I'm a maintenance technician at our local Hilton and we use and Astra M3 in our restaurant, I am trying to find a service manual for the machine, how would I go about this?

Don't install tankless, because repairs, installation, operation, maintenance, replacement etc are expensive.
And the thing can be unreliable since you are connecting a computer chip and sensors to gas line, water line, electric line, and to a gas burner with vent to outdoors ... and each of these systems has problems on its own without a computer chip that can fail at the first electrical surge.

Otherwise call Rinni for specs.
1-800-621-9419

Ask about the 4 year warranty for residential installation.
Ask about deliming each year.
Ask about exposure to hard water, and water impurities, and water problems. Tankless cannot be exposed to more than 11 grains hardness.
Ask about recommended yearly maintenance requirement.
Ask why there are no service manuals, and how technicians know how to fix the thing without detailed service manuals.
Call around locally and see who services the thing, and ask if there are service problems.

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 links for Rheem troubleshooting resources, including generic error codes and contact information for Rheem and recent manuals:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://www.fixya.com/tags/rtg_74pvn

Error codes are start place for troubleshoot, and multiple error codes can indicate bad PC board.
Same error code on one machine can be different cause and solution than another machine.

Error 76 is usually communication problem with remote control.
Disconnect wires from machine, and clean terminals is a start point.
Wire between machine and remote might be bad, or remote control is bad, or PC board or wiring connections are bad.

The problem might be solved by having technician catch the machine up on yearly maintenance.

Manual for same-similar product shows maintenance and troubleshooting and a few error codes, but not 76:
For the most part, the manual recommends calling service technician instead of offering DIY details.
http://waterheatertimer.org/pdf/DirectVentManualRTG-74PVN-1.pdf

Add a comment for more help.

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

I don't have good news.
You are attempting a conversion that local technicians are refusing to do.
There is a reason why we service guys steer clear of some things: liability and continual problems afterward.

Contact Bosch and find the service bulletin for that procedure.
After all this is a highly technical and finicky machine hooked to a gas line located in proximity to people.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

If this procedure is going to cost more than $400-600, and remember there is a risk it will not work afterward or will be a continual problem, then consider installing tank-type LP water heater from local box store. I recommend this.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

For others in same situation: NG to LP conversions are not supported or recommended by every manufacturer. This applies to tank and tankless.
Also check there is a qualified service technician in your area before installing tankless.
Read the manual before buying.
Tankless gas savings are $1-8 per month, but all costs are energy.
Tankless cannot be exposed to more than 11 grains hardness in the water.
Tankless are maintenance heavy, and with expected repairs, maintenance and cost of parts, cost of machine and replacement cost, a simple tank-type water heater is cheaper by far.

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Before calling service technician, open following links for Bosch troubleshooting resources:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
Resources at Bosch
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/tabid/370/Default.aspx
Typical Bosch Aquastar manual:
http://waterheatertimer.org/pdf/Bosch-aquastar-170vp-manual.pdf

All tankless and tank-type gas water heaters require regular cleaning and maintenance.
It is unknown if your unit needs maintenance or repair parts, but catching up on maintenance using information from manual is good place to start.
Gas appliances are increasingly complicated and experienced technician is good resource to have in local area before installing tankless.

1) Rheem offers on-line chat, which you rarely see for typical tank-type water heater
http://www.rheem.com/products/tankless_water_heaters/customer_support/
http://www.rheem.com/hrc.aspx

2) Call local service technician, and put that person in your rolodex-wallet for future help.
When technician arrives, ask that person for tips and tricks keeping unit working with regular maintenance.
They will gladly offer yearly service.

3) Read manual several times get accustom to yearly maintenance required for tankless water heaters. Tankless water heaters are maintenance heavy. All costs are energy.

4) Read manual several times and notice word troubleshoot appears many times.
Since tankless are complicated machines, many internal and external factors can affect operation, especially as unit ages.

5) Look at label on unit to identify exact product number. Identify same/similar product at Rheem, and become familiar with Rheem website
http://www.rheem.com/products/tankless_water_heaters/

6) Read other question/answers at fixya about Rheem tankless to become familiar with the tankless era
http://www.fixya.com/tags/rtg_74pvn

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

8) Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

1) Rheem offers on-line chat, which you rarely see for typical tank-type water heater
http://www.rheem.com/products/tankless_water_heaters/customer_support/
http://www.rheem.com/hrc.aspx

2) Call local service technician, and put that person in your rolodex-wallet for future help.
Ask technician for tips and tricks for keeping unit working and needed maintenance.
They will gladly offer a yearly service.

3) Read manual several times to get accustom to yearly maintenance required for tankless water heaters. Tankless water heaters are maintenance heavy. All costs are energy.

4) Read manual several times and notice word troubleshoot appears many times.
Since tankless are complicated machines, many internal and external factors can affect operation, especially as unit ages.

5) Look at label on unit to identify exact product number. Identify same/similar product at Rheem, and become familiar with Rheem website
http://www.rheem.com/products/tankless_water_heaters/

6) Read other question/answers at fixya about Rheem tankless to become familiar with the tankless era
http://www.fixya.com/tags/rtg_74pvn

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

8) Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Before calling service technician, open following link for Rheem troubleshoot resources including error codes and typical manuals.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
C7 76 is problem with remote control. Following link might help.
http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf

Read manual cover-to-cover to make sure your tankless water heater receives yearly maintenance required for unit to continue working.
Tankless are maintenance heavy which can quickly eliminate any promised energy savings unless you de-lime and troubleshoot and read tips-and-tricks others have written and glean information from service technician who visits your house. Mostly glean how much tankless repairs and parts cost vrs buying whole new tank-type gas water heater at local box store, since all costs are energy.

Before calling service technician, open following link for Rheem troubleshoot resources including error codes and typical manuals.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
C7 76 is problem with remote control. Following link might help.
http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf

Read manual cover-to-cover to make sure your tankless water heater receives yearly maintenance required for unit to continue working.
Tankless are maintenance heavy which can quickly eliminate any promised energy savings unless you de-lime and troubleshoot and read tips-and-tricks others have written and glean information from service technician who visits your house. Mostly glean how much tankless repairs and parts cost vrs buying whole new tank-type gas water heater at local box store, since all costs are energy.

Before calling service technician, open following link for Rheem troubleshoot resources including error codes and typical manuals.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Recall website:
http://www.tankless-recall.com/tanklessSearch.aspx
Error code C2 13 call Rheem.

Read manual cover-to-cover to make sure your tankless water heater receives yearly maintenance required for unit to continue working.
Tankless are maintenance heavy which can quickly eliminate any promised energy savings unless you de-lime and troubleshoot and read tips-and-tricks others have written and glean information from service technician who visits your house. Mostly glean how much tankless repairs and parts cost vrs buying whole new tank-type gas water heater at local box store, since all costs are energy.

manual can be ordered a local merchants