To size a hydronic heating system under the floor, calculate the heat load based on room dimensions, insulation, and desired temperature, then select appropriate tubing and boiler capacity.
Hydronic floor heating systems provide unmatched comfort by radiating heat evenly across your floors. But sizing the system correctly is crucial for performance and energy efficiency. This guide covers everything from heat loss calculations to tube spacing and flow rates.
Understanding Heat Loss in Your Space
The first step in sizing your hydronic system is calculating heat loss. This determines how much warmth your floors need to produce. Heat loss varies based on insulation quality and temperature differences.
Heat Loss Categories
| Insulation Level | BTUs/hr/sq ft/°F |
|---|---|
| Super Insulated | 0.1 |
| Good | 0.3 |
| Fair | 0.6 |
| Poor | 2.0 |
Example: A 1,000 sq ft home with good insulation (0.3 BTU factor) in -10°F weather with 65°F indoors would have:
0.3 x 1,000 x 75° difference = 22,500 BTU/hr heat loss
Determining Floor Temperature Requirements
Your floor surface temperature directly impacts heat output. Follow these guidelines:
- Typical floor temp: 75-80°F
- Maximum temp: 85°F (never exceed)
- Each 1°F increase adds ~2 BTU/hr/sq ft
For our example home needing 22,500 BTU/hr, an 80°F floor temperature would provide 27 BTU/hr/sq ft – more than enough.
Selecting the Right Tubing Layout
Proper tube spacing ensures even heat distribution. Common configurations:
Tube Spacing Options
- 6″ spacing: High heat output areas (bathrooms, tile floors)
- 8″ spacing: Standard residential areas
- 12″ spacing: Well-insulated spaces or supplemental heat
Consider using modern PEX-AL-PEX tubing for durability and efficient heat transfer.
Sizing Your Heat Source
Your boiler or water heater must match your system’s demands:
- Calculate total BTU requirement (heat loss x sq footage)
- Add 10-15% buffer capacity
- Consider high-efficiency tankless options for smaller systems
For our 1,000 sq ft example, you’d need about a 25,000 BTU/hr heat source (22,500 + 10%).
Optimizing Flow Rates
Proper water flow ensures consistent temperatures throughout the system:
| Pipe Size | Max Flow Rate (GPM) | Max Loop Length |
|---|---|---|
| 1/2″ | 2.0 | 300 ft |
| 5/8″ | 3.0 | 400 ft |
| 3/4″ | 4.0 | 500 ft |
According to Radiant Professionals Alliance, maintaining flow velocity between 1-3 ft/sec prevents noise and air entrapment.
Manifold Placement and Zoning
Proper manifold installation is critical for system performance:
- Central location minimizes pipe runs
- Keep within 100 ft of farthest loop
- Install at accessible height (1-1.5m)
- Consider separate manifolds for different floors
The American Council for Heating and Refrigeration recommends zoning by room type (bedrooms vs. living areas) for optimal comfort control.
When to Consider Supplemental Heat
In some cases, you may need additional heat sources:
- Poorly insulated spaces exceeding 45 BTU/sq ft output
- Areas with large glass surfaces (sunrooms)
- Spaces requiring rapid heat response times
For these situations, consider pairing with supplemental heating units that match your decor.
