Water – Buffer tank on input or output of tankless water heater

energy efficiencytanklesswater-heater

As I understand it, in residential systems, tankless water heater are commonly paired with a buffer tank to deal with the "cold sandwich" and finite capacity of tankless heater1. This seems reasonable, but I'm curious as to why the only config I see is a buffer tank at the output of the tankless. That is, why is a buffer at input of tankless heater seemingly not an option?

It seems to me that keeping a buffer of hot water at the output negates one primary benefit of a tankless system: energy-efficiency. As such, the system is obligated to keep the reserve hot 24/7, thereby generating energy waste.

On the other hand, keeping a reserve at the input, the system can selectively heat the reserve when the demand is expected to be the highest (a couple of hrs a day?) and neglect it at other times. And in the worst case, if demand is unpredictable, the system could simply be programmed to maintain a hot input buffer 24/7, just as with buffer at the output. But in either case, an additional opportunity for efficiency with an input buffer is by maintaining a lower reserve temperature.

Under limited circumstances, I think an input buffer tank may not be an adequate solution to the "cold sandwich", but assuming the system could be programmed to minimize the problem during certain time of day, in a typical residential system, is there a really compelling reason to keep buffer at tankless output? Is there historical or empirical data to back the choice to put buffer on output?

Best Answer

A buffer tank must be on the output side of a tankless water heater. If you put the tank on the input side a bunch of unintended consequences happen:

  • The buffer tank will do the bulk of the water heating, until it is overwhelmed.
  • As long as hot water use continues, the buffer tank will continue running as it tries to keep up with raising the incoming water to its set temperature.
  • You might still get a "cold water sandwich" as the buffer's hot water supply runs out. That's because your tankless will be sitting around doing nothing at all while the hot water runs, because the water's already heated! When the buffer runs out the temperature drops and the tankless kicks on (but might need more time to warm up fully).

On the other hand, when you put a buffer tank on the output side of the tankless, sustained use of hot water will shift most of the work to the tankless. It will be heating the water before it reaches the buffer, and the buffer will do nothing during those extended uses of hot water where the tankless is most efficient.

If you want to add a timer on the buffer, the way to do that would be to both shut off the buffer tank's heating and route the water around the buffer (rather than through it), using some electronic 3-way valves.

Please note, however, that there are better solutions to the cold water delay problem. For example:

  • Use a demand-activated recirculation system. You have to train your family to press a button a minute or two before they want to start a shower, but it's pretty nice having the water hot immediately after.
  • Configure your tankless water heater to use a recirculating mode that keeps its heat exchanger warm. This will consume more energy, but is designed to ensure it's pre-heated during periods of likely demand. My Navien CH-240, for example, can be configured to run in recirculating mode during specified 30-minute intervals via its remote control panel.
Related Topic