Broken RV Water Heater Sight Glass
The Short Answer: On a Suburban or Atwood/Dometic, the “sight glass” is the burner observation window in the exterior access door — not a boiler water-level glass. Cracked glass or a cooked gasket dumps LP and spark into a small bay. Shut the bottles. Do not hit gas or reset. Electric only if the tank is full and the box is dry.
Licensed local technicians
RV water heater sight glass cracked?
Connect with a licensed 24/7 technician in your zip code right now.
Search Intent & The Real Problem
You opened the exterior water-heater door and the round or rectangular viewing window is starred, missing, or the gasket is ash. Some inner combustion doors have a smaller observation port. Either way you are looking into a propane burner with a DSI spark a few inches away. Glass fails from UV, road vibration, and rollout heat. People tape it and keep camping. Tape is not a combustion seal.
Gas path when the window is intact: Suburban not working on gas. Reset only on a full tank: how to reset a Suburban RV. Residential tanks do not have boiler-style sight glasses; if you have a glass tube on a hydronic boiler, that is a different vessel.
What other guides get wrong
They treat the sight glass as a cosmetic RV door part. It is the observation port on a combustion chamber. They recommend packing the hole with foil or RTV and running electric-only “until you get home.” Electric is 120 V in that same wet, metal box — fine only if the tank is full and water is not in the thermostat. They mix Atwood boards with Suburban doors. Wrong gasket, leak.
Step-by-Step Troubleshooting
Step 1: What broke — door window vs inner port vs “the tank”
| What you see | What it is | Next |
|---|---|---|
| Cracked/missing window in the exterior access door | Observation sight glass (Suburban / Atwood common) | LP off. Replace glass + gasket or the door |
| Inner combustion door port melted, gasket gone, soot | Rollout / overheat ate the inner sight | Do not fire. Licensed RV tech — possible chamber damage |
| Wet insulation, rust, water at the tank jacket | Tank leak, not glass | Isolate water. Not a window kit |
| Glass tube on a boiler in a mechanical room | Hydronic sight glass | Not this RV article |
- What to look for: LP odor, soot, melted wiring at the DSI board, water in the electric thermostat box.
- Required tools: Flashlight, bottles off. No lighter to “see if gas is coming out.”
Step 2: Make it un-sparkable
- What to look for: Interior gas switch OFF. Electric switch OFF until you know the tank is full. ECO not being mashed. Bypass valves: if you were winterized, the tank may be empty — Suburban reset rules still apply.
- Required tools: Your hands on switches, then the bottle valves.
A DSI that keeps clicking with a hole in the door is dumping unburned LP into the bay. Same “click no light” family as Suburban gas dead — except now the combustion chamber is open to the compartment.
Step 3: Replace the glass, not the heater — when the chamber is intact
- What to look for: Door skin straight, inner chamber not warped, burner tube and electrode still mounted, no hole in the tank. Suburban and Dometic sell sight-glass / window kits (glass + gasket + sometimes a retainer). Match the door: round vs rectangular, Suburban vs Atwood. A cooked inner gasket without a kit is a door or chamber job.
- Required tools: Correct kit, screwdriver. Gas still off. Do not use household window glass or packing tape.
If rollout warped the inner door, a $12 glass kit will not fix the reason the last one failed. Yellow flame, soot, or a vent problem on a house tank is the same physics: heat at the door.
Step 4: Electric side while gas is down
- What to look for: 120 V element and thermostat behind that same door on SW6DE / SW10DE / SW16DE and cousins. Water on 120 V: leave it off. Tank full, box dry, ECO not tripped: electric heat can keep the shower going without LP.
- Required tools: Shore power or generator known good. Do not reset a dry ECO.
Step 5: After the glass is in — combustion check is not a campground DIY
- What to look for: Gasket compression all the way around, no spider web in the burner, electrode gap per the manual, LP on, one ignition trial. Bang or rollout: off. Bottles off.
- Required tools: RV technician with a manometer and an electrode gauge. You may watch through the new glass. You may not keep cycling DSI.
When DIY Ends: Calling a Licensed Pro
Stop if you smell propane, the inner door is melted, wiring is burned, water is in the electric box, or the tank jacket is wet. A door-window kit on an otherwise sound Suburban is legitimate DIY with bottles off. Chamber damage, DSI boards, gas-valve work, and any “tape it for the weekend” plan are an RV tech or a licensed gas fitter. Same-day is cheaper than a bay fire.
Frequently Asked Questions
Q: What do I do if the RV water heater sight glass is broken? A: Shut the LP bottles and leave the gas switch off. Do not spark it. Replace the listed glass and gasket, or the door, before any gas trial. Use electric only if the tank is full and the box is dry.
Q: Can I tape or silicone a cracked water heater sight glass? A: No. Tape is not a combustion gasket. Silicone in a burner door is a fire and carbon monoxide problem. Use the manufacturer window kit.
Q: Does a broken sight glass mean the RV tank is bad? A: No. The window is in the access door or inner combustion door. A wet jacket or a leak at the tank is a different failure. If rollout melted the inner port, have the chamber inspected before you fire LP again.