Tankless water heaters, also known as instantaneous or on-demand water heaters, have become pretty popular lately. I mean, who can resist the allure of "never ending hot water"! But while they may be more efficient than a standard tank-style water heater, do some careful analysis before you go ripping out your current tank to install this solution.
Chances are your current method of heating water is a big insulated tank that either has an electric or natural gas burner. The tank has a thermostat and when the water in the tank gets below a certain temperature, the boiler kicks on and heats up the water. As with any system, heat is constantly lost through the walls of the tank (remember, heat flows from hot --> cold). Although if you have good insulation this heat loss should be minimal. But it is this "stand-by" heat loss that lowers the efficiency of a tank heater compared to a tankless style water heater.
An "on-demand" water heater does just that: heats up the water at the instant it is demanded by you when you turn on the "hot" faucet. The picture above is a great diagram of an instantaneous hot water heater. When you turn on your hot water, the water comes straight out of the pipes in the ground and flows through coils that are in a heat exchanger that surrounds the flame.
This is more efficient than a tank because you don't have the stand-by losses that occur when you have a whole bunch of hot water just hanging around waiting to be used in your shower or dirty dishes. The water is heated only when it is needed, and never any other time. Energy Savers (sponsored by the Department of Energy) has a great analysis of how tankless water heaters work seen here.
In Fall of 2008 Consumer Reports did an analysis on tankless water heaters with the summary: "They're efficient but but not necessarily economical". Some of their drawbacks and a few others are listed below:
- In order to activate the burner, demand water heaters need good pressure at a minimum flow rate. This means that if you just need a tiny bit of hot water running while doing dishes you will need to keep it on a higher flow rate.
- Sometimes water can run hot, then cold, then hot (cold-water sandwich)
- High up-front costs
- Because the burner has a maximum amount of heat it can supply at once, demand water heaters have a maximum flow rate through the burner. Meaning if one person in your house needed to take a shower while the dishwasher was going, the load would be too big on the instantaneous water heater and it would not be able to supply hot water to both! So while technically you may never run out of hot water, you wont be able to use too much of it at any one instant
But doesn't the advertisement "never ending hot-water" really defeat the whole concept of energy efficiency? While there are only two people living in my household (my wife and I) we have only ran out of hot water once. And that was when I experimented with turning down the heat on our standard hot water tank, and resulted in a cold shower for an unhappy wife. We do have a 50 gallon tank, which may be a tad too big for just two people?
This guy has some good recommendations on his blog, A Concord Carpenter, about his experience with his Rinnai tankless water heater:
I have a RINNAI brand on demand heater in my house and properly used these heaters are great. Improperly used they are disappointing and expensive.
To avoid disappointment, make sure to size the unit correctly for your house water usage. The size and number of whole house tankless water heaters you need will be largely driven by flow rate and that is determined by the number and types of fixtures you may have running at one time. My unit can only handle one shower and one faucet or the dishwasher at one time.
The key point to take away from that quote is that if you do decide to go with a tankless water heater, SIZE THE UNIT CORRECTLY!
While tankless water heaters do have a niche, don't assume they will work for every application just because you see them being marketed everywhere. You may be better off with a standard tank with solar thermal back up! You would definitely want to consider a tankless heater if you have a guest house or a cabin that is rarely used.
In later blogs I will do more cost/benefit analysis comparing the different methods of heating water and which one (tank, solar thermal, tankless, hybrid) really works out best in the long run taking into account energy prices, up-front costs, efficiencies, etc.