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Which combi boiler?

OK so you've read about why not to have a combi, and you still want one! There are hundreds about, how do you choose? This page should give you some concrete guidance...

The four features to consider are...

  • Heat output
  • With/without secondary heat exchanger
  • Condensing or not
  • With/without stored hot water reservoir

Heat output first. This one's easy. Most combis are fitted in smaller properties which need way less heat to warm them that even the smallest combis deliver, so any combi will usually do. 

Why are combis apparently so oversized? Because massive heat output is needed to deliver even moderate hot water flow rates. Far more heat is being pumped into a hot tap running than is needed to heat even quite a big house, so even the feeblest combis at 24kW can happily warm up even a big four detached property.

Combis are usually rated at 24kW, 28kW, 34kW or even higher. Hot water performance from a 24kW combi is pathetic. Enough to wash your hands in or run a reasonable shower, but filling a bath will take ages. 28kW is a bit better. Expect a bath to fill (mixing hot and cold water) in six minutes or so in summer. Performance will be significantly poorer in winter because the incoming mains water temperature will be 10 or 15 degrees colder than in summer, so the boiler has to raise the water temp by 30% more than in summertime, and this results in a major reduction in flow rate through the boiler for it to be able to do it.

Stored hot water reservoir.  The latest designs have a few litres of hot water stored inside them. This allows the initial rate of flow to be much higher, and reduces latency (the time it takes for hot water to arrive at the hot tap). Downside is that this makes the boilers even bigger (and more expensive and complicated), and they fire apparently randomly to keep the store hot which surprises some users until they get used to it happening!

Some have a much bigger reservoir and are floor standing. A big reservoir lets them deliver water at something like 18 litres per minute for almost a bath-full before the store runs out. The boiler obviously fires up immediately and starts replenishing the store although not as fast as it is being drawn off. Net result is greatly improved performance for five or six minutes though, until the store is exhausted.

Examples of combis with hot water reservoirs:
Glow Worm Xtrafast
Vokera Linea Max
Worcester Highflow

Secondary heat exchanger This device combats the water scaling problem. Sort of. They still scale up in my experience (despite the manufacturers claims of scale resistance), but without the secondary heat exchanger it's the main heat exchanger that scales up. Changing a scaled up secondary HE is usually quite quick and easy but not so for a main HE, so a secondary HE is generally a Good Thing. They increase latency quite a lot though, so  another balance to draw. Both types of HE can be chemically de-scaled in situ though, but at higher expense.

Examples of combis with secondary HEs:
Worcester 24CDi, 28CDi, 35CDi
Ideal Isar

Examples of combis without secondary HEs:
Worcester 24i, 28i

Condensing. This doesn't really make any difference to the perceived performance of a combi, it just reduces your gas bill and the CO2 emissions from your boiler. It's a bit more technology to go wrong inside an already over-complicated appliance though.

Having said that I installed an Ideal Isar condensing combi the other day and I was actually rather impressed. 30kW, physically smaller than most other combis and despite being a condenser, seemed to be less complicated inside than most non-condensing combis. Worked very nicely too. I'll almost go as far as recommending it!

Examples of condensing combis:
Ideal Isar
Glow Worm Xtrafast

Return to the Combi boilers page.




Page first created 16th January 2004
Last updated 5th November 2007

Copyright 2004-2007 Michael Bryant

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