Hydronic & Radiant Heat Loss Calculator
Calculate the room heating load, then size the radiant floor: required output, floor flux, PEX tubing length and loop count, flow in GPM and the boiler output and fuel input — with every formula shown.
Professional-grade estimate, not a full ACCA Manual J. This uses the recognized transmission and infiltration heat-loss equations. For permit or warranty compliance, a full Manual J by a qualified professional is required.
Your space
Heating load
20,120 BTU/h
- Heating load
- 5.9 kW
- Transmission
- 12,668 BTU/h
- Infiltration
- 7,452 BTU/h
- Radiant flux
- 16.8 BTU/h·ft²
- Tubing
- 1,275 ft (5 loops)
- Flow
- 2.01 GPM
- Boiler output (sized)
- 23,138 BTU/h
- Boiler input
- 24,356 BTU/h
- Professional-grade estimate using the recognized transmission and infiltration heat-loss equations — this is NOT a full ACCA Manual J. For permit or warranty compliance, a full Manual J by a qualified professional is required.
How this is calculated
Exterior walls (U×A×ΔT)
U × area × ΔT
= 0.063 × 640 × 69
2,760 BTU/h
Windows (U×A×ΔT)
U × area × ΔT
= 0.3 × 120 × 69
2,484 BTU/h
Doors (U×A×ΔT)
U × area × ΔT
= 0.2 × 40 × 69
552 BTU/h
Roof / ceiling (U×A×ΔT)
U × area × ΔT
= 0.02 × 1,200 × 69
1,656 BTU/h
Slab-on-grade edge (slab perimeter)
F × perimeter × ΔT
= 0.54 × 140 × 69
5,216 BTU/h
Infiltration (air-change form)
volume × ACH × 0.018 × ΔT
= 12,000 × 0.5 × 0.018 × 69
7,452 BTU/h
Total heating load
Σ transmission + slab + infiltration
= 7,452 + 5,216 + 7,452
20,120 BTU/h = 5.9 kW
Required emitter output
emitter output = room heating load
= 20,120 BTU/h
20,120 BTU/h
Radiant floor flux
load ÷ heated area
= 20,120 ÷ 1,200
16.8 BTU/h·ft²
Loops
ceil(field tubing ÷ (max loop − leader))
= ceil(1,200 ÷ (300 − 15))
5 loop(s)
Tubing length
heated area ÷ spacing + loops × leader
= 1,200 ÷ 1 + 5 × 15
1,275 ft
Required flow
GPM = load ÷ (500 × ΔT_water)
= 20,120 ÷ (500 × 20)
2.01 GPM
Boiler sizing
output = load × (1 + safety); input = output ÷ AFUE
= 20,120 × 1.15 ÷ 0.95
23,138 BTU/h out, 24,356 BTU/h in
How is a heat loss calculation done?
The design temperature difference is ΔT = indoor − outdoor. For a 70 °F indoor and a −7 °F outdoor design that is a 77 °F ΔT. Each surface loses heat by Q = U × area × ΔT, where the U-factor is 1 ÷ R. A wall with U 0.07 over a net 138 ft² loses 0.07 × 138 × 77 ≈ 744 BTU/h. Air leakage adds volume × ACH × 0.018 × ΔT — a 2,816 ft³ room at 1.2 ACH adds ≈ 4,683 BTU/h. The whole-building load is the sum of every surface plus infiltration plus the slab edge.
Slab-on-grade floors use the perimeter method, not area: Q = F × exposed perimeter × ΔT. This is the correct (and more accurate) way to model edge losses — area-based slab estimates are wrong. Results are reported in BTU/h and in W/kW.
Typical assembly R-values and U-factors
| Assembly | R-value | U-factor |
|---|---|---|
| 2×4 wall, R-13 batt | 13 | 0.077 |
| 2×6 wall, R-20 batt | 20 | 0.050 |
| Vented attic, R-50 | 50 | 0.020 |
| Double-pane low-E window | 3.3 | 0.30 |
| Triple-pane low-E window | 5.6 | 0.18 |
| Insulated door, R-5 | 5 | 0.20 |
Two forms of the infiltration equation
| Form | Equation | Use when |
|---|---|---|
| Air-change | volume × ACH × 0.018 × ΔT | You know the ACH |
| Airflow | 1.08 × CFM × ΔT | You know the ventilation CFM |
Cooling load — tons from BTU/h
| Cooling load (BTU/h) | Tons |
|---|---|
| 12,000 | 1.0 |
| 18,000 | 1.5 |
| 24,000 | 2.0 |
| 36,000 | 3.0 |
| 60,000 | 5.0 |
This is an estimate, not a Manual J
This calculator uses the recognized transmission and infiltration heat-loss equations to produce a professional-grade estimate. It is not a full ACCA Manual J load calculation. For permit submissions, equipment warranties or any compliance use, a full Manual J performed by a qualified professional is required.
The cooling estimate carries a stronger caveat than the heating estimate. Cooling loads are driven by solar orientation, shading, humidity (latent load) and internal gains that vary by hour and season and are only coarsely modelled here. Treat any cooling figure as a rough sizing sanity check only — a full Manual J and Manual S are essential before selecting cooling equipment.
Common heat-loss estimating mistakes
- •Estimating slab loss by area. Slab-on-grade loses heat at the edge — use F × perimeter × ΔT, not a U-value over the floor area.
- •Forgetting to subtract windows and doors from the gross wall area. Otherwise the glazing is counted twice.
- •Using cavity R-value instead of whole-assembly R. Framing, sheathing and thermal bridging lower the effective R.
- •Oversizing equipment off a cooling estimate. Short-cycling hurts comfort and efficiency — confirm with a full Manual J / Manual S.
Frequently asked questions
How much PEX tubing do I need for radiant floor heating?+
Tubing length is roughly the heated area divided by the tube spacing in feet, plus a leader run to the manifold. At 12-inch spacing that is about 1 linear foot of tube per square foot; at 6-inch spacing it doubles. Loops are then capped at the maximum loop length (about 300 ft for ½-inch PEX).
How do I calculate GPM for a hydronic loop?+
Flow in gallons per minute is the load divided by (500 × the water temperature drop). A 40,000 BTU/h zone at a 20 °F drop needs 40,000 ÷ (500 × 20) = 4.0 GPM.
What is the maximum radiant floor heat flux?+
Comfort targets keep floor output around 30 BTU/h per square foot; about 40 BTU/h per square foot is the practical ceiling. Above that the floor feels too warm and you need a supplemental emitter or a lower load.
How do I calculate the heat loss of a room?+
Find the design temperature difference (indoor − outdoor), then add the loss through each surface (Q = U × area × ΔT, where U = 1 ÷ R), the slab edge (F × perimeter × ΔT) and air infiltration (volume × ACH × 0.018 × ΔT). The total is the heating load in BTU/h.
How many BTUs do I need to heat a room?+
It depends on the insulation, glazing, air leakage and your climate, not just floor area. This calculator sums the transmission, slab-perimeter and infiltration losses at your design temperatures to give the BTU/h (and kW) you need.
Is this calculator a Manual J?+
No. It is a professional-grade estimate using the recognized transmission and infiltration heat-loss equations. For permit or warranty compliance a full ACCA Manual J by a qualified professional is required. The cooling estimate is rougher still and needs a Manual J / Manual S before sizing equipment.
What is the difference between heat loss and cooling load?+
Heat loss is winter heating demand — conduction and air leakage out of the building. The cooling load adds solar gain through glass and internal gains from people and equipment, and is far more sensitive to orientation, shading and humidity, so it is harder to estimate.
Methodology & sources
Loads use the recognized transmission and infiltration heat-loss equations with full precision: Q = U × area × ΔT per surface, the perimeter F-factor method for slabs, and air leakage as volume × ACH × 0.018 × ΔT. Results are reported in BTU/h and W/kW.
This is a professional-grade estimate, not a full ACCA Manual J. A full Manual J by a qualified professional is required for permit or warranty compliance, and any cooling figure must be confirmed with a Manual J / Manual S before selecting equipment.
- Published effective (whole-assembly) R-values — CA Title 24 Reference Appendices JA4 assembly U-factor tables; NRCan Tables for Calculating Effective Thermal Resistance of Opaque Assemblies — verified July 2, 2026
- NFRC / ENERGY STAR & ASHRAE Fundamentals — typical rated fenestration U-factor and SHGC — verified June 28, 2026
- ASHRAE Climatic Design Conditions (2021) — 99% winter heating & 1% summer cooling dry-bulb design temperatures — verified June 28, 2026
Last reviewed June 28, 2026. Estimates are indicative — verify against current product specs and local requirements before ordering.
We're committed to keeping these tools accurate and improving them over time. If you'd like to contribute to their accuracy, or you run into any issues or errors, please email us at info@tradesppl.com.