Founder | BPI® Energy Auditor Certified Professional
Heat pump water heaters have become increasingly popular in recent years thanks to their energy efficiency and quiet operation.
Getting an efficient heat pump water heater that meets your family’s needs boils down to picking the correct size.
A correctly sized heat pump water heater will meet your household’s hot water needs while operating within its most efficient output range, saving you money on utility bills.
To correctly size your new heat pump water heater, determine the maximum amount of hot water your household needs per hour and match this to the heater’s first-hour rating. These factors will help you get a water heater that efficiently provides sufficient hot water for your household.
In the rest of this article, I’ll take you through the process of correctly sizing a heat pump water heater.
This will ensure you get a heater that can serve your family efficiently and provide plenty of hot water when you need it throughout the day.
Let’s get started!
1. Determine Your Household’s Peak Hour Demand
The maximum amount of hot water needed for your household (peak-hour demand) is crucial for correctly sizing your new heat pump water heater.
You can determine your household’s peak-hour demand by first figuring out the time of the day when you use the highest amount of water.
Here is a simple guide to determining your household’s hot water peak hour demand:
- Determine the time of the day when you use the maximum amount of hot water. It can be in the morning, noon, or evening. You should also consider the number of people in your household and their different activities that involve using hot water.
- Calculate the amount of hot water used in your household in the peak hour you determined above based on gallons consumed by each activity cumulatively.
The table below shows an example of calculating the household’s peak hour water usage.
| Peak Hour Use | Number of Utilities | Consumption Per Utility | Total Amount in Gallons |
| Showering | 2 | 20 | 40 (151.41 liters) |
| Laundry | 1 | 25 | 25 (94.63 liters) |
| Dishwashing | 1 | 15 | 15 (56.78 liters) |
| Totals | 4 | 60 | 80 (302.83 liters) |
Table 1: Determining peak-hour demand by cumulative usage
This household has a peak hour demand of 80 gallons (302.83 liters) from the table above.
Determining Peak Hour Demand From Each End Use Flow Rate
You can also estimate your peak hour demand by checking the flow rate of all your hot water end uses. Some of the end uses to look for are:
- Faucets
- Showers
- Dishwashers
- Washing machines
- Jacuzzis
The flow rate is the gallons per minute (GPM) that a particular activity uses.
For example, an old showerhead may have a flow rate of five gallons (18.93 liters) per minute, while a new low-flow showerhead has a flow rate of just two GPM (7.57 liters per minute).
After determining the different end uses and the amount of water used, add up the water uses that account for your peak hour.
For instance, let’s assume your household has two people who take 8 minutes showering back to back.
The average flow rate of a showerhead is 2 GPM (7.57 LPM). Therefore, you’ll need 2 x (2 x 8) gallons of water for showering alone. This translates to 32 gallons (121.13 liters).
You may also want to shave after showering. Your sink has an average flow rate of 0.5 GPM (1.89 LPM), and shaving takes three minutes. Therefore, you need another 1.5 gallons (5.68 liters).
Now, after having breakfast, you want to wash your utensils. Your dishwasher has a flow rate of 3 GPM (11.36 LPM) and takes 5 minutes to fill. In this case, you need another 15 gallons (56.78 liters).
From the above demonstration, your hot water peak hour demand is (32+1.5+15) = 48.5 gallons (183.6 liters).
2. Determine the Suitable Heat Pump First Hour Rating
After calculating your household’s peak hour demand, it’s time to determine a suitable heat pump water heater for your home.
A suitable heat pump water heater for your family is based on the heater’s first-hour rating.
A heater’s first-hour rating is the amount of hot water it can produce in an hour and is usually labeled as “First Hour Rating” or “FHR.”
The FHR is different from the storage capacity because it considers the rate at which the heater can heat water and deliver it during peak hours.
For example, a 50-gallon (189.27-liter) heat pump water heater with a first-hour rating of 65 gallons (246.05 liters) can provide your household with 65 gallons (246.05 liters) of hot water in an hour.
However, a heater with the same 50-gallon (189.27-liter) storage tank might have a first-hour rating of 80 gallons (302.83 liters) and be able to provide your family with 80 gallons (302.83 liters) in the same one hour. It all depends on the heater’s recovery rate.
As you can see, it’s essential to consider the FHR when sizing your heat pump water heater to ensure an adequate hot water supply during your household’s peak hour.
The FHR is usually printed in the product manual or on the appliance itself. Don’t hesitate to contact the manufacturer if you can’t find it.
To size your heat pump water heater correctly, you must choose a model with an FHR equal to or greater than your household’s peak-hour demand.
For example, in the above case, the household’s peak-hour hot water demand was 48.50 gallons (183.59 liters). Therefore, the correct heat pump water heater for this household is one with an FHR of 50 gallons (189.27 liters).
A heat pump water heater with an FHR of 40 gallons (151.42 liters) will be insufficient for this household. Such a heater is 8 gallons (30.28 liters) less than the household’s needs for their peak hour demand.
On the other hand, going for a 60-gallon (227.13-liter) heat pump water heater will be a waste of money since the family needs only 48.50 gallons (183.59 liters). Remember, the bigger the size, the more the cost.
This demonstration means you should go for a heat pump water heater with a higher first-hour rating if you have a big family or many friends visiting frequently.
Conversely, you can save money by choosing a smaller model with a lower FHR if you live alone or have a small family.
3. Consider the Available Space
Although the space available and the heater’s physical size don’t affect your family’s water availability, it’s essential to consider it for installation.
Heat pump water heaters are usually taller and wider than their electric counterparts.
You also need to consider the availability of servicing space. For example, you need to have 18 inches (0.46 meters) of space at the back and sides of the heater for easy servicing.
Thus, you should consider an electric tankless water heater if you live in a small home or apartment with limited space. These water heaters are compact and can be hung on the wall to save floor space.
The Energy Factor (EF) in Heat Pump Water Heaters
The energy factor of a water heater refers to the ratio of the amount of hot water produced by the heater to the total energy consumed. The higher the EF, the more efficient the heater.
For example, an electric heat pump water heater with an EF of two produces two gallons (7.57 liters) of hot water for every kilowatt-hour (kWh) of electricity consumed.
This means that such a heater will use half the energy of an electric water heater with an EF of one to produce the same amount of hot water.
The Uniform Energy Factor (UEF) is a more recent efficiency metric that considers the energy required to heat the water and the standby and self-cleaning energies needed for some models. The higher the UEF, the more efficient the water heater.
Final Thoughts
A heat pump water heater’s first-hour rating and your household’s peak hour demand are the essential considerations to keep in mind when sizing your new heat pump water heater.
Your household’s peak hour demand considers your hot water usage, including all the utilities and household size.
Ensuring your household’s peak hour demand can be met by your heater (according to its first-hour rating), you get a correctly-sized heat pump water heater that will serve your family efficiently.