As climate change is getting more attention, people are finding new energy-efficient ways to produce hot water. Nowadays, air-to-water heat pumps are the new trend. However, not many people understand how air to water heat pumps work.
Air-to-water heat pumps use the refrigeration cycle to harvest heat energy from the ambient air and transfer it to cold water thereby producing hot water. Because the ambient air is free of charge, air-to-water heat pumps produce hot water at a fraction of the cost of traditional immersion heaters.
Air-to-water heat pumps are often confused with the heat pumps used for room heating. However, they don’t work the same way. So, let’s unpack the working principle of air to water heat pumps.
To better explain it, let’s use “air-to-water heat pump” for hot water and “air-to-air heat pump” for room heating.
Air to Water Heat Pump Working Pressure
Similar to air-to-air heat pumps, air to water heat pumps adopt the refrigeration cycle. However, air-to-water heat pumps are running at a much higher refrigerant pressure.
While R410A air-to-air heat pumps are usually operating at around 280 psi on the discharge line, air-to-water heat pumps are usually running at around 500 psi.
At 500 psi, the temperature of the R410 refrigerant is about 58°C. With a degree Celsius or two differences, the R410A air-to-water heat pumps are able to produce hot water easily at 55°C which is the common temperature of hot water storage tanks in domestic and commercial buildings.
Air-to-water heat pumps also use the condensation process to produce hot water which in turn produces cold air during the evaporation process. Sometimes, engineers divert the cold air to lift motor room and corridor for extra cooling.
Major Components of Air to Water Heat pump
Air-to-water heat pumps are made of the same 4 major components which are the compressor, condenser, evaporator and expansion valve or metering device. However, instead of a finned-tube heat exchanger used by air-to-air heat pumps, the condenser of air-to-water heat pumps often is a brazed plate heat exchanger.
At the start, the compressor compresses the vapor refrigerant and immediately sends it to the brazed plate heat exchanger where the cold water absorbs the heat from the refrigerant thereby condensing the refrigerant.
Then, the refrigerant travels to the thermal expansion valve (or metering device) and gets expanded, causing a rapid drop in pressure and temperature. The heat pump axial fans draw in the ambient air for the cold refrigerant to absorb (or harvest) the heat from the ambient air before heading back to the compressor to repeat the cycle again.
Basically, the refrigeration cycle works the same as air-to-air heat pumps used for room heating as well as the reverse of air conditioners. The common refrigerants used for air-to-water heat pumps are R410A and R134A.
Air-to-water heat pumps often have two fixed-speed compressors. They don’t use variable-speed compressors because it is not necessary, since air-to-water heat pumps are already turning on and off based on the hot water storage temperature anyway.
However, good air-to-water heat pumps can operate at 50% capacity when one of the two compressors is malfunctions.
Many air-to-water heat pumps casing are made of stainless steel because they need to be placed outdoor in order to have good airflow for heat absorption and cold air discharge. However, some of them have mild steel casing and they are suitable to be placed in a room with sufficient mechanical ventilation.
Air to Water Heat Pump System Connection
A typical hot water heat pump system has one air-to-water heat pump connected to one hot water storage tank that is essentially a PU-insulated stainless steel tank with a small hot water circulation pump. A temperature sensor is installed through the insulated tank to contact and read the water temperature inside the tank.
Air-to-water heat pumps and hot water storage tanks are mostly connected by PPR (Polypropylene Random Copolymer plastic) pipes. PPR pipes are excellent in withstanding hot water for the long term. Besides, type L and type M copper pipes are often used to distribute hot water.
Just like other HVAC pipes, hot water pipes require insulation to minimize heat loss. Insulation is important for the hot water system because the 55°C water temperature stored may become 40°C by the time it reaches hotel rooms which the 40°C temperature is not enough hot for most people.
Air to Water Heat Pump System Operation
Most of the time, the hot water storage temperature is 55°C. When the water temperature drops beyond the set point temperature which is usually 5°C below the storage temperature, the air-to-water heat pump is initiated.
First, the air-to-water heat pump sends a signal to call the small hot water circulation pump to run for a few seconds. Once the pump starts running, it pulls the water from the hot water storage tank to the air-to-water heat pump.
Then, the air-to-water heat pump waits for its water flow switch to stabilize before calling its evaporator fan to run. After a minute or two, the compressor finally kicks in to start heating the water.
Usually, the air-to-water heat pump initiates when the hot water storage temperature falls to 50°C. The air-to-water heat pump gradually heats up the water until it reaches the setpoint temperature which is usually 55°C. Then, the air-to-water heat pump stops operating.
Throughout the process, air-to-water heat pumps naturally turn on and off quite frequently depending on the usage of hot water. After midnight, the hot water usage is usually low. With the thick PU insulation, hot water storage tanks are able to keep the hot water temperature high which the heat pumps may only need to operate once or twice before sunrise.
Hot Water Storage Tank and Air to Water Heat Pump
In the heat pump hot water storage system, cold water is coming from the domestic cold water tank straight into the hot water storage tanks, usually by gravity flow. Depending on the location of the storage tanks, they can be either pressurized or non-pressurized.
If hot water storage tanks are located on a lower floor than hotel rooms, they are most likely pressurized tanks. Conversely, if they are located on the roof, they are most likely non-pressurized.
Air-to-water heat pumps require to maintain the hot water storage temperature at 50-60°C to prevent the growth of Legionella disease. Legionella grows best when the water is stagnated at a temperature between 25°C and 45°C [source]. So, the minimum hot water storage temperature is safer to be at 50°C.
However, I’ve seen many hotels try to save energy costs by lowering the hot water storage temperature set point to 45°C and some even down to 42°C which are literally risking the life of their guests.
Air to Water Heat Pump Performance and Efficiency
Generally, air-to-water heat pumps have a COP (coefficient of performance) of about 3.5. Hence, it is 3.5 times more energy-efficient than traditional immersion heaters. However, some hot water storage tanks do have immersion heaters but they only serve as a backup.
While the condenser of the air-to-water heat pump is heating the water, the evaporator is discharging chilling cold air. Some engineers design a duct to divert the cold air into adjacent rooms for extra cooling which is a very good practice to further improve the overall COP of the air-to-water heat pump.
Some people consider air-to-water heat pumps as green technology when the working principle is essentially the same as air-to-air heat pumps that we used for room heating. But, it’s not hard to understand why because they are comparing air-to-water heat pumps to traditional immersion heaters and gas boilers.
The purpose of using air-to-water heat pumps instead of immersion heaters is to improve the energy efficiency of the hot water system. However, some of the hot water systems secretly use immersion heaters to achieve the required heating capacity while physically having lesser heat pumps in order to save cost.
A good heat pump hot water system should interlock the heat pumps with the backup immersion heaters such that at any given moment, only either one can operate. This eliminates the chances of contractors cheating as well as reduces the circuit breaker and power cable size.
Air to Water Heat Pump System Configuration
The hot water heat pump system can have four different configurations; a) one-to-one, b) one-to-many, c) many-to-one or d) many-to-many. One-to-one configuration basically means one heat pump to one storage tank. So, one-to-many means one heat pump to many tanks and many-to-one means many heat pumps to one tank.
Among them, the one-to-one configuration is the most effective and simple solution. Although one-to-many and many-to-one have the benefit of backing up each other when there is a heat pump breakdown, they possess the issue with hot water temperature balancing and heat pump operation control.
With the one-to-one configuration, one heat pump is responsible for one tank. In case the heat pump breakdown, the immersion heaters in the tank are sufficient to produce the same quality of hot water in exchange for a higher energy cost.
In a one-to-many configuration, one heat pump is connected to many tanks in either series or parallel. In a series connection, the hot water supply to and return from the tank must be arranged in a “first-in, last-out” manner so that the hot water production is balanced for all tanks.
It is rare to see the heat pump hot water system in many-to-one configuration unless there is no space for hot water storage tanks. This configuration is easy to control the water temperature but has a higher chance of getting high-pressure trips.
Many-to-many configuration is the most complicated configuration of all. It can be three heat pumps connected to five tanks or six heat pumps connected to 3 tanks. Water balancing, temperature control and sequence of operation must be taken into consideration during design, installation and commissioning.
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