Delta-T in heating mode uses the same formula but different expected ranges.
ΔT = T_supply − T_return
Note: in heating mode, supply is warmer than return (reversed from cooling).
Heating delta-T ranges by equipment type
Equipment ΔT (°F) ΔT (°C)
───────────────────────────────────────────
Gas furnace 40–70 22–39
Electric furnace 40–60 22–33
Heat pump (heating) 25–35 14–19
Hot water coil 20–40 11–22
Compare to cooling:
Residential DX coil 14–22 8–12
Chilled water 10–12 5.5–6.7
Heating delta-T is 2–3x higher than cooling for the same equipment because combustion and electric resistance generate much higher temperatures than refrigeration.
Gas furnace: high delta-T is dangerous
In cooling, high delta-T means restricted airflow — uncomfortable but safe.
In heating on a gas furnace, high delta-T means the heat exchanger is overheating:
ΔT < 40°F → low output (gas pressure, ignition, flame issue)
ΔT 40–70°F → normal range
ΔT > 70°F → DANGER: heat exchanger overheat
→ limit switch trips
→ if limit bypassed: exchanger cracks
→ cracked exchanger = CO in supply air
Most common cause of high delta-T in heating:
1. Dirty filter (restricts airflow)
2. Collapsed return duct
3. Undersized return grille
4. Blower at wrong speed setting
5. Failed blower capacitor
A dirty filter from last winter is the #1 pre-season find.
Heat pump: low delta-T is normal
Gas furnace supply air: 130–150°F
Heat pump supply air: 95–105°F
Body temperature: 98.6°F
Heat pump air feels "cool" compared to a furnace because the delta-T is only 25–35°F vs 40–70°F. The system compensates by running longer:
Gas furnace: 6–8 hrs/day in cold weather
Heat pump: 12–16 hrs/day in cold weather
Total daily heat delivered is similar. Delivery rate per hour is lower.
If heat pump ΔT drops below 20°F:
→ low refrigerant charge
→ defrost cycle running too frequently
→ auxiliary heat strip not energizing
Heating capacity from delta-T
Same formula as cooling:
Q (BTU/hr) = 1.08 × CFM × ΔT
Gas furnace at 1,200 CFM, 55°F delta-T:
Q = 1.08 × 1,200 × 55
Q = 71,280 BTU/hr
If nameplate says 80,000 BTU output:
71,280 / 80,000 = 89% of rated capacity → normal (duct losses)
At 35°F delta-T (something wrong):
Q = 1.08 × 1,200 × 35
Q = 45,360 BTU/hr = 57% of rated → investigate
Pre-season check
1. Replace air filter
2. Run furnace for 15+ minutes (steady state)
3. Measure supply temperature 6 ft from plenum
4. Measure return temperature before filter
5. Subtract: ΔT = supply − return
6. Compare to range for your equipment type
7. If outside range → schedule service before first cold night
