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
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
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
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
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
Examples of condensing combis:
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