Heating
The Lennox furnace could heat a 2,000 sq ft house. For 234 sq ft, it’s a sledgehammer — but at 0°F outdoor, you need the sledgehammer.
Gas dollars, therms burned, and average daily cost over the selected period. In January, 90% of the daily budget is gas. By April, heating nearly disappears from the bill.
Runtime Distribution
The runtime bar chart tells the heating story at a glance. In winter, the gas furnace dominates the equipment list — 9+ hours per day in January. The electric heater runs longer (17+ hours) but at a fraction of the BTU output. Together: 80,120 BTU/hr of combined capacity. Absurd for 234 sq ft, but the polycarbonate’s R-value of 1.639 leaks heat fast when it’s 0°F outside.
Gas Consumption vs Outdoor Temperature
The scaling curve. Gas consumption climbs linearly as outdoor temp drops — roughly 390 BTU/hr lost per degree of indoor-outdoor delta through 640 sq ft of exposed glazing at U-value 0.61. Below 20°F outdoor, the furnace runs continuously overnight. Above 50°F, it doesn’t fire at all.
The Thermal Envelope
Indoor vs outdoor temperature shows the envelope at work. The greenhouse retains 5–8°F above outdoor without heaters, thanks to:
- Polycarbonate insulation — R-1.639, better than single glass (R-0.9)
- House connection — the shared north wall leaks ~68°F house heat all night
- Concrete slab — 234 sq ft of thermal mass storing and releasing solar heat
- Air volume — 2,000+ cu ft of warm air takes time to cool
On moderate nights (50°F+ outdoor), the greenhouse holds above 67°F with zero heater assistance. The electric heater handles mild dips below 58°F. The gas furnace is the heavy artillery — and at 3.9× more cost-effective per BTU than electric, it’s the economical choice when temperatures really drop.
Staging Strategy
| Outdoor Temp | Indoor Response | Heater(s) | Cost |
|---|---|---|---|
| > 50°F | House heat + slab retention handle it | None | $0 |
| 40–50°F | Mild dips below 58°F | Heat1 (electric) | $0.167/hr |
| 20–40°F | Sustained cold | Heat1 + Heat2 (gas) | $0.79/hr |
| < 20°F | Both heaters run most of the night | Heat1 + Heat2 | $0.79/hr |
Day/Night Temperature Differential
DIF — the difference between day and night temperatures — matters for plant growth. Heating creates the night temperature floor. A positive DIF (warm days, cool nights) promotes stem elongation. A negative DIF (cool days, warm nights) produces compact growth. The heating system’s job is maintaining that floor at 55–58°F regardless of what’s happening outside.
Monthly Cost
The seasonal arc is stark. January peak: ~$270/month, almost entirely gas. By March, heating cost drops 60%+. By May, heating is essentially zero and the budget shifts entirely to cooling and lighting.
Equipment
| Unit | Type | Output | Cost/hr | Stage Trigger |
|---|---|---|---|---|
| Heat1 (electric) | Space heater, 1,500W | 5,120 BTU/hr | $0.167 | temp < 58°F |
| Heat2 (Lennox LF24-75A-5) | Gas forced-air furnace | 75,000 BTU/hr | $0.623 | temp < 55°F |
Both controlled via ESP32 → PCF8574 → SSR-25DA relay chain.
⚠️ Heat1 has a physical override switch on the unit. Was accidentally left ON for 24+ hours on 2026-03-25 — ran continuously fighting the cooling system while the ESP32 thought it was off. The override bypasses the relay entirely.
→ See Cooling for what happens when heating season ends and solar gain takes over. → See Climate Overview for the full picture.