Nowadays most householders are aware of the benefits of what is called the combi boiler.  No roof tanks, no copper cylinder and piping hot water on demand.

 

Its certainly a far cry from the old open vented system,  whose hot water pressure was wholly dependent upon how far vertically a draw off point was from the cold water cistern!

 

There's no doubt that modern combi boiler based systems are easier to install than the older type but there are still a few factors that homeowners should be aware of.

 

Firstly, power output.  In normal situations the size of combi boiler tends to be governed by how much hot water the customer is likely to need rather than by heating considerations.

 

A combination boiler works on an “instantaneous” basis. Water drawn from the rising main to the tap is heated as it passes through the boiler’s domestic hot water heat exchanger. The temperature of water obtained is dependent on three factors: -

 

a) Boiler output

The performance of a boiler is directly related to its power output (usually given in kW or Btu/hr)

b) Water flow rate

The speed at which water passes through the heat exchanger will determine how much heat is absorbed. A good analogy is a finger passing through a candle flame – whether the finger gets burnt or not depends on how quickly it is passed through the flame.
Boiler manufacturers will often quote a hot water flow rate for the appliance in l/min or gal/min. However, it is important when assessing the performance of a combi to remember that this flow rate is based on a specific temperature rise – usually 30°C or 35°C. If a greater temperature rise is required, the flow rate must be reduced. Conversely, a greater flow rate can be obtained if the temperature rise required is less.
e.g. A Combi with an output of 23kW gives a flow rate of 9.4 l/min for a temp rise of 35°C but,
A flow rate of 8 l/min gives a temp rise of 41°C
A flow rate of 10 l/min gives a temp rise of 33°C
A flow rate of 15 l/min gives a temp rise of 22°C

 

c) Incoming mains water temperature

We have already seen that the temperature rise depends on the flow rate, but the actual temperature obtained will depend on the initial mains water temperature. Because this varies, the apparent performance of the combi may change according to the season e.g. with the 23kW boiler, at standard water flow rate;
Summer- typical mains temp 15°C, hot water temp 50°C
Winter – typical mains temp 4°C, hot water temp 39°C

 

This explains why an undersized combi boiler can give satisfactory performance in the summer but in the winter can feel inadequate.

 

This effect can be exacerbated where "drench" showers or high flowing taps are used.