Radiators Warming Up When the Hot Tap Is On – Our Experience at Afterglow Plumbing & Heating
At Afterglow Plumbing & Heating, we’ve been repairing boilers across Birmingham for years, and one of the stranger issues our customers call us about is:
“Why do my radiators heat up when I run the hot tap?”
It might sound minor, but this is actually a sign of a deeper problem inside your boiler – usually either a faulty diverter valve or a blocked hot water heat exchanger. We see this often in Birmingham homes, and it’s a problem worth fixing quickly before it causes more serious damage or leaves you without heating and hot water altogether.
Why This Happens
Diverter Valve Problems
Most modern homes in Birmingham run on a combi boiler. Inside every combi boiler is a part called the diverter valve. Its job is simple:
- When you turn on your heating, it sends hot water to the radiators.
- When you run a hot tap, it switches the flow so that all the hot water goes to your taps and showers instead.
When the valve gets worn out or clogged up with sludge, it doesn’t switch properly. We’ve seen countless cases where some heat still slips into the radiator circuit, so every time the hot tap is turned on, radiators start to warm up too.
Blocked Hot Water Heat Exchanger
The other common cause is a blocked plate heat exchanger. This is the part that transfers heat from your boiler to your tap water. In Birmingham, we’re in a hard water area, which means limescale builds up quickly inside the exchanger. Over time, it clogs up with scale and sludge, causing heat to “spill over” into the radiator side of the system.
We once visited a home in Moseley where the radiators would get almost as hot as the taps every time someone washed their hands – all because the plate heat exchanger was choked with limescale. A deep clean sorted it, but in some cases, a full replacement is the only option.
The Different Types of Diverter Valves We See in Birmingham Homes
Over the years repairing boilers across Birmingham, we’ve come across two main styles of diverter valves:
Cartridge Type Diverter Valve
- Found in many modern combi boilers.
- Works a bit like a tap cartridge – a moving spindle or plunger opens and closes internal ports to direct water either to the heating circuit or the hot water circuit.
- These are compact and generally reliable, but they’re prone to sticking if sludge or scale builds up in the system water.
- When cartridge valves fail, the boiler often tries to send heat both ways at once, which is why radiators warm when a hot tap is on.
Paddle (or Flap) Type Diverter Valve
- Older or budget boilers often use this design.
- It uses a simple paddle or flap that physically swings over to block or allow flow depending on whether hot water or heating is being called for.
- Over time, the paddle can wear down, warp, or get jammed by debris.
- When that happens, it doesn’t seal properly, so hot water “leaks” into the radiator circuit.
We’ve replaced dozens of these in Birmingham homes — and in some cases, just cleaning out the paddle mechanism bought the customer another few years of life from their boiler.
The Motor That Drives the Diverter Valve
Whether it’s a cartridge or a paddle type, both need a small electric motor (often called an actuator) to move the valve.
- The boiler’s PCB (circuit board) sends a signal to the motor when you open a tap or switch on the heating.
- The motor then turns gears or drives a spindle, moving the valve into the right position.
- If the motor burns out, the gears strip, or the actuator gets stuck, the valve won’t move properly.
From our experience, sometimes it’s not the valve itself that’s failed, but the motor/actuator. In those cases, replacing just the motor can be a faster and cheaper fix than changing the entire diverter valve.
We always explain this to our customers in Birmingham — no point paying for a whole new valve if just the motor has failed.
Signs You’ve Got This Issue
From our day-to-day call-outs in Birmingham, here are the signs we often see:
- Radiators warm when running a hot tap (even when the heating is off).
- Lukewarm or fluctuating hot water – sometimes piping hot, sometimes barely warm.
- Boiler making unusual noises (gurgling, banging, or humming).
- Error codes or lockouts on the boiler when trying to use hot water.
- Slow hot water flow from taps or showers.
If you’ve noticed more than one of these, it’s time to get it checked.
How We Fix It
Every boiler and every home in Birmingham is a little different, but here’s what we typically do when faced with this problem:
- Thorough inspection – we check the diverter valve, hot water heat exchanger, and system water quality.
- Testing the diverter valve – if it’s stuck, we try cleaning it, but more often than not, it needs replacing.
- Checking or replacing the motor – sometimes only the actuator motor is faulty, and swapping it out fixes the issue.
- Cleaning or replacing the heat exchanger – sometimes a chemical flush or descaling works, but if the exchanger is badly blocked or cracked, we’ll fit a new one.
- System treatment – to prevent the same thing happening again, we add corrosion inhibitor and check if a magnetic filter is installed. If not, we recommend one.
In most cases, the repair takes just a few hours, and your boiler is back to normal the same day.
Real Costs We See in Birmingham
We know customers hate vague answers about costs, so here’s what we see locally:
- Diverter valve replacement: usually £250–£350 depending on boiler model.
- Motor/actuator replacement only: often cheaper, saving the customer money.
- Heat exchanger cleaning: less, if caught early.
- Heat exchanger replacement: £300–£450 including parts and labour.
Of course, if your boiler is very old and parts are hard to source, we’ll always be honest if a full replacement is the more cost-effective option.
How to Prevent This
From our experience, prevention is always cheaper than repair. Here’s what we recommend to all our Birmingham customers:
- Annual boiler servicing – this lets us catch diverter valve or exchanger issues early.
- Magnetic filter installation – stops sludge from circulating.
- System inhibitor – protects against corrosion.
- Water treatment – especially important in hard water areas like Birmingham.
We’ve seen brand-new boilers fail in just a few years because these simple steps were skipped.
Frequently Asked Questions
Why do my radiators heat up when I run the hot tap?
This usually means the diverter valve inside your combi boiler isn’t working properly, or your hot water heat exchanger is blocked. Both issues are common in Birmingham and can be fixed by our engineers. Learn more about our boiler repair services in Birmingham.
Is it dangerous if my radiators get hot with the tap on?
It’s not usually immediately dangerous, but it is a clear sign your boiler is struggling. Leaving it can cause more serious breakdowns. It’s best to get it checked as soon as possible. Book your repair with our Birmingham boiler repair team.
How much does it cost to replace a diverter valve in Birmingham?
On average, diverter valve replacement costs between £250 and £350. If only the actuator motor needs replacing, it’s often cheaper. For an accurate quote, contact our local Birmingham boiler engineers.
Can I fix a diverter valve myself?
No – a diverter valve repair or replacement must be carried out by a Gas Safe registered engineer. DIY work on gas appliances is unsafe and illegal. Call our Gas Safe boiler specialists in Birmingham for help.
Will a new boiler stop this problem happening again?
Yes, if your boiler is very old, it may be more cost-effective to replace it. A new boiler with clean system water and the right protection (like a magnetic filter and inhibitor) will prevent diverter valve issues in future. We can advise whether a repair or replacement is best for your situation.
When to Call Us
If your radiators are heating up when the hot tap is on, don’t ignore it. This isn’t just a quirk – it’s your boiler telling you something’s wrong. Left untreated, it can waste energy, damage your system, and in rare cases, even lead to dangerous faults.
At Afterglow Plumbing & Heating, our Gas Safe registered engineers deal with this issue week in, week out across Birmingham – from Edgbaston to Harborne, Moseley to Selly Oak. We’ll diagnose the problem quickly, explain your options clearly, and get your heating and hot water back to normal.
Need Boiler Repair in Birmingham?
Call Afterglow Plumbing & Heating today for fast, friendly, and expert boiler repairs. Whether it’s a faulty diverter valve, a blocked heat exchanger, or a worn motor, we’ll get it sorted.
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