Field calculator

Hydronic load calculator (GPM, delta-T, tons)

This calculator works the basic hydronic heat-transfer relationship for water: heat rate in BTU per hour equals the flow in gallons per minute times 500 times the temperature difference (delta-T) across the coil, chiller, or boiler. Tons of cooling is that BTU/hr divided by 12,000, which is the same as GPM times delta-T divided by 24. Enter the delta-T and either the flow or the load, and the rest is solved. Use it to check a chiller or boiler against its design flow and delta-T, to size a pump to a load, or to spot a low delta-T problem where the flow is high but the load is not there. The 500 constant is 8.33 lb per gallon times 60 minutes times the specific heat of water; a glycol mix carries less heat per gallon, so drop the constant to roughly 480 to 490 and confirm the fluid properties and the equipment data before you commit.

anvilfield.com/calculators/hydronic-load-gpm-delta-t-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.

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Hydronic load FAQ

What is primary-secondary chilled water pumping?

Primary-secondary pumping splits the plant into a constant-flow primary loop through the chillers, with one pump per chiller, and a variable-flow secondary loop out to the coils. A short decoupler pipe joins them, so the chillers see steady flow while the building flow varies with the cooling load.

What is variable primary flow?

Variable primary flow, or VPF, uses one set of variable-speed pumps to push water through the chillers and the coils, with no secondary loop. The flow varies through the evaporator, so a modulating minimum-flow bypass valve near the chillers opens at low load to keep each running chiller above its minimum flow.

What is a decoupler in a chilled water system?

A decoupler, also called the common pipe or bridge, is a short low-resistance pipe joining the primary and secondary loops in a primary-secondary plant. It lets the two loops run at different flows without fighting each other. It must have almost no pressure drop, so it is sized short and fat with no fittings.

What is low delta-T syndrome?

Low delta-T syndrome is when chilled water returns colder than design, so the temperature rise across the coils is too small. The plant pumps more water to move the same heat, the kW per ton climbs, and the secondary flow can exceed the primary. The cause is at the coils and valves, not the plant.

Primary-secondary vs variable primary flow: which is more efficient?

Variable primary flow usually uses less pump energy, because it drops the secondary pumps and lets the chiller flow ride down with the load. Primary-secondary runs its primary loop at constant flow, which costs more. VPF needs better controls and a minimum-flow bypass to protect the chiller, so the project and the chiller data decide.

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