OUTSIDE GAS MODEL NUMBER c85, WATER IS NOT HOT TAN...

First, I don't even want to know why someone would put a tankless outside as you have found out...really bad idea. In order to help you, more information is needed. Is this a gas or electric unit? Or a gas unit with electric controls? Is it mechanically or electronically controlled ( a make and model would help). Are the water lines to or from the tank exposed? When you say no hot water coming from the tap does that mean when you turn on the hot water faucet you just get cold water or no water?

Question posted under Rheem tankless water heater:

Call Rheem at 800 432 8373
They will need serial number located on label on side of unit.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Usually I recommend that people tear out their tankless water heater and sell it on ebay, and then install tank-type heater from different manufacturer:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

I would be very reluctant to connect used gas appliance without a product manual, and service manual.
http://waterheatertimer.org/images/Exploded-house3.jpg

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

Yes you need to delime tankless each year.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf

Contact Bosch for exact repair information. My records show following number for tankless gas: 800-642-3111
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

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

Please contact your city permit office for construction requirements, they vary from city to city.

Before calling service technician, open following links for troubleshoot resources:
http://waterheatertimer.org/pdf/Bosch_E1_troubleshooting.pdf
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://waterheatertimer.org/Crossover.html

Before installing tankless water heater, read the service manuals and product manual to make sure you have testing skills and equipment needed to repair tankless gas water heater, and that you are prepared to buy expensive parts when needed and do the recommended yearly and monthly maintenance as outlined in manual.
Also check that area has qualified service technician, and that person carries parts on-hand, and is certified for your model number. Keep Tankless manufacturer contact information on-hand, and be skeptical of promises of unlimited hot water, and effortless long-lasting water heating, and saving money. The only folks saying this are the folks selling the product, and not their customers.

Fix most tankless gas water heaters yourself, same day, all brads for about $350.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Call Bosch
Gas Products Technical Support: 800-642-3111

That model number is only shown at one website, and that model number is not shown at Bosch.
Open following link for Bosch:
http://www.boschwaterheater.com/BoschHotWatercomHome/GasProducts/tabid/331/Default.aspx
Search of Bosch for that model number shows no result.

Some water heaters do not have a pilot light. Instead they use hot surface igniter rod that glows red hot
The igniter rod ignites the gas after solenoid valve opens and gas flows to combustion chamber.
The igniter rod and heat sensor have to be operational, and have to be positioned correctly for ignition to take place.
If igniter rod is not working, or if heat sensor does not detect flame, then gas solenoid valve shuts down and water heater does not heat water.

Are you asking about tankless gas or tankless electric?

You posted under Tankless gas water heater: gas type tankless plugs into standard 120Volt household outlet to power circuit board. You need larger gas line than used for tank-type gas water heater.

If you are asking about volt and amp draw of Tankless electric, then that depends on specific model number.
For example, if each tankless element draws 3500Watts, and unit has 3 elements, you will need 3 additional 20 amp 240Volt circuit breakers. Volts x amps = watts. 3500 watts divided by 240Volts = 14.58 amps. At full blast, this unit would draw 43.75 amps and will cost more to operate yearly than tank-type electric water heater. If unit has 4500 watts elements, then it will draw 56 amps at full blast, and use a good percentage of household power.

Tankless water heaters have no hot water in power outage. This is true for gas and electric models.

Additional resources:
http://waterheatertimer.org/images/Compare-water-heater-parts-650.jpg
http://waterheatertimer.org/pro-n-con.html
http://waterheatertimer.org/pdf/Bradford-White-on-demand-tankless-service-manual.pdf

Check that you have power to the unit or that the battery inside the unit isn't dead.
Also check that you have gas at the unit
Hope this helps, if not I'll need more info like Model, age of unit & location e.g. inside - outside etc.

http://www.rheem.com/products/tankless_water_heaters/customer_support/

Make sure breaker is turned on. If that isn't it tell me model number and maybe I can help you more