Sometimes water doesn't get hot.

It is the shower head or plumbing that has a built in mixer or reducer.

The problem is not the water heater. The shower has a mixer or a limiter built into the plumbing so that the water wont get hot. It is a safety feature to prevent burns and protect children from scalding.

Error code 38 doesn't show with typical error codes.
http://waterheatertimer.org/pdf/ErrorcodeTroubleshooting.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start place for troubleshooting.
Contact Rheem:
http://www.rheem.com/products/tankless_water_heaters/customer_support/

This is probably a 16 error code. This code indicates the outlet water spiked above the set temperature for an extended period. It usually means the heater needs to be descaled or the manifold pressures need to be adjusted.

check diaphrapm valve or micro/switch best of luck

if you have build up in the heat exchanger it will cause the unit to use more gas to heat your water, if the gas line isn't sized properly then your going to kick the error codes off. it sounds like you have two problems, first you have build up in your heat exchanger and it needs to be flushed and second you need to get a bigger gas line to it

It might be a question of inadequate flow. All tankless water heaters have a minimum flow rate that must be met, typically .5-.9 gpm, before the heater will operate. In the summer time the incoming ground water temperature is higher, causing you to mix more cold water and less hot water to get a lukewarm temperature. This can reduce your flow rate near or below the minimum flow rate.
Here's a test for you, try running running a sink while taking your next shower. If the hot water remains mostly constant throughout, then your problem is with flow at the showerhead. The flow at a shower head can be reduced by buildup of scale and sediment. Also all showerheads have a flow restrictor that limits flow to 2.5gpm (some even less). Removing the restrictor will increase the showerhead flow rate as well.
Also look for any error code displayed on the digital read out of the heater. Some heaters will flash on their burner indicate light as well.

have you checked the error codes on the heater?
you can do this by powering off the controler and pressing the up and down keys for a few seconds. this should take you into the diagnostics mode scroll using the up and down keys till you find the one that has the letter E on it and that will give you the last 8 error codes write them down and get back to me

I've installed a number of these units and the C4 76 error code is a bad PCB board. The remote unit is not communicating with the PCB board so you need to call customer service at 800 432 8373. You'll need to give them your name, address, model number and serial number. They'll send you a new PCB board with instructions on how to swap it out. It's pretty easy.
Meanwhile, you might be able to get hot water while you wait the day or two for the replacement parts to arrive by simply disconnecting the wires from the remote at the PCB board. This works about 50% of the time. Once they're disconnected try turning on the hot water full blast and see if the unit fires.
Good luck with it.
Paul (Redwood City, CA.)

I had the same problem and called the manufacturer. The error code C-8 was followed by the number 76. They told me to disconnect the thermostat/digital display from the main unit. This was fairly simple to do by removing the front panel. The unit works steady now without shutting down after a few minutes. The help desk told me that the main circuit board needs to be replaced for the unit to work with the display and that they would mail me one asap. At least we have hot water again.

Oakville, Ontario