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To ComBE Or Not To ComBE!?

Firstly, what is a combi boiler? 

In a nutshell, a combi boiler is basically an instantaneous water heater which can also heat your central heating water. It gives priority to hot water which means if your heating is on and someone has a shower or opens a hot tap, the boiler will stop heating your radiators and focus all its attention to heating the hot water, you can’t have both at the same time. 

Do you really need a combi boiler? 

All too often I get asked to quote to rip out an old gravity hot water and heating system and convert it to a combi boiler. Usually this is because one of the customers friends and family have had it done or they have heard combi boilers are better, or they want their old cylinder cupboard as extra storage space. Don’t get me wrong, combi boilers most definitely have their place and in certain situations and dwellings they are the perfect boiler and set up for the job. Other times an alternative boiler and system would have been a better choice.

What size combi do I need?

When MD Plumbing And Heating come to quote for a new combi boiler, we will measure the water flow from your kitchen tap. From there, it’s simple physics and we put the litres per minute into a formula which will tell us exactly how many Kilowatts (KW) is needed to give you a 35 degree rise from your cold water mains coming in, to the hot water coming out of your taps. Some customers and even engineers have the misconception that a bigger size combi will give them more hot water and better flow rate, the bigger the better they think, to a certain degree this is true. However, the reality is that what ever flow rate you have coming In from your mains from the street will be what comes out of your taps. If you have 10 litres per minute coming in from the street and you put a big 35kw combi in, it won’t magically boost your mains up to 20 litres per minute at your taps. The more flow you have the bigger the combi has to be in order to cope with heating the water passing through it, for example a large flow rate like 18 litres per minute with a small size combi like a 24kw will result in the water passing through the boiler too quickly for the heat to be able to transfer and you will be left with luke warm water, even though the boiler is working at maximum rate. A low flow rate like 8 or 9 litres per minute and large size combi like a 35kw will just mean that the water is passing through the boiler too slowly for the boiler to be working at maximum rate, the boiler might only need to work at 40-50% of its maximum capacity to heat the water to the required temperature, so you aren’t getting the full benefit of the larger boiler and have wasted your money buying such a large kw boiler when a 24kw would have been more than enough.

When selecting the correct size combi boiler for a property I describe to the customer as having a length of copper pipe, and there is one blow torch underneath it heating the pipe up, the water will have to be restricted and slowed right down to pass through the pipe really slowly in order to pick up heat from the blow torch. Now imagine you have 20 blow torches underneath the copper pipe heating it up, the water will now not have to be so restricted and will be able to travel through faster, yet still pick up the heat from the torches. If you had 100 blow torches the water can travel even faster whilst still picking up the same amount of heat. Same principle with a combi, if you have a lot of water passing through your mains water pipes, you will need a bigger sized boiler to be able to keep up with the demand. But it’s not a magical fix. I recently went to a job where the previous gas engineer had just installed a 35kw combi (which is nearly the biggest you can get!!) in a property which only had 9 litres per minute coming in from the mains, for this property, to have a 35 degree rise between the cold water going into the boiler and the hot water coming out, all that’s needed is 22kw, so even a 24kw boiler (which is the smallest combi size you can get) still wouldn’t be working at full capacity. The customer wanted a power shower so I ended up having to fit a cold water storage tank in the loft, a powerful shower pump and an unvented cylinder.

So combi boilers are not always the answer, in a 1 bedroom flat that has 10 litres or above flow rate, they are perfect. If you have more than one bathroom, a property with a high hot water demand or you want a power shower, then a vented cylinder with a shower pump or unvented cylinder is your better option. Please contact MD Plumbing and Heating if you would like a new condensing boiler or mega flo installed in your home.