Baxi boiler hot water goes cold when a second tap opens
If a Baxi combi boiler supplies a good hot shower or tap on its own but the water cools when a second outlet opens, the total demand may be exceeding the boiler’s available hot-water flow. A combi heats mains water as it is used; it does not have an unlimited stored supply to feed several outlets at full flow.
Some reduction in flow is expected when two taps share the supply. Water becoming suddenly cold, repeated burner drop-out or the same problem at one low-flow tap can point to a fault or water-supply issue that needs diagnosis.
Why a second tap changes the temperature
A combi boiler has a rated ability to raise a certain flow of incoming cold water by a stated temperature. When another hot tap opens, the available hot water is divided between the outlets. Baxi’s homeowner guidance says two open taps on a combi produce a reduced hot-water flow at both.
If the combined flow remains high, each litre spends less time being heated. The boiler may reduce flow through its own controls, but outlet temperature and pressure can still change. Winter mains water starts colder, so the same boiler may deliver less comfortable simultaneous performance than in summer.
Check whether the boiler is a combi
A combi normally heats tap water directly and has no separate hot-water cylinder. A Baxi system or heat-only boiler usually works with stored hot water, so two-outlet behaviour has different causes involving the cylinder, controls, valves or stored-water recovery.
Check the model label and user guide. Do not assume every wall-mounted Baxi is a combi.
What the model’s flow rate means
Baxi publishes different maximum domestic-hot-water flow rates for different outputs. For example, current Baxi 800 Combi 2 product data lists approximately 10.2, 12.2 and 15 litres per minute at a 35°C temperature rise for the 24, 30 and 36 models. Those numbers are laboratory-style product ratings, not a promise that every installation will deliver the same result at every tap.
Actual performance depends on incoming mains flow and pressure, incoming water temperature, pipe size and length, outlet restrictions and the requested hot-water temperature. Use the figures only after identifying the exact model.
A safe two-outlet test
- Let the boiler return to standby and ensure the heating is not creating confusion.
- Open one hot tap to a comfortable steady flow and allow its temperature to settle.
- Observe the boiler display and flame symbol without removing any covers.
- Open the second hot outlet gradually rather than fully.
- Record whether the first outlet loses pressure, loses temperature or both.
- Close the second outlet and note how quickly the first recovers.
- Repeat with the outlets reversed on another occasion.
Take care with hot water and do not use a thermometer in a way that risks scalding. The pattern is more useful than an exact temperature.
Pattern one: both outlets slow but remain warm
This is often ordinary combi sharing. The same total supply is being divided between two outlets. Reducing the flow at each tap may produce a warmer, steadier result.
If the household regularly needs two showers or a bath and tap at once, the demand may be beyond the design of the appliance and water supply. That is a system-sizing issue rather than necessarily a broken boiler.
Pattern two: the flow stays strong but temperature falls
The combined water flow may be greater than the boiler can heat to the requested rise. A lower flow can give a higher temperature; manufacturer user guides commonly explain this relationship.
Check the domestic-hot-water setting rather than the central-heating flow temperature. Baxi confirms that changing the radiator flow-temperature setting does not change hot-tap temperature. Do not turn every control to maximum as a diagnostic shortcut.
Pattern three: the flame symbol disappears
If the burner goes out while both outlets are still running, record the display and any code. The combined or individual flow may be crossing the boiler’s operating threshold, or a sensor, supply or boiler fault may be interrupting demand.
Close the second tap. If the flame and hot water return reliably at one outlet, tell the engineer. Do not repeatedly reset the boiler or remove its casing.
Pattern four: only the shower turns cold
A thermostatic shower valve mixes hot and cold water and reacts to pressure changes. Opening another tap can disturb that balance. If basin taps remain hot but one shower goes cold, the shower cartridge, filters, check valves or local pipework may need a plumber’s assessment rather than a boiler repair.
Do not dismantle a shower valve without its instructions. Some components retain hot water and must be serviced correctly.
Check the cold-water supply
Baxi explains that combi boilers need adequate mains pressure and flow. If the kitchen cold tap also drops sharply when another outlet opens, the incoming supply or internal plumbing may be the limiting factor.
Ask whether neighbours are affected and check for planned water work. Make sure only an ordinary user-accessible stop tap you know is fully open; do not force a seized valve. A plumber or water supplier can measure dynamic flow and pressure properly.
Tap aerators and outlet restrictions
A blocked tap aerator or shower head can reduce flow, but it usually affects that outlet whether or not another tap is open. Compare outlets and clean only removable user-serviceable parts as their maker describes.
Do not remove flow restrictors permanently. They may be part of the outlet’s safe, efficient operation and can help keep demand within the boiler’s capacity.
What if one tap alone is already lukewarm?
If a single modest-flow tap never becomes properly hot, the issue is not simply two outlets sharing capacity. Possible causes include an incorrect hot-water setting, excessive flow, a temperature or flow-sensing problem, restricted heat transfer, a mixing problem or another boiler fault.
Record whether every hot outlet is affected, whether performance changes from summer to winter and whether the boiler display reaches its target. Internal diagnosis belongs to a Gas Safe registered engineer.
Could preheat solve it?
Some Baxi models have a comfort or preheat function intended to reduce the wait for hot water. It does not increase the boiler’s continuous maximum output or create stored capacity for two high-flow outlets.
Use only the options described in the exact model guide. Preheat may improve initial response, but it is not a cure for water turning cold during sustained simultaneous use.
Why the problem may be worse in winter
Mains water is colder in winter, so the boiler must add more heat to reach the same outlet temperature. A flow that felt acceptable in summer may exceed the comfortable capacity during a cold spell.
This seasonal pattern is useful evidence. It does not justify changing internal output settings or gas controls.
What to tell the engineer
- the full Baxi model and output number;
- which two outlets are involved;
- whether pressure, temperature or both change;
- whether the flame symbol remains on;
- any fault code or unusual sound;
- whether one tap alone is consistently hot;
- whether the problem is seasonal or new; and
- whether cold-water flow also falls.
What not to do
- Do not remove the boiler casing or adjust gas settings.
- Do not confuse central-heating flow temperature with hot-tap temperature.
- Do not keep resetting a boiler that loses its flame.
- Do not remove shower or tap safety devices to increase flow.
- Do not assume a larger boiler alone will overcome a poor mains supply.
- Do not accept scalding water at one tap as the price of feeding two.
When to arrange a repair
Book a professional diagnosis if one outlet alone is also affected, the burner drops out, a fault code appears, temperature swings are severe, the behaviour changed suddenly, or actual performance is well below the model’s expected output despite adequate mains supply.
A competent engineer can measure flow, temperature rise and control response without guessing. Any work inside a gas boiler must be carried out by a suitably qualified Gas Safe registered engineer.
Frequently asked questions
Can a Baxi combi run a bath and shower together?
It can supply two outlets, but both share the available hot-water flow. Whether the result is comfortable depends on the exact model, incoming supply and outlets.
Would turning the hot-water control up fix it?
It may change the target, but it cannot create extra boiler output or mains flow. Excessive settings also increase scald risk.
Why does the shower recover when the tap closes?
The full available flow and heating capacity return to the shower, which strongly suggests simultaneous demand is involved.
Does a higher-output combi always solve the problem?
No. The gas supply, mains water flow, pipework and outlet design must all support the required performance.
Is reduced flow a boiler fault?
Not necessarily. Some reduction at each outlet is expected when a combi divides its supply. Sudden or severe changes still deserve checking.
Related boiler help
- Hot water goes cold when another tap opens
- Baxi hot-water temperature fluctuating
- Combi boiler water is lukewarm
Does the hot water keep cooling when demand increases?
Review the repair service details and arrange a professional hot-water performance check.