On Friday, July 10, 2015 at 1:38:25 AM UTC-4, HerHusband wrote:

Someone other than HerHusband asked:


Does any tankless hot water heater monitor the flow rate and output
temperature and then adjust the flame to keep the temperature constant
regardless of the flow?

Yes, per the site linked to below, they do exist. I quote, but offer no examples:

"Some types of tankless water heaters are thermostatically controlled; they can vary their output temperature according to the water flow rate and inlet temperature."


I have no idea, but a standard tank model doesn't do this either. You set
a maximum temperature and the heater heats the water to that level
(typically 120 degrees). A tankless heater isn't really all that different.
It's just heating the water as it comes in instead of preheating it in a
big tank. Either way the water should always be 120 degrees when it leaves
the heater.

Not so. Flow Rate is known drawback of tankless WH. Well, maybe not a drawback, per se, but a key factor in sizing a unit. It's a drawback in the sense that you may need to purchase a bigger/more expensive unit based of your flow rate/temperature rise requirements.

The more flow you demand, the lower the water temp at the output because the water is not in "contact" with the burners for as long a time.

Granted, flow rate will impact a tank heater also in that you will run out of 120 degree water sooner with a higher flow rate, but the difference is that regardless of how many showers you turn on, you *will* get 120 degree water out of the tank for some period of time. With a tankless heater, you may never get 120 degree water if the flow rate outpaces the heater's ability to impart the required temperature rise.

From: http://energy.gov/energysaver/articl...w-water-heater

SIZING TANKLESS OR DEMAND-TYPE WATER HEATERS
Tankless or demand-type water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you'll need for its application (whole house or a remote application, such as just a bathroom) in your home.