Rheem air conditioner RHSL-HM3617JA how do I change the air filter

Malfunction of Air IntakeFilter Switch

Remedies:
Make sure air filter door is properly seated.
Make sure front panel is properly installed.

https://www.aprilaire.com/...manuals/.../aprilaire-zoned-comfort-model-6202-owners-...

Low water pressure
Clean water filter
Change water filter cartidge
Check water flow to unit
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/Crossover.html

Gene

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 including error codes, Rheem chat and phone number, possible instruction manual, maintenance manual.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start point for troubleshooting.
Error code 21 is "malfunction of air intake filter switch"
Make sure air filter door is properly seated.
Make sure front panel is properly installed.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

What filter are you talking about?
Air filter: Usually with a heat and air unit, the air filter is located behind the return air grill.
Return air grill looks similar to ordinary ceiling or floor grill where the hot or cold air blows into each room. Except the return air grill is larger and is usually put close to the heat-air unit.
Remove the grill and replace filter every 3-6 months.
If no filter is present, then dust and hair get stuck on the coils and unit will not meet temperature setting.
See image of typical air filter:
http://www.instablogsimages.com/images/2009/02/12/air-filter_IYR3W_69.jpg

Error 21 means the air intake cover problem.

Malfunction of Air Intake Filter Switch Ensure air filter door is properly seated.
Ensure front panel is properly installed.
Error codes here: http://www.palomastore.com/paloma_error_codes.html

Richmond is Rheem
Typical manual shows regular maintenance on page 29:
http://waterheatertimer.org/pdf/Rheem-RTG74IndoorOutdoor_owners.pdf
The word maintenance is used throughout manual.
Tankless are prone to lime build-up and should be de-limed each year, and cannot be exposed to more than 11 grains of hardness in the water.

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Use following link for Rheem tankless troubleshoot: http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

I believe error code A1 21 is malfunction of Air intake filter switch
Ensure air filter door is seated.
Ensure front panel properly installed

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.

05 Imperfect Combustion Alarm
Clean air inlet filter.
Clean combustion air fan.
Clean heat exchanger fins.
Check for adequate combustion air ventilation openings and clean if necessary.
Check vent system for partial blockage and correct as necessary.

If this does not help; contact Rheem customer support.

gas valve could be clogged it has a screen in the side of the inlet, check this first if not it could be the gas valve itself,, make sure the gas valve is turned on also, sometimes people turn them off and for get the handle should be in line with the pipe not across the pipe