The Tepid-Tap Test: Why Your Glow-worm Combi Feels Weaker in a Cold Snap
There is a particular November moment in many UK homes. The heating has kicked in, the radiators are warm, and yet the shower feels like a polite suggestion rather than a proper soak. If you have a Glow-worm combi, the first instinct is often to blame the boiler. Fair enough. It is the thing with the display and the pipes. But once the weather turns, the water coming into your house changes too, and that can make a perfectly healthy combi feel underwhelming.
This is not about excuses for a poorly installed or badly maintained boiler. It is about doing a sensible check before you book a call-out. A combi heats water as you use it, so its hot-water performance is tied directly to the temperature and flow rate of the cold mains supply. In autumn, that supply might be around 12–15°C in many parts of the UK. By January, it can be closer to 5–8°C. The boiler has to work harder for every litre you draw, and the result at the shower head can feel like a step down.
The temperature-rise maths, without the algebra headache
A combi does not store a tank of hot water. It raises the temperature of whatever arrives from the mains, up to the hot-water setting you have chosen. The useful number is the temperature rise: incoming water temperature to hot-water outlet temperature.
Suppose your Glow-worm combi is a 24 kW model and you like the hot water at about 50°C. In autumn, with mains water at 15°C, the boiler needs a 35°C rise. In a cold snap, with mains at 5°C, it needs a 45°C rise. Same boiler, same tap, same pipework — but the boiler can only heat so many litres per minute at that bigger rise. In rough terms, the hot-water flow can fall by around a fifth to a quarter when the mains gets that much colder.
That is why the shower can feel weaker even though nothing has broken. The boiler may be running at its maximum hot-water output and still delivering fewer litres per minute than it did in September. If you have a higher-output combi, the effect is less dramatic, but it rarely disappears completely.
What this test is — and what it is not
This is a homeowner-level check. You will need a stopwatch, a measuring jug or a bucket with litre markings, and a thermometer that can read tap temperatures. A digital kitchen thermometer is fine. You are not opening the boiler casing, adjusting gas settings, or fiddling with internal components. Those jobs belong to a Gas Safe registered installer.
The aim is to gather a few honest numbers: flow rate, tap temperature, and what happens when more than one hot outlet is running. That gives you a much better conversation with an installer than “the shower feels rubbish”.
Step one: pick a tap and measure the flow
Use the same tap each time, ideally a kitchen tap with no aerator nonsense and no mixer set to warm. Run the hot tap for 30 seconds to let the boiler settle, then fill a bucket for exactly one minute. Note how many litres you collected. A jug-and-stopwatch method works too, but a bucket is less fiddly.
Do this before the morning rush if you can. Mains water is often coldest first thing, and that is when a combi is most likely to show its winter colours. Write the number down. Six litres a minute is very different from ten.
Step two: take the temperature
Let the hot tap run until the temperature stops climbing, then fill a mug and check it with the thermometer. Compare that with your boiler’s hot-water setting. If you have set the combi to 55°C and the tap is reading 48°C, that is worth noting. Some drop is normal because of pipe runs and heat loss, especially at a low flow rate. A big gap, or a temperature that swings up and down, is a clue.
Do not crank the hot-water setting to maximum in the hope of getting a stronger shower. A higher target temperature means a bigger temperature rise, which usually means fewer litres per minute from a combi. Turning it up can make the shower feel weaker, not stronger.
Step three: run two outlets at once
Turn on the shower at a normal setting, then open the kitchen hot tap. Watch what happens to the shower. A combi serves one hot-water demand at a time in practice, so some drop is expected. If the shower collapses to a dribble, or the temperature at the kitchen tap never really gets hot, make a note. It may point to a flow or pressure issue rather than a boiler fault.
Also check whether the cold supply to the shower is stronger than the hot. A mixer shower can feel weak because the hot side is restricted, not because the boiler has given up.
Step four: rule out the dull stuff
Before you reach for the phone, check the simple things. Is the stopcock fully open? Has a service valve been half-closed and forgotten? Is the boiler pressure where it should be? Are there any error codes on the display? Has the condensate pipe frozen in a cold snap? If there is a leak, a pressure drop, or a smell of gas, stop and call a professional. For a gas smell, ring the National Gas Emergency Service on 0800 111 999.
If the tap has a mesh aerator, give it a clean. If the shower head is scaled, soak it. These are not boiler repairs, but they can absolutely flatten a shower’s performance.
When the numbers say “call an installer”
Book a Glow-worm installer if the hot water is lukewarm regardless of flow, if the temperature fluctuates wildly, if the boiler locks out, if pressure keeps falling, or if you notice leaks or unusual noises. A blocked inlet filter, a scaled plate heat exchanger, a tired diverter valve, or a flow restrictor issue can all affect hot-water performance. An installer can measure dynamic mains pressure and flow, check the boiler’s hot-water output, and tell you whether the combi is doing its best with a cold supply or whether something needs attention.
The winter flow test is not about turning yourself into a heating engineer. It is about separating “my boiler has failed” from “my incoming water is colder and my combi is at its limit”. That distinction saves time, money, and a lot of standing in a tepid shower wondering what went wrong.
