Tankless water heaters are different than storage water heaters.
Living with a gas tankless water heater is not the same as living with a tank type water heater. If your only experience with water heaters has been with storage water heaters, gas or electric, then living with a tankless water heater may take some getting used to.
Storage Water Heaters � Hot Water Temperature Characteristics
With storage water heaters when you turn on the tap the hot water immediately begins flowing through the hot water line on its way to the faucet. As the hot water flows toward the fixture it begins to lose some of its heat energy to the piping material. The more slowly the water travels the more energy is lost from the hot water. As the piping material warms up less and less heat is drawn out of the water.
This means that if your hot water pipe holds a gallon of water, then to get fully hot water at the fixture it could take up to one and a half gallons or more depending what material the pipe is made from, how long the pipe run is, what the diameter of the pipe is, what the flow rate is, and what the ambient temperature is.
Constantly Changing Temperatures
The temperature of the hot water is always changing. If the burner is on the hot water temperature is increasing, and if the burners are off the temperature is decreasing as heat is lost through the tank walls. At the faucet you mix the right amount of cold water and hot water to get the desired temperature.
The burners can’t keep up with even a small flow of water, so when you use the hot water the temperature drops fairly quickly at the bottom of the tank, and as the hottest water at the top of the tank leaves through the outlet and the colder water rises to the top to replace it the temperature begins to drop rapidly . After the hot water temperature begins to drop, you have to begin re-adjusting the hot to cold water mixture, and eventually you run out of hot water.
Since the temperature is constantly changing in the tank, you never know exactly what the temperature is going to be. The truth is the user usually doesn’t care, he or she just adjusts the temperature by mixing hot and cold until if feels right.
Reliability of gas and electric storage water heaters
Gas water heaters are pretty simple devices. They have a tank of course, a burner with pilot light and thermocouple, a combination thermostat gas valve, anode rode, PTR valve, drain valve, and that’s about it. Nothing complicated.
Usually the tank will give out first. A leaking tank often creates a puddle and when you discover the puddle you call the plumber for a replacement. No unexpected cold showers please.
If you are a do-it-yourselfer you can usually find replacement parts at your local appliance store or hardware store. If you aren’t a do it yourselfer then virtually any plumber will be able to fix or replace your water heater quickly.
Tankless Water Heaters
Tankless water heaters are a whole different breed of animal. To better understand some of the characteristics of tankless water heaters we need to examine briefly how they work.
You can picture a gas tankless water heater as a coil of copper pipe suspended over a burner. As you can well imagine, if you run water through the coil of copper tubing with the burner turned on, the water coming out of the tubing will be warmer than the water going in.
How hot the water gets depends on how long the water is exposed to the flames from the burner. A longer coiled pipe will result in hotter water and a shorter coil will provide lower temperature water at the outlet. That is because the water in a longer pipe will be in the flames longer. (Assuming the water is flowing of course)
If you speed up the flow the water will be in the flames for less time and won’t heat up as much and conversely if the flow is reduced the water will be exposed to the heat longer and will get hotter.
Let’s assume that we have a coil of pipe in the flames and as it passes through the flames the water increases from 50 degrees to 100 degrees for a 50 degree rise in temperature. Now if we hold the flow steady and decrease the inlet water temperature to 40 degrees, the heater will still be increasing the temperature of the water by 50 degrees, but the outlet temperature will drop to 90 degrees. Any change in inlet temperature will result in a corresponding change in outlet temperature.
Now think back to the storage water heater… a change in inlet temperature will not change the outlet temperature, nor will a change in flow rate affect the temperature.
Tankless water heaters have a variety of sensors, valves and computer chips so that they can control the size of the flames depending on the flow rate, incoming water temperature, outlet temperature, gas pressure etc. They even monitor the temperature of the exhaust gas. The water heaters computer modulates the gas valve taking into account all the variables and is thus able to produce a very consistent outlet temperature even when water temperature changes and the flow rate changes.
With a tankless water heater you set the temperature you want from the outlet and the water heater modulates the size of the flames and sometimes the flow of water to keep the temperature at the set point temperature. You can run hot water 24 hours a day and the temperature will remain rock steady even with fluctuations in water temperature and flow rate. You can have unlimited hot water forever if you have enough money.
Tankless water heaters have a maximum flow rate which if exceeded will cause the outlet temperature to drop below the set point temperature. The higher the flow above the maximum, the cooler will be the water at the outlet.
Tankless water heaters also have a minimum flow rate. There is a flow switch in the water line that requires a minimum flow rate of typically 1/2″ to 3/4 gallons per minute depending on the make model and brand of water heater. The heater will not turn on if the flow rate is less than the minimum and if the heater is on and the flow rate drops below minimum the heater will shut off.
Remember, the lower the flow rate the higher the temperature of the water leaving the coil. If the flow rate is too low, the water can become too hot and the heat exchanger (coil of tubing) can become damaged and even blow up.
Obtaining the right temperature can sometimes be critical to the application. For instance, let’s assume you have a low-flow shower head and it only has a flow rate of 1 gallon per minute. Let’s further assume that your water heater requires 3/4 gallons per minute flow to stay on. Now if you were to have the tankless outlet temperature set too high the following scenario could occur.
You get in the shower and the temperature is a little too warm for your comfort so you move the mixing valve to supply more cold water and less hot water. However, since there has to be at least 3/4 of the flow coming through the heater if or it will shut off. If you dropped below that then get ready because your shower is about to become instantly cold!
The bathroom sink faucets are often have a flow rate of one gallon per minute. That means that to just keep the heater on you have to run the hot water faucet at 3/4 full. If you want only warm water you have to add more cold water to the mix and you will probably end up with the full 1 gallon per minute flow. This is not a good way to conserve water.
If you have a dish washer you may want to set the water to 140 degrees. Getting warm water at low flow fixtures could be a problem. Perhaps that is why tankless water heater manufactures usually provide a remote control for adjusting the water temperature.
Various conditions can change and require periodic readjustment of the outlet temperature. In many locations the cold water temperature is substantially different between summer and winter and thus the outlet temperature will need to be changed as the seasons change.
Tankless water heaters have a lot more parts and are far more complex than storage water heaters. Typically today’s tankless heaters have dozens of error codes to help you troubleshoot problems. A tankless heater does not give you warning that it doesn’t work. You just suddenly can’t get hot water. There are many things that can cause the water heater to not work, like low gas pressure, high vent temperatures, high heat exchanger temperatures, insufficient air flow etc.
In order to supply continuous hot water tankless hot water heaters have to have much bigger burners than storage heaters and need larger gas supply lines. If the tankless heater is electic it will probably need its own extra heavy wiring and possibly require a service upgrade from your utility company.
With gas tankless water heaters the exhaust vent will need to be substantially larger than the vent for a storage water heater.
Sediment Vs Scale buildup
If you live in a hard water area then you probably get sediment buildup in your water heater. Over a few years you can get quite a bit of sediment and it will begin to reduce the amount of hot water available from the heater. At some point you may have to drain the heater and flush out the sediment.
With a tankless water heater the hard water will cause scale buildup in the heat exchanger. The scale buildup increases the pressure drop through the heater and causes the flow rate to lower which leads to overheating and heat exchanger damage. Tankless water heaters in hard water areas usually need to be flushed out and de-scaled once a year.
Operating a tankless water heater with very hard water will void the warranty. In extremely hard water areas tankless water heaters can only be used if they have a water softener supplying the cold inlet water. Check the owner’s manual for the maximum ph the water can be without voiding the warranty.
Hot Water Circulating Pumps and Hot Water Demand Systems
Many homeowners are installing hot water circulating pumps in their water heating systems. Traditional hot water circulating pumps usually are incompatible with tankless water heaters, but can save tremendous amounts of water when used with a storage tank type water heater.
Hot water demand pumps that require activation each time they are used work with both storage water heaters and tankless water heaters. Tankless hot water heaters typically take 10 to 20 seconds longer to deliver hot water to the faucets than a storage heater.
The tankless water heater has to first heat the water requiring the water to pass completely through the heat exchanger before fully hot water begins its way to the fixture whereas with the storage heater the hot water is waiting and ready to go instantly when the fixture Is opened. This makes tankless heaters excellent candidates for hot water demand systems.
Hot water demand systems do not waste any energy like traditional hot water recirculating systems do.