A combi boiler can feed two showers at the same time, but both usually run weaker and cooler. A combi heats water as it passes through and has no store, so the available hot water is split between the outlets that are open. The limit is rarely the boiler alone. In most homes with two bathrooms, the incoming mains supply decides what is possible.
Why two showers on one combi disappoint
A combi has no hot water cylinder. It heats water on demand and sends it out at mains pressure. Open a second hot outlet and the same flow is shared between them. Pressure at both showers falls, and the temperature can swing.
Ideal Heating puts it plainly. Combi boilers have a limited hot water flow rate and are not ideal where several showers, baths or taps are used together. With two hot outlets running, both temperature and pressure drop.
The honest answer is yes in theory, rarely well in practice. Where two bathrooms are used at the same time most days, the incoming mains supply is almost always the real limit, not the appliance on the wall.
Your mains supply decides what is possible
Every mains-fed hot water system depends on dynamic pressure and flow. Dynamic means measured while the water is actually running, not standing still. The incoming main has to supply the boiler and the cold outlets at the same moment.
Worcester Bosch describes what happens when it cannot. Cold water is drawn away from the boiler, or pinched, as soon as another outlet opens. One remedy is to give the boiler priority by taking its feed as the first branch from the main.
Flow restrictors also cap what arrives at each tap. Worcester Bosch fits a restrictor to every combi so a 40°C temperature rise is reached, following NHBC figures of 6 litres a minute for basins, sinks and showers and 9 litres a minute for baths. Maximum flow still varies with the output of the appliance.
Three realistic ways to get two working showers
- A larger combi, but only if the mains supports it. Dynamic pressure and flow have to be measured on site before any model is chosen.
- A system boiler with an unvented cylinder. Vaillant recommends a system boiler for homes with more than one bathroom, because several outlets can run without pressure dropping or temperature swinging. You need space for the cylinder.
- An electric shower in the second bathroom. Ideal Heating notes that electric showers draw cold water from the mains and heat it themselves, independently of the boiler.
An unvented cylinder is fed straight from the cold main, with no vent pipe to the atmosphere and no cold tank in the loft. Heatrae Sadia describes mains pressure as giving a steadier, more even hot water flow and allowing different outlets to run at once. Our guide on converting a vented system to an unvented cylinder covers what changes in the house, and the Megaflo and unvented page explains the work itself.
What size boiler suits two bathrooms?
Sizing follows radiators, bedrooms and bathrooms. Viessmann gives a rough guide: a three or four bedroom house with around ten radiators suits a 24–30 kW combi, while a larger house with more radiators and bathrooms moves towards 30–35 kW. Viessmann also warns that bigger is not always better, because an oversized boiler brings its own drawbacks.
Output alone does not solve simultaneous use. Vaillant points out that a system boiler can be sized for the heating demand separately from the hot water demand, because the cylinder holds the hot water. If you are still choosing a type, combi, system or regular boiler compares all three.
Can you fit a shower pump to a combi?
No. Shower pumps work with stored hot water, and a combi has no store. Vaillant confirms that a pump cannot be added to a combi, because the boiler has to take its water directly from the main.
Combis do suit pressure-balanced or thermostatic mixer showers. Where a pump is used elsewhere in a home, Viessmann sets out the water regulations position: water pumped from the mains must not exceed 12 litres a minute, and the pump must be approved under the Water Regulations Approval Scheme.
What the law requires in England
Work on a gas appliance is never a DIY job. Under the Gas Safety (Installation and Use) Regulations 1998 as amended, nobody may carry out work on a gas fitting unless competent to do so. Businesses doing gas work in scope must be on the Gas Safe Register. HSE states that engineers carry an ID card with a photo, registration number, expiry date and the qualified work categories on the back.
Approved Document G applies in England to new dwellings and to a material change of use. It requires the hot water supply to a fixed bath to be designed and installed so the water delivered does not exceed 48°C. Part G3 covers unvented hot water storage, which may only be installed by a person competent for that work.
What you can check yourself first
- The boiler pressure gauge, and topping up heating pressure using the manufacturer's instructions.
- The thermostat and programmer settings, including the hot water temperature setting on a combi.
- The reset button, if the boiler has locked out.
- Bleeding radiators that are cold at the top.
That is the full list for a householder. Anything involving gas, the flue, seals or the inside of the boiler case is work for a Gas Safe registered engineer. If the boiler is behaving oddly, what to check before you call an engineer sets out the safe steps.
The next step for a two-bathroom home
This decision starts with measurement, not a brochure. Dynamic pressure and flow at the incoming main show whether a bigger combi is realistic or whether a cylinder makes more sense. FM Power carries out a free home survey and gives a written fixed price before any work starts. You can see how that works on our new boiler installation page.
Sources
- Are Combi Boilers Best For Showers? – Ideal Heating
- How could mains water pressure affect my combi boiler? – Worcester Bosch
- TB 0123 a – Minimum Flow Rates on Combi Boilers – Worcester Bosch
- What is a system boiler? – Vaillant
- What size of boiler do I need? – Viessmann UK
- Back to basics: how does an unvented cylinder work – Heatrae Sadia
- Gas Safety (Installation and Use) Regulations 1998, regulation 3 – legislation.gov.uk
- Gas Safe Register – HSE
- Approved Document G: sanitation, hot water safety and water efficiency – GOV.UK
- How to increase water pressure in a shower with a combi – Viessmann UK