On 30 Mar 2005 16:04:07 -0800, (Antony Gelberg)
wrote:

Hi all,

This post may seem ridiculous. I'm currently researching a new boiler
and radiators for my flat. In order to complete the calculations, I
need to know what the construction of the outside walls in my block
is. I haven't got a clue - is there any way that I can make an
educated guess (using your education!)?

Here are some "clues": the block was purpose-built in 1986. It has a
concrete internal structure. The outside is finished in red brick.
Windows are double-glazed with aluminium frames (not sure if they have
thermal breaks - this is something else I need to establish).

Lastly, when I moved in, with all the household paperwork, there was
some sort of certificate saying something like the property had been
built to highly energy-efficient standards, and the heating thermostat
could be left on around 18. (I have found this last point to be
true, and I like it warm!) Naturally I binned the certificate, so
can't quote verbatim.

Antony

This is going to be, at best, reasonable guesswork.

You would really need to know the construction of the inner leaf of
the wall. You mention concrete... DOes this mean a concrete beam
structure or ??

I would suggest find an unobtrusive place on an outside wall and
removing a small piece of plaster or plasterboard. Then see what is
behind it by way of blocks and ideally the cavity depth and whether
it's filled with insulation. If you drill a hole and push a piece of
metal tube into it and into the cavity, if there is insulating
material then you should pick up a lump of it; plus you can measure
the cavity depth.

Assuming that it is a cavity wall, aerated blocks and insulated, then
from the Building Establishment tables, under best conditions the U
value would be 0.35 W/m^2.K A more typical value is around 0.45 to
0.5

Without cavity insulation, it could be as much as 0.9; so you really
need to know about insulation. On a place of that age, there should
be insulation.

You could assume worst case and overdesign, but that would be a bit
wasteful.

For the windows, double glazing in aluminium frames without low-E
glass or gas filling (reasonable assumption here), the U value is 3.4

You then need to account for floors and ceilings. For a ground floor
on concrete, the loss is reasonably easy to estimate, as it is if you
are the top floor and have to calculate for the roof. It's more
awkward for the floor/ceiling to another flat because you then have to
make some assumptions about heatloss to those.

If you post some details, I can look uo the tables for you. To
calculate for a ground floor, the dimensions are needed to determine
an effective U value because the perimeter dimensions come into it.

Once you have all the U values, the basis for calculation is to
measure the areas in square metres and multiply them by the U value
and the temperature difference through the surface. That gives you
the heat loss in watts for that element. You then add up the elements
for a room to determine the total heat loss through surfaces.

After that, there are heat losses due to air changes. There are
industry standard assumptions for these which are reasonable for
heating calculation purposes and to use them you need the room volumes
in cubic metres.

The two values are then added to get the heat loss for the room, which
determines radiator output requirement from which you can size the
radiators. Finally you add these numbers together, add a margin of
10-20% to determine the boiler requirement. Note that this is usually
not too critical because most boilers are adjustable (range rated) or
automatically modulate to an appropriate output.



--

..andy

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