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Navigating through all the hype about tankless water heaters is not easy. Endless hot water, and energy savings being the two most often touted benefits. I doubt many people need "endless hot water"! With the recent rebate programs being implemented, and rebates of 30% and up to $1,500, saving energy and being green are probably achievable goals in many cases.
Water heater companies, both tankless and storage, recommend sizing
water heaters for multiple fixtures. I don't feel this is necessary.
If I am going to take a shower, my family members know enough not to
start a load of laundry or turn on the dishwasher. I'm sure there
are large families that would find it desirable to be able to have
two showers going at once and maybe a sink too, during occasions
where everyone was getting ready to go on a family outing or
something. But I think most families don't need a water heater big
enough to fill a hot tub, or to run three fixtures at once.
For most families I think running one shower would be sufficient for sizing a tankless water heater, or any other kind of water heater. I suggest that if you are considering the purchase of a tankless heater that you take a 1 gallon milk jug and measure the flow rate of the fixtures you use regularly. Like the shower head and the kitchen sink. The bath tub fixture will have the highest flow rate.
Just measure how long it takes to fill the jug from the hot water
faucet, and then divide the 1 gallon by the number of seconds it
takes to fill it, and then multiply by 60 to get the
gallons/minute. If you want to be able to use more than one
fixture at a time, then just add the flow rates together. This
should provide you with a very conservative figure, since you
usually don't run just hot water for things like showers, it's a
mixture of hot and cold.
The next important detail you need to know is how much temperature rise you need. Measure the temperature of the cold water coming into your house. Do this by running cold water long enough to get water from the source, whether it's the water main or your well. Fill a cup and put a thermometer in it.
Unfortunately it's best to do this both in winter and in summer, in case there is a big difference in cold water temperature between the two seasons. In some areas there is and others there is not.
Measure the hot water in a similar fashion. I would let the hot water run for a couple of minutes to make sure you have reached a stable temperature.
If you want your water hot enough for dishwasher's recommendations, then subtract the cold water temperature from 140 to get your need temperature rise. Otherwise pick the highest temperature you would need from the water heater and use it.
Keep in mind that in many areas the water can be much colder in the winter than in the summer. Use the coldest temperature that you expect to be entering your water heater.
Now armed with your maximum flow rate and maximum required temperature rise you can find a model you like. Unfortunately there is little standardization in the way the various tankless water heater manufacturers present their data. Some provide flow rates for 45 degree rise, some 77 degrees, and some for 100 degrees.
All the brands have roughly equivalent specifications...some are
slightly more efficient than others but that only changes the flow
rate marginally, and of course, the price tag.
One of the most common problems with tankless water heaters is one of regulating the temperature. Most gas tankless water heaters now have modulating gas burners, and produce a good steady temperature at the outlet. But if you exceed the maximum flow rate for the temperature rise you need, the heater won't be able to keep up. At that point any change in flow will result in a change in temperature.
Another problem results if you tankless heater is to big for your application. The reason for this is that the flow of hot water is what turns the unit on. The turn on flow varies with make and model, but generally falls between 1/2 gallon per minute to about .9 gallons per minute.
So if you are taking a shower, and you want to reduce the temperature some, you turn the knob that changes the mixture of hot and cold water. Your shower valve mixes a little more cold water and a little less hot water to get the desired reduction in temperature. But if the heater is too big and producing too much of a temperature rise, then when you reduce the flow of hot water you risk shutting off the burners. A minute or so later the colder water hits, so you turn up the mix and a minute later you again get water that is too hot.
Be sure to look at the minimum flow rate that it takes to turn on
the heater and factor that into the decision.
Typically gas tankless hot water heaters take a little longer to get hot water since the burners don't turn on until hot water is flowing, and then the water has to travel completely through the heat exchanger before actual hot water is flowing through the pipes.
The solution for this is to install a Chilipepper hot water demand pump and not only get your hot water faster, but don't run any down the drain while you wait. By combining a tankless water heater and a Chilipepper demand system you get very green water, i.e. water conservation and reduced green house gas emissions.
More information about tankless water heaters. Tankless Water Heaters