On 20/05/2017 13:35, Terry Casey wrote:

I note it says in the wiki that: "I need a new one, what should I get!" comes
up in some form almost every day on uk.d-i-y, but couldn't find enough
information to clench the deal, one way or the other.

Not sure this will help any ;-)

My wife and I are living in a two bedroomed bungalow - there's nobody else
living there except the cat! The central heating is old - if the pencilled
date on the back of the controller I replaced recently is anything to go by
it dates from 1996.

Not *that* old then...

We had a quote recently from a local plumber who advocated a combi - which
I've always been convinced from various things I've read (probably here) that
they can be problematic - but he said that modern combis are much better than
their predecessors and would be highly suited to our needs. He also pointed
out that he has a combi in his 4 bedroom property and has no problems with
it.

Appreciate that some folks are so biased one way or the other, as to be
incapable of being objective!

We then got another quote from someone who is clearly prejudiced against
combis! Apart from additional strain on the old pipework - which we are well
aware of

This is not a combi issue, but a sealed system issue. The vast majority
of new boilers (combi or other type) are designed for sealed system
operation, although there are a few vented options still out there.

IME having converted several systems from vented to sealed, the problems
whilst frequently mentioned are somewhat overstated. Every system I have
encountered has been fine with the raised pressure. The worst case is
that you may need to replace some rad valves. If a rad springs a leak,
then it was near its end of life anyway.

- he said that the increased pressure can damage the non-return
valve in dual fill washing machines (ours is currently a dual-fill Hotpoint)

Not heard that before. Even if true you may have no actual symptom of a
damaged not return valve anyway. If you were really that worried you
could fit a pressure reduction valve prior to the WM. (check the makers
spec for the model and see what the max pressure is. I would be
surprised if its going to be an issue)

but that we wouldn't be able to run hot water from two taps at the same time!

That depends entirely on the power of the combi you get. A small one
(24kW) will have difficulty delivering more than about 9 lpm in the
depths of winter. More powerful ones ( =35kW) will do much better. The
35kW one I had in my last place would do two concurrent showers with
"adequate" performance, or one with excellent performance.

This could be a worry if we couldn't wash our hands (or whatever) every time
the washing machine was filling or the other is running a bath. Can this
really be true?

What happens when a combi runs out of power to to heat the flow rate of
water being consumed, depends a little on the design. Some include rate
limiting that maintains the water at a reasonable temperature, but limit
the flow rate. Most will simply allow the temperature to fall, but carry
on delivering water at the rate requested.


Quote No. 1 includes an Ideal Logic+ 30kW boiler whilst No. 2 is for a
Worcester Bosch 15ri boiler. No. 1 includes quotes for both combi and non
combi options (but does give any indication of which non-combi boiler would
be fitted) and No. 2, as might be expected, doesn't have a second option!

How do these boilers compare - assume No. 1 would use a non-combi version of
the same boiler?

Internally combi boilers and "system" boilers [1] are very similar and
frequently share the same basic platform.

[1] a heating boiler that includes the pump, expansion vessel and most
of the components required to build a complete system in one box

All comments gratefully received!

Some background things to check. Is your mains flow rate adequate for a
combi? (you need a minimum of 15 lpm really, and more is better).

Is the pressure adequate. 3 bar or better is good.

Is the water supply reliable? (if not, some stored water may be handy).
Have you though about backup water heating if the boiler were in need of
repair?

If looking at stored water systems, have your installers been thinking
about using a conventional gravity fed cylinder, or an unvented one?
(the latter tend to give better overall system performance if there is
adequate mains flow rate availble)


--
Cheers,

John.

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