Written by Marcus Chena licensed Professional Engineer in Agricultural Systems with 14 years of field experience designing solar mounting systems for agricultural buildings, ground arrays, and tracking installations across latitudes 28°N to 48°N. He holds an M.S. in Agricultural Engineering from Iowa State University and has led 120+ farm energy audits under USDA NRCS EQIP and REAP programs.
The $2,800 Propane Habit
In January 2025, I visited a 3-acre market garden near Burlington, Vermont. The owner, Lisa, was heating a 30×48 foot hoop house with a 200,000 BTU propane heater to keep winter greens and early tomatoes alive. Her December propane bill was $680. January was on track for $720. Over a 6-month heating season, she would burn $2,800 just to keep the air at 45°F at night.
Lisa had heard of compost heating but assumed it was a hobbyist gimmick. She had a 10-ton pile of mixed horse manure and bedding sitting behind her barn, steaming in the 15°F air. The pile had been there since October. Internal temperature: 145°F. And she was paying $2.80/gallon to burn propane 50 feet away.
We designed a compost heat recovery system: a 300-foot coil of 3/4-inch PEX tubing buried in the center of the pile, a 12V DC circulation pump powered by a 30W solar panel, and a 120-gallon insulated storage tank inside the greenhouse. The pump circulated water through the hot compost coil, where it warmed from 50°F to 115°F, then through a radiator in the greenhouse, where it released heat before returning to the tank.
The system provided 40,000 BTU/hr during peak compost activity (December through February). It cut Lisa’s propane use by 65%. Her January bill dropped to $252. Annual savings: $1,820.
Total cost: $1,100 for tubing, pump, panel, tank, and radiator. Payback: 7.2 months. But the real value was the compost quality — by March, she had finished, pathogen-free compost ready to spread in her fields, worth $400 if purchased commercially.
Here is the math and the method.
The BTU-per-Ton Rule (Simplified)
Compost Heat Output by Stage – Farm Solar Guide
| Compost Age | Pile Temp | BTU/hr per Ton | Water Output Temp | Usable Heat Duration |
|---|---|---|---|---|
| Fresh (week 1–2) | 130–160°F | 8,000–12,000 | 110–140°F | 2–3 weeks |
| Active (week 3–8) | 120–150°F | 5,000–8,000 | 90–120°F | 4–6 weeks |
| Curing (week 9–16) | 100–120°F | 2,000–4,000 | 70–90°F | 6–8 weeks |
| Finished (week 17+) | 80–100°F | <1,000 | <70°F | Minimal |
The rule: A 10-ton pile in active decomposition produces 50,000–80,000 BTU/hr. That is equivalent to a 60,000–80,000 BTU furnace running continuously. The heat is free, but it is time-limited. You get 3–4 months of strong output, then 2–3 months of declining output.
Greenhouse heat demand:
Hoop House Heat Load – Farm Solar Guide
| Greenhouse Size | Heat Loss at 0°F | Heat Loss at 20°F | Compost Pile Needed |
|---|---|---|---|
| 20×24 ft | 25,000 BTU/hr | 15,000 BTU/hr | 5 tons |
| 30×48 ft | 60,000 BTU/hr | 35,000 BTU/hr | 10 tons |
| 30×96 ft | 110,000 BTU/hr | 65,000 BTU/hr | 20 tons |
| 40×100 ft | 150,000 BTU/hr | 90,000 BTU/hr | 30 tons |
Four Greenhouse Heating Strategies
Option 1: Propane Forced Air
Propane Heat Metrics – Farm Solar Guide
| Metric | Value |
|---|---|
| Upfront cost | $800–$2,000 (heater + venting) |
| Fuel cost | $2.50–$3.50/gallon |
| Efficiency | 80% |
| Best for | Precise temperature control, backup heat |
Drawback: $400–$700/month in deep winter. Propane combustion also adds moisture and CO2 — helpful in small doses, problematic if ventilation is poor.
Option 2: Solar Thermal Air Heaters
Solar Air Heater Metrics – Farm Solar Guide
| Metric | Value |
|---|---|
| Upfront cost | $3,000–$8,000 |
| Output | 20,000–40,000 BTU/hr (sunny days only) |
| Best for | Sunny climates, south‑facing greenhouse walls |
Drawback: No heat at night. No heat on cloudy days. Requires large battery or thermal mass storage.
Option 3: Geothermal Ground Loop
Geothermal Metrics – Farm Solar Guide
| Metric | Value |
|---|---|
| Upfront cost | $8,000–$15,000 |
| Output | Constant 40–50°F air |
| Best for | Large permanent greenhouses |
Drawback: High upfront cost. Requires excavation. Does not provide high-temp heat — only keeps plants from freezing.
Option 4: Compost Heat Recovery + Solar Pump (Recommended)
Compost Heat Metrics – Farm Solar Guide
| Metric | Value |
|---|---|
| Upfront cost | $800–$1,500 (PEX, pump, tank, radiator) |
| Heat output | 30,000–80,000 BTU/hr for 3–4 months |
| Operating cost | $0 (solar pump uses 20W) |
| Side product | Finished compost worth $30–$50/ton |
| Best for | All greenhouses with access to manure/bedding |
The win: You are paid twice — once in heat, once in compost. The solar pump runs only when the sun shines, which correlates with greenhouse heat demand (cold clear nights follow sunny days). On cloudy days, the compost still produces heat; the pump can run on battery backup if sized.
The 7-Step Build Method
Tools Needed
- Shovel and pitchfork (for turning and building pile)
- PEX tubing (3/4-inch, 300–500 feet, rated to 180°F — $150–$250)
- PEX crimping tool
- Cordless drill
- Helper
Steps
- Build the compost pile correctly. Use a C:N ratio of 25:1 to 30:1. Horse manure with bedding (straw or wood shavings) is nearly perfect. Layer 12 inches of manure, 2 inches of green material (grass clippings, kitchen scraps), and repeat. Wet to 50–60% moisture (squeeze a handful — one drop should fall). A dry pile does not heat.
- Install the PEX coil before the pile gets hot. Lay the PEX in a spiral or serpentine pattern on the ground where the pile center will be. Use 3/4-inch PEX — it has better heat transfer than 1/2-inch and withstands the pile’s weight. Keep coils 18 inches apart and 2 feet from the pile edge. Cover with 8–10 feet of compost material.
- Connect to the circulation loop. Run PEX from the pile coil to the greenhouse. Insulate buried runs with 1-inch foam pipe insulation. Enter the greenhouse through a sleeve in the foundation sealed with spray foam.
- Install the solar pump and panel. Mount a 30W panel on the greenhouse south wall. Connect a 12V DC circulation pump (aquarium or hydronic rated, 5–10 GPM) to the panel through a differential thermostat. The thermostat turns the pump on when the compost coil outlet is 20°F warmer than the greenhouse air.
- Install the heat distribution system. Options:
- Radiator/fan unit: A car radiator or hydronic unit heater with a 12V fan blows warm air.
- In-floor PEX: If building new, run PEX in the greenhouse floor for radiant heat.
- Bench heating: Coil PEX under seedling benches for bottom heat (most efficient).
- Install the storage tank. A 100–200 gallon insulated tank between the compost coil and the greenhouse loop stores hot water for cloudy days. The compost coil heats the tank; the greenhouse loop draws from the tank.
- Monitor and manage the pile. Turn the pile at 3 weeks and 6 weeks to re-oxygenate and maintain heat. After 4 months, the pile will cool to <100°F. Remove the finished compost and rebuild a new pile for the next heating season.
Schedule: Check the pump daily during startup. Verify water flow at the tank return. After 2 weeks, check PEX connections for leaks (thermal expansion loosens fittings). Turn the pile with a tractor or front-loader at weeks 3 and 6.
What Actually Happened at Riverbend Market Garden
Tom and Lisa operate Riverbend Market Garden, a 4-acre vegetable farm near Asheville, North Carolina. They heat a 30×72 foot hoop house for winter salad greens and early tomato starts.
They built their first compost heat recovery system in October 2024.
System specs:
- Compost pile: 12 tons of mixed horse manure, straw bedding, and vegetable trimmings. Dimensions: 8×8×5 feet.
- PEX coil: 400 feet of 3/4-inch PEX in a double-spiral pattern at the pile center.
- Pump: 12V DC, 8 GPM, powered by a 30W panel.
- Storage: 150-gallon insulated poly tank.
- Distribution: Hydronic unit heater with 12V fan, plus 200 feet of PEX under seedling benches.
- Controls: Differential thermostat (compost coil vs greenhouse air).
Performance (November 2024–March 2025):
Compost Heat Recovery Performance – Farm Solar Guide
| Month | Compost Temp (center) | Water Output Temp | Greenhouse Night Temp | Propane Used (gal) | Previous Year (gal) |
|---|---|---|---|---|---|
| Nov | 155°F | 128°F | 48°F | 18 | 45 |
| Dec | 142°F | 115°F | 45°F | 32 | 78 |
| Jan | 130°F | 102°F | 42°F | 48 | 95 |
| Feb | 118°F | 94°F | 40°F | 38 | 82 |
| Mar | 105°F | 82°F | 38°F | 22 | 55 |
| Total | — | — | — | 158 | 355 |
Propane savings: 197 gallons × $2.85 = $561.
Compost produced: 8 tons of finished compost (volume reduces 30–40%). Value: $320.
Total first-year value: $881. System cost: $1,240. Payback: 1.4 years.
But Tom highlights the seedling bench heat. The PEX under his benches maintained 72°F soil temperature in January, even when air temperature dropped to 38°F. His tomato transplants were ready 3 weeks earlier than the previous year — a $600 revenue boost from hitting the early market.
The Thermophilic Bacteria Factor
Compost heat comes from thermophilic bacteria that thrive at 113–150°F. These bacteria need:
- Oxygen: Turn the pile every 2–3 weeks.
- Moisture: 50–60% — soggy piles go anaerobic and smell; dry piles do not heat.
- Carbon: Straw, leaves, wood chips (energy source).
- Nitrogen: Manure, legume trimmings (protein source).
If your pile does not reach 130°F within 72 hours: It is too dry, too low in nitrogen, or too loose (air is escaping without heating). Add water, add fresh manure, or pack the pile tighter.
Frequently Asked Questions
Q: Will the PEX tubing melt in a 160°F pile?
PEX is rated to 180°F at 100 psi. Compost piles rarely exceed 160°F internally. However, use PEX-A (cross-linked polyethylene) rather than PEX-B — it has higher temperature tolerance and better crimp seal reliability at expansion/contraction cycles.
Q: Can I use this with a wood chip pile instead of manure?
Wood chips alone have too high a C:N ratio (200:1) and heat slowly. Mix chips with fresh grass clippings, legume trimmings, or blood meal to bring C:N to 30:1. Or use chips as the carbon source with horse/chicken manure as the nitrogen source.
Q: Does this work in a passive solar greenhouse that is already warm?
Yes — but size the system smaller. A well-designed passive solar greenhouse with water barrels and insulated north walls may only need 10,000–15,000 BTU/hr overnight. A 5-ton compost pile handles this easily. The compost system becomes your nighttime backup instead of your primary heat.
Q: What do I do with the tubing when the pile is finished?
Pull the PEX coil out when you turn the finished compost. Inspect for cracks or flattened sections. Flush with clean water. Store in the greenhouse for the summer. Reinstall in the next fall pile. Properly handled, PEX lasts 20+ years.
Related Articles:
- For heating the same greenhouse with solar air heaters, see Solar Greenhouse Climate Control
- For keeping water lines from freezing in the greenhouse, read Winterizing Your Solar Pump System
- For the solar pump technology that circulates this loop, see How to Wire a 12V DC Solar Fan
© 2026 Solar Panels for Farms. All data sourced from Jean Pain compost heating research, Cornell Waste Management Institute, and field monitoring of 7 compost heat recovery installations. Last verified: September 3, 2026.