Boiler 7 September 2026

Combi Boiler Hot Water Not Hot Enough? Why Mid Kent's Hard Water Is Usually Behind It

If your combi boiler’s hot water is not hot enough but the heating still works perfectly, the fault is almost never the boiler as a whole — it is the hot water side of it, and in this part of Kent that usually means limescale. A combi heats water instantly through a small plate heat exchanger, and in a hard water area that exchanger furs up long before anything actually breaks.

That is why the symptom creeps up on people. Nobody rings us on the day their shower goes cold, because it never does — it just gets a little less hot each winter until one morning it is not really hot at all.

Why is my hot water not hot enough on a combi boiler?

A combi has two separate jobs and two separate water circuits. The heating circuit is a closed loop — the same water goes round the radiators for years. The hot water circuit is mains water, fresh from the street, passing through a plate heat exchanger every time you open a tap.

That difference matters. The heating side is protected simply because it is sealed — the same water circulating over and over cannot keep depositing new scale. The hot water side takes fresh mains water every time, and every litre brings more dissolved calcium into a very small, very hot component. So when someone tells us the radiators are fine but the hot water has gone lukewarm, that is a useful clue: it points straight at the part of the boiler that mains water touches.

The mid Kent problem: what the chalk does to your boiler

Maidstone sits on the Medway at the foot of the North Downs, and the water that arrives at your stopcock has come through chalk. South East Water, which supplies Maidstone along with Tonbridge, Tunbridge Wells and Sevenoaks, is straightforward about what that means: most of the water it supplies in southern England comes from underground chalk aquifers, so the water is hard, and that leads to a build-up of limescale in kettles, boilers and hot water pipes.

Everyone in mid Kent already knows the kettle half of that. The boiler half is invisible, which is exactly why it goes unaddressed for years. Hardness varies by supply zone, so if you want the figure for your own address rather than a general statement about the area, South East Water publishes a hardness search you can run against your postcode.

What our engineers see on boiler repairs around Maidstone and the surrounding villages is the predictable end of that process: an ageing combi, never descaled, with a heat exchanger that can no longer move heat into the water fast enough. If you want a Gas Safe registered engineer to look at yours, our plumbers covering Maidstone work out of Tonbridge and cover the whole area.

What scaling actually does inside the boiler

A plate heat exchanger is a stack of thin metal plates with hot boiler water running down one side and your mains water running up the other. It works because the plates are thin and the gaps are narrow.

Limescale attacks both of those things at once. It coats the plates, and scale conducts heat poorly, so less heat crosses over — and it narrows the already narrow channels, so less water gets through. That is why the two symptoms usually arrive together:

• The water is not as hot as it was, particularly when more than one tap is running.
• The flow drops off when you open the tap fully — turn it down a little and the temperature improves, because the water is spending longer against the plates.

If you recognise that second one, you have effectively already diagnosed it: a boiler giving decent hot water at a trickle and lukewarm water at full flow is not short of power, it is short of heat transfer.

What temperature should the hot water be set at on a combi boiler?

Before assuming the worst, check the setting — it is free, and it is wrong more often than you would think. A combi has two dials or two menu settings: one sets the heating flow temperature for the radiators, the other sets the hot water temperature for the taps. They are easy to confuse, and we have been called to boilers where somebody had turned the wrong one down over the summer.

For most homes, somewhere around 50 to 55°C is the sensible middle ground. There is no stored cylinder in a combi, so there is no need to run it scaldingly hot, and running it lower genuinely slows the rate at which new scale forms. If the setting is already sensible and the water still is not hot, the setting was never the problem.

Combi boiler not enough hot water for a bath — is that the same fault?

Not necessarily, and this is worth separating out.

Running out of heat partway through filling a bath, or the shower going cold when someone runs the kitchen tap, can simply be the boiler being too small for how the house is used now. A combi is rated by how many litres a minute it can heat, and a family of five asks more of it than two people did. That is a sizing question rather than a fault, and the answer is a conversation about a new boiler rather than a repair.

Can a combi boiler run out of hot water? In the sense a cylinder does, no — there is no tank to empty. But it can fail to keep up with demand, and that feels identical from inside the shower. The distinction we look for is whether the hot water was ever good enough: if the boiler used to fill a bath happily and now cannot, something has changed and scaling is the likeliest candidate.

Things it might be instead

Not every lukewarm tap is limescale. Before condemning a heat exchanger we rule out the ordinary things:

• A sticking diverter valve, which decides whether the boiler heats the radiators or the taps. Stuck partway, it gives hot water that never quite commits.
• A scaled tap aerator or shower head — if only one outlet is affected, the boiler is innocent.
• A faulty temperature sensor telling the boiler the water is hotter than it is.
• Low incoming mains pressure, which limits what any combi can deliver.

If you have no hot water at all rather than lukewarm hot water, that is a different diagnosis — heating but no hot water walks through that one properly.

What can actually be done about a scaled combi

There are two honest answers, and which applies depends on how far gone the exchanger is.

A scaled plate heat exchanger can sometimes be chemically descaled in place, or removed, flushed and refitted. On a boiler that is otherwise sound, that is well worth doing. On a heavily scaled exchanger the plates have often corroded too, and the sensible repair is replacing the part rather than repeatedly cleaning it.

Either way it is work on a gas appliance and must be done by a Gas Safe registered engineer — we are registered under number 951167. Never attempt to dismantle the appliance yourself; descaling chemicals and an open boiler are not a DIY afternoon.

Our repair work is charged at £95+VAT for the first hour, which is always billable, then £47.50+VAT for each hour after. Materials are confirmed and agreed with you before we fit or order anything.

Keeping it from happening again

In a hard water area you cannot stop scale forming, but you can slow it down:

• Have the boiler serviced annually. A boiler service is a flat £95+VAT, and it is when hot water performance gets checked properly rather than noticed accidentally.
• Fit a scale reducer on the mains feeding the boiler. On a new installation in mid Kent we would generally recommend one, and many manufacturers expect one in hard water areas as a warranty condition.
• Do not run the hot water hotter than you need. Higher temperatures precipitate scale faster.
• Raise it when you replace the boiler. Protection fitted on installation day is far easier than dealing with a scaled exchanger later.

Get it looked at properly

If your hot water has been quietly getting worse and you are in Maidstone or anywhere else across Kent, ring us on 01732 792 454 and we will send a Gas Safe registered engineer to diagnose it rather than guess at it. We will tell you plainly whether it is a setting, a valve, a scaled exchanger or a boiler that was never big enough for the house — and what each of those actually involves.