Heat recovery chillers

A heat recovery chiller is a water-to-water heat pump that produces chilled water and useful hot water at the same time. Instead of rejecting condenser heat to a cooling tower, it delivers that heat to a building heating loop. Every kilowatt of cooling becomes roughly 1.2 to 1.3 kW of heating, because the compressor's work is added to the heat removed from the chilled water.

In Canadian buildings that need cooling and heating at the same time, such as hospitals, laboratories, data-intensive offices and large mixed-use buildings, a heat recovery chiller can displace a large share of boiler fuel. It is also one of the most practical first steps in electrifying an existing gas-fired heating plant.

How a heat recovery chiller works

All chillers move heat from an evaporator to a condenser. A conventional chiller sends condenser water at 29 to 35°C (85 to 95°F) to a cooling tower or dry cooler. A heat recovery chiller raises its condensing temperature so the condenser leaving water is hot enough to use for space heating, reheat or domestic water preheating.

The amount of useful heat equals the cooling delivered plus the compressor input:

1. Chilled water enters the evaporator, for example at 12°C (54°F), and leaves at 6°C (43°F).
2. The compressor raises the refrigerant pressure and temperature, using electrical energy.
3. Hot water enters the condenser, for example at 40°C (104°F), and leaves at 50 to 60°C (122 to 140°F).
4. If there is more heat than the building can use, it is rejected to a cooling tower or fluid cooler through a second condenser or a separate heat rejection loop.

Efficiency in combined mode

A heat recovery chiller's efficiency is often expressed as a combined or total COP: useful cooling plus useful heating, divided by electrical input. Because both outputs are useful, combined COPs of 5 to 7 are achievable when the lift between chilled water and hot water temperatures is modest. The higher the hot water temperature, the higher the lift and the lower the efficiency, so the temperature of the heating loop is one of the most important design variables.

Where heat recovery chillers fit best

The ideal application has a steady cooling load in winter and a heating loop that can accept moderate temperatures.

Buildings with VAV reheat are especially good candidates, because ASHRAE 62.1 ventilation and dehumidification often keep cooling coils active while reheat coils run, even in January.

Sizing: do not size for the peak

The most common mistake is sizing a heat recovery chiller for the building's peak cooling or peak heating load. The machine only saves energy when both a cooling load and a heating load exist at the same time, so it should be sized on simultaneous load.

A practical sizing approach:

1. Build an hourly profile of cooling and heating loads using an energy model or measured trend data.
2. For each hour, take the smaller of the cooling load and the heating load, converted to the same basis.
3. Plot the result as a load duration curve.
4. Select a capacity that runs at a high load factor for thousands of hours per year, typically the base load rather than the peak.

A smaller machine that runs most of the year usually beats a larger machine that cycles or sits idle. Conventional chillers and boilers, or air-to-water heat pumps, carry the remaining peaks.

Heating loop temperature: the key design decision

Older Canadian buildings often have heating systems designed for 82°C (180°F) supply water. Many heat recovery chillers can produce 55 to 65°C (131 to 149°F) efficiently; some machines using newer refrigerants or multi-stage compression can go higher, at a cost in efficiency. Matching the two usually takes one or more of these steps:

• Reset the heating supply temperature with outdoor air temperature, so high temperatures are only needed on the coldest days
• Verify that existing coils deliver adequate capacity at lower water temperatures, and replace undersized coils in critical zones
• Use the heat recovery chiller to preheat boiler return water, so it operates at its low temperature while boilers top up
• Dedicate the heat recovery chiller to low-temperature loads such as reheat, radiant floors or domestic water preheat
• For new buildings, design the heating system around 40 to 55°C (104 to 131°F) supply from the start

Our guide to heating hot water system design covers low-temperature design in more detail.

Piping arrangements

Heat recovery chillers are usually installed in a "sidestream" or parallel arrangement rather than as the main chiller.

Sidestream on the return

The heat recovery chiller draws water from the chilled water return and the heating water return. Because it sees the warmest chilled water and the coolest heating water, its lift is minimized and efficiency is highest. This is the most common arrangement for retrofits.

Dedicated heat recovery loop

Some designs place the heat recovery chiller on a dedicated loop serving a specific load, such as domestic water preheat or a low-temperature reheat system. This simplifies control at the cost of flexibility.

Heat rejection and source backup

When cooling exceeds useful heating, the excess must go somewhere. When heating exceeds available cooling, the machine needs another heat source. Options include a cooling tower connection, a ground loop, an outdoor air-source coil or a connection to a district energy network.

Controls

A heat recovery chiller is usually controlled to follow the heating load, with the chilled water side "taking what it can get," or vice versa. Common control rules include:

• Enable the machine only when both chilled water and heating water demand exist above a minimum threshold
• Control capacity to hold heating supply temperature, with chilled water temperature limits as a constraint
• Stage conventional chillers and boilers around the heat recovery machine, so it runs first and longest
• Maintain minimum flow through both heat exchangers, and protect the evaporator against freezing during low-load operation

Poor sequencing is a frequent reason heat recovery chillers underperform. If boilers are allowed to satisfy heating before the heat recovery machine loads up, the savings disappear. Functional testing during commissioning, followed by trend review in the first heating season, should confirm that it runs as the lead heat source.

Refrigerant considerations

Heat recovery chillers commonly use HFC, HFO or blended refrigerants, and some use CO2 (R-744) or ammonia for high-temperature applications. Federal rules now limit the global warming potential of refrigerants in new chillers manufactured in or imported into Canada, and the machine room must comply with CSA B52. See our guide to refrigerant regulations and low-GWP options for details.

Frequently asked questions

How much can a heat recovery chiller save?

Savings depend on simultaneous load hours, utility rates and the heating loop temperature. An hourly energy model using local weather data and utility rates gives a defensible estimate. Rules of thumb are not reliable here.

Can a heat recovery chiller replace boilers entirely?

Rarely on its own. In most Canadian climates it handles base load heating, while boilers or air-source heat pumps cover peak days when there is little cooling load to recover.

Key takeaways

• A heat recovery chiller delivers chilled water and hot water at the same time, recovering heat that would otherwise go to a cooling tower.
• Size it on simultaneous heating and cooling load, using hourly data, not on peak load.
• Lower heating water temperatures raise efficiency; temperature reset and coil verification are often required in older buildings.
• Sequence it as the lead heat source, and verify that operation with trend data after start-up.
• Hospitals, labs, buildings with VAV reheat and buildings near data loads are usually the strongest candidates; energy modeling is the best way to confirm the case.