Boiler heating works but no hot water (and the reverse)

If the radiators heat up but the hot water has stopped, the gas, ignition and burner are working: the fault is on the hot water side. The most common causes are too little flow at the tap, a scaled flow sensor, a stuck diverter valve or a scaled heat exchanger. The reverse often points to the same valve or the thermostat.

For a repair at home, see our heating engineers in Paris.

How a boiler chooses between heating and hot water

A combi boiler (chaudière mixte) makes both heating and hot water with the same burner. When you open a hot tap, a flow sensor tells the control board. A diverter valve, called a three-way valve (vanne trois voies), then sends the heated water to the domestic hot water heat exchanger instead of the radiators. Hot water takes priority, and heating resumes when the tap closes.

So three things must work: enough flow to be detected, a diverter valve that switches, and a heat exchanger that passes the heat on. If one fails, you get heating without hot water.

First: what heats your water?

In Paris this is not always obvious. In many buildings, hot water comes from a central system run by the syndic (the building manager), or from an electric water heater separate from the boiler. If your neighbours have no hot water either, it is a building fault: tell the syndic. If your hot water comes from an electric tank, the boiler is not involved: see our guide on emergency electric water heater repair.

Checks you can do yourself

1. Mode and temperature setting

Some boilers let you turn hot water off, or set its temperature. On Saunier Duval's ThemaPlus Condens, the hot water setting goes from 38 to 60 °C. A setting at the minimum gives lukewarm water, especially in winter when the incoming cold water is colder.

2. One tap or every tap?

If only one tap stays cold, the boiler is not the problem: it is the mixer tap, its cartridge or its aerator. Our tap, shower and toilet service handles that. If every tap is cold, read on.

3. The one-litre test

A combi boiler only fires if the water runs fast enough. The ThemaPlus Condens manual sets this minimum flow at 1.5 litres a minute. Open the hot tap fully and fill a one-litre jug: if it takes more than 40 seconds, the flow is below that threshold and the boiler may not fire. The usual causes are a scaled aerator or shower head, a blocked filter on the boiler's cold inlet, a half-closed isolation valve, or low pressure throughout the flat, covered in our article on low water pressure in a flat. The threshold varies by brand: look for "minimum flow" (débit minimal) in your manual.

4. The display when you open the tap

Many boilers show a tap or flame symbol while making hot water. If nothing changes on the display when you open the tap fully, the boiler is not seeing the demand: flow sensor or board. If the symbol appears but the water stays cold, the diverter valve or the heat exchanger is more likely. Note any code shown.

Heating yes, hot water no: causes inside the boiler

• The flow sensor, scaled up or faulty: the boiler does not know a tap is open.
• The diverter valve, stuck in the heating position, or its motor dead: the heat goes to the radiators instead of the hot water exchanger.
• A scaled domestic hot water heat exchanger: water comes out lukewarm or uneven. The Saunier Duval manual calls for this exchanger to be checked when hot water is insufficient or too cool. Paris limescale plays a big part: the 588 public hardness measurements in Paris between July 2025 and June 2026 give a median of 26.2 °f (French degrees of hardness).
• The hot water temperature sensor, sending a false reading and cutting the burner too early.

The reverse: hot water yes, heating no

The burner and gas work, since hot water comes out. The suspects are then:

• Summer mode, left on: the boiler only makes hot water.
• The room thermostat: flat batteries, setting too low, away mode.
• The diverter valve, this time stuck in the hot water position.
• The circulation pump, which pushes water to the radiators, seized or worn.

If only some radiators stay cold, the problem is in the radiator circuit (air, sludge, a thermostatic valve). If nothing works at all, neither heating nor hot water, our guide on what to do when the heating fails is the place to start.

What not to do

• Open the cover to move the diverter valve or its motor.
• Keep resetting the boiler when a code comes back.
• Turn the hot water setting to maximum to make up for lukewarm water: if the exchanger is scaled, the problem stays, and the scalding risk at the tap goes up.

When to call a heating engineer

Call if every tap stays cold although the flow passes the one-litre test, if the water is lukewarm despite a correct setting, or if the heating does not restart after checking the mode and thermostat. Repairs are quoted after diagnosis, with the price given before any work, and the call-out charge inside Paris is 90 euros excluding VAT: see our prices page.

Frequently asked questions

Why does my boiler not fire when I turn on the hot tap?

The boiler only starts if it detects enough flow at the tap: 1.5 litres a minute on the Saunier Duval ThemaPlus Condens, for example. A scaled aerator, a blocked filter or low water pressure can stop it from firing. If the flow is good, the flow sensor or the diverter valve is to blame.

Why is my hot water only lukewarm?

First check the hot water setting on the boiler, adjustable from 38 to 60 °C on some models. If that is right, the domestic hot water heat exchanger is often scaled: Paris water is hard, around 26 °f median. A thermostatic mixer tap set badly can also add too much cold water.

The shower runs hot, then cold. Why?

Most often the flow drops below the firing threshold when you turn the mixer down, and the boiler stops. Open the hot side further. If it happens at full flow, a scaled heat exchanger overheating or a faulty sensor can cut the burner.

Can I free the diverter valve myself?

No. It sits inside the boiler, next to the gas and electrics, and its motor is removed with the cover off. It is a job for a heating engineer, usually quick once the fault is identified.

How much does the repair cost?

It depends on the part: flow sensor, diverter valve motor, sensor or heat exchanger. At Réseau Tubulure, boiler repairs are quoted after diagnosis, and the call-out charge inside Paris is 90 euros excluding VAT.

In short

Heating but no hot water: first check what heats your water, then the setting, then the flow with the one-litre test. If every tap is cold despite good flow, the flow sensor, diverter valve or hot water heat exchanger is at fault, and it is a job for a heating engineer. Hot water but no heating: summer mode, thermostat, diverter valve or pump.

No hot water in Paris? We repair your boiler

Tell us the boiler brand and the result of the one-litre test, and the engineer arrives with the right idea of the fault. Fast response 7 days a week, 24 hours a day, with priority for emergencies. Réseau Tubulure works across Paris and the inner suburbs (92, 93, 94). Many of our plumbers speak English.

• Phone: +33 1 89 70 15 36
• Form: contact us
• WhatsApp: use the button on the site

Sources: Saunier Duval ThemaPlus Condens installation and maintenance manual (0020238207_08), hot water specifications and maintenance table; Hub'Eau, drinking water quality API, water hardness in Paris, measurements from 1 July 2025 to 30 June 2026. Accessed 27 October 2026.