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The Complete Guide to Unit Heater Sizing
Everything you need to know about BTU calculations, heat loss, and selecting the right heater for any space.
What Is a Unit Heater?
A unit heater is a self-contained heating appliance that combines a heat source (gas burner, electric element, or hot water coil) with an air-moving fan in a single cabinet. Unlike central furnaces that distribute heat through ductwork, unit heaters deliver warm air directly to the space they serve. They are the workhorse of industrial and commercial heating — found in garages, warehouses, workshops, factories, gyms, barns, and retail stores worldwide.
Unit heaters are rated in BTU per hour (BTU/hr) or kilowatts (kW). Choosing the right size is the most critical decision: an undersized heater will never reach the set temperature; an oversized one short-cycles, wastes fuel, and costs more upfront than necessary.
OSRS Combat Calculator | Complete Old School RuneScape GuideTypes of Unit Heaters
- Gas-Fired Unit Heaters — The most common type. Burns natural gas or propane. Available in 30,000–400,000+ BTU/hr. Efficiency ranges from 80% (standard) to 93%+ (condensing). Best for large spaces where gas is available.
- Electric Unit Heaters — 100% efficient at point of use. No combustion, no flue required. Ratings typically 1.5–25 kW. Higher operating cost than gas in most areas, but lower installation cost and zero maintenance. Ideal for small spaces or where gas is unavailable.
- Propane (LP) Unit Heaters — Operate identically to natural gas heaters but use propane. One gallon of propane contains 91,452 BTU. Common where natural gas service is unavailable.
- Hydronic (Hot Water) Unit Heaters — A coil of hot water piping replaces the gas burner. Quieter, safer, and integrates with existing boiler systems. Popular in multi-zone commercial buildings.
- Infrared/Radiant Heaters — Do not heat the air; they heat objects and people directly using radiant energy (like sunlight). 20–40% more efficient for spot heating in drafty spaces like loading docks and outdoor patios. Not suited for fully enclosed, well-insulated spaces.
- Fan-Forced Heaters — A subset of electric heaters that use a fan to blow air over heating elements. Faster response time, more even heat distribution than baseboard heaters.
How to Size a Unit Heater — The BTU Method
The fundamental formula for heating load (BTU/hr) is derived from heat loss through the building envelope:
Where ΔT is the difference between your desired indoor temperature and the outdoor design temperature for your area.
Insulation Factors
| Insulation Quality | Factor | Typical Example |
|---|---|---|
| Poor | 0.50 | Uninsulated metal building, old barn |
| Average | 0.35 | Standard residential walls (R-13), single-pane windows |
| Good | 0.25 | R-19 walls, double-pane windows, well-sealed |
| Excellent | 0.18 | R-30+ walls, triple-pane, spray foam |
BTU per Square Foot Rules of Thumb
| Climate | BTU/sq ft | Examples |
|---|---|---|
| Mild (Zone 1-2) | 10–20 | Southern Florida, Hawaii |
| Moderate (Zone 3-4) | 25–35 | Mid-Atlantic, Pacific Northwest |
| Cold (Zone 5-6) | 35–45 | Chicago, Denver, New England |
| Very Cold (Zone 7) | 45–60 | Northern Minnesota, Montana, Alaska |
Heat Loss Calculation Method (Manual J Simplified)
For more accurate sizing, calculate heat loss through each building component separately using Q = A × U × ΔT, where A is area (sq ft), U is the U-value (thermal transmittance = 1/R-value), and ΔT is the temperature difference.
Then add infiltration loss: Q_inf = Volume (cu ft) × ACH × 0.018 × ΔT, where ACH is air changes per hour (typically 0.5 for tight construction, 1.0–2.0 for older or poorly sealed buildings).
Gas vs. Electric vs. Propane — Cost Comparison
Fuel cost comparison is essential. Use the cost per 1,000,000 BTU (1 MMBtu) as a common denominator:
- Natural Gas at $1.20/therm: $12.00 per MMBtu (÷ efficiency)
- Propane at $2.50/gallon: $27.34 per MMBtu (÷ efficiency)
- Electricity at $0.15/kWh: $43.96 per MMBtu (but 100% efficient)
Natural gas is almost always the lowest-cost option where available. Electric heaters make financial sense in mild climates with low rates, or where the lower installation cost offsets higher running costs.
Altitude Derating for Gas Heaters
At high altitudes, thinner air reduces gas combustion efficiency. De-rate gas heater output by 4% for every 1,000 feet above 2,000 feet elevation. A 100,000 BTU heater at 6,000 ft elevation effectively produces 84,000 BTU output (4% × 4 = 16% de-rate).
Recommended Unit Heater Sizes by Application
| Application | Typical Size Range | Fuel Recommendation |
|---|---|---|
| 1-car garage (600 sq ft) | 15,000–30,000 BTU | Electric or gas |
| 2-car garage (500–600 sq ft) | 30,000–45,000 BTU | Gas or propane |
| Workshop (1,000–2,000 sq ft) | 45,000–75,000 BTU | Gas preferred |
| Small warehouse (5,000 sq ft) | 100,000–150,000 BTU | Gas, 1–2 units |
| Large warehouse (20,000+ sq ft) | 400,000–800,000 BTU | Gas, multiple units |
| Barn / agricultural | 50,000–150,000 BTU | Propane or gas |
Energy Efficiency Tips
- Add insulation before upsizing your heater — reducing heat loss is always cheaper than burning more fuel.
- Install a programmable or smart thermostat to reduce runtime during unoccupied hours.
- Seal air infiltration points (gaps, cracks, unsealed doors) — infiltration often accounts for 20–40% of total heat loss.
- Consider a destratification fan in high-ceiling spaces — warm air pools at the ceiling; fans push it back down, reducing the heater's workload by 10–20%.
- For warehouses with large dock doors, consider insulated dock seals and levelers that stop cold air infiltration when doors are closed.
- Regular maintenance (cleaning burners, checking heat exchangers, replacing filters) keeps efficiency at rated levels.