Understanding Combi Boiler Hot Water Flow Rates

What does this mean?

Well what it means quite simply is how much hot water you get out of your hot water tap from your combi boiler.

Combi boilers installed about fifteen years or more ago were typically offered in two ‘sizes’ although customers were as baffled then as they are now, probably even more so what with the numerous combi boiler ranges available from almost every single boiler manufacture in the UK.

For the most part combi boilers then and now are more than adequate to heat the radiators sufficiently in all but the very largest house so in that sense the heating output of the boiler is not really important, just be aware that a typical combi boiler will heat your house be it a small one bedroom flat in Shawlands to a large semi-detached Victorian sandstone house in Giffnock, Clarkston or Broomhill.

Normally a boiler manufacturer will describe their combi boiler as a ' Worcester Bosch Greenstar 37 CDi condensing combi boiler' which means that the boiler has a maximum output to hot water of 37KW (15/1 litre per/minute) and on further reading you will see that the heating output of that particular boiler is described as '30KW p/hr' or approx 105,000 BTU p/hr.

Nowadays boiler manufacturers employ sophisticated sensors and fully modulating gas valves to manage how boilers heat the radiators in a house therefore  30 KW output to the central heating radiators will only use that amount of gas if the boiler gas-valve and sensors (Thermistors) demand that output. Similarly, if the heating load for a small flat is only 12 KW then the boiler will only use that amount of gas to heat that small flat.

In the past combi boilers were usually either 24 or 28 KW output models for central heating and hot water which translated to approx 9 or 11 litres of hot water delivered by the boiler to the hot water taps with a temp rise of 35 degree C. So if the temperature of the incoming water main to your house is 10 degree C then the boiler will raise that temperature by 35 degree C or if the incoming water main temperature temp is 18 degree C then the boiler will raise that temperature by 35 degrees too but will use less energy to do so because the cold water main temperature is warmer to begin with.

Now, however, there is an enormous choice in combi boiler hot water outputs,which would be determined by your particular hot water requirements (number of bathrooms etc), from 24 KW output to an enormous 54KW output which obviously affects the amount of hot water you can get from your hot water taps.

As said previously, a 24 KW combi boiler will provide a hot water flow rate at 35 degree C of 9 litres per minute whilst a 54 KW output combi boiler will provide a hot water flow rate of approx 25 litres per minute but the average hot water output from a combi boiler these days is around the 15 litres per minute mark which is what a typical 35KW combi will produce.

There are however some provisos. In order to achieve these flow rates the cold water supply in your house has to have sufficient water flow and pressure and ideally should be around 4 BAR pressure with a cold water mains flow rate of around 20 litres per minute.  The gas supply to the boiler also has to be of sufficient size to provide the boiler with enough gas to produce the stated manufacturer flow rates.

Often when a 24 or 28KW combi boiler was installed years ago they were replacing existing conventional gas boilers and the existing gas supply was sufficient for those types of boilers but totally inadequate for the new combi boiler however a lot of installers simply connected the new combi boiler to the existing gas supply (typically 15MM pipe work) which resulted in very poor hot water flow rates, particularly in winter when the incoming cold water main was at a much lower temperature than in the summer.

Some of these poorly installed installations resulted in hot water flow rates as little as 6 litres per minute, almost 50% less than the boiler was designed for, resulting in the extremely bad reputation combi boilers still have in relation to hot water production which has been difficult to eradicate and is often unwarranted.

I wish I had a pound for every time I’ve heard the remark “combi boilers are rubbish they take ages to fill a bath” which sadly in an awful lot of cases was true but combi boilers have improved enormously since those early days of 1988 when I first started installing them.

Personally I have had combi boilers installed in the last three houses I’ve lived in (29 and 35 KW models) since 1992 with very little problems at all and especially in relation to hot water flow rates. For showering combi boilers are excellent and are only bettered by unvented hot water cylinders which are much more expensive to install.

If you are changing from an old 24 KW Combi to a new Worcester Bosch Greenstar 37 CDi Combi boiler, for example, you may have to have the gas supply to the boiler upgraded to at least 28MM pipe work depending on how far away from the gas meter the new boiler is.

Also, there is little point in installing a combi boiler in a house with poor cold water mains pressure and a low flow rate in which case a 24 or 28KW combi boiler may be the only options open to you or you may have to opt for a traditional set up of conventional boiler with hot water cylinder or unvented hot water cylinder and a fully pumped set up of a twin channel programmer,room stat,cylinder stat and motorised zone valves.

I like to use a computer analogy in relation to combi boilers which is really a bit too simple however the bigger the number on the boiler the more hot water that particular boiler will produce, subject of course to pressure, flow rates and gas supply pipe-work. 

So think of the number on a boiler like you would the RAM in a computer, the more you have the faster the computer will run so the bigger the number on the boiler the more hot water it will produce.

Hopefully this post has helped you to understand what combi boiler flow rates and those confusing numbers on combi and most other boilers really mean for you.