A combi boiler heats water directly from the mains, on demand, with no stored hot-water tank – which means shower performance depends on what is coming into the house, not just which boiler is fitted. Two identical boilers can perform very differently in two different properties for exactly this reason.

Pressure and flow rate are different measurements

Worcester Bosch’s own technical guidance is explicit on this point: it is important not to confuse pressure with flow. Pressure is the force pushing the water through the pipe, usually measured in bar; flow rate is the volume of water actually delivered in a given time, usually measured in litres per minute. A property can have adequate pressure but still deliver a disappointing flow rate if the incoming supply pipe is narrow, old, or partially restricted – because flow depends on the size, type and condition of the incoming main, not on pressure alone.

It is also worth knowing that dynamic pressure – the pressure while water is actually running – will always be lower than the static pressure measured with a tap closed. A pressure reading taken with everything off does not tell you what will actually be delivered to a shower while it is running.

Why multiple outlets at once can cause problems

Because a combi boiler heats water on demand rather than from a stored tank, the incoming main has to supply enough flow for hot and cold simultaneously, for every outlet in use at once. On an undersized main, running a washing machine, flushing a toilet or opening another tap while someone is in the shower can genuinely “pinch” flow away from the shower, because the incoming pipe cannot deliver enough water to every point at the same time. This is a common, genuine cause of a shower that performs well most of the time but drops off when other water is being used elsewhere in the house.

What limits the maximum incoming pressure

At the other end of the scale, incoming mains pressure that is too high can also be a problem. Worcester Bosch’s guidance notes that maximum incoming water pressure for a combi boiler is normally 10 bar, and that a pressure-reducing valve may be needed to protect the appliance where mains pressure exceeds what it is designed to handle. This is a reason to have supply pressure and flow properly assessed as part of any combi boiler installation or quote, rather than assumed.

Questions to ask before choosing a combi

When comparing combi boiler quotes, ask the installer to confirm, ideally based on an actual measurement at your property rather than an assumption:

• the measured incoming mains flow rate and dynamic pressure at your property, not just the boiler’s rated maximum output;
• the manufacturer’s stated flow rate for the specific model being quoted, since this varies between models within the same range;
• whether your household’s typical simultaneous demand – for example, two showers used close together, or a shower alongside a washing machine cycle – has been taken into account; and
• whether the existing supply pipework is adequate, or whether an upgrade to the incoming main would be needed to get the flow rate the boiler is capable of delivering.

When a combi may not be the right fit

If a property has a genuinely low mains flow rate that cannot practically be improved, or a household with high simultaneous hot-water demand – several bathrooms used at similar times, for instance – a system boiler with a stored, pressurised hot-water cylinder can deliver strong flow to multiple outlets at once in a way a mains-fed combi cannot, because the cylinder, not the incoming main, is the limiting factor. This is a property-and-household-specific comparison rather than a fixed rule, which is why an on-site assessment matters more than a generic recommendation.

Frequently asked questions

Will a bigger combi boiler fix low water pressure?

Not on its own. The boiler’s output rating describes how much heat it can add to the water passing through it; it cannot deliver more flow than the incoming mains supply is physically able to provide. If flow rate at the property is the limiting factor, a larger boiler does not solve that.

Why does my shower get worse when someone flushes a toilet?

This is the simultaneous-demand issue described above: on some supplies, drawing water at a second outlet reduces what is available to the shower at the same moment, particularly where the incoming main is narrower or older.

Can I add a pump to improve a combi boiler shower?

Sometimes, but this needs proper assessment by a qualified engineer for your specific system and mains supply – not every combi installation is suitable for a booster pump, and any equipment used must be appropriate for use with a mains-fed system. Ask an installer to assess this rather than fitting one speculatively.

Sources

• How could mains water pressure affect my combi boiler? – Worcester Bosch
• Find or check a Gas Safe registered engineer – Gas Safe Register

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