Vaillant Boiler Not Heating? Four Scenarios — And What to Do About Each

I've learned to slow down when someone says "my Vaillant boiler isn't heating." That sentence covers at least four different situations, and the fix for one is usually a waste of money on the others.

Quick note on who's talking: I'm a quality and brand compliance manager at an HVAC service company. I review every service report before it reaches the customer — roughly 300 a quarter. In 2024 I sent back about 12% of the first drafts, mostly because the tech jumped to a fix before confirming the symptom. Not a knock on the techs. Just that "not heating" is a description, not a diagnosis.

Here are the four scenarios, in the order they usually walk in the door:

  1. The boiler fires, but the radiators stay cold. (Water-side problem.)
  2. The boiler won't fire, or it's showing a fault code. (Boiler-side problem.)
  3. The boiler heats fine — you just can't control it. (Control problem.)
  4. It's not the boiler at all. (Comfort-load problem.)

Find yours, or skip to the last section if you're not sure.

Scenario 1: The boiler fires, but the radiators stay cold

This is the one I see diagnosed wrong most often. The boiler is running — you can hear it, the flue is warm — but the rooms don't heat up.

Most of the time, the fault is on the water side, not the boiler: air in the system, a stuck thermostatic radiator valve (TRV), or radiators that were never balanced to begin with. The TRV issue is sneaky. The spring and pin inside the valve body seize up, and it usually happens over summer, when nobody touches the heating for four months. Come October, one radiator just sits cold and everyone blames the boiler.

I made this exact mistake in 2023. I assumed a cold radiator meant a failing heat exchanger. Didn't verify. Turned out the TRV head on the affected radiator had a seized spring and that one room hadn't heated since the previous spring. The customer had been paying for a "hot water problem" that was actually a £40 valve.

Quick checks you can do without touching gas:

  • Feel the pipes. Both cold = no water reaching the radiator. One hot, one cold = likely a stuck valve or airlock.
  • Bleed the radiator. If air hisses out, that's your answer (at least partly).
  • Press the TRV pin. Pop the head off and press the small pin underneath. It should move down and spring back. If it's stiff, that's your problem.
  • Check the balance. If the radiator nearest the boiler is roasting and the furthest one is cold, the system probably needs balancing — a job for a heating engineer, not a DIY fix.

None of this requires opening the boiler. Good — because you shouldn't.

Scenario 2: The boiler won't fire, or it's throwing a fault code

Now it is the boiler. And here the rules change.

On a Vaillant Ecotec Pro boiler, a few fault codes cover most of what you'll see. F22 means low water pressure. F28 or F29 usually means an ignition failure. F75 points at a pressure sensor or pump issue. The exact list is in Vaillant's Ecotec Pro installation and servicing manual — check the revision that matches your model, since the codes have shifted slightly across generations.

Here's what you can safely do yourself: check the pressure gauge on the front of the boiler. As of the current Ecotec Pro documentation, cold system pressure should sit around 1.0 to 1.5 bar. If it reads below 1.0, you can top up using the filling loop — that's a legitimate owner-level task on most models.

Here's what you can't: anything past the case. Gas, flue, combustion, the sealed components — that's Gas Safe territory (or your local equivalent). Not a suggestion. It's the law in the UK, and it's the difference between a £90 service visit and a very different kind of phone call.

I watched a colleague weigh this one up. The upside of a DIY top-up was saving the £90 callout. The risk was over-pressurizing a sealed system past 2.5 bar and popping the pressure relief valve — which dumps water, corrodes the discharge pipe, and turns a simple top-up into a repair. The expected value said "just do it yourself." The downside felt catastrophic. They called the engineer. Smart call.

So glad I checked the gauge on one job before authorizing a top-up. It was already at 2.4 bar — one more fill and the PRV would have discharged. Almost turned a £0 fix into a £200 clean-up.

One more thing: if the pressure keeps dropping after you top up, you have a leak somewhere. Stop topping up. Every top-up adds fresh oxygenated water to the system, and that's what actually eats heat exchangers over time. A slowly leaking system isn't a maintenance annoyance — it's a countdown.

Scenario 3: The boiler heats fine — you just can't control it

The radiators get hot. The hot water works. But the house is either too warm, too cold, or "comes on whenever it feels like it." That's a controls problem, and it's the most common "boiler fault" that isn't one.

If you're using a Honeywell thermostat, three things cause 80% of the complaints I see:

  1. Dead batteries. Sounds obvious. It isn't — a thermostat with a low battery sits there looking fine while the schedule quietly stops running.
  2. A "hold" or "permanent override" setting. Someone pressed a button in November, set the temperature to 15°C "just for now," and never turned it off.
  3. A schedule that was never actually set. Plenty of thermostats ship with a default schedule nobody ever changed.

The general sequence is: check the battery icon first, then check whether the display says HOLD or PERM, then walk the schedule day by day. If none of that works, the thermostat may not be paired to the boiler correctly — and that's a job for whoever installed it.

Honestly, the number of callouts I've seen where the "fix" was two AA batteries is embarrassing. (For us, not the customer.)

Scenario 4: It's not the boiler at all

This is the one nobody wants to hear, and it's the one I care most about catching before anyone pays a callout fee.

A lot of "my heating isn't working" calls are really comfort-load complaints. The room is cold because of drafts, bad insulation, or a ventilation setup pulling heat out faster than the boiler can put it in. No amount of Vaillant tweaking fixes that. You can raise the flow temperature, replace the boiler, and the room still won't feel right.

The flip side shows up seasonally. In summer, the same commercial clients who complain about cold workshops in January call up about heat in July — and the answer isn't anything on the boiler. In an industrial or warehouse setting, a misting fan on the load side does more for comfort than any heating adjustment. Different problem, different tool. I'd rather tell a customer that up front than sell them a "system health check" that can't possibly address what's actually bothering them.

So before you call anyone: is the problem that the system isn't working, or that the space isn't comfortable? Those are basically two different conversations.

How to tell which scenario you're in

Run this in order. Stop at the first "yes."

  • Is the boiler firing? Listen for it. Warm flue = firing. If no → Scenario 2.
  • Is the boiler firing but at least one radiator cold? If yes → Scenario 1.
  • Are all radiators hot but the room temperature doesn't match the thermostat? If yes → Scenario 3.
  • Is the system working as designed, but the space still feels wrong? If yes → Scenario 4.

If two of those are true at once, you probably have two problems. That happens more than you'd think, and it's why a technician who diagnoses over the phone is worth less than one who shows up and measures.

Before you book anyone

Whatever scenario you're in, ask two questions before you agree to anything:

"What's included in the callout fee?" — not "how much is the callout?"

And then:

"What's not included?"

The vendor who answers those plainly — even if the number looks higher than the cheap one — usually costs you less by the end. A £50 callout that turns into a mandatory £120 diagnostic, plus parts, plus VAT, isn't cheaper. It just feels cheaper at the start.

On that note: in most markets, advertised price claims have to be substantiated. The FTC is explicit about this for US advertisers — a headline price must be truthful and not misleading, which is why "boiler fix from £50" with a hidden diagnostic charge is the kind of thing that draws complaints. If a company won't give you the all-in number in writing, that tells you something useful before anyone shows up.

You don't need a universal answer to "why isn't my Vaillant heating working." You need to know which of the four situations you're actually in. That's the whole game.