Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater LOW TEMP OUTPUT Error Fix: Insufficient Temperature Rise — Unit Undersized

The water coming out of the unit is not hot enough despite the temperature dial being set to maximum. This is NOT a malfunction — it is a performance limitation inherent to the unit size. The RTEX-13 at 13kW can raise water temperature approximately 20-25°F at a flow rate of 2 GPM. In cold northern climates with groundwater temperatures around 40°F, the maximum output may only reach 60-65°F — insufficient for a comfortable shower. To increase output temperature, reduce the flow rate by partially closing the hot water fixture or installing a low-flow showerhead. The RTEX-13 is designed as a point-of-use unit for a single fixture in mild to moderate climates, not as a whole-house water heater in cold regions. For higher temperature rise, a larger unit (RTEX-18 or RTEX-24) or a gas tankless unit is needed.

info2 fixes available✨ Free fix available

Quick Answer

Error LOW TEMP OUTPUT on the Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater means: Insufficient Temperature Rise — Unit Undersized. The top-ranked community fix is “Upgrade to a Higher Capacity Unit for Adequate Temperature Rise” (hard difficulty, ~120 min, DIY cost: $0). 2 community-ranked fixes available below.

Published February 2026 · 2 fixes · water heater

Content is based on manufacturer service documentation, community repair experience, and OEM technical bulletins. Always verify with your owner's manual.Learn about our editorial standards.

When to Call a Professional

While many water heater repairs can be done safely at home, consider hiring a qualified technician if:

The repair involves electrical wiring, gas lines, or refrigerant
You've tried the top-ranked fix and the error persists
You smell gas, see sparking, or notice water leaking
The repair requires specialized tools you don't have
Your appliance is still under manufacturer warranty

Average professional repair cost for this issue: $600-$900.

Other Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater Error Codes

DISPLAY DASHES

Temperature Sensor Error

2 fixes

ELEMENT LED OFF

Elements Not Firing — No Demand or No Power

2 fixes

ELEMENT LED ON

Heating Elements Active — Normal

2 fixes

NO DISPLAY

No Power to Unit — Display Dark

2 fixes

People Also Ask About Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater Error LOW TEMP OUTPUT

What does error code LOW TEMP OUTPUT mean on a Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater?

Error code LOW TEMP OUTPUT on the Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater means: Insufficient Temperature Rise — Unit Undersized. The water coming out of the unit is not hot enough despite the temperature dial being set to maximum. This is NOT a malfunction — it is a performance limitation inherent to the unit size. The RTEX-13 at 13kW can raise water temperature approximately 20-25°F at a flow rate of 2 GPM. In cold northern climates with groundwater temperatures around 40°F, the maximum output may only reach 60-65°F — insufficient for a comfortable shower. To increase output temperature, reduce the flow rate by partially closing the hot water fixture or installing a low-flow showerhead. The RTEX-13 is designed as a point-of-use unit for a single fixture in mild to moderate climates, not as a whole-house water heater in cold regions. For higher temperature rise, a larger unit (RTEX-18 or RTEX-24) or a gas tankless unit is needed.

How do I fix error code LOW TEMP OUTPUT on my Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater?

The most successful fix is "Upgrade to a Higher Capacity Unit for Adequate Temperature Rise" (hard difficulty, ~120 minutes). This fix costs nothing. HomeMD has 2 community-ranked fixes for this error.

Is error code LOW TEMP OUTPUT on a Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater serious?

This error is rated info severity. It is generally not urgent and can often be resolved with a simple DIY fix.

Can I fix Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater error LOW TEMP OUTPUT myself?

Yes — the top-ranked community fix ("Upgrade to a Higher Capacity Unit for Adequate Temperature Rise") is rated hard difficulty and takes about 120 minutes. Tools needed: T20 Torx screwdriver, Adjustable wrench, Multimeter, Drill with wall anchors.

How much does it cost to repair Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater error LOW TEMP OUTPUT?

DIY cost: $0. Professional repair typically costs $600-$900. Many homeowners save money by trying the top-ranked community fix first.

What other error codes does the Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater have?

The Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater has 4 other known error codes, including: DISPLAY DASHES (Temperature Sensor Error), ELEMENT LED OFF (Elements Not Firing — No Demand or No Power), ELEMENT LED ON (Heating Elements Active — Normal), NO DISPLAY (No Power to Unit — Display Dark).

What other error codes does the Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater have?

The Rheem Rheem RTEX-13 13kW Point-of-Use Electric Tankless Water Heater has 4 other known error codes:

DISPLAY DASHES — Temperature Sensor Error (2 fixes)
ELEMENT LED OFF — Elements Not Firing — No Demand or No Power (2 fixes)
ELEMENT LED ON — Heating Elements Active — Normal (2 fixes)
NO DISPLAY — No Power to Unit — Display Dark (2 fixes)

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