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Evgenii Konkin
Evgenii Konkin

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Heating Season Delta-T: The Ranges That Change When You Switch From Cooling to Heating

#hvac #engineering #heating #diagnostics

Delta-T in heating mode uses the same formula but different expected ranges.

ΔT = T_supply − T_return
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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
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Compare to cooling:

Residential DX coil    14–22       8–12
Chilled water          10–12       5.5–6.7
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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
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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
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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
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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
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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
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Heating capacity from delta-T

Same formula as cooling:

Q (BTU/hr) = 1.08 × CFM × ΔT
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Gas furnace at 1,200 CFM, 55°F delta-T:

Q = 1.08 × 1,200 × 55
Q = 71,280 BTU/hr
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If nameplate says 80,000 BTU output:

71,280 / 80,000 = 89% of rated capacity → normal (duct losses)
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At 35°F delta-T (something wrong):

Q = 1.08 × 1,200 × 35
Q = 45,360 BTU/hr = 57% of rated → investigate
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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
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