Valve will not mix enough hot water with cold...shower is luke warm even though there is plenty of hot water. Can not locate screw, etc to adjust the mixing valve.

Check two things, 1- check that the hot water heater bypass valve is completely closed so that it is not bypassing at all and mixing cold water . 2- check your outside shower valves, make sure both of those are turned off...if they are turned on but the valve on the head is turned off...this will also allow cold water to mix in the hot water line. If those check out ok...then the mixing valve is likely clogged, you could take it off and try soaking it in boiling white vinegar, or try to adjust it, there is an adjustment screw under the black cap, I think its an allen screw...but they fill the center with glue so you can't adjust it...you have to dig that glue out so you can get the allen wrench in...may even be a torx screw...either way it can be adjusted, you just have to dig the glue out first.

If all the other faucets work, then the problem is the mixing valve in your shower. You will need a new shower fixture.

the problem is the hot/cold mix is set wrong ---
remove the water control handle to expose the mix valve -- you should see a small set screw that will either increase or decrease the travel of the hot water valve. --- sometimes instead of a set screw there is a little pop off plastic set ring,, pop off and set to a more advanced travel.
this will increase the amount of hot water that flows to the shower head and the bath tap.

c...

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.

if the water is that hot then lower [adjust] the temp of your hot water heater -if need to adjust the shower you can adjust the mixing by removing the handle and on the valve stem are the adjusting clips the red [hot] and blue [cold]plastic clips can move them around to adjust the mixing of hot and cold water

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.

Check cylinder stat , if fitted check 3 way valve is fully open on hw side . Check water level in header tank

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.

you could be having a problem elsewhere in house system! do you have a garden hose hooked up with a "Y" hose? or something in system were hot and cold connect...then a valve after? what happens is the hot water can back siphon through cold lines..its rare, but this happens, and is dangerous! water from hot water tank is full of bacteria, and if you drink it , well thats not good!

its the mixing slide in there its stuck u can take it apart at the handle it pulls out shake it if it doesnt move its bad. what happens is over time sediment gets in there and seazes it up. if you take it apart and its not stuck then its ur mixing valve on your boiler is bad good luck