Your Vaillant ecoTEC has gone cold at the tap. Here are the usual culprits, the checks you can safely make yourself, and the point at which it becomes a job for a Gas Safe registered engineer.
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.
If you have a combi boiler (heating and hot water on demand, no separate cylinder), the usual suspects are:
Safety first. Never attempt work on the gas valve, gas pipework, the flue, the sealed combustion circuit, the pressure-relief valve, or anything behind the boiler casing. These are jobs for a Gas Safe registered engineer only. If you ever smell gas, call the National Gas Emergency line on 0800 111 999 and leave the boiler alone.
These three checks are homeowner-safe and resolve a good proportion of "no hot water" calls.
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.
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, while F22 points to low water pressure. Don't try to "work around" a code; it's the boiler protecting itself.
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.
| 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 |
| 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 sensor / thermostat | Gas Safe engineer |
| F-code on screen | Lockout (various) | Reset once; engineer if it returns |
If you've checked the pressure and tried a single reset and you still have no hot water, the cause is almost certainly internal — a diverter valve, a DHW sensor, or scale in the heat exchanger. All three 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, or flush and descale the heat exchanger safely.
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.
Worried about the bill? A diverter valve or heat exchanger repair on a Vaillant can run into several hundred pounds in parts and labour. This is exactly where boiler cover earns its keep — a policy with a breakdown component can mean you pay only a fixed call-out or excess. See whether boiler cover is worth it before you decide.
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.
A diverter valve or heat exchanger repair can be costly out of the blue. Compare cover from a selected panel of UK providers and see what's protected.
Compare boiler coverThis pattern usually points to a stuck diverter valve, which switches the boiler between heating and hot water. The valve sits inside the casing and is a job for a Gas Safe registered engineer to test and replace.
No. The diverter valve is part of the sealed internal system behind the casing. Removing the casing or working on internal components is gas work and must be done by a Gas Safe registered engineer.
Water that starts hot and then cools, or never gets properly hot, often indicates limescale in the plate heat exchanger, especially in hard-water areas. An engineer can flush, descale or replace the part.
Reset only once. If the boiler locks out again straight away, stop and call an engineer. Repeated resets can hide a genuine fault that the boiler is trying to protect you from.
Around 1 to 1.5 bar when the system is cold, rising towards 2 bar when hot. Below 1 bar is too low and can stop the boiler firing — you can top up via the filling loop, but call an engineer if the pressure keeps falling.
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.