I've installed a new 80,000 BTU gas furnace.

Checking Temperature Rise:

1. Turn off power to the furnace.
2. Remove filter and ensure blower is clean.
3. Connect a digital multimeter with a temperature probe or use a non-contact thermometer.
4. Measure supply air temperature at the furnace outlet (plenum) and return air temperature at the filter rack or return duct.
5. Calculate ΔT: Supply temp minus return temp.
6. Compare to manufacturer’s data—typically 40–70°F for 80,000 BTU furnaces, but check your specific model’s rating plate or installation manual.

If Out of Range:

• Too high (overheating):
• Check airflow—clean/replace filter, verify blower wheel is clean and spinning correctly.
• Confirm blower motor is wired for correct speed (high heat tap).
• Check ductwork for restrictions or closed dampers.
• Verify gas pressure per manufacturer specs (requires manometer).

• Too low:
• Check for restricted airflow (dirty filter, closed registers).
• Verify blower is running at proper speed.
• Check for duct leakage or disconnected ducts in unconditioned spaces.

Safety Note: Gas pressure adjustments and internal component testing should only be done by a licensed HVAC technician. Improper adjustments can cause carbon monoxide production or equipment damage.

Code Reference: Per IRC G2405.2, furnaces must be installed per manufacturer’s instructions, including proper airflow and temperature rise. NFPA 110 requires safe operating conditions.

If issues persist after basic checks, contact a licensed HVAC contractor.

Checking Temperature Rise (ΔT):

1. Prep: Run furnace 10–15 mins for steady state. Confirm manifold pressure matches nameplate (typically 3.5" WC natural, 10" WC LP).
2. Measure: Use calibrated digital probes. Return: Probe in return plenum before filter/coil. Supply: Probe in supply plenum after heat exchanger, before A-coil (drill 3/8" hole, seal after).
3. Calculate: ΔT = Supply Temp – Return Temp.
4. Compare: Check nameplate for Rated Rise Range (e.g., 30–60°F, 40–70°F). Your measured ΔT must fall within this range.

If Out of Range:

• ΔT Too HIGH (Low Airflow) — Most Common:
• Risks: Overheating, cracked heat exchanger, high-limit trips, short cycling.
• Fixes: 1. Check filter (clean/new, MERV ≤8 unless system rated higher). 2. Verify blower speed tap: Move to higher heating speed tap (check wiring diagram; often "Heat" tap is separate from "Cool"). Do not exceed 0.5" WC TESP (Total External Static Pressure) per nameplate. 3. Check duct sizing/restrictions: Undersized returns, closed dampers, crushed flex, dirty A-coil. 4. Verify gas input: Clock meter (IFGC 402.4) or check manifold pressure. Derate only if overfired (max 10% per NFPA 54/IFGC).

• ΔT Too LOW (High Airflow/Underfired):
• Risks: Condensation in primary heat exchanger (non-condensing), poor comfort, potential corrosion.
• Fixes: 1. Lower blower speed tap (if above min CFM). 2. Verify gas input — increase pressure only to nameplate spec. 3. Check for excessive bypass air / zoning bypass damper open.

Critical: After any adjustment, re-measure ΔT and TESP. Document final values. If ΔT cannot be brought into range with clean filter & correct speed taps, duct system is deficient — requires redesign.