Why is my hot tap slower than my cold tap?
Why a difference between hot and cold is normal on some systems and a fault on others, and how to tell which you have.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 3 min
Short answer
On a gravity system, hot comes from a loft tank a few metres up and cold comes from the mains, so hot is genuinely slower and always will be. On a combi or unvented cylinder both are at mains pressure, so a slow hot tap is a fault — usually a scaled aerator, a partly closed valve, or a blocked flexible tail.
The first question is not what is wrong but what your system is. A gravity-fed hot supply running slower than the mains cold is physics, not a fault, and no amount of cleaning will change it.
What you'll need
- A bucket of known volume and a timer
- Descaler
- A spanner
Step by step
- Identify the system.Cold tank in the loft plus a cylinder means gravity hot and mains cold — a difference is expected. No tanks at all means a combi, and both should be similar.
- Measure both.Time how long each tap takes to fill a known volume. A difference of a few per cent is nothing; hot at half the cold rate on a combi is a fault.
- Check whether it affects one tap or all of them.One slow hot tap is local — the aerator, the valve or the tail. Every hot tap slow points at the cylinder, the boiler or the main hot distribution.
- Clean the aerator.Unscrew the nozzle from the spout and rinse the gauze. Hot water deposits scale faster than cold, so the hot side blocks first — this is the single most common cause.
- Check the isolating valve under the basin.The hot valve nudged closed during other work is very common. The slot should be in line with the pipe.
- Check the flexible tail.A kinked, twisted or internally collapsed flexible tap connector on the hot side strangles the flow. Straighten it, or replace it if it is old.
- Consider scale in the pipework on a combi.In hard water areas the hot side of a combi's heat exchanger and the first stretch of pipe scale up. That needs a plumber and possibly a descale.
- Consider a pumped or accumulator solution on gravity.If the system is gravity and the hot is genuinely too slow to be usable, the answer is a pump or a change of system, not a repair.
- Look at the cylinder outlet on a gravity system.Old cylinders and their gate valves scale and corrode internally, and a seized gate valve part-open is a classic slow-hot cause.
Tips
- Clean the aerator first. It takes two minutes and solves a large share of slow hot taps.
- Compare with another hot tap in the house before you take anything apart — it localises the problem instantly.
- On a gravity system, the higher the tap, the slower the hot. An upstairs basin close to the tank will always be feeble.
Common mistakes
- Assuming a gravity hot supply is faulty — A metre or two of head produces a fraction of mains pressure. Slower hot is the correct behaviour for that system and no cleaning will change it.
- Removing the aerator permanently to gain flow — It splashes badly, and the gauze also catches grit that would otherwise reach the cartridge. Clean it rather than discarding it.
If it doesn't work
One hot tap slow, others fine
Cause: Aerator, isolating valve or flexible tail at that tap — Fix: Clean the aerator, check the valve is open, inspect the tail.
All hot taps slow, combi system
Cause: Scale in the boiler's hot circuit or the distribution pipework — Fix: Needs a heating engineer; a descale or a filter may be recommended.
All hot taps slow, gravity system
Cause: Normal, or a seized cylinder gate valve — Fix: Check the gate valve moves fully; otherwise consider a pump.
Got slower after other plumbing work
Cause: Debris carried into the aerator or the tail — Fix: Clean the aerator; flush the tail through into a bucket.
Questions people ask
Should hot and cold run at the same rate?
On a combi or unvented cylinder, yes, near enough. On a gravity system with a loft tank, no — the hot will always be slower and that is normal.
Can I speed up a gravity hot supply?
Only by adding a pump, raising the tank, or increasing the pipe size. Cleaning and adjusting will not overcome a lack of head.