Tank Vs. Tankless Update
With “green” the latest buzzword, there has been a resurrection of the debate on tank-type water heaters versus tankless water heaters.
I have an engineering friend who reduces the debate to a simple statement: “It doesn’t matter. One Btu heats 1 pound of water 1 degree F, whether it is a tank-type or tankless water heater; you can’t cheat the laws of physics.”
In many ways, he is correct. It doesn’t matter how you apply the Btus; it takes the same amount to raise a given amount of water a given temperature. The difference would be in the efficiency of the water heater. If the tank-type water heater had an efficiency of 70 percent, it would take 1.43 Btus of input to raise 1 pound of water 1 degree F. If the tankless water heater had an efficiency of 80 percent, it would take 1.25 Btus of input. (I just made up these efficiencies to demonstrate the difference.)
No matter what type of water heater, the higher the efficiency, the lower the amount of fuel used to heat the water. This is completely independent of tank versus tankless.
Another claim made in support of tankless water heaters is that there is no standby loss. If hot water is not being used, a tank-type water heater has a heat loss through the jacket of the water heater. This energy loss is minimal, but it still exists.
The System CountsWhat really needs to be evaluated is the design of the hot water distribution system and the use of the system. This has more to do with the pros and cons of the two types of water heaters than does standby loss. Plus, when you compare the two types of water heaters, you must properly size the water heaters for the intended use.
In many buildings, a hot water recirculation system is installed. The recirculation system provides the user with a faster arrival of hot water from the plumbing fixtures. In many McMansions, recirculation is necessary, if not required, by the plumbing code. In many commercial buildings, hot water recirculation also is installed.
When you circulate hot water continuously in a building, the standby loss is in the piping system. This accounts for the majority of heat loss. This piping heat loss is the same for a tank-type water heater as it is for a tankless water heater.
When you continuously run water through a tankless water heater, the water heater burner will continuously operate. Hence, for a hot water distribution system with a recirculation loop, there is no savings in energy between a tank-type and a tankless water heater, other than the efficiency of the unit.
Many of my engineering colleagues think it is dumb to install a tankless water heater for these types of installations. I wouldn’t go that far. I would say that you can make a choice. If you are wondering about the economics, a tank-type water heater for this installation will blow away a tankless.
When it comes to use of the water heater, this can have a significant impact. DINKs (double income, no kids) will save more energy by using a tankless water heater than a stay-at-home mother or father with children of younger ages. In the second scenario, water is being used throughout the day. This minimizes standby loss.
For very intermittent use, such as a football stadium, a tankless water heater really makes sense. The stadium is only being used during football games. If a tank-type water heater is installed, it will store all of that hot water for long periods of time without use.
From a purely energy standpoint, a tankless water heater will use less energy when compared to a tank-type water heater. But in many cases, it is more dependent on the higher efficiency than on the concern for standby losses.
It's The EconomyWe need to look at the overall economy of the two types of water heaters, as opposed to the energy use. Unless you have customers that are independently wealthy, you always have to look at the overall cost of the system.
A tankless water heater will always be more expensive to install. The units cost more, the installation is more involved, and two trades are required - a plumbing contractor and an electrician. Most gas-fired tankless water heaters have a special vent system. The units cannot connect to a type B vent.
On the plus side, tankless water heaters have a life expectancy of double that of a tank-type water heater. That being said, the double life expectancy does not make up for the difference in initial costs.
The cost difference between installing a tank type water heater compared to a tankless ranges from two-and-a-half to four times the cost. Hence, if it costs $1,000 to install a tank-type water heater, it will cost $2,500-$4,000 for a tankless water heater installation. Of course, the actual price depends on the model selected and the labor rate you charge.
If you are looking for an immediate payback for a tankless water heater, it isn’t going to happen. Any payback will take many years to be realized, if ever. However, if I had to guess, I would say that the price of tankless water heaters will get more competitive.
Benefits To BothThere are benefits to both types of water heaters. Don’t select the water heater purely on the economics or energy savings of the water heater. Analyze the system installation, the application, the fuel costs and the demand on the water heater. Using all of these parameters, decide what is best for the installation.
I try to avoid general statements of one water heater being better than another. Both tank-type and tankless water heaters have a future in this country. Know the pros and cons of each water heater, and select the best water heater for your customer’s installation.