Shower faucet only lukewarm even though the fixture feels hot
Anonymous user 21/08/2026 - 9:52 AM
Hi everyone,
I’m having some issues with my shower and would really appreciate any advice on what might be going wrong. I’m not sure if the problem is with the water heater, the shower faucet, or possibly both.
I have a thermostatic shower faucet, and it’s only giving lukewarm water. The fixture itself gets very hot to the touch where it connects to the hot water line.
The hot water comes from a combi water heater (tankless unit), which is located in the same room. Sometimes the shower water even goes cold for about 10 seconds before going back to lukewarm — it seems to happen when the unit cycles off and then kicks back on.
All the other hot water faucets in the condo work fine, and the heating system isn’t having any problems.
Could this be a faulty cartridge in the shower faucet? Or with the water heater, could there be a build-up or cycling issue that’s causing it to shut down?
This issue is characteristic of a temperature-limiting adjustment failure or a minimum flow-rate conflict with a tankless combi water heater, rather than a general water heater failure. Since other faucets work fine, your tankless unit is producing hot water, but the shower mixing valve or your combi unit's flow parameters are disrupting delivery.
Key Diagnostic & Code-Compliant Steps:
Adjust the Anti-Scald Limiter (The Rotational Stop): Most modern pressure-balancing and thermostatic shower cartridges under the Ontario Building Code (OBC) feature an internal maximum temperature stop (limit ring). If this is misadjusted or slipped, it physically blocks the handle from turning far enough into the hot position.
Check the Thermostatic Cartridge Calibration: Sediment or mineral scale can jam the internal wax thermal motor inside a thermostatic cartridge, preventing it from correctly blending hot and cold water.
Tankless Cycling (Low-Flow Cutoff): Tankless combi units require a specific minimum flow rate to stay ignited. If your showerhead is a high-efficiency low-flow model, it may not draw enough water volume to keep the tankless burner continuously active, causing the 10-second cold drops you described.
A licensed plumber can inspect and recalibrate your thermostatic cartridge or check the flow switch settings on your combi water heater to ensure consistent hot water delivery.
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