No Hot Water Upstairs or at One Tap? What It Means
If some of your taps run hot and others do not, the boiler or cylinder is almost certainly working, because it is already making hot water for the rest of the house. The fault is somewhere between the heat source and the cold outlets. At a single tap it is usually a closed or scaled isolation valve, a blocked aerator or a failed tap cartridge. When everything upstairs is cold or weak, and you have a tank in the loft, it is usually an airlock or a lack of height between the tank and the taps. On a combi, a tap that will not run hot is often simply not drawing enough flow for the boiler to fire. Work out which pattern you have, and you have most of the answer.
First, map which taps are affected
Run the hot at every tap in the house for a minute, one at a time, and write down which get hot, which stay cold, and which only trickle. The pattern is the diagnosis.
- One tap cold, the rest fine: the fault is at that tap or the pipe just before it.
- One room cold, such as a bathroom or the kitchen: a valve or blockage on the branch that feeds that room.
- Everything upstairs cold or weak, downstairs fine: typical of a tank fed system with an airlock, or not enough height from the loft tank.
- Hot eventually, but only after a long wait: a long pipe run, or a failed circulation pump in a larger house.
- Every tap cold: that is not this problem. Start with our no hot water checklist instead.
One tap running cold
Look under the basin or sink first. Most taps have a small isolation valve on each supply pipe, with a slot for a flat screwdriver. When the slot runs along the pipe, the valve is open. Across the pipe, it is closed. These get knocked half shut when someone clears out the cupboard, and a half closed valve on the hot side gives you a weak, lukewarm trickle.
Next, unscrew the aerator from the spout and run the tap. If the flow jumps, the aerator was blocked with scale or debris. In London water this is common, especially after work on the pipes has stirred things up. Soak it in descaler or vinegar and refit it.
Modern mixer taps use cartridges, and a cartridge can fail or scale up on the hot side while the cold side is fine. A lever that turns to hot and gives you cold, or a tap that only gets warm, points to the cartridge. It is a small part, but the right one for that tap has to be found.
A cold kitchen tap on a combi
A combi only fires when water flows through it fast enough. A kitchen mixer with a restrictive aerator, or one that is only opened a little, may not draw enough to start the boiler, so you get cold water while the shower upstairs runs hot. Open the tap fully, towards hot, and see whether the boiler fires. If it does, the tap is the problem, not the boiler.
If no tap in the house gets the boiler firing, even fully open, it is a boiler fault, usually the flow sensor. Our guide to no hot water from a combi boiler covers that.
Showers and baths: the thermostatic valve
A thermostatic shower or bath mixer contains a cartridge that blends hot and cold to a set temperature. When it fails, it usually fails safe, towards cold, which feels exactly like losing the hot water. If the basin in the same bathroom runs hot and the shower does not, the cartridge is the likely cause. Our page on no hot water in the shower goes through that in detail.
Some baths and basins, particularly in newer flats and anywhere built for landlords, are fed through a separate blending valve under the bath panel or in a cupboard. When that valve fails or scales up, every outlet it feeds runs cold or lukewarm. Our guide on whether you need a thermostatic mixing valve explains what they do.
No hot water upstairs on a tank fed system
In a house with a cold water tank in the loft and a vented cylinder, the hot water to your taps is pushed by gravity: the only pressure is the height of the tank above the tap. Upstairs taps and showers sit closest to the tank, so they get the least pressure, and anything that interrupts the flow shows up there first.
An airlock is the most common cause. Air collects at a high point in the pipework, often after the water has been drained down or the loft tank has run low, and the small pressure available upstairs cannot push it through. The tap spits, hisses, then gives little or nothing. Our guide to airlocks in hot water pipes explains how they are cleared.
Check the loft tank too. If it is low, or its float valve is stuck, the hot side will fade upstairs before anywhere else. If it is overflowing or warm, that points to a different fault, covered in our page on the header tank overflowing.
Hot water that takes ages to arrive
If a far tap does eventually run hot, but only after a minute or more, the hot water is working and the pipe run is long. The cold water sitting in the pipe has to be drawn off first. That is normal in a large house, or where the cylinder is at one end and the kitchen at the other.
Some larger houses have a secondary return, a loop of pipe with a small pump that keeps hot water moving so it arrives at the taps straight away. If hot water used to be instant and now takes a long time, that pump or its timer has probably failed.
After work on the pipes
If the problem started straight after a new tap, a new kitchen or a bathroom refit, suspect the work first. Hot and cold swapped over, an isolation valve left closed, or debris lodged in a new tap are all common. On an unvented cylinder or combi, a mixer tap with a faulty check valve can also let cold water push across into the hot pipe, so the hot runs lukewarm. Our guide to warm water from the cold tap covers the same crossflow from the other side.
When to call us
Call us on 020 4652 5111 or send an enquiry if the isolation valves are open, the aerator is clear and the tap is still cold, if a whole floor has lost its hot water, or if a blending valve or shower cartridge needs replacing. Tell us which taps are affected and what kind of system you have, combi or cylinder, tank in the loft or not. We have 20 engineers across London, and our rate is on the pricing page, agreed before anyone attends.
Questions we get asked
- Why do I have hot water downstairs but not upstairs?
- On a system with a loft tank, the upstairs taps have the least pressure, so an airlock or a low tank shows up there first. On a pressurised system it is more often a closed valve, a failed blending valve or a scaled tap on that floor.
- Why is there no hot water from my kitchen tap only?
- Check the isolation valve under the sink and the aerator on the spout. On a combi, open the tap fully, because a low flow may not be enough to start the boiler. If those are fine, the tap cartridge is the usual cause.
- Can one tap have no hot water if the boiler is fine?
- Yes, and it is the most common reason for a single cold tap. If any other outlet runs hot, the boiler or cylinder is making hot water and the fault is local to that tap or its pipe.
- Why is my bath hot tap cold when the basin is hot?
- Look for a blending valve under the bath panel, which can fail or scale up, and check the isolation valve on the bath feed. On a mixer, the cartridge may have failed on the hot side.
- How do I fix an airlock in my upstairs hot tap?
- An airlock usually needs mains pressure pushed back through the hot pipe to shift it, which our airlock guide explains. If it keeps coming back, the cause is often a low loft tank or a fault in the pipework layout.