The Pros and Cons of Combi Boilers

Combination boiler croydonA lot of Kuco’s customers in Croydon which have regular boilers (the ones with a separate hot water cylinder) ask about changing to a combination boiler, they’re not for everybody however, these are the pro’s and con’s as we see them…

What are the Combi Boiler Pro’s?

You don’t need cold water storage tanks or a hot water cylinder. A big selling point for combi’s, they free up space and you can do away with tanks and cylinders.

Water pressure can be improved as the system is pressurised. As the combination boiler heats water directly from the mains water coming into the property rather than storing hot water in a hot water cylinder you will have greater pressure. People who install combination boilers will see an improvement in the pressure of their shower for example.

Your hot water will never run out. As the system runs off the mains and heats for hot water on demand, you will never run out and have to wait for the cylinder to heat up again! Long winter showers here we come!

Combination boilers can be more efficient. A regular boiler generally heats water more efficiently as it will heat up at certain times, heated hot water is stored in a hot water cylinder however, where heat is gradually lost, so a combination boiler may be more efficient overall.

What are the Combi Boiler Con’s?

May not be suitable if more than 1 bathroom or for large families. Combination Boilers heat hot water on demand, depending on the size of boiler, generally the greater KW output the more water a boiler can heat, measured in litres per minute. If one person is taking a shower and another turns on the hot water tap then the amount of hot water the boiler can heat up is shared. If the boiler is big enough it can satisfy both, if however there is another bathroom, or hot water being used elsewhere, the more of that capacity is used up again, so you may find your shower interrupted briefly by cooler water…

Your Gas pipework may need replacing. Combination boilers need more power to heat on demand, so your 12kw regular boiler maybe replaced with a 24kw combination boiler for example. This means that the boiler needs greater gas pressure to get the amount of gas it needs. In order to ensure this, you may need to have existing gas pipework shortened or replaced with wider pipes to deliver more pressure. Your Heating Engineer will be able to make calculations to understand if your existing pipework is suitable. In most cases this is not a factor, but can add costs if pipework needs replacing.

No Immersion. If your Boiler did stop working for whatever reason then you have no hot water backup with a combination boiler, (apart from the kettle that is!) If you have a hot water cylinder then an immersion can be used to heat the water using electricity.

Need good water pressure. As we said, your combination boiler will heat water on demand. Two factors affect how much hot water you will get out over a period of time (1) the power of the boiler to heat an amount of water in litres per minute, and (2) the flow of the water coming into the property, your mains pressure. If you’re at the top of a hill for example, your mains pressure may not be good enough, there is an easy test for Heating Engineer to carry out though using a weir cup to check the flow from a mains tap.

Should you switch to a combination boiler?

As a general rule Kuco would recommend combination boilers to smaller households which use less hot water at a given time, or where space is an issue. For larger households or big families using more hot water more frequently we would likely recommend a regular system.

However… there are large combination boilers available which can heat a lot of hot water in one go, suitable for properties with two bathrooms, so Kuco do sometimes install combination boilers for larger properties. As long as the boiler is specified correctly, considering mains pressure and the length and size of gas pipework in relation to the gas appliances at a property alongside the heating/hot water requirements of the householders themselves, then a combination boiler can be suitable.

For more information on Combi Boilers and their suitability, or an assessment of your property, please call the team on 0208 406 7038 or email  

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