A combi boiler copes with a new en-suite if the second shower is rarely used at the same moment as the first, and if the incoming main delivers a strong flow. A system or regular boiler with an unvented cylinder suits you if two showers run together most mornings, or if the mains flow is modest and the household is larger. The figure that settles it is the flow rate in litres per minute at your incoming main, measured at the property rather than guessed from the boiler badge.
Combi boiler or unvented cylinder for a second bathroom: what actually changes?
A combi heats water as it flows, so two open outlets share one supply. A system boiler with an unvented cylinder heats a store in advance at mains pressure, so two outlets draw from the same hot volume.
| Criterion | Combi boiler | System boiler with unvented cylinder |
|---|---|---|
| Two outlets at once | The boiler splits the water, so the flow drops at both the shower and the tap | Both outlets draw from the stored hot water at mains pressure |
| What sets the limit | Mains flow rate and boiler output in kW decide litres per minute | Cylinder volume decides how much is ready, heat input in kW decides how fast it returns |
| Running out | Hot water keeps coming as long as the main keeps flowing | Long back-to-back showers can empty the store, which then has to reheat |
| Space in the house | No cylinder needed, so the airing cupboard stays free | Needs a cylinder, often in the cupboard where the en-suite was planned |
| Rules and upkeep | Annual boiler service by a Gas Safe registered engineer | Part G competent person for the install, plus yearly checks of safety valves and expansion vessel |
Both routes get fitted into the same kind of house for different reasons. A combi keeps the cupboard space and the simpler pipework. A cylinder at mains pressure gives two outlets a steady flow together. How much water your incoming main can deliver, and how often two showers run at the same time, decides which one fits.
When a combi boiler still copes with a new en-suite
A combi boiler still copes with a new en-suite when the two showers are rarely used at the same minute. A couple with an en-suite off the main bedroom is the usual case, with the family bathroom used in the evening instead of the morning rush. Because a combi heats water as it passes through, nothing runs out part-way through a long shower.
The limit shows up the moment two outlets open together. Baxi explains that a combi then divides the water between them, and the flow falls at both. Output matters as well: Baxi gives roughly 9.8 litres per minute at 24 kW and 16.4 litres per minute at 40 kW, each at a 35 degree temperature rise.
Viessmann suggests around 12 to 15 litres per minute as the sensible minimum for a decent shower from a combi. Where the main sits comfortably above that, a second shower room used at different times stays within the boiler's reach. The pattern of use matters more than the number of bathrooms. There is more on this in our guide to whether a combi can serve two showers.
When a system boiler and unvented cylinder suit a second bathroom
A system boiler with an unvented cylinder suits a second bathroom when two showers run at the same time most mornings. A family of four or five with staggered school and work starts is the typical household. It also suits homes where the main delivers only a modest flow, because the cylinder stores the water in advance instead of heating it on demand.
An unvented cylinder is fed directly from the mains, without a vent pipe open to the atmosphere. Heatrae Sadia notes that mains pressure gives a more even flow, widens the choice of taps and showers, and allows several outlets to run at once. That is the practical difference from an old vented tank fed by gravity from a loft cistern.
Size is set by people, not just bathrooms. Baxi works on roughly 45 to 60 litres per occupant, putting a three-bedroom house in the region of 180 to 240 litres. Heatrae Sadia uses about 60 litres per wet room and gives 210 litres as an example for a two-bathroom home. Volume is only half the answer, since reheat speed follows the kW input to the cylinder, and the cupboard space the cylinder occupies is the trade-off. Our guide on what size hot water cylinder you need goes through the sums.
What flow rate does a second shower need, and how is it measured?
Flow rate is the number of litres per minute your incoming main can deliver, and it decides whether a second bathroom works at all. Viessmann rates more than 15 litres per minute as good, 10 to 15 as acceptable and under 10 as low. Pressure and flow are not the same thing: a narrow or shared supply pipe can hold back the flow even when the pressure gauge looks healthy.
Worcester Bosch describes what happens when the main cannot keep up with a combi. Open a second outlet and cold water is effectively drawn away from the boiler, so the shower weakens or swings in temperature. An engineer measures the flow at the property during the free home survey, usually at the kitchen cold tap, before any boiler or cylinder is specified.
One measurement carries the whole decision. Where the main gives a strong flow and the showers are used at different times, a combi can stay. Where the flow is modest, or two showers must run together, stored hot water at mains pressure is the route that answers it.
Which approvals does a new en-suite or second bathroom need in England?
An additional bathroom or en-suite in England normally needs building regulations approval. You apply through your local authority building control or a registered building control approver. The work touches drainage, ventilation and hot water safety, which is why it is notifiable.
Approved Document G covers the hot water. An unvented cylinder may only be installed by a person competent to do so. The system has to prevent the water exceeding 100 degrees Celsius, with at least two independent safety devices, and any discharge from them must be visible and safely routed. Systems up to 500 litres and under 45 kW should be a packaged or unit system meeting a recognised standard.
Building control must be notified before the work starts if the installer is not registered under a competent person scheme. Approved Document G also limits the hot water supplied to a fixed bath to 48 degrees Celsius, normally through a thermostatic mixing valve, for new dwellings and dwellings created by a change of use. Who may carry out the installation is set out in our guide on installing an unvented hot water cylinder.
Waste water falls under Approved Document H, so the new WC, basin and shower connect to the foul drainage, not to a rainwater pipe. In England and Wales, the Water Supply (Water Fittings) Regulations 1999 also require advance notice to your water supplier for certain fittings. Regulation 5 lists a single shower unit, a bath holding more than 230 litres to the overflow centreline, and a bidet with an ascending spray or flexible hose.
Can the boiler stay where it is, and who may do the work?
A gas appliance may only sit in a room used, or intended to be used, as a bathroom or shower room if it is room-sealed. That comes from regulation 30(1) of the Gas Safety (Installation and Use) Regulations 1998 as amended, which applies in Great Britain. Room-sealed means the appliance draws its air from outside and discharges its products outside, sealed off from the room.
The rule reaches into cupboards too. A cupboard inside the new en-suite counts, and so does an adjoining cupboard or void with a ventilation opening into the room. If your boiler sits in a landing cupboard and a new stud wall moves around it, the position needs checking before the walls go up.
HSE sets out the split on the work itself. Someone who is not Gas Safe registered may do wet work, such as water pipes and radiators. Any work on the boiler itself, and the final connection of the water pipe to the boiler, must be done by a Gas Safe registered engineer. Before an engineer arrives, four things are safe for you to look at:
- The boiler pressure gauge, which manufacturers put at roughly 1 to 1.5 bar with the heating off
- The room thermostat and programmer, including whether the hot water times still match the new routine
- The reset button on the boiler, following the manufacturer's instructions for your model
- Bleeding radiators that are cold at the top
Everything beyond those four belongs to a qualified engineer. The gas connection, the flue, seals, condensate inside the appliance and anything behind the boiler casing is work for a Gas Safe registered engineer. The safety valves and expansion vessel on an unvented cylinder are work for a person qualified for unvented systems.
Which option is right for your home: combi or cylinder?
Keep the combi if your incoming main measures comfortably above about 12 to 15 litres per minute and the second shower will mostly be used at a different time from the first. A couple adding an en-suite to a two-person household usually sits in that group, and the airing cupboard stays free.
Move to a system boiler with an unvented cylinder if two showers will run together most mornings, if a bath and a shower need to work at once, or if the measured flow sits in the lower band. Larger households and homes still on an old gravity-fed tank fall here most often, and the price is the cupboard space the cylinder takes.
If stored hot water looks like the answer for your second bathroom, our page on Megaflo and unvented systems explains how the free home survey and the written fixed price work.
Sources
- Baxi – Can I run a bath and shower at the same time
- Baxi – A simple guide to matching the right boiler to the right customer
- Viessmann – How to increase water pressure in your shower with a combi
- Worcester Bosch – How could mains water pressure affect my combi boiler?
- Heatrae Sadia – Recovery time vs size of a cylinder
- Baxi – Hot water cylinder for a two or three bedroom house
- GOV.UK – Building regulations approval: when you need approval
- GOV.UK – Approved Document G with 2024 amendments (PDF)
- GOV.UK – Drainage and waste disposal: Approved Document H
- legislation.gov.uk – Water Supply (Water Fittings) Regulations 1999
- legislation.gov.uk – Gas Safety (Installation and Use) Regulations 1998, regulation 30
- HSE – Domestic gas: frequently asked questions
- Heatrae Sadia – PremierPlus unvented cylinder installation and user manual (PDF)