Boiler not producing hot water: six causes and the matching part

Short answer: if the heating works but the tap stays cold, it is rarely the boiler as a whole and almost always one of six parts. With a combi boiler, a clogged plate heat exchanger is the most common cause, followed by a hot water sensor giving incorrect readings and a diverter valve that stays stuck in the heating position. Work through the complaints below from top to bottom; they are in order of likelihood.

First rule out: is it the boiler or the tap?

Before you go to the boiler: run hot water from another tap, preferably the kitchen tap without a thermostat. If only the shower stays cold, the problem is probably in the thermostatic mixer itself, which may be scaled up or blocked. If the kitchen tap also stays cold, it is the boiler.

Then check whether the boiler display shows a code. If there is a code, skip the rest and look it up; we list all codes for almost ninety brands in the fault code database. Staat er geen code, dan hieronder verder.

1. Plate heat exchanger clogged or scaled up

By far the most common cause with combi boilers. The plate heat exchanger transfers the heat from the heating water to the tap water. If it fills up with limescale, less and less heat gets through: the water is lukewarm at first, later cold. Typical picture: the boiler does fire up when you ask for hot water and the radiators work fine.

In hard water areas this often happens after just eight to twelve years. Sometimes an engineer can still descale the heat exchanger chemically; if it is leaking, you will see the heating pressure rise as soon as you run hot water. Replacement usually costs between two hundred and four hundred euros for the part, depending on the brand and output. More on this in our guide on replacing a heat exchanger. Parts: heat exchangers and plate heat exchangers.

2. Hot water sensor giving incorrect readings

The hot water sensor tells the boiler how hot the water going to the tap is. If the sensor reads too high, the boiler stops heating while the water is still cold. The result feels exactly like a faulty heat exchanger, but the cause is a part costing a few tens of euros.

Many appliances report this with a code. On Vaillant it is F.09 (short circuit or open circuit in the hot water sensor), on Bosch A7 (hot water sensor open circuit or short-circuited) and on Remeha E.00.16 of E.00.17 for the hot water cylinder sensor. Intergas reports a fault in the hot water sensor as code 40 for the cylinder sensor. Parts: temperature sensors and NTC sensors.

3. Diverter valve stuck in the heating position

In a combi boiler, a diverter valve switches the water from the radiators to the hot water heat exchanger when hot water is requested. If that valve sticks, all the heat goes to the heating and the tap stays cold. You can recognise it because the radiators get warm while only the hot tap is open.

Sometimes only the motor or the internals are stuck and the whole valve body does not need to come out. Prices vary widely: a separate valve motor costs around a hundred euros, while a complete valve from a premium brand can approach four hundred euros. Parts: diverter valves and hydroblocks.

4. Flow sensor does not detect the flow

The boiler only knows you want hot water when the flow sensor or flow switch detects flow. If it does not work, simply nothing happens: no burner noise, no hot water, no fault. This is often overlooked because the boiler stays completely quiet.

A dirty strainer in front of the sensor gives the same picture and can sometimes be fixed just by cleaning it. Parts: flow sensors and flow switches, usually between fifty and a hundred euros.

5. Water pressure too low or a locked-out boiler

If the heating pressure is too low, the boiler switches off the hot water function first. Check the pressure gauge: when cold, the pressure should be between 1.5 and 2 bar. Top up if necessary and then bleed the radiators. If the pressure drops again within a few days, the expansion vessel has often lost its pre-charge pressure.

If the boiler is in lockout, reset it once. If the same message comes back immediately, leave the reset button alone and find the cause first.

6. Cylinder, cylinder sensor or anode (only with a storage cylinder)

If you do not have a combi boiler but a system with a separate cylinder, the list shifts. The cylinder sensor is then the first suspect; it is reported on Vaillant as F.03, on Bosch as A5 and on AWB as 08. In addition, the condition of the cylinder determines how much hot water you still have: a depleted anode eventually leads to corrosion of the cylinder. Parts: anodes.

If you notice that you are running out of hot water faster and faster, that is usually a sign that the cylinder is dirty or the sensor is no longer accurate, not that the cylinder has become too small.

What can you do yourself and what not?

You can do the following yourself: test another tap, descale the thermostatic mixer, top up the water pressure and bleed, and clean the strainer in front of the flow sensor if you can reach it without opening the boiler casing. Everything behind that casing is work for a certified gas engineer, in line with local regulations. That also applies to replacing the plate heat exchanger, the diverter valve or a sensor inside the boiler.

Feel free to order the part yourself and have it fitted by a qualified installer; that saves the mark-up an installer charges on parts. Do check first that the part fits your exact model. General information on safe heating and servicing can be found under Milieu Centraal.

Frequently asked questions

Waarom heb ik wel verwarming maar geen warm water?

Because heating and hot water use different parts in a combi boiler. The heating circuit can still work fine while the hot water side (plate heat exchanger, hot water sensor, diverter valve or flow sensor) fails. That is exactly why this complaint can almost always be traced back to a single part.

The water gets hot at first and then cold again. What is that?

That usually points to a partly scaled-up plate heat exchanger: the first litres are still hot, after which the heat exchanger can no longer transfer the heat quickly enough. A hot water sensor that reads too high gives a similar picture.

Can low water pressure block hot water while the heating still works?

Yes. Many appliances switch off the hot water function first when the pressure is too low, and only later the heating. So always take a quick look at the pressure gauge before looking further.

Which part should I order if I know the code?

Look up the code in our fault code database. Each code shows the cause and the matching part, with the manufacturer number and the appliances it fits. If you only know the model of your boiler, search in the spare parts catalogue by make and model.

Is having no hot water dangerous?

Not in itself. But it is a reason not to keep resetting endlessly: if a safety device intervenes, there is a reason for it. If you smell gas, see soot marks or the carbon monoxide alarm goes off, switch off the boiler, open a window and call a certified gas engineer immediately.