D3 · HVAC and Refrigeration Distribution and Systems

Foundation lesson · 25 minute study session

Balance water duty and pump head separately

Thermal capacity sets hydronic flow; the pressure-loss network sets pump head. Elevation means different things in open and closed systems.

By the end of this lesson

Calculate water flow from thermal duty.
Identify series losses and the controlling parallel route.
Explain why closed-loop elevation does not equal pump lift.

Thermal duty sets flow

For ordinary liquid water near conventional HVAC temperatures, the familiar coefficient of about 500 connects Btu/h, gpm and °F. It assumes a suitable water density and heat capacity. Glycol, unusual temperatures or supplied properties require the general mass-flow relationship. The thermal relation uses $\Delta T$ ; pump head $H$ follows the hydraulic boundary.

The network sets head

Pipe friction and component losses consume pump head. Parallel branches share the pressure difference between their junctions; flows add. The controlling route and balancing requirements set the design operating point. The pump operating point lies where the pump curve meets the system curve.

Distinguish open from closed

In a filled closed loop, upward and downward static elevation changes balance around the circuit. The pump must overcome flow losses, not continuously lift all water through the full building height. Static fill pressure is still needed to maintain acceptable pressure at high points and avoid vapor formation. In an open system, a net elevation difference may contribute directly to required pump head.

A decision map for this lesson

Separate thermal flow from circulation head

The closed-loop illustration uses the worked example: flow comes from heat duty, while head comes from the controlling hydraulic route.

A realistic closed hydronic circuit connects one low circulation pump to an elevated coil and a return riser, with an expansion vessel near suction. — **Elevation cancels around a filled closed loop**Connect the equipment to the calculation

A pump circulates 25 gallons per minute through a filled closed loop with an elevated coil 40 feet above it. Pipe and fitting losses of 18 feet plus coil and valve losses of 12 feet require 30 feet circulation head. — **Elevation cancels around a filled closed loop**Connect the equipment to the calculation

Choose the governing relationship and catch the wrong route
Condition or question	Governing decision	Error to avoid
Required water flow	Use $\dot V=\dot Q/(500\Delta T)$ only for the stated ordinary-water approximation.	Carrying the same factor into a different fluid-property basis without checking it.
A filled, closed circulation loop	Sum friction and component losses; the net hydrostatic rise cancels around the loop.	Adding the building height to circulation head.
An open system or imposed pressure difference	Include the net elevation and pressure terms as well as losses.	Applying the closed-loop cancellation to unequal free surfaces.
Pump suction performance	Use absolute pressure head, liquid level, vapor pressure and suction loss for NPSH.	Treating circulation head or fill pressure as a complete NPSH check.

On a narrow screen, scroll across the table to compare the decision and the error.

Relationships and their limits

General thermal balance

\dot Q=\dot m c_p\Delta T

Use actual fluid density and heat capacity when provided.

Water approximation

\dot Q\approx500\dot V_{gpm}\Delta T

Btu/h, gpm, °F; appropriate ordinary-water approximation only.

Symbols and unit basis
Symbol	Meaning	Units or basis
$\dot Q$	Thermal duty	Btu/h
$\dot m$	Fluid mass flow	lb/h
$c_p$	Fluid specific heat	Btu/(lb·°F)
$\dot V_{gpm}$	Liquid volumetric flow	gpm
$H$	Required pump head	ft of stated liquid
$\Delta T$	Fluid temperature change	°F difference

One decision, fully worked

Apply the method.

A closed chilled-water loop serves a 150,000 Btu/h coil at a 12°F water-temperature rise. Use the ordinary-water factor 500. At this flow, the controlling route has 18 ft of pipe/fitting loss and 12 ft of coil/valve loss. The building elevation difference is 40 ft; the loop is filled and closed.

Calculate water flow from the coil heat balance.
Add losses on the controlling route.
Do not add the closed-loop elevation difference to operating head.

\dot V=150{,}000/[500(12)]=25\ \mathrm{gpm}

H=18+12=30\ \mathrm{ft}

Decision

The stated operating requirement is 25 gpm at 30 ft of head, before any additional explicitly required allowances.

Independent check

Adding 40 ft of closed-loop elevation would mix fill-pressure requirements with circulating head.

Specialist extension

Check NPSH margin at the selected operating flow

For an open reservoir, available NPSH is absolute surface-pressure head plus signed surface elevation above the pump datum, minus vapor-pressure head and suction losses. Use all terms as head of the pumped liquid. Required NPSH is the manufacturer’s characteristic at the selected flow and speed. Rising temperature increases vapor pressure and can reduce availability. A positive difference from NPSHR does not automatically satisfy every manufacturer’s recommended margin.

A new applied case

At the selected flow, absolute surface-pressure head is 33.9 ft of the liquid, its surface is 4.0 ft above the pump datum, vapor-pressure head is 5.0 ft and suction loss is 3.0 ft. Supplied NPSHR is 8.0 ft and the required additional margin is 3.0 ft.

\mathrm{NPSH_A}=33.9+4.0-5.0-3.0=29.9\ \mathrm{ft}

\mathrm{margin}=29.9-8.0=21.9\ \mathrm{ft}

Decision. The stated 3.0 ft additional-margin criterion is satisfied.

Cross-check. For suction lift, the surface-elevation term is negative. Do not substitute gauge surface pressure for the supplied absolute head.

Unit basis. All pressure, elevation and loss terms are ft of the pumped liquid; NPSHR is at the selected flow and speed.

ASHRAE Pocket Guide for Air Conditioning, Heating, Ventilation, Refrigeration, SI, 9th ed. (2017), printed pp. 70–71, supporting fundamentals. This original USCS example converts the pressure basis to head explicitly; the guide is not an examination reference.

Open targeted practice in the study studio →

Transfer the method

HVAC and Refrigeration Distribution and Systems · HV-LC-D3

Try a new decision.

Select one

A filled closed hydronic loop transfers 240,000 Btu/h with a 16°F water temperature change. Use the thermal factor 500 Btu/(h·gpm·°F). At the required flow, the entire friction and component loss is 24 ft of water. The highest pipe is 50 ft above the pump; there are no free surfaces or other net pressure requirements. The required flow and pump head, respectively are most nearly:

A30 gpm; 24 ftB30 gpm; 74 ftC15 gpm; 50 ftD48 gpm; 24 ft

A lesson check supports practice, not a readiness score. Your response stays in this page session and clears on reload.

Catch the common errors

Adding closed-loop building height to circulating pump head.
Summing unrelated parallel branch losses.
Using the water coefficient for glycol without checking properties.

Handbook navigation drill

Find it, then explain it.

Search: Centrifugal Pump Characteristics

Locate pump and system-curve concepts and the pump power equation.

Check your navigation

You can explain the distinction between static pressure, head loss and the operating point.

Reference path: §3.3; §3.7.4–3.7.6, pp. 255–259; §9.6, pp. 533–535. These are printed handbook page numbers; section names and search terms remain the primary navigation method. See technical review status.

Theory sources

NCEES handbook v2.1, §§3.3, 3.7.4–3.7.6 and 9.6.
Grondzik and Kwok, Mechanical and Electrical Equipment for Buildings, 12th ed., §§12.10 and 12.12.

Worked examples and lesson prose are original ReasonSpan^™ material. Obtain authorized publications separately; this preview does not reproduce the supplied books or handbook.