Hot Water Takes Ages To Reach The Tap

What's happening?

When you open a hot tap, the water that comes first is the cold water that has been sitting in the pipe since the last time. The hot water behind it cannot arrive until that has run out, so the wait is the volume of pipe between the heat source and the tap divided by the flow. A tap two metres from the boiler barely waits. A kitchen tap at the other end of the house, on a long run of wide pipe, can wait for minutes. That is not a fault, but it is worth fixing because it wastes water and patience. Hot water that arrives lukewarm and then warms up slowly points at the pipe losing heat on the way, usually because it is uninsulated or buried in a cold floor. A combi adds a few seconds as the boiler lights and the plate heats. Hot water that never gets hot is a different problem.

What usually causes it?

• A long run of pipe between the boiler or cylinder and the tap
• Oversized pipework, such as 22 mm pipe where 15 mm would do, holding more cold water
• Uninsulated hot pipes that lose heat to a cold floor, wall or loft
• A dead leg, a branch pipe holding stagnant water, before the tap
• A low flow from a tap or aerator, which stretches the wait
• On a combi, the few seconds for the boiler to light and the plate heat exchanger to warm
• A cylinder system with no circulation loop to keep hot water close to the taps

Is it dangerous?

No. A very long wait is a waste rather than a risk. Stored hot water that sits warm in a long pipe is a separate matter for hygiene, which is why unused outlets should be run.

Can I fix it myself?

Insulate any hot pipe you can reach, such as in the airing cupboard, under the kitchen sink or in the loft, with foam lagging. Check the tap or aerator is not clogged with limescale, which cuts the flow. On a combi, check whether a pre-heat or comfort mode is available in the settings. Use the hot tap that is nearest first if a short draw is all you need.

When should you call a heating engineer?

Call a heating engineer or plumber if you want the wait cut properly. Options include shortening or resizing a pipe run, a small pump that circulates hot water so it is always near the taps, or a small point-of-use heater at a distant tap. If the delay has suddenly got worse, that points to a fault, and the no-hot-water page applies.

What will the heating engineer actually do?

• Trace the hot pipe run and measure the length and diameter to the slow tap
• Check whether pipes are insulated and whether there is a dead leg
• Fit a secondary circulation loop with a pump and controls where the run is long
• Fit a local instantaneous heater at a remote sink where a loop is impractical
• Check the combi settings for a pre-heat or keep-hot option

Questions people ask

Sources & References

1. Vaillant: what is a combi boiler?
2. Vaillant: what is a system boiler?
3. HSE: legionnaires' disease