Hot-only pressure problems point at the cylinder, the coil, a blending valve, a pump or scaling in the hot distribution. If cold is weak too, the cause is upstream and it is a different page.
First, establish that it really is hot-only
This one question splits the diagnosis in half and it takes a minute.
| What you find | Where the problem is |
|---|---|
| Hot weak, cold fine, at the same tap | The hot side. This page |
| Hot and cold both weak at the same tap | That outlet – aerator, isolation valve, flexible tail |
| Hot and cold both weak everywhere | The incoming main or a pressure reducing valve – see low water pressure |
| Hot weak upstairs, fine downstairs | Gravity system with limited head – a pump is the likely answer |
| Hot weak at one outlet only | That outlet, or its blending valve |
| Hot fine at the kitchen, weak elsewhere | Unusual – worth investigating the distribution |
In most UK houses the kitchen cold is fed directly from the incoming main. If it runs strongly and the hot is weak, you have proved the mains is fine and the problem is downstream on the hot side. That single comparison saves a lot of guessing.
What causes hot-only weakness
A gravity system, working as designed
If you have a cold tank in the loft and a cylinder in a cupboard, the hot water arrives by gravity – its pressure comes purely from the height of the tank above the outlet. A first-floor shower sitting a metre or so below the tank has very little head, and no amount of adjustment changes that. It is not a fault; it is the system.
The answers are a correctly specified pump, or a conversion to unvented if the incoming main supports it – see vented cylinders.
A scaled coil or heat exchanger
In hard water, the surfaces that heat your water scale up. On a combi, a scaled plate heat exchanger both insulates and restricts, so hot water gets cooler and slower together – which is the defining hard water complaint in this county. On a cylinder, a scaled coil reduces reheat rate rather than flow, so you run out rather than getting a weak flow.
A blending or mixing valve
Thermostatic mixing valves at outlets, or a blending valve on the cylinder, restrict flow by design and can restrict it further as they scale or fail. A TMV that has partially failed is a common and easily missed cause – see thermostatic mixing valves.
Scaling in the hot distribution
The hot pipework itself scales over decades, reducing bore. It is slow and it is real, and it affects hot only because the cold side deposits nothing.
A failing or wrongly specified pump
Where a pump is fitted, it may be failing, running on one impeller, drawing air from a badly taken feed, or simply be the wrong type for the head available – see shower pumps.
A partly closed valve
Mundane and worth eliminating. A gate valve on the cylinder outlet that has been nudged, or an isolation valve never fully reopened after work.
The checks worth doing first
- Compare hot and cold at the same tap. The fundamental test.
- Compare upstairs and downstairs. A gravity system will show a clear difference.
- Check the kitchen cold as your mains reference.
- Descale the shower head and tap aerators. Free, and it resolves a surprising proportion.
- Check the cylinder outlet valve is fully open.
- Listen for a pump, if you have one, and check it is running when hot water is drawn.
- Look at the cylinder thermostat setting, because lukewarm is frequently mistaken for weak.
Between them these eliminate the easy causes and tell us a great deal before anybody arrives.
What we do about it
| Cause | Fix |
|---|---|
| Gravity system, insufficient head | A correctly specified and correctly fed pump, or conversion to unvented if the main supports it |
| Scaled combi plate exchanger | Chemical descale, plus scale protection so it does not simply recur |
| Scaled cylinder coil | Descale, or cylinder replacement where it is far gone – see cylinder replacement |
| Failed or scaled blending valve | Service or replace, and set it correctly |
| Scaled hot distribution | Replace the affected section, and treat the water |
| Failing pump | Diagnose the feed and the head type before replacing – the wrong pump is a common expensive mistake |
| Partly closed valve | Open it. Genuinely, this is sometimes the answer |
| Undersized hot pipework | Upsize the affected run |
The pattern worth noticing: in this county, several of those rows come back to hard water. If hot water performance is declining gradually rather than suddenly, scale is the first thing to suspect – see what limescale actually does.
Hot weak, cold fine? Descale the head first, then compare upstairs with downstairs. Those two checks narrow it a long way.
Questions we get asked about this
Why is only my hot water weak?
Because the problem is on the hot side – the cylinder, its coil, a blending valve, a pump, or scaling in the hot distribution. If the cold were weak too, the cause would be upstream.
Will a pump fix it?
If you have a gravity system with a loft tank, yes, and it will transform it. On a combi or unvented cylinder, no – and pumping directly off the mains is not permitted.
My hot water has got gradually worse over years.
Scale, most likely. On a combi that is the plate heat exchanger; on a cylinder it is the coil; in the pipework it is bore reduction. Gradual decline is the signature of scaling rather than a fault.
The hot water is weak and lukewarm.
Check the cylinder thermostat first – lukewarm is frequently mistaken for weak, and they have different causes. If it is genuinely both, a scaled coil explains both at once.
Is this the same as low water pressure?
No. Low water pressure affecting hot and cold together is a mains or whole-house problem. Hot-only is a different shortlist entirely.
Could it be a mixing valve?
Yes, and it is commonly missed. Thermostatic mixing valves restrict flow by design and restrict it further as they scale or partially fail.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.