To Combi or not to Combi – that is the question?

To Combi or not to Combi – that is the question?


Hi, I am Martyn Fowler, Technical Director at Vast Group. I manage the plumbing and electrical division at Vast Group.

It was not so long ago that Combination boilers were considered to be the salvation of all things and the best thing since sliced bread. However, reality has proven that they are only one option and that they do have drawbacks which need to be pointed out to potential clients.

I remove quite a few in the course of our business because clients become frustrated about their performance, so why is that?
There is no doubt that their biggest advantage is the extra space gained as a result of removing water storage tanks and cylinders, in addition the ability to only heat the water required on demand. However, the delivery of hot water via a Combi is primarily only as good as the incoming water mains pressure and flow rate and with the recent habits of Water Utility companies reducing pressure to minimise leakage in their systems, water delivery is not what it used to be.
Consequently a shower may not highlight the problem, but try to run a bath and 20 minutes later frustrations are bubbling to the surface. Multi bathroom homes also discover that in spite of having 2 showers, they still can’t use both at concurrent times as there is insufficient water available; hence they have to plan the use of hot water – not very convenient if the family are all trying to get ready at once.
We also need to appreciate that Combi boilers supplying hot water on and off do suffer from a loss of efficiency due to the burners coming in and out on full power but the question is does that more than compensate from the heat loss from a tank of stored hot water? Oh why is life not straightforward?



Stored water may have issues in the past as regards ingress of impurities, insects etc, but at least if you wanted a bath, two 22mm pipes could fill a bath pretty quickly. If you wanted a power shower, fit a booster pump and away you went – even two showers at once were not a problem. The advent of better tanks, by Law kits and better plumbing have all increased the purity of water available so gravity systems can still work well with a regular heat boiler. An unvented hot water system is also a happy medium with just the one tank, a regular boiler and good pressure, although the client is still dependant of incoming water pressure. That said, this can be improved with booster pumps or an accumulator which is a very “green” solution to suspect or variable water pressure.

In summary what is best? The answer is to ask your client what they want to achieve and be honest with them as regards the benefits and drawbacks of all the options then you can be sure of a satisfied client.

Vast Group can provide this advice free of charge, Our maintenance team, carry out boiler replacements throughout the South East. Remember, use a Corgi or Gas Safe registered installer like Vast Group.

Please call me personally on 0207 147 9952 if I could be of any assistance.