Combi VS Unvented
The Battle is On!

Written by James Tranham

Well, Sort of ...

The SCD Team are often out surveying for new boilers and new boiler installations and we constantly hear the words “ we want a combination boiler”. The first response from us is why? This is not to say that combi’s are not a great solution, however, in most places there is a demand for them without any rational decision behind them.

The most common response we receive is “I want to be able to have a really powerful shower” – well that’s a good reason who doesn’t like a nice powerful shower?! The trouble is you are reliant on the mains pressure and flow rate to deliver this – no matter how big your combi is, it will not improve the mains pressure or flow rate from your taps. We see all too often combi boilers being sized incorrectly and either are too large for the requirement or even worse too small to cope with the mains flow rate.

SC Duncan always carryout a flow rate test AND pressure test when specifying a combination boiler as these are the only ways to establish what boiler size is required. The biggest mistake made is that pressure and flow rate are NOT the same thing. Flow rate may be 15 litres a minute, but if you can stop the water at an outlet with your hand without drowning yourself then when you open two outlets together you will only have 7 litres a minute through both.

Alternatively you may have a low flow rate because of a restriction or small inlet, but up to 6 bar of pressure pushing it through. In this case you would be able to run two outlets at once of 15 litres a minute, BUT this would not work with a combi this is when it is time to step up to an unvented system. An unvented system holds a buffer of water at mains pressure up to 3 bar (still dependent on that mains) and can support multiple bathroom outlets at one time, these are a far more expensive option, but all of a sudden not as expensive when you consider a 40KW combination boiler is around £2k!

It is always important to investigate all avenues that suit your needs and not what Bob in the pub says – if you would like free advice or an appraisal of your boiler or heating system call SC Duncan Heating Plumbing and Electrical now!