When you’re looking around for a new boiler, it’s important to make sure that it’s going to be able to heat your home and provide enough hot water for your needs. This is where flow rates come in. Combi boilers are the most popular type of heating system in the UK, and so there’s a strong likelihood that it’s going to be a contender for your home.

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But that said, not all combi boilers are suitable for all homes. It might be that the flow rate of a heat only or system boiler might be a better fit. So that means that you need to understand what all the technical terms mean and how they relate to hot water supply and heating systems.

To give you a helping hand, everything that you need to know about flow rate is found right here including how much a new boiler cost.

Combi boilers with highest domestic hot water flow rate (DHW)

Viessmann Vitodens 111-w 32kw storage combi	21.2 l/min
Worcester Greenstar 8000 Lifestyle 50	20.3 l/min
Vaillant ecoTEC Plus 38kw Combi Store	20.1 l/min

What is the boiler flow rate?

There are two things to consider here, firstly the boiler flow rate and then the boiler output. Both are important considerations to ensure that your boiler can provide the heating and hot water that you need. After all, a new boiler that can’t heat the whole house is going to be an expensive mistake.

Combi boiler flow rate

This measures the volume of water that the boiler is able to process in 60 seconds. It’s measured in litres, so when a boiler is advertised as having a flow rate of 8 litres, then it can heat 8 litres of water every minute.

While that may seem like a lot of hot water, it’s important to remember that if you have more than one tap running then that supply is going to be divided between the two outlets. An average tap provides 6 litres of water each minute that it’s on full power.

But, when you have two taps on, that maximum flow rate of 6 litres per minute isn’t going to be able to provide the same hot water performance. In fact, you may find that flow rates have reduced dramatically.

This really becomes important if you have two or more bathrooms or need to run two showers simultaneously. If this is a regular requirement, then it might be that combi boilers aren’t the right solution for your home. Instead, a system boiler with a hot water cylinder is a better option.

What else affects flow rates?

The more powerful a boiler is then the quicker it will be at heating the mains supply of water. However, it’s important to remember that if you only have a flow rate of 6 litres per second as mains pressure coming into your home, your boiler isn’t going to be able to increase that to give a higher rate coming out of the tap.

That means that it’s important to understand what the flow coming into the house is to begin with as that pressure is going to be a big factor in finding the right type of boiler for your home. If you’re not sure what the water pressure is in the house, then a local installer might be able to help out.

Boiler output

The output of the boiler and flow rates are closely linked. This is because flow rate refers to how much water the boiler can push through to the tap, while the output decides how much water the boiler can heat.

This means that larger homes needed a higher heating output. And that should really be no surprise, a five bedroom home is going to have a heating system with many more radiators than a much smaller living space. With more radiators, the system water is going to have to travel around a larger house and that’s going to need a more powerful boiler.

Then, with more bedrooms, comes more people living in the room and so there are going to be much higher hot water requirements.

The kW rating and high flow rate

The kW rating of the boiler determines how much hot water and the temperature rise it can deliver. While each home has slightly different requirements, as a starting point, you should be looking at:

• Up to 28kW for smaller homes and flats with one bathroom and up to 12 radiators
• 28-36kW for larger properties with 2 bathrooms and up to 17 radiators
• 36kW+ for large homes with 2 or more bathrooms and in excess of 18 radiators.

If you end up with a boiler whose heating output rate is too low, then it’s going to struggle to provide water at the temperature you need. If though, the output rate is too high, then the running costs will be higher than they should be and that new boiler is no longer a cost-effective choice.

Can a combi boiler supply my heating system?

Absolutely yes, as long as the right size of boiler for your needs and home is fitted. While we’re talking about the right combi boiler size, this relates to its kW capabilities rather than its actual physical size.

As long as you follow our guidance in the previous section then a combi boiler can supply heating systems with no problems at all.

Do I need a hot water cylinder?

Well, if you have a larger home or more than one bathroom, then you might find that a combi boiler just isn’t able to supply the hot water and heat that you need. You might also need a heating system with a hot water cylinder if you mains water pressure is low.

Very often homeowners choose to keep their existing boiler type rather than looking at new boilers because changing over to a regular or system boiler is a bigger, and more expensive, installation process.

Combi boilers heat the water directly from the mains and on demand, giving you instantaneous hot water. The boiler system itself is often installed on the ground floor with many new boiler being designed to fit into a kitchen cupboard.

The hot water flow is then delivered to your taps and showers at mains pressure. That then means that if you have strong mains pressure and not too many demands on the hot water system at the same time, then you might be able to have a reasonably powerful shower.

But, if the mains power is low then a combi boiler is going to struggle to deliver enough hot water at the power you need. That might mean that that the water flow drops when another tap is put on or that the shower goes cold or hot unexpectedly.

Combi boilers with hot water storage

Now, while we’ve said that combi boilers don’t need a hot water cylinder, there are now boiler ranges that have built in storage for hot water. This might mean that a combi is the right boiler for you after all! The Viessmann Vitodens 111-w combi store is the perfect example.

With storage combi boilers, you don’t lose the space needed for a hot water cylinder and you get the benefits of both worlds, instant hot water and a supply ready heated for when you need hot water at different outlets at the same time. This might be the perfect solution when there are problems with mains water flow rates.

Getting boiler quotes is the first step for getting a new boiler replacement that meets all your needs!

Can you have an electric shower with a combi boiler?

When you have an electric shower, the supply of cold water is pulled from the mains. That then mean that they can be used no matter what type of boiler that you have. In the same way that a kettle heats water, an electric shower has a heating element over which the water passes and then the hot water flow is delivered to the shower.

Now it has to be said that you’re not going to get the power shower experience with high water pressure. However, you will get a reliable flow of water even if someone turns on a tap in the kitchen at the same time as you’re in the shower. That then means that they can be a good option for homes with a combi boiler and where there are two or more bathrooms.

Do remember to check out the kilowatt rating of the shower. Powerful water flows also come from a shower with a higher kilowatt rating . As ever though, this does depend on the water pressure from the mains.

Can combi boilers run a power shower?

Unfortunately not. If you’re looking for the power shower experience then you’re going to need a boiler that’s connected to a hot water cylinder. This is a requirement for the pump within the shower to be able to work.

That means that if a power shower is a must, then you may need to move away from a combi boiler. That then means either a system boiler which has a separate hot water storage cylinder or a heat only boiler that has both a separate hot water cylinder and a cold water storage tank in the roof space. Generally regular boilers are found in older properties.

How about a mixer shower with combi boilers?

Now this option will work with combi boilers. That’s because they connect with the hot water and the cold water to the deliver water at the perfect temperature.

They are however going to have some of the problems we’ve already mentioned in that the hot water flow is being delivered by your combi boiler rather than the shower itself. That then means that if someone else turns on a hot water tap then your shower might go cold.

Is an electric shower or a power shower cheaper to run?

Because the flow is at a lower rate within the electric shower, they are usually the most efficient forms of shower to run. Because the flow in an electric shower is lower than in a power shower, it’s likely to mean that they are more efficient.

This means that you may be able to get lower energy bills by having an electric shower rather than a power shower.

Are system boilers a better option for higher flow rates?

Although combi boilers are incredibly popular, different different boiler types might offer you more flexibility to meet your heating and hot water requirements. If you have times of day where there is high demand for hot water then take a look at system boilers.

You are going to need space for a hot water tank, and that will need to be put in place when the installer is putting in the boiler. System boilers are mains fed for the water supply but the water is heated in advance and stored ready for use. That then means that this type of boiler is less affected by the mains water flow rates, hence its ability to work with a power shower.

If though you currently have a combi boiler do be aware that it’s going to be a much bigger job to switch over to a system boiler so you’ll need to factor in additional boiler installation costs.

How about a conventional boiler for high flow rate?

Also known as a regular boiler, conventional boilers have both a cold water tank and hot water storage and that takes up a fair bit of space. If you live in an area where the cold mains water pressure is low then a conventional boiler can provide with hot water to several bathrooms all at the same time.

Do bear in mind though that in these systems water isn’t heated instantly. That means that you’ll need to program the thermostat to ensure that your hot water requirements are met. Then if you use all the hot water during times of high demand, you’ll then have to wait until another tank full is heated.

The open vented system of this type of boiler would need to be converted to a ‘sealed system’ if you change it over to a combi boiler. But, that puts the system under pressure and as a result leaks can develop.

This type of boiler is ideally suited when there is routine to your hot water needs and you want a high flow rate.

What are the best combi boilers with higher flow rates?

While we’d love to direct you to that one perfect combi boiler, it’s just not that straightforward. That’s because it all comes down to your unique requirements for your heating and hot water. Then you’re going to have requirements around whether you need a five year warranty or one that’s much longer. You can research a bit for information to potentially find the best combi boiler for you with higher flow rates.

Thankfully there is an easy way to get information on the right boiler for your home, and that’s by getting a free quote. And all you need to do is provide a few details about your needs. As part of your new boiler quote you’ll also be able to book installation all at one fixed price with no surprises!