Tankless & GPM

Notes

• A tankless heater has no capacity, only a curve. two showers means 4 GPM, and lifting that from 55°F to 105°F takes 100,000 BTU/hr delivered to the water. Ask for GPM at a stated rise, never for "how many bathrooms".
• Geography changes the answer more than the house does. The same unit gives 5.7 GPM in the south and 3.8 GPM in the north. The box did not change — the incoming water did. Marketing that promises "3 bathrooms" is quoting the southern number.
• The cold-water sandwich is real and it is not a fault. A tankless fires when flow starts, so there is a slug of cooled water between one draw and the next. Low-flow trickles below the minimum activation rate — a bathroom tap barely open — may not fire it at all.
• Gas line and venting are usually the real cost. A 120k BTU unit often needs a larger gas line than the old tank had, plus dedicated stainless or PVC venting. On a retrofit those two lines regularly exceed the price of the heater itself, and they are what turns a cheap quote into an expensive one.
• Hard water shortens tankless life more than anything else. The heat exchanger scales, and scale is what kills them. Annual descaling, or a softener ahead of it, is not optional above about 7 gpg.