Hi, to check the performance of a combi boiler you will need both the boiler heat specs and to do a quick flow test.
i) What is the rated max. output to water. (in kW). This will be in the documentation.
ii) Does it have any hot water storage? If so, the power test will take a little longer.
Make sure the boiler is working at full capacity. While running hot water into a bath or sink, the boiler should be on, and at full gas flow if it has a variable gas-flow valve. Make sure that the hot water temperature control is at maximum to ensure the boiler is running flat out.
Leave the hot water running at a constant rate and make sure no other taps are on, etc. The central heating should be OFF for the test.
Measure the temperature of the water from the tap. It should arrive at a constant temp. within a few minutes. Make a note of this water temperature.
While still running the tap, measure the flow rate of the water, in ml (millilitres) per second. (Time the filling of a 1 litre jug or whatever.)
Next, turn off the hot water and turn on the cold tap. Leave to run for a minute or two.
Measure the temperature of the cold water after ensuring it is constant. Make a note of this.
Then calculate the output wattage of the boiler to water:
The arithmetic is: Hot tap flow rate (in ml/sec) x Temp difference in degrees C between hot and cold water x 4.2. This will give you the boiler wattage (in Watts) going into the hot water flow. Divide by 1,000 to get the kW figure.
Compare this with the boiler output to water in the specification manual. They should be pretty close.
For example, say your combi is 30kW output. This will heat 200ml/sec of cold water by 35C (note 'by', not 'to'). So with an input cold water temp. of 5C you will fill a 150 litre bath-full to about 40C in 12-13 minutes. An important thing to remember is that the hot water temperature from a combi depends upon the temp. of the incoming cold water feed. The colder the water (in winter, say), the lower the temp. of your hot water will be at any particular flow rate.
You will, of course, be able to raise the hot water temp. by reducing the flow rate, as you have noted.
Hope ths helps. Regards,
George.