Eli5 – why is the first few moments of every shower cold, even if I already warmed up the water via the bath first?

Question

Don’t understand how the plumbing works. I keep thinking the water should already be hot.

In: Engineering

Answer 1

To clarify, you mean if you run the tub to warm the water up, then pull the lever (or use whatever method your tub uses to switch from the faucet to the shower head), why does the water initially come out cold?

If accurate – the shower head is usually a few feet, at minimum, above the faucet…so while the the water and pipes up to the faucet level is warm, it cools a little when it suddenly has to flow through cold pipes up to the shower head. In my experience though this cooling doesn’t last very long, like a second or two max before it warms back up.