Enter Radiant Zone Data
Example Data Table
Use these values as starting points. Replace them with project data.
| Input | Example Value | Notes |
|---|---|---|
| Indoor design temperature | 70 F | Comfort target. |
| Outdoor design temperature | 10 F | Local winter design condition. |
| Wall U value | 0.055 | Use assembly data when known. |
| Window U value | 0.30 | Use product rating. |
| Safety factor | 15% | Covers uncertainty. |
| Tube spacing | 9 in | Hydronic early layout check. |
Formula Used
- Temperature difference: ΔT = indoor design temperature − outdoor design temperature.
- Transmission loss: Q = area × U value × ΔT.
- Infiltration CFM: CFM = room volume × ACH ÷ 60 + extra ventilation CFM.
- Infiltration load: Q = 1.08 × CFM × ΔT × heat recovery factor.
- Base load: wall + window + door + ceiling + floor + infiltration losses.
- Design load: base load × (1 + safety factor ÷ 100).
- Input capacity: design load ÷ system efficiency.
- Required floor flux: design load ÷ heated panel area.
- Surface temperature needed: indoor temperature + required flux ÷ output coefficient.
- Hydronic flow: GPM = design load ÷ (500 × water temperature drop).
- Electric demand: watts = design load ÷ 3.412.
How To Use This Calculator
- Enter the room size and heated floor coverage.
- Add indoor and outdoor design temperatures.
- Enter envelope areas and U values for each loss path.
- Include air leakage, ventilation, and heat recovery data.
- Set floor covering resistance and surface temperature limits.
- Add hydronic tubing details or electric circuit details.
- Press calculate and review the load summary above the form.
- Check notes before changing insulation, panel area, or water temperature.
Radiant Load Planning Guide
Radiant Load Planning Basics
Radiant heating works best when loads are known early. The floor, wall, ceiling, glass, and air leakage all affect required heat. A small room can still need high output when glazing is large. A slab over cold ground can also steal heat. This calculator separates each loss path. That makes weak design points easier to see.
Why Floor Output Matters
Radiant floors do not behave like simple heaters. The finish layer adds resistance. Carpet, thick wood, and heavy underlayment reduce useful output. Tile and concrete move heat faster. Surface temperature limits also matter. A design may meet boiler capacity, yet fail at the floor. The output check compares required load with heated panel area. It also estimates the surface temperature needed for comfort.
Hydronic And Electric Choices
Hydronic systems depend on flow, water temperature, loop spacing, and loop length. Too little flow causes uneven floors. Excess loop length increases pumping resistance. Electric mats need enough wattage and safe circuit sizing. Continuous loads may require extra circuit margin. The calculator gives hydronic flow, loop count, and electric amperage. These values help with early coordination.
Construction Details Change Results
Envelope numbers should reflect the real assembly. Use tested U values when available. Include exposed walls only. Add windows, doors, ceilings, and floors that lose heat. Infiltration can be important in older buildings. Heat recovery can lower that portion. Safety factors cover weather swings and estimating error. They should not hide poor insulation.
Using Results In Design
Compare the design load with the maximum floor output. If required flux is too high, increase heated area or improve insulation. You can also reduce floor resistance. Hydronic designs may need closer spacing or warmer water. Electric designs may need additional zones. Final plans still need local code review. A qualified designer should verify controls, limits, and installation details.
Field Checks Before Installation
Measure rooms after framing changes. Recheck glass sizes after ordering. Confirm manifold locations before tubing starts. Keep tubing away from cabinets and drains. Record actual spacing during installation. Take photos before the floor is covered. Balance each loop after startup. Revisit settings after occupants move in. Good records make service easier later. They also protect comfort during future renovations. They help owners understand why room performance changes over time.
FAQs
What does this radiant load calculator estimate?
It estimates room heat loss, design load, floor output, hydronic flow, loop count, electric wattage, and circuit needs. It is made for early construction planning and comparison.
Can I use it for hydronic radiant floors?
Yes. Enter supply temperature, return temperature, tube spacing, and loop limits. The tool estimates flow rate, total tube length, loop count, and flow per loop.
Can I use it for electric radiant systems?
Yes. Enter voltage, breaker size, and mat watt density. The calculator estimates watt demand, current, circuit count, and mat area needed for the design load.
Why does heated floor coverage matter?
Radiant output comes from available heated surface. Cabinets, fixtures, islands, and tubs reduce area. Less area means higher required heat flux and higher surface temperature.
What U value should I enter?
Use tested assembly data or product ratings when available. Lower U values mean better insulation. Enter separate U values for walls, glass, doors, ceilings, and floors.
How is infiltration heat loss handled?
The calculator uses room volume, air changes per hour, extra ventilation CFM, temperature difference, and heat recovery efficiency to estimate air leakage load.
Why is my floor capacity ratio low?
A low ratio means the available heated area may not meet the design load. Improve insulation, increase panel area, reduce floor resistance, or review water temperature.
What surface temperature limit should I use?
Use a limit suitable for the room, floor finish, and comfort requirement. Many occupied spaces use conservative limits. Always verify with product guidance and local rules.
Does floor covering resistance change output?
Yes. Carpet, thick wood, and underlayment resist heat flow. Higher resistance reduces useful output and may require closer spacing, more area, or higher water temperature.
Is this a replacement for a formal heat loss report?
No. It is an estimating tool. Final construction documents should be reviewed by qualified professionals using local climate data, codes, product limits, and complete drawings.
Why add a safety factor?
A safety factor covers uncertain inputs, weather swings, and field variation. Avoid using it to mask poor insulation, missing air sealing, or undersized heated area.