Gas heater thermostat installation.

http://waterheatertimer.org/How-to-install-gas-water-heater.html

First suspect is always to clean combustion parts.
I don't have that specific manual, but assume unit has digital gas control valve thermostat.
http://www.reliancewaterheaters.com/lit/om.html
As a general rule, the gas control thermostat activates the blower and turns on the gas, and controls flow of gas to pilot.

If the ECO is tripped, or there was FV event, or the gas valve thermostat is bad, or the combustion parts and tubes need maintenance and cleaning, then the pilot will not relight.
Open following link for older Reliance troubleshoot manual, plus some troubleshooting resources and illustrations:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Add a comment and say what you did to restore functionality.

Hello Skyhalliday,

I understand that you are having a problem with you central heat/air. You do not have a thermostat installed and have the red and yellow crossed in order to make the unit run.

The wires you have crossed are not to the heat if they are to the common color code. Here is the color code for HVAC units:
Thermostat / Wire / what is controlled
W / White / Heat gas valve or heat elements
Rc(Rh) / Red / 24 volts transformer
G / Green / Fan
Y / Yellow / Air Conditioner Outside unit
C / Blue or Black / Common (energizes thermostats that do not have batteries)

You may not have the proper wires shorted in order to have the burners engage.

Thank you for choosing FixYa.com

Justin

It seems unprofessional that Bosch, or any tankless manufacturer, would suggest self-installment when each tankless manufacturer pointedly says their product should be install and maintained by person specifically trained by manufacturer. Especially when it is widely known that self-installed units can void the warranty.

That being said.
Cover on your unit should come off. Unit is attached to wall with screws/toggles at each corner.
Gas line (assuming gas heater) should have a union located on gas line near tankless unit.
Hot and cold pipes are located on bottom of unit.
Vent stack is attached to top.

Specific plumbing techniques are used for each item shown above.
If you are not fully qualified to work with gas appliances, or have no plumbing experience, I suggest taking advantage of fixya paid service where expert walks you through each step while you work on unit.
But you need the tools and vocabulary, and remember the expert cannot see what you are doing or know exactly what you are talking about.

Frankly, if you are not qualified, call a service technician.

Install DIY tank-type gas water heater and avoid the whole headache
http://waterheatertimer.org/How-to-install-gas-water-heater.html

It's a thermostat issue. Replaced the thermostat and check operation

Solution 1: Contact your apartment manager and clear it up whether it had previous gas leakage issues. Have the person assist you in turning on the gas valve on the heater unit. (Most wall gas heater units have the red knob.)
Solution 2: If you don't hear the solenoid "clicking" on. (Solenoids control the opening/shutting off of the gas that is fed into the burner, and are actuated by the thermostat), then there is a problem with either your thermostat (i.e. paint got inside, or just plain broken). Have it replaced.

Make sure the switch on the thermostat is on heat.make the power to the heater is turned on.
if you can hear a click at the gas valve then it is stuck from sitting with out use.

Check to make sure the gas valve is opened. I've turned mine off for the summer months; when I went to use the heater for the first time...I realized I had turned the gas off.

check the thermal fuse or switch. and replace it with the value., check the felament or the the main heating part. maybe its already shorted/ burn.

on the back side of the door you took off it sould tell you what the flashes mean, make sure you put door back right or it will not run,