HVS wrote:
On 26 Oct 2020, Vir Campestris wrote

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!

I grew up in Canada in the 1950s and 1960s with oil-fired, forced hot
air heating, with a furnace and large oil tank in the basement, and a
fan forcing the hot air through ductwork running to each room.

It worked fine[1], but I can think of a couple of disadvantages for a
lot of the UK housing stock: it would be a bugger to retrofit in
solid-wall house (rather than building it in too a stick-built/balloon-
framed house), and the furnace, tank, and ductwork took up a lot of
room. That's not much of a problem if you have a full-size basement,
but otherwise it needs a lot more space than a boiler in a cupboard
feeding CH pipes and radiators.

[1] Our cat certainly liked it: she'd lie on top of the hot-air
outlet.


In places that still have oil heating, the tank goes outdoors now.
The house I was born in, the tank is still inside, and it's
been highly reliable compared to the track record of
outdoor tanks. But if you want to save space on a 200 gallon tank,
you can do it.

The places using oil heating, would not be using it if natural
gas was available. The price of oil is astronomical. And electric
heating is similarly stratospheric pricing, so a non-starter
as an option. Electric heating was tried here (because, well, we
got a bunch of nuclear reactors and what are you doing to do?),
and a few people at work were always bitching about theirs.
The elements used to fail in the electric furnace. You won't
find many electric furnaces today (Bill Gates maybe?).

The biggest liability with oil heat, is leaking oil and cleanup cost.

And before we got oil, some of the houses were still using coal.
I was lucky as a kid, I was down at my friends house when
a coal truck came up and delivered a load of coal. And we
watched while my friends dad shoveled coal through a basement
window, into the basement :-) Some of the members of the
family, use to have arguments about which person was
supposed to be cleaning out clinkers. Well, all that
changed when the oil came along. The coal room was re-finished
and became my friends bedroom. And we were careful to *never*
mention he was sleeping in the coal room. It's not like
he had a choice in the matter (big family).

*******

As for the routing of ductwork, my house has a 10"x20" rectangular
cross section pipe running along the spine of the house (in
the basement). There is also a steel I-beam that runs the length
of the house and it's near that pipe. Whereas other houses used
sistered wooden beams nailed to one another for the spin, this
house series they used a steel beam instead.

Smaller round pipes (6" diameter) feed from the 10"x20" galvanized
sheet metal pipe, and the round pipes run between floor joists.
This reduced the impact of the heating distribution in the
majority of the room area. The basement has the penalty of headroom
dropping to 6'3" below the rectangular pipe. Whereas the rest of
the basement has a ton of headroom.

In two story houses, the round pipes may be replaced with a
rectangular cross section pipe running vertically up the
walls. Which makes for a long run of pipe, and makes it
hard to balance the system. Each run of pipes has a "vane"
inside the pipe, and a lever on the outside which you can
rotate. You adjust the lever to get the degree of heat
you need in the room (with the second floor ones being
at a disadvantage because of the flow resistance). In the
dead of winter, the people on the second floor are freezing
to death :-) That's why they get extra blankets up there.
In late fall and spring, everything is fine.

The hot air vents go on the outside walls. The cold air return
are on the inside walls. The cold air returns get little
attention, so it's hard to say whether the flow rates
are really all that balanced between the two sides.
If you don't get that part done correctly, the furnace
starts sucking air through any available crack, from
basement air. Which isn't always the best thing.

The only draft in the house, comes from wall sockets. And
todays R2000 techniques (sealant and proper boxes for outlets)
helps control that kind of leakage. In the old days, the
build quality wasn't all that good on that sort of detail.

The combustion furnace has a range of delta_T it can handle.
You can damage the heat exchanger if the conditions are
not properly met. The speed of the motor (four speed motors
being typical) helps set the delta_T. The door of thr furnace
states how much temperature difference is allowed between
the "heat" central pipe and the cold air return pipe
(aka "ambient"). The speed requirements for heat and AC
are different. In some cases, the same speed used for
both, in other cases, one season runs the motor faster
than the other season. If you get this wrong, you can
crack the heat exchanger. (You would think moar air
for heat distribution would always be good, but
that's not the case.) Part of checking an air circulating
furnace, is making sure some wacko hasn't buggered the
delta_T. (Two furnace techs can get into a fight about
which wacko did it :-) That's why they work in pairs.)

Paul