How to Fit a Power Shower with a Combi Boiler

Few bathroom additions are more desirable than a power shower, giving you a refreshing blast of high-pressure hot water whenever you step in the shower. But if you live in a house with a combi boiler, there’s one bit of information that you might have received conflicting advice on. And that’s whether you can fit a power shower at all. Well, yes and no is the answer, but there’s a lot of compatibility between power showers and combi boilers that’s important to understand before you begin the installation.

In this complete guide, we’ll give you all the information you need on power showers, combi boilers and the ultimate solution for showering in your house.

Understanding Combi Boilers and Power Showers

Before we talk about how you can install a power shower with a combi boiler, it’s important to note what a combi boiler is and why a traditional power shower does not work with it.

A combination boiler (combi) is a boiler that heats mains cold water as and when it is needed. There is no requirement for a hot water tank and a cold water tank like a traditional gravity-fed boiler system. A combi boiler system is on mains pressure, which is normally 1 to 3 bar depending on your property’s location and mains water supply.

A traditional power shower is built to be used with a gravity-fed boiler system that has a hot water tank and a cold water tank. They have an internal pump which pressurises water that comes from low-pressure hot and cold water storage tanks to give you a powerful showering experience. This is why they are incompatible, the power shower requires low-pressure to pump, but the combi boiler supplies mains pressure water that would damage the pump inside the power shower.

Why You Cannot Fit a Traditional Power Shower to a Combi Boiler

The technical incompatibility between power showers and combi boilers stems from several critical factors:

• Pressure Dynamics: Power showers are engineered to increase pressure from low-pressure systems. When connected to a high-pressure combi boiler, the pump can be overwhelmed, leading to mechanical failure, leaks, or even complete breakdown of the unit.
• Flow Rate Limitations: Combi boilers have a maximum flow rate measured in litres per minute. Adding a pump to an already pressurised system doesn’t increase this flow rate—it’s determined by the boiler’s capacity and the diameter of your incoming water pipe.
• Warranty Voidance: Installing a power shower on a combi boiler system will almost certainly void both the shower and boiler warranties, leaving you financially exposed if anything goes wrong.
• Potential Damage: The back pressure created by attempting to pump already pressurised water can cause serious damage to your boiler’s internal components, potentially resulting in costly repairs or complete boiler replacement.

Alternative Solutions for Better Shower Performance with a Combi Boiler

Whilst you cannot install a traditional power shower with your combi boiler, several excellent alternatives can deliver the shower experience you’re seeking.

Electric Showers: The Independent Option

Electric showers operate completely independently from your central heating system. They heat cold water instantly using an internal heating element, making them compatible with any boiler type, including combis.

Advantages of electric showers:

• Complete independence from your boiler system
• Consistent temperature control regardless of other water usage
• Energy efficient as they only heat water when needed
• Available in various power ratings from 7.5kW to 10.5kW
• Relatively straightforward installation
• Lower initial purchase cost compared to other options

Considerations:

• Require a dedicated electrical circuit and proper earthing
• Flow rate depends on the power rating and incoming water temperature
• May produce a less powerful spray compared to high-pressure showers
• Installation requires a qualified electrician

High-Pressure Mixer Showers: The Optimal Choice

High-pressure mixer showers, also called thermostatic mixer showers, are specifically designed for combi boiler systems. These showers blend hot and cold water at mains pressure without any pumping mechanism, taking full advantage of your combi boiler’s existing pressure.

Benefits of mixer showers:

• Utilise the full mains pressure from your combi boiler
• Thermostatic valves maintain consistent temperature
• Safer operation with anti-scald protection
• Wide range of styles and features available
• No electrical work required beyond the boiler itself
• Compatible with various showerhead types including rainfall and massage jets

Important factors:

• Performance depends on your boiler’s flow rate and mains pressure
• May experience temperature fluctuation if other taps are used simultaneously
• Requires adequate boiler capacity (typically 24kW minimum for good performance)
• Best results with a higher specification combi boiler

Digital and Smart Showers: The Premium Solution

Digital showers represent the cutting edge of shower technology, offering precise temperature control and programmable settings. These systems work excellently with combi boilers and provide a luxurious showering experience.

Features of digital showers:

• Exact temperature control to 0.1 degrees
• Programmable user preferences
• Remote operation via smartphone apps
• Multiple outlet control (overhead, body jets, handset)
• Consistent performance regardless of pressure fluctuations
• Sleek, modern aesthetics

Assessing Your Combi Boiler’s Capability

Before selecting any shower solution, you need to evaluate whether your existing combi boiler can deliver the performance you want. The two critical specifications are flow rate and boiler capacity.

Flow Rate Requirements

Combi boilers are rated by their hot water flow rate, typically measured in litres per minute at a 35-degree temperature rise. For a satisfying shower experience, you should aim for:

• Minimum acceptable: 10 litres per minute
• Good performance: 12-15 litres per minute
• Excellent performance: 15+ litres per minute

If your current boiler provides less than 10 litres per minute, even the best mixer shower will feel disappointing. In such cases, you might consider upgrading to a higher-capacity combi boiler.

Boiler Power Output

The kilowatt rating of your boiler affects how quickly it can heat water:

• Small properties (1 bathroom): 24-28kW typically sufficient
• Medium properties (2 bathrooms): 28-35kW recommended
• Larger properties (3+ bathrooms): 35kW+ for simultaneous use

Mains Water Pressure

Check your mains water pressure, as this directly impacts shower performance. You can purchase an inexpensive pressure gauge from any DIY store. Ideal pressure ranges between 1.5 and 3 bar. If your pressure falls below 1 bar, you may need to consider a mains water pressure booster pump, which is different from a power shower pump and can be safely installed on mains water supplies.

Step-by-Step Guide to Installing a Mixer Shower with Your Combi Boiler

If you’ve determined that a high-pressure mixer shower is the right choice for your combi boiler system, here’s what the installation process typically involves. However, remember that this work must be carried out by qualified professionals.

Planning and Preparation

Begin by selecting a shower that matches your boiler’s specifications and your aesthetic preferences. Consider the following:

• Thermostatic vs manual mixing valve
• Concealed or exposed pipework
• Single or dual outlet capability
• Accessibility for maintenance
• Compliance with building regulations

Professional Installation Requirements

In the UK, any work involving water supply systems and particularly anything connected to your boiler must comply with Water Supply Regulations. Additionally, if you’re installing in a new location or significantly altering existing plumbing, you may need building regulation approval.

Installation typically involves:

1. Isolating the water supply and draining relevant sections
2. Running appropriate hot and cold supply pipes to the shower location
3. Installing isolation valves for future maintenance
4. Mounting the mixer valve at the correct height (typically 1100mm from shower tray)
5. Connecting hot and cold supplies ensuring correct orientation
6. Installing the showerhead and any additional outlets
7. Pressure testing all connections
8. Commissioning and temperature checking

Optimising Performance

Once installed, proper setup ensures optimal performance:

• Set the thermostatic mixer to your preferred maximum temperature
• Test the anti-scald function works correctly
• Check flow rate at various settings
• Ensure no pressure drop occurs when other fixtures are used
• Verify all connections remain leak-free under operating pressure

How Local Boiler Helps Guide Your Installation Decisions

If you are not confident, you can be forgiven for getting cold feet when looking at how to fit a shower with a combi boiler. However, to help with this Local Boiler has the information and advice you need to assist with the needs of UK homeowners who want to know more about upgrading their bathroom and what options they have that will not conflict with their heating system.

Whether you need help finding out whether your boiler is suitable, which shower is right for you, or if you want a professional to come and fit it for you, having expert help and knowledge means you can have confidence that it will be done right the first time. There’s no need to rush into these important decisions, and with the right information, you can avoid making expensive errors and have the performance and reliability you know and expect from a new bathroom appliance.

Common Mistakes to Avoid

Understanding what not to do is just as important as knowing the correct approach:

1. Ignoring boiler capacity: Installing a high-flow shower without checking if your boiler can support it leads to disappointing performance and lukewarm showers.
2. Neglecting pipe sizing: Using undersized pipes (typically 15mm instead of 22mm) restricts flow and negates the benefits of a high-pressure system.
3. Improper isolation valves: Failing to install isolation valves makes future maintenance difficult and potentially disruptive.
4. Temperature limiting: Not setting or testing the maximum temperature limiter can create scalding risks, especially in homes with children or elderly residents.
5. DIY electrical work: Attempting to install electric showers without proper electrical qualifications is illegal, dangerous, and will fail building inspection.

Upgrading Your Combi Boiler for Better Shower Performance

If your assessment reveals that your current combi boiler cannot deliver adequate performance for your desired shower experience, upgrading may be the most effective solution.

Modern high-output combi boilers offer significantly improved flow rates compared to older models. A new boiler installation also provides:

• Improved energy efficiency reducing running costs
• Better pressure and flow rate management
• Enhanced reliability and warranty protection
• Compatibility with smart home systems
• Reduced carbon footprint

Upgrading your boiler is a serious decision. Local Boiler will guide you through the correct specification for your family needs, so your new boiler provides a great shower AND is also well matched to be as efficient as possible in meeting your heating and hot water needs around the rest of your home.

Maintaining Your Shower and Boiler System

Proper maintenance extends the life of both your shower and boiler whilst ensuring consistent performance:

1. Regular descaling: In hard water areas, limescale buildup affects both boiler heat exchangers and shower valves. Annual descaling maintains efficiency and prevents failures.
2. Annual boiler service: Professional servicing keeps your boiler operating safely and efficiently, preventing breakdowns that would leave you without hot water.
3. Shower head cleaning: Remove and clean shower heads quarterly to prevent reduced flow from limescale and bacterial buildup.
4. Checking for leaks: Regularly inspect all visible connections for signs of moisture or dripping, addressing issues promptly, preventing water damage and maintaining system pressure.
5. Filter maintenance: If your system includes filters on the shower or boiler supply, clean or replace them according to manufacturer recommendations.

Understanding Building Regulations and Compliance

Any installation work must comply with relevant UK building regulations:

• Water Supply Regulations 1999: These govern all work on water fittings, requiring competent persons and approved materials.
• Building Regulations Part G: Covers hot water safety, requiring temperature limiting devices to prevent scalding.
• Electrical Safety: Any electrical work must comply with Part P of Building Regulations and be certified by a qualified electrician.
• Notification requirements: Certain works require notification to local authority building control, particularly new installations or significant alterations.

Using registered professionals ensures compliance and provides necessary certification for insurance and property sale purposes.

Cost Considerations

Understanding the financial investment helps with planning:

• Electric shower installation: £150-£400 for the unit plus £150-£300 for electrical installation by a qualified electrician.
• Mixer shower installation: £100-£600 for the unit plus £200-£500 for plumbing installation, depending on complexity.
• Digital shower systems: £500-£2000+ for the unit plus £300-£600 for professional installation.
• Boiler upgrade: £1500-£3500 including installation, depending on boiler specification and installation complexity.

These figures are approximate and vary based on your location, property specifics, and chosen products. Always obtain multiple quotes from qualified professionals.

Conclusion

Although you can’t install a traditional power shower on a combi boiler, there are plenty of other options that can provide a great showering experience. The high-pressure mixer shower is the easiest solution, as it can take full advantage of your combi boiler’s mains pressure. The electric shower is fully independent from your heating system. The digital shower gives you precision control, as well as luxury features.

The most important aspect is understanding what your boiler is capable of, what types of shower will work best and ensuring that it is installed professionally and to code. With some careful consideration and expert advice when required, you can take your daily shower from something of a chore to something you can really look forward to, whilst retaining the efficiency and convenience that originally drew you to a combi boiler system.

Whether you are simply replacing your shower or carrying out a full bathroom refurbishment, taking the time to understand the technical considerations and weighing up the options available will ensure that your investment provides the performance, reliability and enjoyment you are looking for.

Frequently Asked Questions

Can I install a power shower pump on my combi boiler system?

No, you cannot and should not install a traditional power shower pump on a combi boiler system. Power shower pumps are designed exclusively for low-pressure gravity-fed systems with hot water cylinders and cold water tanks. Combi boilers supply water at mains pressure, which would damage the pump and potentially harm your boiler.

Why is my shower temperature fluctuating even with a thermostatic mixer?

Temperature fluctuation can occur for several reasons: inadequate boiler capacity when multiple outlets are used simultaneously, incorrect thermostatic cartridge calibration, limescale buildup affecting the mixing valve, or insufficient mains pressure. If you have a smaller combi boiler (below 28kW), using other hot water outlets whilst showering will affect temperature. 

What flow rate do I need from my combi boiler for a good shower?

For a satisfying shower experience, your combi boiler should deliver at least 12-15 litres per minute at a 35-degree temperature rise. Higher-specification boilers providing 15+ litres per minute offer excellent performance, even when other taps are in use. You can check your boiler’s specifications in the manual or on the manufacturer’s website.

How much does it cost to install a shower suitable for a combi boiler?

Installation costs vary depending on the shower type and complexity. A basic mixer shower installation typically costs £300-£1100 total (including the unit and professional installation). Electric showers range from £300-£700 including electrical work. Premium digital shower systems can cost £800-£2600 or more. These prices assume straightforward installation with existing pipework nearby. 

Do I need building regulations approval to install a new shower?

Most shower replacements in existing locations don’t require formal building regulations approval, but the work must still comply with Water Supply Regulations and relevant building standards. However, if you’re creating a new bathroom, installing an electric shower requiring new electrical circuits, or making significant alterations to plumbing, you may need to notify building control. Professional installers registered with competent person schemes can self-certify certain works.