I have the exact same problem, house is only 6 years old, two handel faucet, Ive turned off the cold side to eliminate it, turn on hot and have to run at full blast and it takes a full 10 minutes to get to 117 degress and i have the hot water heater set om 135. this is a slab home in Florida and the plumbing is copper running thru the slab. Plumber put on a circulating pump under sink and said it would cure all problems. next day same problem 3 minutes before water temp evenn rises 1 degree. Any thoughts on this one? any suggestions would be much appreciated.

It sounds like you have a scale problem which can seriously affect your water flow. If the water heater is older than 10 yrs, then this may be your problem.
If so, try draining the htr. First off, turn your heater off. If electric, turn off 30 amp two-pole breaker in your elec panel. Your elements may also be scaled up, thereby reducing their effectiveness. If you have little hot water, your bottom element and t-stat may be bad. If the electric heater is older than 10 yrs old, then you may want to consider replacing it as the average lifespan of a heater is about ten years. If gas, just turn the control knob all the way down. Attach a garden hose to your heater drain valve. Run the hose outside and open the drain valve with the water supply to the htr still on. This uses water pressure to blow out the scale that is likely clogging the drain valve. Blow it out for ten seconds, and then shut the valve feeding water to the htr.
Shutting the valve creates a vacuum so the draining will slow way down. To speed this up, turn on hot water all around the house. Do not do this with single handle faucets because cold water will cross over. Only the hot on two handle faucets should be opened.
If you don't have any two handle faucets, you can either turn off the water main and open the single handle faucets, or flip the lever up on the relief valve located on top or on the side near the top. Pull it so that it sticks straight out. This locks it in the open position and allows air to enter, and water to exit. Drain until no more water comes out.
Please keep in mind that if your heater is old, the scale may be the only thing keeping it from leaking, and this may expose that leak.
If none of this works, your inlet and/or outlet on the heater may be scaled up. This requires the removal of piping and scraping out the scale. Once again, if the htr is old, (over 10) you may want to consider just replacing it.
GOOD LUCK!

turn off gas
turn off cold water to heater
connect hose to spickot
place hose end outside
turn on spickot
open hot water faucets in house
wait.
when water stops, turn on cold water for 30-60 sec or so to try to stir up any sentiment in tank
turn off cold water and let drain.
close spickot
turn on cold water
let fill till water start coming out hot water faucets then turn off hot water faucets.
turn on gas and relight pilot.
finish bleeding air from hot water faucets.
set water heater thermostat

http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html#flow
1) Tankless require minimum flow rate before burner will activate.
2) Sounds like the water pressure drops quickly when faucet turns on. This implies combination of problems including sediment build up inside lines, crossover at single-handled faucet, or sediment filter clogged or water softener clogged.
3) Low flow faucets can reduce needed flow rate.
4) Other problems can contribute: sensor failure, loose wires, PC board failure, high temperature setting, cold incoming water temperature during winter months.

may be overheat stat or fan check little hose at fan area for pinhole best of luck

you will need to have a 115 curent for electric, run a line off the panel box, turn off water to the house drain it down, disconnect the lp gas line and cap it off, remove the water heater, install the new heater run the cold side water to the cold side {blue on the heater} the hot side to the red, take the cover plate off on the water heater run the wires, hook up the groung wire then the red and white wires to the wires comming off the panel box,, fill the water heater with water have a open faucet on the hot side, and bleed the air out, DO NOT TURN ON ELECTRIC TILL THE AIR IS OUT OF TANK AND WATER IS FLOWING THRU COMPLETELY, after you get water commming out of the faucet turn it off and turn the electric on to the water heater ,, wait about 30 min, you will have warm after an hour it will be hot and you are done,, good luck write me if you have questions, [email protected]

you might also want to check into the piping in your house is it gavinized, cpvc or copper, this could also make a diffrence, you could also be pulling mud from the bottom of your well, the anoid rod in the water heater will keep you from seeing this in the hot side lines,suggest to have a test done on the water take a jar with a lid and take it to your local plumbing supply house and see if they can't have it tested then you will have all your answers, the test i'm in md. it cost us aprox 40.00 to have the complete water test,

I had a customer a few weeks ago that had the same problem. When the water heater was installed they crossed the supply lines. They had the inlet as the outlet. Esentially, the water heater was plumbed in backwards. Also there could be a miss piping somewhere in the house but it would probably be near impossible to find it right now without tearing up foundations. Also check that the faucet is not plumbed backwards as well. Especially if this is a single handle faucet that you are referring to.

There is something in the line or in the faucet.Are your pipes going to the faucet black iron or galvinze.Some of the pipe will rust up and stop a full flow of water.But I would check the faucet first to see if the screw is loose or somethng in it.

I found a way to get warm water in the shower, but I'm not sure it qualifies as a solution. After trying everything else I thought that maybe when I was feathering the faucet in the shower toward the cold side so I would get warm water that at a certain point it blocked the flow on the hot water side enough so that the sensor in the hot water heater sensed there was no demand in the line and it shut the water heater off. To test this, I went to the kitchen and turned the kitchen sink faucet all the way to the hot side and then turned it on so that I got a small flow of water. I waited for the water to get hot to be sure the hot water heater was working. Once it was hot, I left it running to keep demand in the line and then I went to the shower and turned the faucet on. When the water got hot I started feathering the control towards the cold side and soon I had warm water. So I think I'm right in that the sensor in the hot water heater was shutting the water heater off when the demand in the shower was lowered to a certain point when I was trying to adjust the water temp in the shower.

So now I can get warm water, but it means I have to leave the hot water running at another fixture in the house to create a false demand in the system so the hot water heater doesn't shut down. Is there a way to adjust the senor in the hot water heater so that it will stay on when I'm using the shower only, so I don't have to waste water by running another faucet when I want to take a shower?