Yes, a combi boiler can run two showers at the same time, but both showers only feel good if three things line up: the boiler’s hot-water flow rate, your incoming mains water flow, and how much water each shower uses.
A 24kW combi will usually struggle. A 35kW or 40kW combi can work in a home with a strong mains supply. If two people shower together every morning, a system boiler with an unvented cylinder is often the more reliable choice.
This guide shows you how to work out which case you are in. It covers boiler size, litres per minute (L/min), why the second shower goes cold, and what your options are in a London flat or house.
Can a Combi Boiler Run Two Showers at the Same Time?
It can, but not every combi can, and not every home can support it. A combi (combination) boiler heats water on demand. It has no tank of stored hot water. Every litre you use is heated as it flows through the boiler.
That is why the answer is “it depends” rather than a flat yes or no. Some websites say a combi can’t do it. Others say a 40kW model can. Both are partly right:
- A small combi in a home with average mains flow will usually fail with two showers.
- A high-output combi (35kW to 42kW) can manage two showers if the mains supply is strong and the showers are not very hungry.
- A system boiler and cylinder avoids the problem by storing hot water in advance.
Quick answer by situation:
| Your situation | Likely outcome with a combi |
|---|---|
| Two showers, used at different times | Fine with most combis |
| Two showers, sometimes together, strong mains | A 35kW to 40kW combi may cope |
| Two showers, together every day, average mains | A combi will often disappoint |
| Two power showers or large rain heads | A combi is usually the wrong tool |
Why Does a Combi Boiler Struggle With Two Showers?
A combi struggles because it makes hot water on the spot and must share a limited flow between both showers. When the second shower opens, the same hot water is split in two.
A combi heats water on demand
Here is what happens when you turn on a hot tap:
- Cold water enters the boiler from the mains.
- The boiler fires up and a heat exchanger warms the water.
- Hot water travels straight to the outlet.
There is no reserve. The boiler can only heat so many litres per minute by a set amount. That limit is fixed by its power.
The available flow has to be shared
Say your combi delivers 12 L/min of hot water at a comfortable temperature. One shower uses 8 L/min, so it works well. Turn on a second 8 L/min shower and the demand is 16 L/min. The boiler can’t heat that much water fast enough, so one of two things happens:
- The water gets cooler (the boiler heats more water but less each litre), or
- The flow drops (the boiler limits the flow to keep the temperature right).
This is why the second shower goes cold, or why the first one loses pressure when someone else turns on a tap.
Your mains water supply matters
A combi doesn’t have its own water source. It relies on your incoming mains water, which is why your supply is often the real limit. Three terms matter here:
- Static pressure: the pressure when no water is running.
- Dynamic pressure: the pressure while water is running. This is the number that counts.
- Flow rate: how many litres per minute the supply can deliver.
Old or narrow pipework, a shared supply in a block of flats, or a long run from the street can all cut flow. Many older London homes, such as Victorian terraces, still have small-bore pipes that were never designed for two showers.
How Much Water Do Two Showers Need?
Two showers usually need between 12 and 20 L/min combined, depending on the type of shower. Add the flow of each shower to get the total demand.
| Shower type | Typical flow per shower | Two showers together |
|---|---|---|
| Low-flow or eco shower | About 6 L/min | About 12 L/min |
| Standard mixer or thermostatic shower | About 8 L/min | About 16 L/min |
| Powerful or large-head shower | About 10 L/min or more | About 20 L/min or more |
These figures are typical, not exact. Check your shower’s manual for its real flow rate.
A worked example: two standard showers at 8 L/min need 16 L/min in total. But that 16 L/min is not all hot. A thermostatic shower mixes hot and cold, so the boiler supplies only part of it. Even so, both the mains and the boiler must deliver enough flow. If either falls short, performance drops.
The rule to remember:
Boiler hot-water output + mains flow + shower demand = real-world performance.
A strong boiler on a weak supply gives weak showers. A strong supply with a small boiler gives lukewarm ones.
What Size Combi Boiler Do I Need for Two Showers?
For two showers at once, look at a 35kW to 42kW combi, and check your mains flow first. A 24kW or 28kW combi is rarely enough.
Boiler size (kW) is only part of the picture. What matters more is the hot-water flow rate, shown in L/min at a set temperature rise (usually 35°C). Check the manufacturer’s data sheet.
| Combi size | Typical hot-water capability | Two showers at once? |
|---|---|---|
| 24kW | Lower flow | Usually not comfortable |
| 28kW to 30kW | Moderate flow | Can struggle, depends on showers and mains |
| 35kW | Higher flow | Possible in the right home |
| 40kW | High flow | Often suitable with a strong mains supply |
| 42kW and above | Very high output | Still limited by mains flow and shower demand |
Treat this as a guide, not a guarantee. Flow rates vary by model, by the season (cold winter mains water needs more heating) and by the temperature you like.
Can a 30kW combi boiler run two showers?
Sometimes, but usually with compromises. A 30kW combi can manage two low-flow showers if your mains supply is good. With two standard or powerful showers, you will probably notice a drop in flow or temperature.
Can a 35kW or 40kW combi boiler run two showers?
Often yes, if your mains supply can keep up. These high-output combis heat more water per minute, which suits homes with two bathrooms. But a bigger boiler doesn’t fix a weak supply. That leads to the most common myth.
Does a Bigger Combi Boiler Give You More Water Pressure?
No. A bigger combi heats more water per minute, but it does not increase your water pressure or your mains flow. Boiler output and mains pressure are separate things.
Think of it like a kettle and a tap. A more powerful kettle heats water faster, but if the tap only trickles, you still get a trickle. A 40kW boiler cannot create flow that the mains isn’t delivering.
So before you pay for a bigger boiler, find out what your supply can do. If the mains is the bottleneck, a new combi may change very little.
What Mains Water Flow Do You Need for Two Showers?
As a rough guide, you want at least 16 to 20 L/min of mains flow at a decent pressure, measured while other taps are running. Lower flow can still work with low-flow showers, but you will feel it.
Here is what affects your supply:
- Incoming flow rate in L/min
- Dynamic pressure under demand, often described in bar
- Pipe size, such as 15mm or 22mm pipework
- Shower flow restrictors built into some shower heads
- Other water use, such as a washing machine or dishwasher running at the same time
How to check your mains flow
The simple “bucket test” gives a rough idea:
- Turn off other taps and appliances.
- Open the kitchen cold tap fully (the kitchen tap is usually plumbed straight from the mains).
- Fill a 10-litre bucket and time it.
- Divide 10 by the seconds, then multiply by 60 to get L/min.
For example, 10 litres in 40 seconds is 15 L/min.
This is only a rough check. A Gas Safe registered engineer can measure flow and dynamic pressure with proper gauges, which gives you numbers you can trust.
Combi Boiler vs System Boiler for Two Showers
A system boiler with an unvented cylinder is usually better for two showers at once, because it stores hot water instead of heating it on demand. A combi is better when space is tight and demand is low.
| Feature | Combi boiler | System boiler + cylinder |
|---|---|---|
| How hot water is made | On demand | Stored in a cylinder |
| Cylinder needed | No | Yes |
| Space needed | Less | More |
| One shower at a time | Excellent | Excellent |
| Two showers at once | Depends on flow | Better suited |
| Large family | Can be limiting | Often more suitable |
| Suits small London flats | Yes | Often hard, due to cylinder space |
| Hot water storage | None | Yes |
A system boiler heats a stored unvented cylinder (also called a pressurised or mains-fed cylinder). That cylinder can supply several outlets at good pressure, which is why installers often recommend it for homes with two or more bathrooms. The catch is space: you need room for the cylinder, often in an airing cupboard or loft.
If you like the idea, see our page on system boiler installation in London.
What If I Already Have a Combi Boiler?
If your existing combi can’t handle two showers, the fault may be your boiler, your water supply or your showers. Work through these checks before you replace anything.
1. Check your boiler’s hot-water flow rate
Look up your boiler’s model number and find its hot-water flow rate in the manual. If it’s around 10 to 12 L/min, two showers will always be a stretch.
2. Measure your incoming mains flow
Use the bucket test above, or ask an engineer for a proper flow and pressure test.
3. Check your shower flow rates
Some showers use much more water than others. A large rain head can use double what a standard shower does.
4. Check what else is running
A washing machine, dishwasher or garden tap can steal flow from the showers.
5. Work out whether the boiler or the supply is the problem
- Temperature drops, but pressure stays fine: the boiler is probably running out of heating capacity.
- Pressure drops and temperature holds: the mains supply is likely the limit.
- Both drop: you may have a supply and boiler issue together.
Some fixes don’t need a new boiler at all. Swapping to a lower-flow shower head, cleaning a blocked filter or replacing old supply pipework can help.
Can You Add a Shower Pump to a Combi Boiler?
Usually no. A standard shower pump is designed for stored tanks, where water is fed by gravity. On a combi, which runs at mains pressure, a normal pump won’t help and can cause problems.
Some special mains-fed booster pumps exist, but they depend on your mains flow being high enough in the first place. A pump can’t create water that the mains doesn’t supply. Speak to an engineer before you buy one, as some water suppliers restrict pumping directly from the mains.
Is an Electric Shower Better for the Second Bathroom?
It can be, and it’s often the cheapest fix. An electric shower heats its own water using electricity, so it doesn’t draw on your combi’s hot-water output at all.
Pros:
- Doesn’t compete with the boiler for hot water
- Lower installation cost than a new boiler
- Works well in an en-suite or a bathroom far from the boiler
Cons:
- Flow is often lower than a good thermostatic mixer shower
- Needs its own electrical circuit, installed by a qualified electrician
- Needs a good cold mains supply to work well
For a household that is happy with one great shower and one decent second shower, this option can save thousands.
What Is the Best Boiler for a London Home With Two Bathrooms?
It depends on how the home is used, not just how many bathrooms it has. The right answer changes by property type.
- London flat with one bathroom: A combi usually makes sense. It saves space and works well for one shower at a time.
- Conversion flat or small house with two bathrooms, rarely used together: A properly sized combi (35kW or 40kW) may be enough.
- Family home where two showers run together most mornings: A system boiler with an unvented cylinder deserves serious thought.
- Victorian terrace or Edwardian house with older pipework: Check the incoming supply first. Small-bore or lead-free-replaced pipes may limit any boiler.
- Larger home with a loft conversion and several en-suites: Look at the whole hot-water system, not just the boiler kW.
A good engineer will ask how many people live in the home, when they shower, and how many outlets run at once. Those answers matter more than a number on a boiler.
A Simple Decision Guide
Use these six steps to choose the right solution.
- Count your showers. How many could run at once?
- Find each shower’s flow rate in L/min.
- Test your mains flow with the bucket test or an engineer’s gauge.
- Check your boiler’s hot-water flow rate in its manual.
- Think about real use. Do both showers actually run together, and how often?
- Choose your route:
- Simultaneous use is rare: keep your current combi.
- Sometimes together, and mains flow is strong: consider a 35kW to 40kW combi.
- Often together, or mains flow is limited: consider a system boiler and unvented cylinder.
- Only one bathroom is the problem: consider an electric shower for the second.
How Much Does It Cost to Upgrade a Boiler for Two Showers in London?
Costs vary widely depending on what you choose, so get a written quote for your specific home. Here is what affects the price:
- Combi replacement: the cost of a new boiler, labour and any flue changes
- High-output combi (35kW to 42kW): usually costs more than a standard combi, and may need larger gas pipework
- System boiler and unvented cylinder: the highest cost, since it includes the cylinder and extra pipework
- Combi-to-system conversion: extra work to fit a cylinder, remove old pipework and rearrange the system
- Pipework changes: upgrading to 22mm gas or water pipe adds cost
- London factors: access, parking and building type
A high-output combi is often the more affordable route. A system boiler with a cylinder costs more up front but can be the better long-term choice for busy homes. If you’re weighing these options, our guide to converting from a combi to a system boiler explains what is involved.
How an Engineer Checks If Your Boiler Can Run Two Showers
A proper assessment measures your real supply and demand, rather than guessing from boiler size. A Gas Safe registered engineer will usually check:
- Your existing boiler model and hot-water flow rate
- Incoming mains flow and dynamic pressure
- The flow rate of each shower
- The number of bathrooms and people in the home
- How often showers run together
- The condition and size of your pipework
- Space available for a cylinder, if needed
Every installation must also comply with Building Regulations, including Approved Document G on hot water safety, and must be carried out by a Gas Safe registered engineer.
Illustrative examples (replace with your genuine anonymised jobs before publishing):
- Three-bedroom house with two bathrooms: an older 28kW combi struggled whenever two showers ran together. It was replaced with a higher-output combi after a mains flow test showed the supply could support it.
- Four-bedroom house with two busy bathrooms: a system boiler and unvented cylinder was chosen because both showers ran together most mornings.
FAQs About Running Two Showers From a Combi Boiler
1. Can a combi boiler run two showers at the same time?
Yes, but performance depends on the boiler’s hot-water flow rate, your mains water flow and each shower’s flow rate. High-output combis do best.
2. Can a 24kW combi boiler run two showers?
Rarely. A 24kW combi has limited hot-water flow, so two showers will usually cause cold or weak water.
3. Can a 30kW combi boiler run two showers?
Sometimes, with low-flow showers and strong mains supply. Expect compromises with standard or powerful showers.
4. Can a 35kW or 40kW combi boiler run two showers?
Often yes, if your mains flow is strong enough. The boiler heats more water per minute, but it can’t increase your mains supply.
5. What size boiler do I need for two showers?
A 35kW to 42kW combi may work for two showers. For frequent simultaneous use, a system boiler with an unvented cylinder is usually more reliable.
6. How many litres per minute do two showers need?
Usually 12 to 20 L/min combined. Two standard 8 L/min showers need about 16 L/min.
7. Does a bigger combi boiler increase water pressure?
No. A bigger combi heats more water per minute, but water pressure and flow come from your mains supply.
8. Why does my second shower go cold?
The boiler can’t heat enough water for both showers, so it shares limited hot water. Poor mains flow can make this worse.
9. Is a system boiler better than a combi for two bathrooms?
For simultaneous use, usually yes. A system boiler with a cylinder stores hot water, so it can supply two outlets at once more reliably.
10. Is an electric shower a good option for a second bathroom?
Yes, in many homes. It makes its own hot water, so it doesn’t compete with your combi, and it can cost far less than a new boiler.
Conclusion: Test Your Supply Before You Buy a Bigger Boiler
A combi boiler can run two showers, but only when the boiler’s hot-water flow, your mains supply and your shower demand all match. A 35kW to 40kW combi works in the right home. A system boiler with an unvented cylinder is the safer choice if two showers run together every day.
Don’t guess. Measure your mains flow, check your boiler’s hot-water rating, and think about how your household really showers. That saves you from paying for a bigger boiler that doesn’t fix the problem.
Not sure which route suits your London home? Book a survey with a Gas Safe registered engineer. They can test your flow and pressure, look at your pipework, and give you a clear, written recommendation.