Choosing a Pump for a Chiller Loop

Choose a pump by the flow it delivers at your loop's real head, not by its free-flow rating. Add the lift, the hose losses and the filter resistance, then check the pump curve at that duty point and confirm a duty rating that matches how long it will run.

Most "the chiller is not working" complaints are pump problems wearing a disguise. Flow is invisible, so the chiller gets blamed for a loop that never delivered what it needed.

Start with the chiller's requirement

Every chiller has a flow window: a minimum and a maximum. Below the minimum, the evaporator can freeze or the unit trips on protection. Above the maximum, erosion and noise.

Find those two numbers first, then design a loop that lands in the middle of the range at real conditions.

		What the flow window tells you
Situation	Symptom	Consequence
Below minimum flow	Chiller short-cycles or frosts	Protection trips, no cooling
At the low end	Slow recovery, poor mixing	Tub warms unevenly
Mid range	Stable performance	Target
Above maximum	Noisy loop, high pressure	Wear, possible warranty issue

Head is not just height

"Head" is the total resistance the pump must overcome, and height is only part of it.

Static lift: the vertical distance from the water line to the highest point.
Friction: hose length, diameter and roughness.
Fittings: every elbow, valve and strainer adds resistance.
Equipment: the filter and the chiller's own pressure drop.
Filter loading: resistance rises as the cartridge clogs.

A loop with two metres of lift and a loaded filter can present more resistance than one with five metres of lift and a clean filter.

Reading a pump curve

A pump curve plots flow against head. Flow falls as head rises. The point where your system's resistance curve crosses the pump curve is your duty point.

Practically, that means:

Estimate the total head of your loop.
Find that head on the pump curve.
Read the flow at that point.
Compare it with the chiller's window.

If the flow is below the minimum, you need a bigger pump, a shorter loop, larger hoses or a less restrictive filter. All four are legitimate fixes; changing the chiller is not.

Choosing between pump types

		Pump types for chiller loops
Type	Best for	Notes
Submersible	Simple tubs, low head	Heat goes into the water it is pumping
Inline centrifugal	Most chiller loops	Easy to isolate and service
Magnetic drive	Small, quiet loops	Low head capability
Self-priming	Suction lift from a tub	Useful when the pump sits above water
Variable speed	Variable load	Quieter and more efficient at part load

For a plunge tub loop with a filter, an inline centrifugal pump with a strainer is usually the most serviceable choice.

Duty rating and location

A pump that runs 24 hours a day needs a continuous duty rating. A pond pump rated for intermittent use will fail early if left on permanently.

Also consider location:

Submersible pumps dump their motor heat into the tub, which the chiller then has to remove.
Inline pumps are easier to service and do not heat the water, but they need a flooded suction or self-priming design.
Noise travels through the floor; a rubber mat under an inline pump helps.

Symptoms of an undersized pump

Chiller runs constantly without reaching set point.
Frost on the evaporator or frequent protection trips.
Water at the return feels only slightly colder than the tub.
Recovery after a session takes much longer than expected.
Filter clogs quickly as flow drops and debris settles.

Before buying a bigger pump, check the strainer, the filter and the hoses. A partially blocked strainer looks exactly like an undersized pump.

Installation checklist

Fit a strainer before the pump inlet.
Use the largest hose diameter that fits the fittings.
Keep the suction run short and rising.
Add isolation valves either side of the pump and filter.
Support the pump so its weight is not carried by the hoses.
Fill and prime before switching on; never run dry.
Check flow at the return, then check it again with a loaded filter.

If your chiller has no strainer

Some chillers arrive with only a hose barb, which leaves the pump unprotected. Add one:

Inline strainer on the suction side, sized to the hose.
Filter bag over the suction end, for a quick fix.
Pre-filter housing if you also need fine filtration.

A strainer is cheaper than an impeller, and much cheaper than a service call.

Winterising and long pauses

If the loop will sit idle in a cold place, drain it. Water left in a pump or a chiller's exchanger can freeze and crack the housing, and that damage usually is not covered.

Drain the tub, the hoses, the filter and the pump.
Blow through the lines if water traps in a low section.
Store the pump dry, with the impeller free to turn.
Check the manual for the chiller's storage temperature.

Frequently asked questions

What size pump do I need for a 1 HP chiller?

Match the chiller's stated flow window at your loop head rather than the chiller's power. A 1 HP unit often needs a mid-range flow that a small pond pump cannot deliver through a filter and a long hose.

Can I use a pond pump for a chiller loop?

Sometimes, if it delivers the required flow at your head and is rated for continuous duty. Many pond pumps are intermittent-rated and lose flow quickly as resistance rises.

Why does my pump get hot?

Restriction, air locking, or a pump running outside its curve. Check the strainer, confirm the loop is primed, and verify the discharge is not throttled.

Does a bigger pump use more electricity?

Usually yes, but a pump that is too small runs the chiller longer, which costs more overall. The right size is the one that meets the chiller's flow window with margin.

Should the pump run all the time?

Continuous circulation keeps temperature even and filtration working. If noise or energy is a concern, a variable-speed pump can run slower outside session hours.