On 7/8/2015 9:30 PM, HerHusband wrote:

Tankless heaters are generally rated for a given temperature rise at a
given flow rate. If you draw water faster than it can heat it, you'll get
cooler water.

This isn't his problem. The problem is that tankless heater is not
heating to a specific temperature regardless of the flow rate so at low
flow the water is too hot. Changing the mix at the faucet for more cold
and less hot doesn't work because then the hot water, while lower in
volume, is much hotter.

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? It would not be difficult to do this but a good
flow meter that works for hot water is not cheap.

Why would anyone put in a tankless water heater in a residential
property? Even for vacation homes it's easy to fit something that turns
the hot water heater off when not in use.