A water heater booster is a small inline heater that raises already-warm water by 20–40°F. Unlike a main water heater, it doesn't heat from cold — it boosts. Useful for specific applications where main-tank water cools or loses temperature before reaching a fixture. Rheem's answer is the RTEX-AB7; competing options include the Eemax EX55T and similar models.
When you need a booster
1. Dishwasher input temperature
Modern dishwashers spec 120°F input water. If your main tank is set to 120°F (scald-safety default), the long pipe run from tank to dishwasher cools the water — by the time it reaches the dishwasher, it's 110°F. Result: insufficient water heat for sanitization cycle, longer dishwasher run time, less effective cleaning.
Adding an inline booster just before the dishwasher input restores the water to the dishwasher's required 130°F+ without raising the whole-house tank temperature. Cost: ~$300 RTEX-AB7 + $400–$700 install.
2. Distant-shower temperature
If your master shower is 30–50 ft of pipe from the water heater, the water arrives 10–15°F cooler than at the tank. For mild climates this is fine; for cold-climate winters, it makes the shower lukewarm. Booster at the master-shower wet wall restores temperature.
Alternative: install a recirculation pump on the main hot-water loop. Sometimes cheaper than the booster + install, but the pump runs continuously and uses real electricity. For occasional-use distant fixtures, booster is more efficient.
3. Light-commercial sanitizer applications
Janitor closets, dental offices, restaurants. Code often requires 140°F+ at the fixture for sanitation. Whole-tank at 140°F + mixing valves at all other fixtures is the traditional answer; booster at the specific fixture is the cheaper modern alternative.
4. Garage / shop hand-wash with degreaser
Mechanic's shops sometimes need 130–140°F water for proper degreaser activation. Main house tank stays at 120°F; booster at the shop sink provides the higher temperature on-demand.
When you DON'T need a booster — alternatives
Booster economics fail in several common scenarios:
- Whole-house "not hot enough": if your overall water isn't hot, the problem isn't a single fixture — it's the main tank. Diagnose with our troubleshooting hub first.
- Single fixture with no hot water: usually a closed valve, blocked aerator, or failed local supply line — not a heat problem.
- Multiple distant fixtures: if 3+ fixtures are too cool, a recirculation pump on the main loop is cheaper than 3 separate boosters.
- Just need slightly warmer: raising the main tank from 120°F to 125°F is a 5-minute adjustment and free.
Sizing the booster — temperature rise math
Boosters are sized by the temperature rise they can deliver at a given flow rate:
- 4kW booster (low-end): 25°F rise at 1.0 GPM, 12°F rise at 2.0 GPM
- 7kW booster (RTEX-AB7 standard): 25°F rise at 2.4 GPM, 40°F rise at 1.5 GPM
- 13kW booster (commercial): 25°F rise at 4.4 GPM
Pick by your application's peak flow + required rise. Dishwasher: ~1.5 GPM at 20°F rise → 7kW booster sufficient. Shower at 2.5 GPM at 15°F rise → 7kW also works. Restaurant 3-compartment sink at 5 GPM: needs commercial-tier 13kW+.
Install requirements
- 120V or 240V dedicated circuit (depends on wattage)
- Inline plumbing modification at the fixture inlet — interrupt the hot-water supply line, install the booster, reconnect
- Wall-mount or under-counter location — compact units fit in vanity cabinets or behind appliances
- Permit typically required for the electrical work
Booster vs raising the main tank temperature
The cheapest "solution" to a too-cool fixture is raising the main tank from 120°F to 140°F. Trade-offs:
- Scald risk at all fixtures — solves one fixture, creates risk at others
- Energy cost increase — 3–5% per 10°F
- Anode rod faster consumption — replace anodes earlier
- Tank longevity reduction — high-temp operation accelerates corrosion
For a single problem fixture, the booster + main-tank-stays-at-120°F approach often wins on total economics over 10 years.
Bottom line
The Rheem RTEX-AB7 is the right pick for dishwasher booster, distant-fixture boost, and light-commercial sanitizer applications. For most residential whole-house issues, fix the main tank instead. For Rheem main-tank issues see our troubleshooting hub. For full Rheem context see our Rheem water heater lineup.