A Rinnai tankless install is more involved than a tank-to-tank swap. Skipping any item below voids the warranty and creates downstream problems that Rinnai support sees daily.
Gas line sizing
The #1 install mistake: undersized gas line. A 199 kBTU Sensei or RL94 needs ¾" black iron all the way back to the meter in most homes. ½" line will hold 5" WC at idle but drop below 4" WC under fire, throwing Code 11 or short-cycling. Have a plumber do a gas pressure test under full load; this is non-negotiable.
| Model | BTU | Min gas line (back to meter) |
|---|---|---|
| V53, V65 | 120-150 kBTU | ½" up to 30 ft / ¾" over 30 ft |
| V75, V94 | 180-199 kBTU | ¾" required |
| RL75, RL94 | 180-199 kBTU | ¾" required |
| RU98, RU130 | 180-199 kBTU | ¾" required |
| RU150-RU199, RX130-RX199, RUR199 | 199 kBTU | ¾" required |
Venting
- Condensing (Sensei, Sensei RX, RUC, RSC): 2" or 3" Schedule 40 PVC / CPVC concentric or two-pipe up to 65 ft equivalent length. Use Rinnai-approved manufacturers (IPEX, Heat-Fab)
- Non-condensing (RL, V): Rinnai-specific stainless Category III venting (not the same as B-vent — do not mix). 4" diameter typical.
- Outdoor (V94eN, V53DeN, V65iP): no venting; cabinet is weather-sealed and vents directly to the exterior
Condensate drainage (condensing models only)
Condensing tankless units produce 1-2 gallons of acidic condensate per day. Required setup:
- Condensate trap at the unit outlet (Rinnai includes this)
- PVC drain line to floor drain, sink trap, or condensate pump
- If running into iron drain pipe or septic, neutralizer is required (pH 3-5 condensate eats iron)
Water softener / scale filter
If incoming hardness exceeds 11 grains per gallon, Rinnai requires either a softener or scale filter — otherwise the warranty doesn't apply to the heat exchanger. Even at 7-11 gpg, soft water dramatically extends time between flushes.
Electrical
Standard 120V 15A circuit. Sensei RX and RUR units with built-in pump draw slightly higher peak amperage when the pump runs — still fine on 15A. Dedicated circuit recommended but not required.
Commissioning
- Purge air from gas line
- Open inlet/outlet isolation valves
- Open hot tap downstream to verify water flow
- Power on; verify display
- Set temperature (120°F default)
- If condensing: verify condensate drain flowing
- If recirc: program Circ-Logic schedule
- Pair to ControlR app
- Register warranty
Permits
Most US jurisdictions require a permit and inspection for tankless install — gas line, venting, and condensate disposal are all inspected. Skipping the permit voids the warranty in some states and triggers insurance issues if there's ever a gas leak.
Bottom line
The "tankless install was a nightmare" Reddit threads almost always trace to an installer who did a tank-to-tank swap without re-sizing the gas line, re-piping for proper venting, or installing a condensate trap. A Rinnai-trained installer ($300-500 more upfront) saves these problems. See our dealers hub for finding one.