Hydronic Calculator
Calculate water-side heat transfer, required flow, and temperature differential for chilled water, hot water, and boiler loops. Built on the industry-standard formula Q = 500 x GPM x delta-T.
Use Calculator in AppWhat is Hydronic Heat Transfer?
Hydronic heat transfer is how water-based systems move heat around a building - chilled water for cooling, hot water for heating, and condenser water rejecting heat to a cooling tower. Because water carries roughly 3,400 times more heat per unit volume than air, hydronic loops move large loads through comparatively small pipes.
The heat carried by any water loop is found with Q = 500 x GPM x delta-T, where Q is heat transfer in BTU/hr, GPM is the flow rate in gallons per minute, and delta-T is the temperature difference between supply and return in °F. Rearrange it to solve for the flow you need or to back out the delta-T a system is actually achieving in the field.
GPM per Ton at Common Delta-T
| Design Delta-T | GPM per Ton | BTU/hr per GPM | Typical Application |
|---|---|---|---|
| 10°F | 2.4 GPM/ton | 5,000 BTU/hr | Standard chilled water & condenser water design |
| 12°F | 2.0 GPM/ton | 6,000 BTU/hr | Higher delta-T chilled water, reduced pump energy |
| 16°F | 1.5 GPM/ton | 8,000 BTU/hr | Low-flow chilled water, larger coils |
| 20°F | 1.2 GPM/ton | 10,000 BTU/hr | Hot water heating & boiler loops |
GPM per ton = (12,000 x Tons) / (500 x delta-T). Watch for low delta-T syndrome: when a coil runs below its design delta-T, flow climbs, pumps work harder, and plant efficiency drops.
Where the 500 Constant Comes From
The 500 constant isn't magic - it bundles three properties of water into one number so you don't have to convert units every time. It only holds for plain water; glycol antifreeze changes the fluid's weight and specific heat, so the constant shifts.
500 = 8.33 x 60 x 1.0
- 8.33 lb/gal: the weight of one gallon of water
- 60 min/hr: converts per-minute flow (GPM) to per-hour heat (BTU/hr)
- 1.0 BTU/lb/°F: the specific heat of water
- Glycol mixes: use ~485 for 30% propylene glycol - lower specific heat means more flow for the same load
Key Hydronic Variables
Flow Rate (GPM)
Gallons per minute moving through the loop, read from a flow meter, pump curve, or balancing valve.
Temperature Differential (delta-T)
Difference between supply and return water temperature in °F - the heart of a balanced hydronic system.
Heat Transfer (Q)
Total load moved by the loop in BTU/hr; divide by 12,000 to express it in tons of cooling.
Fluid Type
Plain water uses the 500 constant; glycol antifreeze lowers specific heat and changes the multiplier.
Hydronic Heat Transfer Formula
One equation solves three ways. Know any two of heat load, flow, and delta-T, and you can find the third:
- Heat Load: Q = 500 x GPM x delta-T (BTU/hr)
- Required Flow: GPM = Q / (500 x delta-T)
- Actual Delta-T: delta-T = Q / (500 x GPM)
For cooling, tie it to tonnage with GPM = (12,000 x Tons) / (500 x delta-T) - about 2.4 GPM/ton at the standard 10°F chilled water delta-T. Swap the 500 for ~485 when the loop runs 30% propylene glycol.
Typical Hydronic Delta-T Ranges
| System Type | Design Delta-T |
|---|---|
| Chilled water (comfort cooling) | 10 - 16°F |
| Condenser water (cooling tower loop) | 10°F |
| Hot water baseboard / fin-tube | 20°F |
| Radiant floor heating | 10 - 20°F |
These are design references. Always verify against the equipment nameplate, balancing report, and local mechanical code (IMC/UMC and ASHRAE) - field results are estimates until confirmed on the actual system.
Run Hydronic Numbers on the Job with Ventora
Solve BTU/hr, GPM, or delta-T in seconds. Enter any two values and get instant heat transfer, required flow, GPM-per-ton, and tonnage results - right from the mechanical room.
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