Changing to a combination boiler may be the start of a expensive nightmare if your incoming water supply cannot supply enough water at a pressure of 10 metres head, and you might only know if it works for you after you replace your gravity system with a Combination Boiler.
A Gravity System Stores Water
A typical gravity system will have a 225 litre cold tank which stores enough water to supply a 6 litre a minute shower for 40 minutes even if there is no water replenishing the tank. You can happily be using water for multiple showers and be totally oblivious to a supply problem. A gravity system masks problems with your supply and you cannot assume you have enough water to run a Combi Boiler until you do a flow test.
Flow Tests
A flow test will determine what pressure you have when you take water from a pipe and most plumbers will only measure the static pressure.
Static Pressure Test
The static pressure test is a measure of the pressure when there is no flow taken.
Dynamic Pressure
The dynamic pressure is a pressure you get when you are taking water and it is only when you start taking water that you know what pressure you will be getting. When water flows in a pipe slowly there is very little head loss and when there is no flow there is no head loss.
Dynamic Pressure = Static Pressure – Head loss in pipe.
Predicting the head loss using theoretical calculations is a risk and unless you run the water at the maximum rate you will need, you will not know. Your pipe could have restrictions within it which creates turbulence which effectively caps the maximum flow you will get.
A Combination Boiler is a ‘Instantaneous Heater’
A combination boiler will heat water on demand and its output will be determined by how fast it can heat water – you can expect these outputs from a combination boilers according to its rating.
24 KW Combi Boiler gives 11.5 litres a minute (2 showers)
30 KW Combi Boiler gives 14.5 litres a minute (2.4 showers)
42 KW Combi Boiler gives 20.0 litres a minute (3 showers)
A power shower with a rain shower head may take 20 litres so a 42 KW Combi Boiler will only give enough water for one such shower (that is assuming that your water service can cope with 20 litres a minute).
Security of Supply
With a gravity system in the event of a power cut you still have 225 litres of stored hot water which mixed with cold water should allow you to run a good shower for 60 minutes.
If you boiler breaks down simply switch on your emersion heater.
Why is Alan Wright so against Combination Boilers?
I have visited and spoken to consumers who have been abused by plumbers who simply fit combi boilers without doing flow checks and explaining the limitations of any instantaneous heating device.
A shower power booster can and will reprioritise the flow to a shower when other taps are opened in a house.A shower which would otherwise be killed will run strong and true when protected by a shower power booster, but even with this, a combi boiler is not right for everybody.
Alan Wright
BSc(Hons) CEng MICE
