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Small Cars and Tankless Water Heaters - A Comparison

We are trying to "think green" and many of us are buying products that consume less energy. Are economy automobiles and tankless water heaters similar?

The ways in which consumers select them are similar and the answer is often "yes".


We are thinking of tankless heaters in respect to the fact that they use substantially less energy simply because they only use energy when they are making hot water. As tankless water heaters are sized by their abilities to raise the temperature of water and send it out of the heater in a specified gallons per minute capacity (Or liters per minute) they are a different concept than the typical tank type heater. Once a person determines the gallons per minute (GPM) that are needed for an application, a tankless heater can be selected. Tankless heaters come in many sizes and different types of design and construction.

Selecting a tankless heater is in some ways similar to buying a new car. Many people are looking at small cars because they consume less fuel for each mile driven. Maybe they are trading-in the "gas guzzler"? But certainly most people realize that they are reducing the size and power when they switch to a small car? For example; when one wants a car that gets tremendous miles per gallon, typically they have a very small engine and the car itself is also very small. As the horsepower and size are enlarged, then the economy goes down. This is the trade-off. Certainly one cannot expect a small inexpensive economy car to have comforts, space and performance of a much larger automobile. When one purchases an economy car they should realize that most are intended for commuting to and from work. When operating a small economy car while driving alone, the performance may in fact be impressive, but as passengers and materials are loaded into the interior, the weight increases and the performance rapidly drops. If a person takes the family and all their travelling necessities on vacation and expects to drive this little car across the continent in comfort, they are often disappointed.

The same basic rules apply to tankless heaters. Smaller ones can work well, but when they are tested to their capacity they sometimes disappoint the user. But this is the same for a tank-type heater as well. For example, a 40 gallon tank-type heater holds that water in reserve pre-heated. If a person uses three showers at once and each shower-head consumes 2 GPM, then there is a 6 GPM draw on the tank-type heater.. The math is simple, that big 40 gallon tank of water will be expended in only six and one half minutes! And you may have thought that your current 40 gallon heater was big! But use two showers and there is only a 4 GPM draw and the supply will last ten minutes. A 4 GPM tankless heater may in fact run two showers, but this depends upon the ground water temperature. As the way in which tankless heaters are sized also depends on how many degrees in "temperature rise" the heater must achieve. People who live in Northern climates (especially in the winter) receive less GPM from a tankless heater than those who live in Southern climates.

This is the way it is, but many may not realize that this rule applies to tank-type heaters as well. Even though a tank type heater must recover the full tank of water and reheat it to the desired temperature which takes time. The fact is that like a tankless heater, this older tank-type technology also changes the temperature from the incoming ground water and raises it to the pre-set desired temperature. And again, there is that same calculation of temperature rise. So, this means that people in Southern climates can expect that their tank-type heater will recover faster than those who live in the North. This is the reality and there is nothing that can change it. For this reason, the ground water temperature is a constant variable that must be considered for those who are shopping for any type of water heater.

What about this temperature rise thing, why is it important?

All manufacturers specify the capacity of their tankless heaters in GPMs with a specific temperature rise. Most companies use the "45 degree rise" calculation which makes the GPM number much higher. But the reality is that typical ground water temperature in the middle of the continent is 55 degrees. A 45 degree rise will only deliver 100 degree water to the user. As 105 degrees is considered comfortable for showering and 120 degrees is the typical desired temperature of users in North America. A 45 degree rise is usually insufficient. A 70 degree rise is more realistic but with this calculation the GPM capacity will be reduced substantially. (This is the reason that we have placed temperature rise and GPM charts on our website.)

What does all this really mean when I am shopping for a heater?

Once again we can take a lesson from the automobile manufacturers. A typical economy car may have a combined rating of 38 MPG city/highway. But we know that real world users rarely see numbers this high. Since this variable is industry wide, it can be assumed that all cars and trucks actually get lower miles per gallon than specified which makes things relatively equal.

If a person wants to buy a less expensive economy model tankless heater that produces GPMs close to the needs of the user, then it can be assumed that there will be times when the capacity is exceeded by unexpected demands or that it simply does not perform in the ways in which the buyer visualized. If an economy car is driven in the way in which it was designed and for the purpose intended, this economy car delivers the desired results. But it is not a large car capable of hauling large loads and delivering higher performance. After all, it is an "economy" car and it should be driven in an economical way. If a consumer really needs a large car because of passenger and load needs or wants high performance, then buying a small "econobox" car will only lead to occasional or possibly frequent disappointment. These same basic rules apply to any type of water heater, tank or tankless.

Elsewhere on our website we explain in detail the ways in which to select a model to fit your specific needs. We explain in detail how to measure the GPM needs, what are the parameters of installation and typical municipal, state and federal rules and regulations.