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Heating Systems flashcards as text
What does the AFUE rating of 96% mean for a residential gas furnace?
Answer: 96% of fuel energy is converted to usable heat over the heating season
AFUE (Annual Fuel Utilization Efficiency) measures what percentage of fuel input is converted to useful heat annually; 96% means only 4% is lost.
When sizing a residential heating system using Manual J, which factor has the greatest impact on the calculated heating load?
Answer: Envelope insulation levels and window U-values
Manual J calculations are most sensitive to the building envelope — insulation R-values, window U-values, and infiltration rates drive the majority of heat loss.
A gas burner with a yellow-orange flame instead of a blue flame indicates what condition?
Answer: Incomplete combustion due to insufficient combustion air
A yellow or orange flame indicates incomplete combustion, often caused by restricted air supply, which can produce carbon monoxide.
What is the role of an aquastat in a hydronic heating system?
Answer: It controls boiler water temperature by cycling the burner on and off
The aquastat is a temperature controller that energizes and de-energizes the burner to maintain boiler water within a set temperature range.
Which type of gas valve is considered the safest for residential furnaces because it requires two independent failures to allow uncontrolled gas flow?
Answer: Redundant dual-seat combination gas valve
Redundant (dual-seat) combination gas valves contain two independent operators in series; both must fail simultaneously for gas to flow uncontrolled.
In a forced-air system, what is 'heat rise' and why does it matter?
Answer: The temperature difference between supply air and return air across the heat exchanger
Heat rise is the supply air temperature minus the return air temperature; it must stay within the furnace manufacturer's specified range to prevent overheating and damage.
A homeowner notices the furnace short-cycles (turns on and off rapidly). What is the most common cause?
Answer: Oversized furnace heating the space too quickly and triggering the high-limit switch
An oversized furnace heats the space rapidly, causing the supply air temperature to exceed the high-limit set point, which shuts the burner off before the thermostat is satisfied.