Cant get a mix of cold and hot water, it is either cold or scalding hot?

well since u didn't mention the exact model u have or include a photo so I can positively ident it, id say u need to look around base of faucet for lever , this is the mixing valve control , it controls how much cold water mixes with hot water to prevent a scalding occurrence in use , this lever may have gotten moved and a new position will need to be explored to find a comfortable temp u like it at again

If you are referring to limiting the temperature of the hot water to prevent scalding, then a tempering or mixing valve can be plumbed in between the cold and hot water lines. This type of valve adds cold water to the hot water and it is adjustable.

Pull apart the front of the valve and there is a limiter that is set to limit hte hot water, you need to adjust that limiter. I find many valves are shipped with teh setting too cold.

The Delta shower has a rotational limit stop designed so that when another fixture is activated, such as a toilet or dishwaser, the water does not go to extreme hot or cold. The reason for this is that the Delta shower unit is not pressure balanced or does not contain an anti scald device. The pdf document below will show how to adjust the water tempature if you feel it is too cold. Please use caution as it is probably set the way it is to prevent scalding.

http://www.deltafaucet.com/media/Product_Tech_Docs/Maint_Installation/50001to80000/51975RevH_4.15_15e05.pdf

If you aren't getting any error codes, then the heater is probably doing its job. Your problem is probably somewhere in your plumbing system. The most common reason you get this sort of problem is due to cold water mixing in the hot water system. There are a number of causes for a "cross connection" :

-Faulty check valve
-Missing check valve on reciculation return line
-Faulty shower cartridge
-Thermostatic mixing valve set too low
-Faulty thermostatic mixing valve
-Faulty anti-scald valve

The easiest way to confirm a cross connection is to: 1) shutoff the cold water entering the tankless heater 2) open hot water valves around the house. If you still have water flowing at your hot water fixtures, then you have a cross connection.

When you filled the water heater did you get all the air out of lines and water flowing before you turned power onto heater. It sound's like the bottom element on the heater is not working. That is if it's the same throughout the house hot then get's cold.
Check the power to the lower element and thermostat.

As hot water leavs the heater it mixes with cold water so what you feel at the pool side is barley warmer than the pool. This is ment to keep from scalding people in the pool or spa. 7-11 degrees is the max on the outlet.

Your valve has an anti-scald feature which automatically adjusts the water temp if cold water is diverted to another location, e.g., a flushed toilet, washing machine filling, etc. Most valves have an adjustment screw to allow you to set the temp. Since each manufacturer is different you'll need the manual or contact the manufacturers support. BTW, these anti-scald valves are being added to building codes in many parts of the U.S.

Sounds like you have a problem with water flow. Since your ground water is warmer in the summer, you mix less hot and more cold in order to get the same warm temperature. If your hot water flow gets low enough, your tankless will start to "short cycle" in order to prevent scalding. Use one of these three methods to increase your water flow at the shower:

1) Clean your shower head and descale
2) Widen the hole in the showerhead's flow restrictor
3) Remove your flow restrictor entirely

Good luck!

try this.
if you have anti scald device installed in the valve, take it out. the anti-scald device is to keep the hot water from pouring out in case the cold water pipe is not functioning. In general sense, it is a good idea, however, it also mixes too much of cold water, therefore you don't get eough hot water.