Vaillant Boiler: No Hot Water (Causes & Fixes)

Vaillant's ecoTEC range is one of the most common boilers in UK homes, and "no hot water" is one of the most common faults owners report. The good news is that the cause is often something simple.

The important thing is knowing which checks are genuinely safe for a homeowner and which ones cross the line into gas work that only a Gas Safe registered engineer should touch.

This guide walks through the likely causes on a combi ecoTEC, what you can check in a couple of minutes, and when to book an engineer.

How a combi makes hot water on demand

It helps to know what should happen when you open a hot tap. A combi has no stored cylinder — it heats water on demand. When you run a hot tap, a flow sensor detects the moving water and tells the boiler there's a hot-water demand.

The boiler fires and a diverter valve redirects the flow of heated water away from the radiators and through a separate plate heat exchanger, where heat is transferred to the cold mains water passing through to your tap.

The moment you turn the tap off, the diverter springs back to its heating position.

That chain — flow sensor detects demand → boiler fires → diverter switches → plate heat exchanger heats the mains water — is why "no hot water" on a combi usually traces back to one of those parts, or to the boiler not firing at all.

Common causes of no hot water on a Vaillant ecoTEC

If you have a combi boiler (heating and hot water on demand, no separate cylinder), the usual suspects are below. The first few you can sometimes check or clear yourself; the rest sit inside the sealed system and are engineer-only.

• Low system pressure — if the pressure drops below around 1 bar, the boiler may refuse to fire, often showing a fault code such as F22. (Homeowner-checkable.)
• Thermostat or programmer not calling for hot water — if the controls or timer have been knocked to "off", set to heating-only, or the room thermostat has nothing scheduled, the boiler may simply not be asked to run. Worth ruling out before anything else. (Homeowner-checkable.)
• A frozen condensate pipe — in cold snaps the white plastic waste pipe outside can freeze and block, locking the boiler out (often with an F28). This is one you can sometimes thaw yourself. (Homeowner-checkable.)
• A gas-supply interruption — if other gas appliances (hob, fire) have also stopped, or your meter's emergency control valve or a prepayment balance has run out, the boiler simply has no gas to burn. Check those before assuming a boiler fault. (Homeowner-checkable, but never touch the gas pipework or meter internals.)
• A stuck diverter valve — this valve switches the boiler between heating and hot water. If it jams, you can end up with heating but no hot water (or the other way round). (Engineer-only.)
• A faulty DHW / flow sensor or thermostat — the domestic hot water (DHW) flow sensor tells the boiler the tap is running and how hot the water is. If it fails, the boiler may not register the demand and won't heat water. (Engineer-only.)
• Scale or sludge in the plate heat exchanger — in hard-water areas, limescale builds up in the plate heat exchanger and reduces or blocks hot water flow. You may notice lukewarm water that runs hot then cold. (Engineer-only.)
• An airlock or trapped air — air drawn into the system (often after draining, a repair or a top-up) can stop water circulating properly and starve the hot-water side. Clearing it correctly is part of an engineer's diagnosis. (Engineer-only on a sealed combi.)
• A seized or faulty heating pump — if the internal pump fails or seizes, water won't circulate and the boiler can't transfer heat. It sits inside the casing. (Engineer-only.)
• An F-code lockout — the boiler has detected a fault and shut itself down for safety, displaying an F-code on the screen. (Reset once; engineer if it returns.)

Safe checks you can do yourself

These checks are homeowner-safe and resolve a good proportion of "no hot water" calls. None of them involves removing the casing or touching gas, the flue or any internal part.

1. Check the programmer and thermostat

Before anything else, make sure the boiler is actually being asked to make hot water. On the boiler or a wireless control, check there's a hot-water tap symbol or the hot-water mode is on (not set to heating-only or "off"), and that any timer or programmer isn't sitting in a scheduled off-period.

It's an easy thing to knock by accident, and it costs nothing to rule out.

2. Check the pressure gauge

Look at the pressure gauge or digital pressure reading on the front of the boiler. Cold, it should sit at roughly 1 to 1.5 bar, rising towards 2 bar when the heating is hot. If it reads below 1 bar, the pressure is too low and the boiler may have locked out.

You can top it up yourself using the filling loop — the small braided hose with a valve or tap underneath the boiler. Open it slowly until the gauge reaches about 1.5 bar, then close it firmly.

If the pressure keeps dropping after you top it up, there is a leak somewhere and you'll need an engineer to find it.

3. Read the fault code

Vaillant boilers display an F-code on the screen when something is wrong. Note it down exactly — it tells the engineer a lot before they even arrive.

For example, F28 relates to a failed ignition or no flame (and can be triggered by a frozen condensate pipe — see below), while F22 points to low water pressure. Don't try to "work around" a code; it's the boiler protecting itself.

4. In a cold snap, check the condensate pipe

If the temperature has dropped below freezing and the boiler has locked out, a frozen condensate pipe is a common, homeowner-fixable cause. This is the white plastic pipe that runs from the boiler to an outside drain.

Vaillant's own advice is to pour warm — not boiling — water along the frozen section to melt the blockage; boiling water from the kettle can crack the pipe, so let a kettle cool for about ten minutes first, or use a hot-water bottle held against the pipe. Once it's clear, reset the boiler once.

Lagging the pipe afterwards helps stop it happening again. If you can't safely reach the pipe, leave it for an engineer.

5. Reset the boiler — once

If there's a lockout code, you can press the reset button on the front panel once to clear it. Wait a couple of minutes to see whether the boiler fires and hot water returns. If it locks out again, stop. Repeatedly resetting a boiler that keeps failing can mask a genuine fault — book an engineer instead.

No fault code at all? Read the live status when you run a tap

If there's no F-code but still no hot water, you can do one more read-only check. With the boiler in its information/live-monitor mode, open a hot tap and watch the screen.

On many Vaillant ecoTEC models the display shows a flow reading or an SE status value: a low value (often shown as SE05 or near zero) means the boiler isn't detecting any flow when it should — pointing towards the flow sensor or a flow problem — whereas a higher value (such as SE12–SE14) means flow is being detected and the fault lies elsewhere.

This is purely observing the screen, so it's safe to do, and the value is exactly the kind of detail worth giving the engineer. (Indicative — the exact codes and how to enter monitor mode vary by model and year; always check your boiler's manual, and don't act on the reading beyond noting it down.)

Symptom	Likely cause	Who fixes it
Pressure below 1 bar, no heating or hot water	Low pressure / lockout	You can top up; engineer if it keeps dropping
Nothing fires at all, controls look off	Programmer / thermostat not calling for hot water	You can check and switch back on
Lockout in freezing weather (often F28)	Frozen condensate pipe	You can thaw with warm water; engineer if unreachable
Boiler and other gas appliances all dead	Gas-supply interruption / prepay run out	Check supply; engineer/supplier for the gas side
Heating works but no hot water	Stuck diverter valve	Gas Safe engineer
Water runs hot then cold, or only lukewarm	Scale in plate heat exchanger	Gas Safe engineer
No hot water on demand, no obvious code	Faulty DHW / flow sensor or thermostat	Gas Safe engineer
Gurgling, poor circulation, no hot water	Airlock / trapped air or seized pump	Gas Safe engineer
F-code on screen	Lockout (various)	Reset once; engineer if it returns

What a Vaillant repair typically costs

If the fault is internal, it helps to know roughly what you're looking at before the engineer arrives. The figures below are indicative 2026 UK ranges for the fitted job (part plus Gas Safe labour) and vary by region, model and how accessible the part is — always get a written quote.

Repair	Indicative fitted cost (2026)	Notes
Diverter valve replacement	~£150–£300	The part is fairly cheap; most of the cost is labour.
DHW / flow sensor replacement	~£100–£200	Sensor itself is inexpensive; labour-led.
Plate heat exchanger replacement	~£400–£600+	Bigger strip-down; flush/descale may be cheaper if caught early.

Costs indicative only, last checked 2026; your engineer's quote is what counts.

When it's a job for an engineer

If you've checked the programmer, the pressure and (in cold weather) the condensate pipe, and a single reset still leaves you with no hot water, the cause is almost certainly internal — a diverter valve, a DHW/flow sensor, scale in the heat exchanger, an airlock or a seized pump.

These all sit inside the boiler casing and involve the sealed system, so they're firmly engineer territory. A Gas Safe registered engineer can test the diverter, swap a failed sensor, bleed an airlock, free or replace the pump, or flush and descale the heat exchanger safely.

One thing to check first if the water is merely lukewarm rather than cold: on the boiler's controls, make sure the hot-water temperature isn't set too low.

Vaillant recommends a hot-water setting of around 50°C (most guidance suggests 50–55°C), which is hot enough for taps and showers — above about 45°C at the tap risks scalding, so it's mixed with cold at the point of use. Combis heat on demand, so there's no Legionella risk in setting it here.

If it's already at 50–55°C and the water is still only lukewarm, that points back to scale in the plate heat exchanger or a sensor fault — an engineer's job.

You can confirm any engineer is qualified by checking their registration on the Gas Safe Register. Anyone working on your gas boiler must be on it — always ask to see the card.

Could it be the same fault as no hot water on any boiler?

Many of these causes aren't unique to Vaillant. If you'd like the general version of this troubleshooting, our guide to no hot water from a boiler covers the same ground across combi, system and heat-only models. And if your screen is showing a specific Vaillant code, our Vaillant F28 explainer breaks down what that one means and what to do.

Frequently asked questions

This article is general information, not advice. We compare a selected panel of providers, not the whole market, and may earn a commission. Always use a Gas Safe registered engineer for gas work.