Inputs
Use real measurements when possible. Defaults are typical for small hydro or reservoir setups.
Example data table
| Volume (L) | Start → Target (°C) | Pull-down (h) | Pump (W) | Lights (W @ %) | Insulation | Suggested (BTU/hr) | Suggested (W) |
|---|---|---|---|---|---|---|---|
| 200 | 26 → 18 | 6 | 120 | 0 @ 20% | Basic | ~2,000 | ~585 |
| 500 | 28 → 18 | 8 | 250 | 400 @ 25% | Good | ~6,000 | ~1,760 |
| 1000 | 30 → 18 | 10 | 400 | 800 @ 30% | Basic | ~12,500 | ~3,660 |
Formula used
1) Pull-down load (average):
Q_pull (W) = (m · cp · ΔT) / t
Where m is water mass (kg), cp ≈ 4186 J/kg·°C, ΔT is (start − target), and t is pull-down time (seconds).
2) Internal heat gains:
Q_internal = Pump_W + (Lights_W · Heat% ) + Other_W
3) Ambient heat gain (quick estimate):
Q_env = A · U · (T_air − T_target)
Surface area A is estimated from volume (cube assumption). U depends on insulation selection.
4) Total required chiller capacity:
Q_required = (Q_pull + Q_internal + Q_env) · (1+Fouling%) · Safety · Altitude_Adjust
Conversions: BTU/hr = W × 3.412 and Tons = BTU/hr ÷ 12000.
How to use this calculator
- Enter total system water volume, including lines and trays.
- Set your target water temperature for roots or irrigation.
- Use your warmest starting water temperature for sizing.
- Pick a pull-down time that matches your control goals.
- Add pump watts and any other equipment heat into water.
- If lights warm the zone, enter watts and a transfer percent.
- Choose insulation level and set safety and fouling allowance.
- Click Calculate, then use the suggested rated capacity values.
- Download CSV or PDF to document, compare, or share results.
Sizing goals for garden water systems
Water chillers stabilize root-zone and reservoir temperatures when hot air pushes water above the crop’s comfort band. Correct sizing limits swings that stress plants, reduce dissolved oxygen, and invite pathogens. Define the target temperature, peak air temperature, and how quickly you need recovery after heat spikes. Short pull-down times need higher capacity; longer pull-down times favor efficiency and cost. Aim for stable water, not constant cycling overall.
Estimating pull‑down load from volume
Pull‑down load is the energy to cool stored water from its starting temperature to the target. The calculator uses water mass (volume × 8.34 lb/gal) and specific heat of 1 BTU/lb‑°F. Divide total BTUs by pull‑down hours to get BTU/hr. This term dominates for big reservoirs, warm top‑ups, or operations needing fast recovery after irrigation. If you store water outdoors, use the warmest starting value.
Accounting for internal heat sources
Anything that warms the water is entered as watts and converted to BTU/hr using 3.412. Pumps, UV units, mixers, dosing skids, and other submerged equipment add steady load while running. Grow lights may contribute indirectly; apply a transfer percent to represent heat absorbed by benches, troughs, or return lines. Use measured power draw when possible, because motor efficiency and speed control change heat to water.
Ambient heat gain and insulation choices
After pull‑down, heat still flows from warm air into cool water through tanks, lids, and exposed piping. The tool estimates surface area from volume and applies a U‑value based on insulation choice. Better insulation, reflective covers, shade, and protected lines can cut this continuous load. Seal lids and reduce splashing to avoid additional heat pickup. Treat this term as the baseline daily work.
Selecting capacity, margin, and verification
After summing loads, add allowances for fouling, safety margin, and altitude. Margin helps cover extreme days, shade loss, or future expansion, without oversizing excessively. Compare required BTU/hr to manufacturer ratings at your leaving‑water temperature and flow. Verify performance by logging inlet/outlet temperatures, flow, and run time; a reasonable duty cycle and steady setpoint indicate a good match. Recheck inputs after system change.
FAQs
Q: What chiller capacity unit should I shop with?
A: Use BTU/hr for direct comparison, then convert to tons by dividing by 12,000. If a brand lists watts, multiply watts by 3.412 to convert to BTU/hr at the same rating conditions.
Q: How do I choose pull‑down time?
A: Pick the time you need to recover after the hottest part of the day or after large warm-water additions. Faster recovery improves control but increases required capacity and usually raises upfront cost.
Q: Do pumps always become heat in the water?
A: Most electrical input to submerged or in-line pumps ends up as heat. Some is lost to the air through the motor casing, but for sizing, treating pump watts as water heat is a practical, safe assumption.
Q: How much safety margin is reasonable?
A: A 10–25% margin is common for outdoor gardens, seasonal heat waves, and minor future growth. Avoid extreme oversizing, which can short-cycle and reduce efficiency unless the unit has good capacity modulation.
Q: Why does insulation change required capacity so much?
A: Insulation reduces continuous ambient heat gain through tank walls and piping. Lower heat gain means the chiller runs less often to hold setpoint, improving stability, energy use, and equipment life.
Q: What field checks confirm the chiller is sized correctly?
A: Log inlet and outlet water temperatures, flow rate, and run time across a hot day. If the unit reaches setpoint and cycles with a reasonable duty cycle, the capacity match is usually acceptable.