The correct sized water heater will operate efficiently while meeting all of the hot water needs for your household. Therefore, before you buy a water heater, make certain it’s the proper size first.
In this article we will be discussing how to size the following systems:
Tankless water heaters (demand-type)
Solar Water Heating System
Heat Pump (with tank) And Storage Water Heaters
If you still haven’t decided what kind of water heater may be the best one for your house, keep reading to learn more about choosing a new water heater.
How To Size A Demand-Type Or Tankless Water Heater
The ratings for demand-type or tankless water heaters are based on the maximum temperature rise at a certain flow rate. So in order to determine the correct size for a demand water heater, you will first need to determine the temperature rise and flow rate that is needed for its application (remote application like a just a bathroom or an entire house) in your home.
First of all, list how many hot water devices you are expecting to use at one point in time. Next, add their flow rates up (gallons per minute). That gives you the desired flow rate for a demand water heater. For instance, say you are expecting to run simultaneously a shower head having a 2.5 gallon per minute flow rate and hot water faucet having a 0.75 gallon per minute flow rate. The demand water heater’s flow rate would have to be 3.25 gallons per minute at least. Install low-flow water fixtures in order to reduce flow rates.
In order to calculate temperature rise, incoming water temperature needs to be subtracted from desired output temperature. Assume an incoming water temperature of 50 degrees F (10 degrees C) unless you have more accurate information. For a majority of uses, you are going to want the water to heat to 120 degrees F (49 degrees C). In our example, your demand water heater will need to produce a 70 degree F (39 degree C) temperature rise for a majority of uses. For applications like dishwashers that don’t have internal heaters, you may want to heat your water to 140 degrees F (60 degrees C). In this case, you are going to need to have a 90 degree F (50 degree C) temperature rise.
A majority of demand water heaters have been rated for various inlet temperatures. Usually it is possible to have a 70 degree F (39 degree C) water temperature rise at a 5 gallon per minute flow rate through a gas-fired demand water heat, and through electric ones at 2 gallons per minute. Cooler inlet temperatures or faster flow rates at times can reduce water temperature at the faucet that is the farthest away. Some kinds of tankless water heaters are controlled by a thermostat. Their output temperature can be varied according to inlet temperature and water flow rate.
How To Size A Solar Water Heating System
To size a solar water heating system involves figuring out the storage volume and total collector area you will need for meeting 90% to 100% of the hot water needs of your household during the summer. Computer programs and worksheets are used by solar system contractors to help with determining collector sizing and system requirements.
Usually contractors follow a 20 square feet (or 2 square meters) guideline for a collector area for each of the initial two family members. Then for each additional individual, 8 square feet (or 0.7 square meters) is added for residents within the Sun Belt area of the U.S. or 12-14 square feet for northern U.S. residents.
Usually a small storage tank (50-60 gallon) is sufficient for one to three individuals. A medium storage tank (80 gallon) is suitable for three or four individuals. For four to six individuals a large tank is needed.
For an active system, the solar storage tank size increases along with the collector size – usually 1.5 gallons for every square foot of collector. It helps prevent your system from overheating at times when there is low demand for hot water. In sunny, warm climates, it is suggested by some experts that this ratio should be increase by up to 2 gallons storage to a collector area of 1 square foot.
Some of the other calculations that are involved in getting your solar water heating system sized include determining the proper tilt and orientation of the solar collector and evaluating the solar resources of your building site. For more about these calculations, visit our solar water heaters page.
How To Size Heat Pump (With Tank) And Storage Water Heaters
In order to correctly size your home’s storage water heater – including a heat pump water heater that comes with a tank – the first hour rating on the water heater should be used. This first hour rating is how many gallon of hot water can be supplied per hour by the heater (beginning with a tank that is full of hot water). This will depend on what the capacity of the tank is, the size of the element or burner and the source of heat (element or burner).
The first hour rate is listed on the EnergyGuide label located in the top left hand corner as either first hour rating or Capacity. The EnergyGuide label is required by the Federal Trade Commission for all new conventional storage water heaters. However, they aren’t required for heat pump water heaters. The manufacturer’s product literature might also supply you with the first hour rating. Search for a water heater model that comes with a first hour rating matching within 1 to 2 galls of what your peak hour demand is – which is your home’s 1-hour daily peak hot water demand.
Estimating Peak Hour Demand:
Figure out what time (morning, afternoon or evening) of day that your home uses the most hot water. Remember to consider how many people reside in your house.
Use our worksheet to estimate maximum hot water usage during that one hour – that is the peak hour demand. Just note that daily total hot water usage is not estimated by the worksheet.
In our worksheet example, the total peak hour demand is 36 gallons. That means that the household will need to have a water heat model that has a 34 to 38 gallon first hour rating.