On Sat, 11 Sep 2004 00:06:50 +0000 (UTC), (Fred)
wrote:

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.


No it doesn't.

I have a 200 litre HW cylinder with fast recovery from the boiler.

I can get 25 litres/minute of 60 degree water for filling the bath,
plus the same of cold. You won't get those figures out of a
domestic combi.

I can have 25 litres/min of 40 degree water out of two showers
simultaneously.

All of this is independent of the cold mains temperature.




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.

Not where I live.

Seriously, 60 degrees needs a fairly long time to
kill of most (not all!) bacteria.

It is what is recommended for control of Legionella etc.

You are assuming that there is a population of harmfull bacteria in
the first place. The water in the loft tank is replaced with
chlorinated water from the mains as it is used. Most households get
through several tanks a day.

If what you say were an issue, there would be massive outbreaks of
water borne, disease and poisoning in the UK. There isn't, so it's
a non issue.


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.

Why would you want drinking water at 95 degrees?

VERY handy to wash veg in deep
winter.

You wash vegetables in hot water?

Oh dear. First of all this is unnecessary. Secondly it begins the
cooking process earlier than necessary which results in many of the
vitamins in them being broken down.



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.

No. The only issue between boiler and consumer is the length of
time it takes for the water to travel - i.e. how long it takes to get
hot water at the tap.

There will be little temperature drop along a pipe run unless the flow
is a tiny trickle. Even the poorest combi can manage that.

For significant temperature drop to occur, there would need to be
significant heat loss. In calculations for central heating, heat
losses from the pipes (where running internally) are not taken into
account when sizing the radiator and boiler requirements.



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.

Anything up to a 5 bedroom, 2-300m^2 house - where pipe runs are a few
metres internally.


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.

If there is one person living in it and they take showers only, and
the combi is a 15lpm+ model, possibly. However, if the house is
populated with 4 people and two bathrooms and they take baths and
showers simultaneously, in the winter one one be very hard pressed to
satisfy the requirement with a single domestic combi.


Unless you know the customs of the household (shower rather than bath)
and the preferences for low heating bills, you cannot possibly decide.

Exactly. So the requirements of the possible population of the house
over the lifetime of the heating system should be the factor.
THerefore, for a larger house with two bathrooms, the assumption of a
lot of use should be made.

If somebody wants to fit a combi in this type of environment, knowing
the limitations, then fine, but they should be aware of them.

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.

It doesn't matter how "great" they are. Thy still can't defy physics.

The basic prinicple of mass x specific heat x temperature rise still
applies, and this is a fundamental limitation on any form of instant
heating using the fuel supply directly. The only way to improve on
this is to store the hot water directly, or via a heat bank.

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

That would depend on your definition of "marvellously" and how many
bathrooms. I suppose if they had used electric showers before, it
would seem good.

Even quite large combis can only deliver 15-20lpm of water at 40
degrees in the winter. THis is marginal for one shower, let alone
two.


(there are limits: would
like to know the heating system of Beckhams ;-))

Fred

..andy

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