How much Domestic Hot Water can an Air to Water Heat Pump produce?
How much Domestic Hot Water can an Air to Water Heat Pump produce?
Air to Water Heat Pump energy output depends on multiple factors such as outdoor air temperature, DHW supply and return Temperatures, Control Algorithm, DHW flow / Temperature Set point as well as DCW entering temperature which is colder in winter and hotter in the summer.
Introduction
Traditional electric or gas fired boilers have capacities ranging from 85 MBH (25 KW) to millions of BTUs and Thousands of KWs. Beside the relatively limitless heating capacity, their hot water supply temperature can be as low as 95°F (35°C) and as high as 212°F (100°C). Single Phase Air to Water (ATW) Heat Pumps are available in capacities ranging from 30 MBH (8.8 KW) to 80 MBH (23.5 KW). Their capacities decrease with the outdoor air temperature and their hot water supply temperature is limited to 140°F (60°C). Boilers suppplied heating capacity is completely independant of outdoor air temperatures which makes them great as a backup, emergency or even a secondary heat sources.
Producing Domestic Hot Water (DHW) using Air to Water Heat pumps, reduces the overal cost of DHW prodcution, however the amount of heat available for DHW and the time of DHW usage and recovery rate is not the same as in traditional boilers.
This blog is to better inform Designers, Contractors and Homeowners about the limitations and constraints of using ATW heat pumps for DHW hot water heating production.
Case Analysis of Dual Tanks ATW heat pump system
A typical ATW heat pump kit has two thermal storage tanks: One tank dedicated for either space heating or space cooling (depending on the season) and another tank dedicated for DHW pre-heating. DHW can be pre-heated via the DHW tank's indirect coil or via the tank directly. Always check with applicable local regulations/codes/laws/by-laws if going directly through the tank is allowed since it's allowed in some places and not allowed in others.
ATW Heat Pumps have Air Cooled Condenser installed outdoor. The amount of Heat a condenser (Condenser becomes an evaporator in heating mode because HP reverses its cycle) can extract from ambient air, decreases when it's colder outside. Below is the heating capaciy table of our HSS080V2LS Split ATW heat pump:
At extreme winter temperatures -15°C (5°F) and lower and a Supply Temperature of 55°C (131°F), an 80MBH ATW heat pumps produces only 51 to 36 MBH of thermal energy. This means that DHW tank will take longer time to heat to setpoint and consequently showers with water at temperatures higher than 43.33°C (110°F) will have to be shorter when the energy produced by the heat pump is less than energy required to heat DCW to DHW setpoint.
A shower requires 2.5 US GPM of DHW water at a minimum temperature of 43.33°C (110°F). In a typical Canadian winter when DCW enters the DHW tank at 4.44°C (40°F) and outdoor air temperature is at -25°C (-13°F), we need 87 500 Btu/hr to heat DCW from 4.44°C (40°F) to 43.33°C (110°F).
The heating output of our HSS080V2LS ATW heat pump at -25°C (-13°F) outdoor air temperature is 36 643 Btu/hr. This is less than the required energy to maintain the DHW tank at its setpoint. This means that the longer the DHW usage is, the colder the DHW tank it gets. In technical language, and according to the second law of thermodynamics, the energy deficit (87 500 - 36 643)=50 587 Btu/hr is translated into a reduction of internal energy of water contained in the DHW tank.
Thermal Energy Deficit
As you can see in the table at the right, when outdoor air temperature is too cold (less than 12°C (53.6°F) and incoming domestic cold water temperature is very low as well, the energy supplied by the ATW heat pump is less than the required energy for heating DCW from entering temperature to 43.33°C (110°F).
For exterme winter temperatures, such as -20°C (-4°F) to -25°C (-13°F), there is an energy deficit is regardless of incoming DCW temperature. This is mainly due to the fact that ATW heat pumps have their lowest performance at their lowest operating outdoor temperatures.
N.B: Red Numbers are negative numbers and Black ones are positive.
DHW Tank Cooling time from 131°F to 110°F
It's important to properly size the DHW pre-heating tank to match demand and HP capacity. Tank Volume shall not be too small to run out of DHW too quick and not too big so HP will take too long to heat the tank.
For a 300L (79.3 US Gallons) DHW thermal storage tank and only one shower (2.5 US GPM) happening at a time, the table at the right shows the time (in minutes) it takes the temperature of the water in the tank to drop from 55° (131°F) to 43.33°C (110°F). Below 110°F, DHW water won't be useful for showering.
Ex: When Outdoor temperature is -20°C (-4°F), it will take 16.92 min for the tank temperature to drop from 131°F to 110°F when DCW is entering tank at 35°F.
Volume of DHW extracted from the tank at Temperatures above 110°F
The amount of DHW in litres that can be extracted from a 300L tank, when ATW HP is running is 151.67 Litre when outdoor air temperature is -25° (-13F) and DCW is entering the tank at 4.44°C (40°F).
The higher the outdoor temperature is the more DHW can be extracted from the 300L tank.
Larger Volume tank offer more DHW for usage since the internal energy of water contained in the tank is higher.
DHW Tank Temperature Recovery Time from 110°F to 131°F
Recovery time is defined by the time it takes to heat up the tank from entering DCW temperature to 55°C (131°F).
We assume that tank has been cooled down to the same temperature as the entering DCW temperature.
When outdoor air temperature is 7°C (19.4°F) and entering DCW temperature is 10°C (50°F), it takes 52.49 minutes for the heat pump to heat the tank up to 55°C (131°F).
Conclusion.
Click on the icon on the left to view the performance of our three ATW heat pump models with 100, 200, 300, 400 and 500L thermal storage tanks.
HP and Tank Capacity combinations determine how much DHW can be used in a specific timeframce. Properly sizing both tank and HP is crucial for saving energy and money.
For more complex applications, please contact our engineering design team either by email or at our toll free number +1 (888) 686 7652 (Extension 1).
Further readings
- Enhanced Vapor Injection for Air to Water Heat Pumps
- Air To Water Heat Pumps Savings And Paybacks
- The Return of Hydronic Heating and Cooling to Small and Medium Size Buildings
- How to Prevent Condensation in Radiant Cooling Applications?
- How do Solar Thermal Heating Works?
- Vacuum Tube and Hybrid PVT Panels Comparison for North America