Heat Recovery Chillers: Cooling and Free Hot Water

A heat recovery chiller captures the heat it would otherwise reject and uses it to make hot water. It pays back where a site needs cooling and hot water at the same time; it is a poor fit where either demand is intermittent.

A conventional chiller spends its working life throwing heat away. A heat recovery chiller keeps that heat and puts it somewhere useful, usually hot water. Whether that is worth doing depends entirely on whether the site needs both things at once.

How recovery works

A chiller rejects heat at the condenser. In a recovery unit, that heat is captured at a higher temperature and transferred to a water circuit rather than dumped to the air.

• Cooling side: unchanged, the evaporator still removes heat from the chilled loop.
• Recovery side: the condenser rejects heat into a hot water circuit at a useful temperature.
• Extra hardware: a recovery heat exchanger, a second pump, controls, and safety devices.

The thermodynamics do not change. The same compressor does the same work; the question is where the waste heat goes.

Where it pays back

Recovery makes sense when three conditions line up:

1. Cooling runs for long hours, so there is a steady heat source.
2. Hot water demand is simultaneous, such as a hotel, hospital, food plant or pool.
3. The hot water temperature you need is achievable with the recovery circuit's temperatures.

If cooling runs in summer and hot water is needed in winter, the overlap may be too small to justify the extra capital.

The design questions that decide it

• What hot water temperature do you need? Higher temperatures reduce what the recovery circuit can deliver.
• How many hours do cooling and hot water overlap? That overlap is the actual energy you recover.
• What is the backup? You still need a boiler or another heat source when cooling stops.
• How is it controlled? Priority logic matters: cooling must not be compromised to make hot water.

A recovery system that is badly controlled either stops recovering or lets chilled water drift.

Sizing and storage

Recovery output follows cooling load, not hot water demand. That mismatch is why storage matters.

• Size the hot water tank to bridge the gap between cooling hours and demand hours.
• Do not size the recovery circuit for peak hot water draw; size it for what cooling can supply.
• Keep a conventional heat source for the rest.

Maintenance implications

A recovery chiller has more parts: a second pump, a second heat exchanger, additional valves and sensors. It needs a maintenance routine that covers the recovery circuit, not just the refrigeration side.

Plan for:

• Cleaning the recovery exchanger.
• Checking the hot water circuit's water treatment.
• Testing recovery controls and changeover logic.
• Logging recovery output, so you can prove the saving.

A worked example

A site with 60 kW of cooling running 3,000 hours a year and a simultaneous hot water demand.

• 60 kW of cooling rejects roughly 60 kW plus compressor power at the condenser.
• If 40% of that rejected heat can be captured at a usable temperature, the recovery circuit supplies about 24–28 kW of heat while cooling runs.
• Over 3,000 hours, that is a substantial quantity of heat not bought from a boiler.

Whether it pays back depends on the boiler fuel price, the capital premium, and whether the hot water demand actually exists during those hours. The arithmetic is site-specific; the structure of the calculation is not.

Retrofit or new build

Recovery is easiest to justify at the design stage, when pipework and space can be planned.

• New build: the recovery circuit is designed in, with storage and controls.
• Retrofit: possible only if the chiller was built to support it, and if there is room for the extra exchanger and pipework.
• Unsupported retrofit: usually voids the warranty and rarely controls well.

Measuring the saving

You cannot manage what you do not meter.

• Install a heat meter on the recovery circuit.
• Log recovered kWh alongside cooling kWh.
• Compare against the boiler fuel the site would otherwise have used.
• Review after a full season, not a month.

A recovery system that nobody measures becomes a recovery system that nobody maintains.

When not to bother

• Cooling is intermittent or seasonal, and hot water runs year round.
• Hot water demand is small relative to the cooling load.
• Nobody will maintain an extra circuit.
• The site cannot accommodate the extra pipework and storage.

In those cases a standard chiller plus a separate water heater is simpler and often cheaper over its life.

A recovery system that nobody measures becomes a recovery system that nobody maintains. If the site cannot commit to a seasonal review, skip the recovery circuit and keep the simpler plant.

Frequently asked questions

How much heat can a chiller recover?

In principle, most of the heat rejected at the condenser. In practice, the useful amount is limited by how much of it matches a simultaneous hot water demand at a usable temperature.

Does heat recovery reduce cooling capacity?

It can, depending on design and control. A recovery unit must be selected so that condensing conditions stay within limits while it makes hot water.

Is a heat recovery chiller worth the extra cost?

Only where cooling and hot water demand overlap for many hours a year. Without that overlap, the extra capital and complexity rarely pay back.

Can any chiller be retrofitted for heat recovery?

Some can, with a recovery heat exchanger and controls, if the manufacturer supports it. Many cannot, and retro-fitting an unsupported unit voids warranties.

What temperature hot water can it produce?

It depends on the refrigerant and the compressor's limits. Higher recovery temperatures reduce cooling capacity, so the design balance matters.