Short answer: If your combi gives heating but no hot water, the usual causes are a stuck diverter valve, a plate heat exchanger blocked with limescale, a faulty flow sensor, or low water flow into the boiler. If you have neither, check the pressure, the gas supply and any fault code first. Repairs need a Gas Safe registered engineer.
A combi boiler heats water straight from the mains when you open a hot tap. There is no hot water cylinder. That makes it the usual choice for flats, and in Tower Hamlets most homes are flats. The borough profile says 81% of households live in purpose built flats. So when a combi stops giving hot water in Bethnal Green, Bow or Whitechapel, there is no backup tank to fall back on.
First, is it hot water only, or everything?
This tells you a lot.
| What you have | Likely area of the fault |
|---|---|
| Heating works, no hot water | Diverter valve, flow sensor, plate heat exchanger |
| Hot water is only warm | Scale in the plate heat exchanger, low flow, gas supply |
| Hot water comes and goes | Low flow at the tap, sensor, scale |
| No heating and no hot water | Pressure, gas supply, ignition, power, frozen condensate |
The common causes explained
Diverter valve. A combi has one burner. A diverter valve sends the hot water from the burner either to the radiators or to the plate heat exchanger for your taps. If it sticks, you can get heating but no hot water.
Flow sensor. When you open a hot tap, a sensor sees the water moving and tells the boiler to fire. If the sensor is faulty or blocked with debris, the boiler does not know you want hot water.
Plate heat exchanger. This is a small stack of plates where heat passes from the boiler’s water to your tap water. In hard water it fills with limescale. Tower Hamlets is hard water. Thames Water’s 2025 reports give averages of 276 mg/l for Bethnal Green, 264 mg/l for Bow and 273 mg/l for Whitechapel. Thames Water’s scale puts 200 to 300 mg/l in the hard band. Our guide on hard water and your boiler explains the full picture.
Low water flow. A combi needs a minimum flow of mains water to fire for hot water. In upper floor flats, or where a stop tap is partly closed, flow can drop below that point. The boiler then fires and stops, or does not fire at all.
If you have no heating and no hot water
Work through these safe checks. Never open the boiler casing.
- Power. Is the display on? Check the fused spur, the switch next to the boiler, and the fuse box.
- Pressure. Look at the gauge. If it is below the level in your manual, top up through the filling loop as the manual explains.
- Gas. Does the hob work, if you have one? If not, the gas supply may be off. Check the emergency control valve near the meter is open. The handle is in line with the pipe when on.
- Prepayment meter. If you have one, check there is credit.
- Fault code. Note it. Reset once if the manual allows.
- Cold weather. If it is freezing outside, the condensate pipe may be frozen. Our guide to a frozen condensate pipe covers that.
If you smell gas at any point, stop. Open windows, do not touch switches, turn off at the emergency control valve if safe, leave and call 0800 111 999.
Flats and access
Flats in east London bring a few extra things to plan for. Boilers are often in a kitchen wall cupboard or a hall cupboard. The flue may run up through a ceiling void to the outside wall. If an engineer cannot see the flue along its length, rules introduced in 2013 mean they will class the boiler as “At Risk” and, with your permission, turn it off until inspection hatches are fitted. HSE explains this in its flues in voids bulletin. Our post on boiler flue rules for flats goes into this.
In tower blocks and older council blocks, parking and lift access also affect how long a visit takes. Let the engineer know about entry codes and parking when you book.
What happens on a repair visit
On a boiler repair visit, a Gas Safe registered engineer will run the boiler on heating and hot water and check where it fails. They test the gas pressure at the boiler, the flow rate at a hot tap, and the parts involved in switching to hot water. If the plate heat exchanger is scaled, they can flush or replace it.
On older combis, a scaled heat exchanger may be one problem among several. The engineer can tell you what they found and what the options are. The price is confirmed after a look at the job.
Gas work in the UK must be done by a Gas Safe registered engineer. Ask to see the ID card. The back lists what work the engineer is qualified to do, and a domestic boiler is one category.
Renting in Bethnal Green
Private renting is high here. The Census 2021 shows 38.2% of Tower Hamlets households renting privately, against 20.5% in England. If you rent, report a hot water fault to your landlord or agent in writing. HSE says landlords are responsible for repairs needed through general wear and tear on gas appliances they provide. See our Bethnal Green area page for more on the area’s homes.
Keeping hot water working
- Have the boiler serviced every year.
- Ask whether a scale reducer is fitted on the cold supply. Guidance says one should be used where hardness is 200 ppm or more.
- Do not leave the boiler’s hot water temperature set higher than you need. Thames Water notes scale forms faster at higher temperatures and suggests about 60 degrees.
Quick questions
Why does my combi give heating but no hot water?
A combi uses a diverter valve to switch between heating and hot water. If the valve sticks on heating, taps stay cold. A flow sensor that does not detect the tap opening has the same effect.
Why is my hot water only warm?
In hard water areas the plate heat exchanger, which heats tap water, often scales up. Water then cannot pick up enough heat. A low gas pressure or a worn part can also cause it.
Should I reset the boiler?
If there is a fault code, you can reset once as your manual explains. If the fault comes back, leave it switched off and book a repair.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.