Tankless units fail quietly when undersized. Homeowners feel it as lukewarm showers when the dishwasher runs. This issue breaks down the sizing mistakes we see most often.
Start with simultaneous demand
Count fixtures that can run together, not the square footage of the house. A three-bath home with laundry on the same loop often needs more GPM than the sticker on a “whole-home” unit suggests.
Gas and venting are half the job
Many “failed installs” are actually undersized gas lines or improper venting — not bad heat exchangers. Ask for:
- BTU load calculation
- Manometer reading after install
- Condensate management plan for high-efficiency units
Sponsored inclusion · Sample slot
Copper & Flow Mechanical — Denver metro · copperandflow.example · tankless replacements & recirculation loops. Format demo for sponsored Roundup placements.
Recirculation: comfort vs. waste
Dedicated return lines beat timer-only pumps on long runs. If a quote skips recirculation entirely in a two-story home, ask why.
Bottom line
Buy GPM for your peak hour, not the marketing brochure. Pair the unit with the gas and vent work it actually needs.
