Do This Now (Safe Checks)
You’re aiming to confirm it’s “hot water only” and not a controls / settings issue. Keep it simple. No boiler covers off.
- 1) Check the thermostat is actually calling for heat Raise it above room temperature. If it’s a wireless stat, check the batteries. Sounds basic — but this catches a surprising number of callouts.
- 2) Look at the boiler pressure Most systems behave best around 1.0–1.5 bar when cold. If it’s low, topping up may get you going — but if your boiler keeps losing pressure, don’t keep feeding it forever. That’s when you want leak detection that avoids pointless digging.
- 3) Try one reset only One reset, then leave it. If it immediately drops back out, don’t keep hammering reset like a vending machine.
- 4) Feel the heating flow/return pipes (if accessible and safe) If you’ve got access under the boiler area and it’s safe to touch (not scalding), this can hint whether the boiler is attempting heating at all. If you’re unsure, skip it.
- 5) Check radiator valves aren’t all turned down If you’ve got TRVs, make sure a couple are open to demand heat. Again: seems obvious. Panic makes people miss obvious.
If you want the fastest diagnosis on the phone, tell me three things: “pressure reading”, “any fault code”, and whether the boiler sounds like it’s trying to run heating. Call: 07706 889 614.
Why This Happens (Most Likely Causes)
In this exact scenario — hot water works but heating doesn’t — we’re usually looking at a few repeat offenders. I’ll keep it honest: some are “simple”, some aren’t, and most aren’t solved by guessing.
If only some radiators are staying cold, you’re usually dealing with distribution (TRVs, bleeding, balancing, circulation). If now nothing is working at all, that’s a different failure path and needs faster triage.
If your boiler pressure keeps falling for no reason, don’t keep topping up endlessly — it’s a leak pattern until proven otherwise. And if you’ve received a Section 75 leak warning letter, handle the notice properly so you don’t end up paying for the wrong repair.
Diverter valve / actuator issues
In many combis, a diverter sends heat either to hot water (taps) or to central heating (rads). If it’s stuck or the motor/actuator is failing, you can get perfect showers… and freezing radiators. This is one of the most common “hot water but no heating” callouts.
Heating demand not reaching the boiler
Thermostat issues, wiring faults, programmer settings, zone valve problems (if you’ve got zones). The boiler can still fire for hot water because that demand is direct when you open a tap.
Circulation / pump / blockage symptoms
Sometimes heating tries to engage but can’t circulate properly. Pressure, debris, sludge, pump issues. You might hear the boiler “try”, then back off.
Boiler pressure too high (yes, that’s a thing)
People top up repeatedly and end up with pressure climbing too high, which can trigger safety behaviour, leaks from PRVs, and odd symptoms. If you suspect the boiler pressure is too high, don’t start draining randomly — call and we’ll handle it properly.
What this looks like in real life
I had one in BR6 where the customer said: “boiler not working… but the shower’s fine.” They’d reset it six times, topped up twice, and were ready to call it a write-off. It wasn’t a new boiler. It was a heating-side issue that needed proper diagnosis — and once it was fixed, the heating came back immediately. That’s why I keep the DIY steps short on purpose.
When to Stop DIY (Hard line)
If you’re about to start opening panels, turning random screws, or endlessly topping up — stop here.
- If you smell gas Leave the property and call the National Gas Emergency number: 0800 111 999.
- If you need to remove the boiler casing Don’t. Gas appliances are not a “have a go” job. This is for a registered engineer.
- If pressure is swinging low or too high Low pressure can lock out; high pressure can dump water and cause leaks. Both need proper handling. If the boiler keeps losing pressure, start with leak detection in Orpington & Bromley.
FAQ
Is “hot water but no heating” usually a boiler replacement?
Why do the taps work if the boiler is “broken”?
I topped up pressure and now it’s high — what should I do?
How do I know if it’s linked to a leak?
What if radiators are the issue rather than the boiler?
If you’re in Orpington or Bromley and you’ve got hot water but no heating, call 07706 889 614. If you’re stuck near Bromley South / The Glades area or you’re coming off the A232, mention it — it helps me plan the visit properly.