No hot water

That's the next step. Make sure the flame sensor on the pilot assembly is in the blue part of the pilot flame. If it isn't the gas valve will not open the main burner.

there is no pilot- it is lit with a ignitor. If you have problems the unit needs a service cleaning of the flame sensor rods.

Quit screwing around and shut gas off at tank valve and call a professional ASAP!!!!!!!!! This is a serious and quite dangerous situation. Fixya is full of people who think they know. This is a situation needing a hands on experienced professional,not a bunch of whining incompetent egocentric morons.

Reliance water heater?
Thermocouple is good if the pilot stays lit continuously.

501 is older model. Unknown if this unit has FVIR protection and sealed combustion chamber or open air intake on front of unit.
http://waterheatertimer.org/pdf/Reliance-2006-Service-Handbook.pdf

With FVIR protection:
Get more fresh air to the unit.
Clean the flame arrestor screen.
Test and replace the thermocouple:
http://waterheatertimer.org/How-to-test-water-heater-thermocouple.html
Inspect and clean burner and combustion parts
Test gas pressure, bleed air out of gas line.
Replace gas control valve.
Test draft air by lighting match and see if smoke is pulled into the vent pipe at draft hood on top of tank.

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

You might have crossover problem caused by bad faucet cartridge.
http://waterheatertimer.org/Crossover.html

The green standby light should come on when 240Volt power is applied. Page 6 manual.
Both red lights should come on when tankless is activated. Page 6 manual

RTE 13 requires 60 amp breaker and 6 gauge wire
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html
http://waterheatertimer.org/pdf/Rheem-TanklessElectricUC_AP15447.pdf

Factory temperature setting is usually default 120 degrees.
Adjust temps page 11 manual
Thermostat calibration is always approximate.
120 degrees is not hot enough to burn the skin.
Exceptionally cold incoming water temperature can cause tankless not to heat enough hot water: install tempering tank to preheat water to room temperature.
Tankless cannot be exposed to more than 11 grains hardness in water. Delime tankless yearly, clean water filter monthly, do not install low-flow restrictions or conserve water since the flow rate must remain high for tankless to activate.
Rheem does not post service manuals for their tankless water heaters.
Call Rheem 800 432-8373

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

Hello Mike,
Check gas pressure, burner parts need to be cleaned, check electrical outlet, hot surface igniter is bad

White Rodgers intellivent error code:
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html#Intellivent
http://waterheatertimer.org/pdf/Intelli-vent-gas-valve2.pdf

Error code says: system in lockout, but you know that.
1) Low gas pressure, no gas to water heater: call gas company to check pressure
2) Hot surface igniter not positioned correctly: inspect and clean burner parts.
http://waterheatertimer.org/images/Reliance-hot-surface-ignite.jpg
3) Low voltage to water heater/ outlet has wrong polarity: Use circuit analyzer on electrical outlet to make sure polarity is correct

Typical power vent manual:
http://waterheatertimer.org/pdf/Reliance-40-50-power-vent.pdf
Cleaning water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
http://waterheatertimer.org/pdf/How-to-clean-combustion-chamber.pdf

Well you didn't indicate what kind. Gas or electric? If it's electric, check for open fuses or circuit breaker. Could be bad element(S) If it is gas, is the pilot lit, if it doesn't have a standing pilot, is the ignitor getting red hot to light it. Need more model info to help

Clean the air filter even if it looks clean. The air filter is located in the front panel(unscrew knob to open door)...filter looked ok to me but cleaned it under water and that fixed the problem. Note: I cleaned the water filter first and that did not solve the problem. A simple test to see if the air filter is the problem...take the panel door off, press the filter door button and test hot water usage.

Hi - hope someone can help. I have a VOKERA EXCEL 80SP combi boiler and been without hot water/central heating for two days. The pilot light is on, the fan is on, but boiler not triggering once the hot water tap is turned on.
Hopefully someone come to look at it today but hoping for some advice. Anyone else had this problem?
Thank you.

resetting your lock out for richmond power vent