Andy Hall wrote:

On Fri, 10 Sep 2004 16:56:07 +0000 (UTC), (Fred)
wrote:

While I have seen combis in various locations in the UK where
the hot water at mains pressure. This results in a high flow at least
in areas suitable for combis, not to true for other areas where combis
should not be installed - ask your plumber. I usually take a bath
after doing the dishes in the evening, my bloody tank is not big
enough and with the heating on, it takes forever and a bit to fill the
tank. Not an issue with the combi, where hot water is on demand.

No guys, where combis can be done (depending on mains pressure), they
usually beat a comparable tank system flat out.

That is demonstrably nonsense. There is no physical way that even
quite a large combi system can deliver hot water at a greater rate
than a tank storage system provided that the pipework is adequate on
the storage system.

Ok, point taken that I kind of compare unequally sophisticated systems
with my cranky old tank. I accept what you say about the flow rate.
The problem of limited supply remains. Doing the dishes, taking a
bath, taking another bath, and another set of dishes - easy with
children.

Well, to be honest, I
have never seen a new tank system, but plenty of new combi systems
here. And the thing with the cold water tanks in the loft dribbling
through your house makes you feel as if your buddy has a chronic
prostate problem ;-) Oh and for hygienic apostles amongst us all, take
a look in the cold water tank, then think about the hot water tank,
where hot water stays for days. And then count the bacteria.

If the hot water cylinder stores the water at 60 degrees there will
not be a bacteria problem.

That contradicts the fact that we all have to boil water to get rid of
the nastier bugs. Seriously, 60 degrees needs a fairly long time to
kill of most (not all!) bacteria. To kill the lot you need to go above
95 or so. Nice touch about the combi: it produces drinking water
regardless of hot or cold water. VERY handy to wash veg in deep
winter. Up here in the North it gets really cold indeed!

Right. Final issue: water temperature. The combi's performance depends
on the distance between boiler and bath. The closer the hotter the
water. The tank has the same issue (hence its usual location in the
bathroom) but, BUT, a mildly old tank system will loose heat = the hot
water is not of the same temperature. Kind of sucks when you keep your
hot water at a low temperature only to find out that your bath is
lukewarm because the water was sitting in the tank for a day or so.

This is complete nonsense.

Of course you are right that the incoming mains temp is the main
determinant, but if you look at the installation then distance between
boiler and consumer becomes crucial. Same for your tanks, hence having
the tank close to the bath.

Neither will give any significant temperature drop between the point
of heating and point of delivery in a normal house.

That will depend on what you call normal.

A combi is suitable for a flat or very small house, or where the
occupants have modest requirements for hot water delivery.

For a 3-4 bedroom house wiht multiple occupants, the performance
becomes increasingly inferior to storage. Storage can be by directly
storing the DHW in a cylinder, or via a heatbank, but is the only way
to get good results where multiple bathrooms are in use.

Yes that is the point. With en-suite being the dream component of an
English household I see your point. Nowhere else are en-suites seen as
so desirable, but I fully take your point. But to quote you: it's of
course nonsense to say that a combi cannot serve a four bed house.
Unless you know the customs of the household (shower rather than bath)
and the preferences for low heating bills, you cannot possibly decide.
Also, with combis, you get what you pay for. The cheapest oldest
combis had problems, but the ones I know that have been installed here
in the last five years, work great - even in big houses. I accept
that the English use the bath much more than our continental
neighbours, but for many people the combi works marvellously more or
less regardless of the size of the household (there are limits: would
like to know the heating system of Beckhams ;-))

Fred