Hot Water Pressure Low but Cold Fine? Causes and Fixes
By Emergency Plumb,
If the cold taps run strongly and the hot ones only trickle, the fault, or the limit, is somewhere on the hot side of the house. The first job is to work out whether you are looking at a fault at all, or at how your system was built.
Short answer: If you have a tank in the loft and a hot water cylinder, weaker hot pressure is normal, because the hot water has only gravity behind it while the cold kitchen tap is on the mains. If the hot has got worse recently, check for a part-closed valve on the cylinder feed, an airlock, a low loft tank, or scale in tap aerators and shower heads.
Step one: which system do you have?
Loft tank and hot water cylinder (vented). The cylinder is filled from a cold tank in the loft. Hot water pressure depends on how high that tank is above the tap, so upstairs outlets are weak and downstairs ones are a little better. The mains cold tap in the kitchen will always feel far stronger.
Combi boiler. There is no cylinder or loft tank. Hot water is heated as it flows, so its strength depends on the mains and on how much water the boiler can heat at once.
Unvented cylinder. A sealed cylinder fed directly from the mains, so hot and cold should be close to each other.
If you have a vented system and the hot has always been gentler, nothing is broken. If any system has changed, read on.
Checks to make, in order
- One tap or all of them? If only one hot tap is weak, the problem is at that tap: a scaled aerator, a blocked filter in a mixer, or a worn cartridge. Unscrew and clean the aerator first.
- Valves. Find the cold feed into the top or bottom of the cylinder. It usually has a gate valve with a wheel handle. If it has been turned partly closed during other work, the whole hot supply suffers.
- The loft tank. A tank that is not refilling properly lowers the pressure and can let air into the pipes. Check the level and that the float valve opens.
- Airlock. Spitting, hissing hot taps straight after the water was off point to trapped air. Our guide to clearing an airlock explains how.
- Scale. In hard-water London, aerators, shower heads and mixer cartridges slowly close up with limescale. A soak in descaler restores most of them.
If you have a combi boiler
When every hot outlet is weak on a combi, the restriction is usually at the boiler: scale in its heat exchanger, a blocked inlet filter, or a flow setting turned down. Those are inside a gas appliance, so they need a heating engineer. We do no gas work, but we can check the pipework and taps either side of it.
If you have an unvented cylinder
An unvented cylinder has a pressure-reducing valve and a strainer on the incoming main. A blocked strainer or a failing valve can drop the hot pressure while the cold taps upstream of it stay strong. Those parts are serviceable; see our unvented cylinder service.
Making a tank-fed system stronger
On a vented system, the usual fix for weak hot water at a shower is a pump fed from the tank and cylinder, which our guide to low pressure in the shower covers in detail. Replacing the whole system with an unvented cylinder raises hot water pressure everywhere, at a much higher cost.
Questions people ask
Why is my hot water pressure low but the cold is fine?
In many UK homes the cold kitchen tap comes straight off the mains while the hot water comes from a cylinder fed by a tank in the loft, so the hot side only ever has gravity behind it. If that has always been the case, it is the design. If the hot has recently got weaker, something on the hot side is restricting it.
Why has my hot water pressure dropped all of a sudden?
A sudden change points to something that has just happened, such as a valve partly closed during other work, an airlock after the water was off, a loft tank running low, or a combi boiler's water filter blocking. Gradual decline over months is more often scale.
My combi boiler gives weak hot water but the cold is strong. What should I check?
Check whether every hot tap is weak or just one. One tap means the tap or its filter. Every tap means the problem is at the boiler, where scale or a blocked inlet filter reduces flow. That is a job for a heating engineer; we do no gas work.
Is low hot water pressure normal in the UK?
In older homes with a loft tank and a vented cylinder, yes, hot water pressure is noticeably lower than mains cold. Homes with a combi boiler or an unvented cylinder should have hot water at close to mains pressure.