Hi, I don't know anything about tankless hot water heaters but here are a couple threads discussing them. One contains links to even more information and comments from people who have them. Good luck!

https://www.houzz.com/discussions/tankless-gas-water-heatersgood-or-not-dsvw-vd~970216

https://www.houzz.com/discussions/tankless-wate-heater-or-not-dsvw-vd~526745

You have not really given enough information. Did you buy a properly sized on demand heater? Many folks don't, and instead purchase units that are simply not designed for whole house applications. Is yours electric or gas? Typically on demand heaters are not a problem if they are sized properly.

Thanks all. We had our furnace replaced in 2006. The prior furnace also had tankless which we had issues with but they convinced us that the new ones were better. Not for more than a few years. We have space constraints. The house was built in 1938. For whatever reason, the water supply to the tub & shower is on an outside wall. Had it been on the INSIDE, we could have a supplemental tank or heater installed in the bedroom closet. It appears 99% sure that we w/b replacing with a 50 gallon electric tank. Currently, water will be plenty hot in the bathroom sink but cold in the shower which is why Symmons has supplied us with replacement valves several times. At our whit's end. Tired of wasting gallons of water waiting for hot water to come up only to have it turn cold b/4 the shower is finished. This is basically winter issue. In the summertime things seem to work properly.

If the water is hot at the sink but cold in the shower...then the issue is not the tankless heater. If your outside walls are so cold that they are cooling the hot water before it gets to the shower then that is an issue by itself that should be changed if you possibly can. I cannot see why a 50 gallon tank will make that issue go away. Tankless heaters don't run out of hot water!

Our walls are well insulated. Had the State/power company out for an energy audit. NOT an issue in our downstairs bathroom which is also on an outside wall. (no shower). Symmons states that it is an issue with tankless. Found this: http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/water-heaters/tankless-water-heaters/overview/tankless-water-heaters-ov.htm

Thanks all. Guess we have made our decision. Not another winter of uncomfortable showers and water going down the drain. Consumer Reports appears to confirm that the "hot and cold" issue is common, as have 3 different plumbers. :(