Adrian wrote:
In message , Vir Campestris
writes
It used to be we had open fires. But then hot water radiators came in
- much cleaner, and easier to manage.

Nowadays we seem to be moving towards underfloor heating - where
effectively the entire floor is a radiator. Efficient, but slow to
respond.

But there was a time when hot air systems were popular, and I think
still are in the USA.

Does anyone know why they went out of fashion? It can't just be
because they made a great place for spiders to hide!

We used to live in an early 60s place that had it. To describe it as
heating was getting close to misleading advertising, more like a slight
warm draught. Even if it had worked anything like, it was very
difficult to control compared to a radiator with a thermostatic valve,
basically vent open/vent closed. It got replaced with a conventional
gas fired CH system.

Adrian

One thing I've noticed, is the home heating people aren't
very good at design.

The ductwork on houses is seldom all that good. A long narrow
pipe is expected to deliver the same air as a short larger pipe.
All sorts of non-intuitive stuff going on.

If you had a saleman come into your house today, he'd
sell you a 60,000 BTU furnace, because he'd tell you
that the "longer run time gives more efficient heating"
and would save you a tiny bit of money. But at the
expense that if you put your toes over the register,
you can't really "feel" the heat.

I've had both a 60,000 BTU furnace (current one) and
an 80,000 BTU furnace, and the 80,000 one really
did "warm toes".

You can't go too high though, because the ductwork
is designed with a certain size of furnace in mind.
If you connected a 120,000 BTU furnace, the delta_T
between the hot air output and the cold air return
would be too high, and the heat exchanger would melt.
The furnace has a four speed motor, to allow some
adjustment to flow rates (most of the time it runs
on high or just-below-high). Each pipe arm has at
least one damper on it, a vane with a rotating lever
to set the airflow in the pipe. But this is not designed
to make large corrections, only small ones. For example,
if you had two identical runs of pipe, the dampers
could make fine adjustments so the airflow was
made equal. But you can't balance 10sqin of pipe
with 40sqin of pipe, just using dampers. If you
damp down the 40sqin pipe to 10sqin, now there's
too much overall resistance to airflow and the
furnace overheats.

After a while, you'll realize that every analogy
in the air circulation system, has an equivalent
one to a water based system. Many knobs. Maths.
And occasionally, a result.

To show you how stupid people can be, on a hot air
system, the hot vents go on the outside walls, the
cold air return is on internal walls. Yet, one house
I was in, they reversed that, and put the wrong
pipes on the respective walls. Naturally, the
results are far from ideal. Miserable even. Any time
the exterior walls get cold (due to the weather
conditions outside), those walls will suck the
life out of you. If you're sitting in a chair, you
move away from the wall :-) Because cold air will
be streaming down the wall at you.

Paul