What you’re describing almost certainly isn’t air—it’s restriction. The pattern (strong flow for a second, then dropping to a trickle) is classic for debris clogging the hot side after the tank was drained and disturbed.
Here’s what likely happened and how to fix it:
What’s probably going on
A 20-year-old Rheem tank will have a lot of sediment (calcium, rust flakes). Draining and moving it stirred that up, and when it was refilled, debris got pushed into:
- Faucet aerators
- Cartridge valves (especially single-handle faucets)
- Angle stops under sinks
- Possibly sections where pipe size reduces (like your 3/4" → 1/2")
That’s why:
- You get brief normal flow (line pressure)
- Then it chokes down (restriction fills with debris)
Most effective fixes (in order)
1. Clean the easy choke points first
Before doing anything major:
- Remove and clean all faucet aerators
- Check shower heads
- Flush toilet hot supply (if any mixing valves exist)
You’ll often find grit or flakes there.
2. Check faucet cartridges
Single-handle faucets commonly trap debris:
- Shut off water
- Pull cartridge
- Rinse it out
If multiple fixtures are affected, this is very likely part of the problem.
3. Flush the hot water lines (controlled way)
Your handyman’s idea is on the right track, but it needs to be done carefully:
Safer approach:
- Turn off water to the heater
- Disconnect the hot outlet at the heater
- Use cold water pressure to flush through the hot lines only
- Open one fixture at a time
This prevents pushing debris deeper into smaller valves all at once.
4. Reverse flush (your idea — actually very good)
What you suggested is often the most effective method:
- Hook a garden hose to an outside faucet
- Use an adapter to connect to a hot-side faucet indoors
- Push water backward through the hot system
This can dislodge debris stuck at:
- Valves
- Size reductions
- Elbows
Just be careful:
- Don’t exceed normal household pressure
- Make sure heater is isolated so you don’t push debris back into it
5. Check shutoff valves under sinks
Debris often gets stuck in:
Sometimes you’ll need to:
- Disconnect supply lines
- Briefly open valve to flush into a bucket
6. Last resort: partial pipe cleaning or replacement
If flow is still restricted:
- A section of pipe may be packed with sediment
- Especially common in older galvanized piping
At that point:
- Mechanical cleaning or replacement may be needed
Important reality check
At 20 years old, that water heater is at or beyond typical lifespan (8–12 years). Even if you fix this:
- More sediment problems are likely
- Internal components may fail soon
You might want to consider replacement rather than investing too much time.
Bottom line
- Start simple (aerators + cartridges)
- Then flush lines (forward or reverse)
- Your reverse-flush idea is solid and often works best
- If issues persist → likely deeper blockage or aging plumbing
If you want, tell me what type of pipes you have (copper, PEX, galvanized), and I can narrow down the most likely blockage points and best flushing method.
https://www.reddit.com/r/Plumbing/comments/1sxql68/hot_water_lost_pressure_after_water_heater_was/
