A whole house tankless water heater works in the same way as a point of use model, but it has more powerful gas burners or electric heating elements (in the case of a whole house tankless electric water heater). Because of its more powerful heating system, it can cope with higher flow rates of cold water through its heat exchanger, and consequently can supply hot water to more than one hot water fixture at the same time.
The heating systems of some models are more powerful than others. For example, a whole house water heater that is rated to handle 7.5 gallons per minute (gpm), will be able to provide sufficient hot water for a household to run two showers and a dishwasher at the same time. A 2.5 gpm water heater would only be able to heat sufficient water for one shower at a time. If two showers are turned on it will deliver lukewarm water.
Under-sizing a whole house water heater is a very common mistake that buyers make. If you choose a tankless water heater whose flow rate capacity is less than your peak household demand, you will not have sufficiently hot water during peak usage times. If you purchase a tankless water heater that exceeds your needs, you will be paying more for your water heater than you need to.
Determining Your Hot Water Demand
To determine the right size whole house tankless water heater, you first need to know what your maximum hot water demand is at peak usage times. This demand is the sum of the flow rate requirements of all the hot water fixtures you want to use simultaneously. If the flow rate of any fixture is not inscribed on the outside of the fixture, you can easily determine its flow rate by measuring how many seconds it takes to fill a gallon container, with its faucet opened to its normal position. Then divide 60 by the number of seconds, to determine the flow rate in gallons per minute (gpm). If you need assistance you should consult your plumber.
Selecting Your Hot Water Temperature
Many tankless water heaters can heat water to at least 145 degrees Fahrenheit. However, it is desirable to select the lowest hot water temperature to meet your needs, for you will then be able to purchase a smaller unit, and your operating costs will be less.
A comfortable shower or bath temperature is around 104 or 105 degrees Fahrenheit. For hand washing you won’t need water that is hotter than 110 degrees Fahrenheit. If a hot water temperature of 110 degrees satisfies your needs, then it does not make sense to set your hot water temperature to 125 degrees and then add cold water.
Your Groundwater Temperature
The colder the groundwater coming into your water heater, the more powerful the whole house tankless water heater must be, to raise the temperature to your desired hot water temperature. Therefore, if you live in a cold climate, you will need a higher capacity water heater than if you live in a warm climate. Conversely a particular tankless model will be able to serve less hot water fixtures at the same time in a cold climate than in a warm climate.
To ensure that you do not run out of hot water during the coldest time of the year, you should select your water heater based on your lowest groundwater temperature.
Whole House Tankless Water Heater Rating System
The difference between the temperature of the hot water leaving the water heater, and the temperature of the cold water entering the water heater, is called the “temperature rise”.
If you want 110 degree hot water and you live in Florida where the groundwater temperature is 75 degrees, then you need your water heater to be able to produce a 35 degree temperature rise. If, however, you live in Wisconsin, and the coldest groundwater temperature is 40 degrees, you will need a 70 degree rise to bring the water to 110 degrees.
Tankless water heater capacities are rated in gallons per minute (gpm) for a particular temperature rise.
So, in selecting a whole house tankless water heater, you need to find a model that can handle your peak demand (in gpm) for the maximum temperature rise in your area.
For example, the Stiebel Eltron Tempra 36 has a rating of 7.0 gpm for a temperature rise of 35 degrees. This means that in a warm climate where the groundwater temperature is 75 degrees, and the desired hot water temperature is 110 degrees, this electric tankless water heater would be able to simultaneously serve 3 hot water fixtures with a flow rate of 2.3 gpm each. For example, these 3 fixtures could be 2 showers and a dishwasher, or 1 shower, a dishwasher and a kitchen sink.
At the average US groundwater temperature of 55 degrees, the Tempra 36 would be required to produce a temperature rise of 55 degrees to deliver 110 degree hot water. For a 55 degree temperature rise this water heater is rated at 4.6 gpm. It will therefore only be able to simultaneously serve 1 shower and a bathroom sink at this groundwater temperature. In the snowbelt, it would only be able to serve 1 shower at a time in mid-winter.
The Stiebel Eltron Tempra 36 is one of the highest capacity electric whole house tankless water heaters. Tankless gas models have much higher capacities.