Heat on demand, regardless of weather.

Output does not fall as air temperature drops.

Bookings and unpredictable use need that.

Recovery from cold is where it excels.

Heat pumps and solar cannot match it.

Running cost is the trade accepted for it.

From the temperature rise and the time allowed.

Maintenance duty and recovery duty differ greatly.

A maintenance-sized unit holds but does not recover.

Decide which job it is for first.

Then quote output against that requirement.

Pool surface area and cover use both feed in.

Where the heater runs for long periods, yes.

It recovers heat from the flue gases.

Efficiency is noticeably higher across a season.

It produces an acidic condensate.

That must be drained and usually neutralised.

Treat the drain as a real installation constraint.

Water chemistry, more than anything else.

Aggressive water attacks copper exchangers.

Salt-chlorinated pools are harder on them still.

Exchanger material must suit the treatment system.

This is assumed far more often than confirmed.

Check it before ordering, not after a failure.

Usually inadequate flow rather than a fault.

Heaters require a minimum flow through them.

A variable-speed pump at low speed may not deliver it.

The flow switch then shuts the heater down.

It reads as a fault and is protective behaviour.

Raise pump speed during heating periods.