Blinking red light,

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!

Here is what should be going on.
There is a length of pipe between the hot water that is in your tank and the nozzle you are expecting to get it out of.
It is really possible the are nearly 3 gallons of water in those pipes, especially if the water heater is many feet from the faucet.
Now about the drop in water pressure.
Try the cold water.
Does it do the same as the hot? Does the pressure drop after 2 - 5 minutes?
If so, suspect your pipes are clogged with sediment. How old are the pipes and the house/apartment?
Does the water contain minerals that clog pipes?
If the cold water is good, and the hot not, I should suspect there is blockage in the hot water pipes, especially going in to or coming out of the water heater., or perhaps the faucet is clogged with rust or deposits.
God bless your efforts.

Hard water deposits from the water heater in the supply lines to the faucets. Sounds like your water heater needs to be cleaned out -- not just drained of water. You need to get inside and clear out the deposits in the bottom of the tank. You also need to remove the hot water supply lines to your faucets and see of you can get the lines to run free from the deposits -AFTER you clean out the water heater. Remove the drain -put on a 3/4 in. ball valve for the drain and a hose bibb on that - you can hook up a 5/8 - or 3/4 inch hose to get the particles out of the drain of the water tank. You may have to blow back the water line, from the faucets, the hot water lines- to get the deposits out of the water lines. You also may have to blow out the faucets with the water line disconnected to clear the calcium out of the hot side of the faucet.

Feb 2013
1) Copy following links for Intellivent gas control service manual, low hot water pressure, and crossover:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/Crossover.html

2) Unknown if tank has hot water > that is first thing to check.

3) Unknown if air intake screen located under burner is clogged with dust that cause combustion chamber to overheat, tripping FV lockout and breaker TRD glass vial.
Requires removal of combustion parts, replacement of TRD, blow out air intake screen, reset intellivent

4) Unknown if there is error code.

5) Rheem does not post service manuals.
However this unit can probably be repaired.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Before calling service technician, open following link for Rheem troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
Tankless fuses and control board.
Water flow rate too low. Minimum .8 gallon required (depending on model)
Tankless inlet filter clogged.
Household aerators clogged at faucets.
Crossover at single-handled faucet
If installed, there can be crossover at mixing valve.
Faulty household check valve, including check valve on re-circulation system if that system is installed.
Restriction in household water line, such as clog at elbow.
Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow and tankless will not turn on.
Loose wire connection inside tankless.
Unit temperature set too high.
Unit temperature set too low.
Water temperature is very hot coming out of tap, so more cold is added, but that reduces amount of hot water flow, so tankless turns off because flow rate is too low.
On some tankless, if water is preheated by solar or other indirect source, the burner will not activate.
High mineral content in water > delime tankless yearly
Misaligned temperature sensor
Low gas pressure. Wrong type of gas. Undersized gas line.
Too many faucets on at same time.
Faulty tankless water valve. Water valve stuck.
Fluctuating household water pressure, for example MUD and PUD district water pressure problems or water well pressure.
Faulty household pressure reducing/regulating valve if installed.
Faulty household pressure tank, if installed -or- pressure tank needs to be installed.

Information below is for tank-type electric or gas heater only.

There is a condition called stacking.
This might apply to your water heater or not.
Having authorized service people to your house 3 times suggests more complicated mix of problems.

Stacking is caused when there are repeated short draws of hot water.
As you know, each time any hot water is drawn out of faucet, cold water immediately enters bottom of tank through the dip tube. The dip tube is connected to cold water inlet and runs to bottom of tank.
So all cold water enters bottom of tank close to the gas water heater thermostat.
See illustration of dip tube, that also shows copper thermostat sticking into tank that is connected to gas valve.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#flushelectric

Let's say your water heater if fully heated and burner is off.
A short draw of hot water causes cold water to enter tank.
Another short draw. And then another short draw, each draw happens close to same time as other draws.
Water at top of tank is plenty hot, but bursts of cold water get near thermostat and cause thermostat to turn on burner.
Burner heats water and more hot water rises to top of tank. Water at top of tank becomes overheated each time stacking occurs.

Again, a stacking situation may not be the cause of your problem.
However, remember if single-handled faucet is opened straight upward, then that draws both hot and cold water. And cold water immediately enters tank.
Only when the single-handled faucet is lifted with the handle all the way to the right, does it draw only cold water. Even so, some single-handled faucets have bleed-over between hot and cold sides, so hot water is drawn anyway.

Another possibility is that the dip tube ends at or near thermostat, instead of going all the way to bottom of tank. The dip tube should go straight down from cold water inlet, but dip tubes are made of plastic, and some self-cleaning tanks have a curved dip tube or dip tube with perforations that create currents designed to stir sediments off bottom of tank (although this is not proven to be effective deterrent to sediment formation).

Add a comment for more free help.
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Fixya is always less expensive than a service call.

how many times does the light blink? this is a code

Have you thought about possible calcium buildup in your hot water tank/pipes? I am helping a friend in Palm Springs CA (California not Canada) and we are going to pour a really huge pile of vinegar into the hot water tank (let it sit for a day) then open up the hot water taps to the kitchen (where we don't have hot water flowing very well), then going to let everything sit for another day. Remove the aerator from the kitchen faucet after letting the vinegar/water solution sit in the pipes for a day, and turn on the kitchen hot water tap. Once all the **** is drained out, you must of course drain out the vinegar/water solution from your hot water tank or you will smell like French Fries (in Canada we put vinegar on French Fries) when you shower for a few days.

If you don't understand what the basic chemistry of dissolving calcium via this method is all about, then please call a professional plumber. Hope this helps, and remember that Google is your friend (sometimes), but be careful...

You have a 16 error code, abnormal output temperature. I recommend performing a descale to remove hard water deposits in the heat exchanger. This should resolve your issue.