A Vaillant boiler with no hot water usually has a fault in one of the parts that only work on the hot water side: the flow sensor that spots a tap opening, the diverter valve, the plate heat exchanger or the hot water sensor. Low pressure, a frozen condensate pipe or a gas supply problem can stop it too, though those tend to take the heating out as well. On an older ecoTEC, a new Vaillant boiler is usually the better long-term option for efficiency and lower repair bills.
We’re Clever Energy Boilers, a Gas Safe registered (no. 554138) installer based in Bingley and accredited by Worcester Bosch. Over the last 15 years we’ve completed more than 25,000 installations across Yorkshire, the North West and the Midlands. This guide covers the Vaillant parts behind a hot water fault, a quick test you can do at the tap, what’s safe to check yourself and when replacing the boiler makes more sense.
Why has my Vaillant boiler got no hot water?
Either the boiler isn’t noticing that you’ve opened a hot tap, or it notices and can’t get the heat through to the water. On an ecoTEC combi, the usual suspects are:
- A flow sensor that doesn’t pick up the tap opening, often because of scale or a blocked inlet filter
- A diverter valve stuck on the heating side, or a damaged wiring harness to it
- A scaled plate heat exchanger
- A faulty hot water sensor (NTC)
- A fault on the printed circuit board (PCB), which does happen even when every part checks out
- Something more general, like low pressure, a frozen condensate pipe or no gas
Start with the display. An F code tells you the boiler has found a fault and stopped, so deal with that first using our Vaillant boiler error codes guide. No code at all points to one of the hot water parts above. For other makes, our general guide to heating that works but no hot water covers the same problem.
Vaillant no hot water at a glance
This is how the symptoms usually line up with the parts:
| What you notice | Most likely cause | Who deals with it |
|---|---|---|
| No status code appears when you open a hot tap | Flow sensor or its inlet filter | Gas Safe registered engineer |
| Hot water only at full flow | Scale on the flow sensor or a blocked inlet filter | Gas Safe registered engineer |
| Status code appears but the taps stay cold, radiators warm | Diverter valve or its actuator | Gas Safe registered engineer |
| Water only lukewarm | Scaled plate heat exchanger or hot water sensor | Gas Safe registered engineer |
| F22 on the display | Low water pressure | You can top up |
| F28 or F29 in freezing weather | Frozen condensate pipe or gas supply | You can thaw the pipe and check the gas |
How can I tell whether my Vaillant is detecting the hot tap?
Watch the display while you run a tap. Most ecoTEC boilers show a status code, an S followed by a number, that tells you what the boiler is doing at that moment.
- Check there’s no F code showing first.
- Open a hot tap all the way.
- Watch the boiler display for a few seconds.
If a status code appears as the water starts running, the boiler has seen the demand. The problem then sits further along, in the diverter valve, the plate heat exchanger or the hot water sensor.
If nothing changes, the boiler isn’t registering that you want hot water at all. That points straight at flow detection: the flow sensor, its inlet filter or the electronics reading it. The exact S number differs between ecoTEC generations, so check the manual for your model rather than a list you’ve found online.
What stops a Vaillant ecoTEC making hot water?
Flow sensor not picking up the tap
Vaillant combis use a hot water flow sensor to detect a tap opening. DIY forums sometimes call it the “aqua sensor”. If it fails or scales up, the boiler never finds out you want hot water, and no status code appears when the tap runs. A blocked inlet filter does the same thing, and it shows up first at low flow.
Diverter valve stuck or its wiring damaged
The diverter valve switches heat between your radiators and your taps. Stuck on the heating side, it leaves you with warm radiators and cold water. A damaged wiring harness to the valve or its actuator causes the same symptoms. Our guide to a faulty diverter valve goes through the signs, and we explain why freeing one is an engineer’s job in our article on a stuck diverter valve.
Scaled plate heat exchanger
This small exchanger passes heat from the boiler water into the mains water heading for your taps. As scale builds up inside it, less heat gets across, so the water comes out lukewarm before it stops warming at all. Our guide to heat exchanger faults covers the wider picture.
Faulty hot water sensor
The hot water sensor is an NTC thermistor that reads the temperature of the water leaving the boiler. A wrong reading leaves the boiler unable to control your hot water. Testing it means opening the casing.
PCB fault
Sometimes every hot water part checks out and the control board is to blame. In one case on the DIYnot forum, a Vaillant with no hot water was only put right with a new PCB. Our guide to boiler PCB faults explains what’s involved.
Pressure, condensate and gas supply
These stop the hot water and the heating together. Low pressure brings up the Vaillant F22 fault code. A frozen condensate pipe or a gas problem can stop the boiler lighting, which shows as F28, or losing its flame, which shows as F29. Other codes you might see alongside a hot water problem include F75 and F83, and if the display shows an egg timer, our guide to the egg timer on a Vaillant boiler explains it.
System and regular boilers: the zone valves
With a Vaillant system or regular boiler and a hot water cylinder, things work differently. A motorised zone valve sends heat to the cylinder, and a micro-switch on that valve tells the boiler there’s a demand. If the micro-switch fails, the cylinder stays cold while the heating carries on. A DIYnot thread showed exactly this on a Vaillant system boiler.
What can you safely check yourself?
Work through these in order. None of them involve taking the casing off.
- Any F code on the display? Look it up before anything else.
- Pressure. Vaillant’s manual gives 1 to 2 bar on the gauge with the boiler cold. If it’s lower, top the pressure up, and our guide to what boiler pressure should be explains the gauge.
- Gas. Make sure the hob or another gas appliance works, and that a prepayment meter has credit.
- Condensate pipe. In freezing weather, find the plastic pipe running outside and thaw any frozen section with warm water, never boiling. Our guide to defrosting a condensate pipe shows how.
- A hot tap at full flow. Open it all the way and watch the display, as described above.
- One reset. Press reset, then try the hot tap again.
Frozen condensate pipes are a regular winter problem round here. Boilers on the exposed Pennine hillsides around Queensbury and Thornton tend to be the first to go, because their outside pipe runs catch the wind.
Is a Vaillant with no hot water dangerous?
Usually not. Cold taps are a nuisance rather than a safety risk. Gas is the exception: if you smell it, open the windows, don’t touch any switches, turn the gas off at the meter and call the National Gas Emergency Service on 0800 111 999.
When should you stop and call a Gas Safe engineer?
If one reset doesn’t bring the hot water back, stop there. Every likely cause from this point on, the flow sensor, diverter valve, plate heat exchanger, hot water sensor and PCB, sits inside the boiler, and that work is for a Gas Safe registered engineer. Never take the casing off yourself.
Stop as well if the pressure keeps falling after you’ve topped it up. That usually means a leak, and a boiler losing pressure needs finding properly.
How much does a Vaillant hot water repair cost?
These are typical UK prices from a third-party guide (247homerescue), so treat them as a rough guide:
| Vaillant hot water repair | Typical UK cost |
|---|---|
| Diverter valve | £250 to £375 |
| Heat exchanger | £400 to £700 |
| PCB | £250 to £500 |
Should I repair or replace a Vaillant with no hot water?
On a young boiler, one failed part is often the whole story, and repairing it is fine. An older ecoTEC is different. The hot water parts tend to wear out together, and forum threads are full of owners who swapped the diverter valve, then the flow sensor, and still had cold taps at the end of it. Think about replacing the boiler if:
- It’s more than 8 years old, as boilers lose efficiency with age
- You’ve already paid for one hot water repair and the problem came back
- The quote involves the heat exchanger or PCB, which pushes the bill towards the price of a new boiler
- You’ve needed more than one engineer visit this year
Our boiler replacement service comes with a fixed price, and a long, market-leading warranty is available through Clever Energy on every new boiler. You can spread the cost with our boiler finance options. To weigh it up, read how to know when your boiler needs replacing and our new boiler cost guide.
Get a new Vaillant boiler with Clever Energy
We fit new Vaillant boilers in Bingley, Bradford, Leeds, Keighley, Huddersfield and Halifax, and across the North West and the Midlands from Manchester and Liverpool down to Birmingham and Leicester. See our Vaillant boiler installation page, get your fixed online boiler quote in a few minutes, or request a callback. Once it’s in, a Clever Care plan keeps your new boiler protected.
Vaillant boiler no hot water FAQs
Why is my Vaillant heating working but not the hot water?
Because one of the hot water parts has a problem, most often the flow sensor, the diverter valve or the plate heat exchanger. The radiators don’t rely on those parts, so they can stay warm while the taps run cold.
Why does my Vaillant only give hot water at full flow?
It isn’t detecting lower flow rates. Scale on the flow sensor or a blocked inlet filter are the usual causes, and both need a Gas Safe registered engineer.
Is it the diverter valve?
Often, yes, if a status code appears when you open the tap but the water stays cold. If no status code appears at all, flow detection is the more likely culprit.
Does warm start affect Vaillant hot water?
Not when you’ve got no hot water at all. Warm start is one of Vaillant’s hot water comfort settings, so check it against your manual if someone has changed it, but a total loss of hot water points to the flow sensor, diverter valve or plate heat exchanger.
Should I repair or replace a Vaillant with no hot water?
Repair a young boiler with one failed part. Replace an older one, especially if it’s over 8 years old or the fix involves the heat exchanger or PCB.
Different make, or a boiler that’s stopped altogether? Read our guides to Worcester boiler no hot water, combi hot water that goes cold and a Vaillant boiler not working with no fault code.