Text 10553, 92 rader
Skriven 2007-01-15 12:38:19 av Glen Jamieson
     Kommentar till en text av Dave Sacerdote
Ärende: IN-FLOOR HEATING 70115
==============================
 -=> Quoting Dave Sacerdote to Glen Jamieson <=-

 GJ> The main advantage of the more expensive and less efficient hot water
 GJ> system is that takes up less space.  

 DS> "Less efficient"?  Please explain.

"Efficiency" as measured by the %-age of the heat value of the fuel
which is absorbed by the occupants of the building, depends on the
process by which the heat is generated, and where heat is lost from
the system.

The most efficient is the "heat pump", or reverse cycle refrigerator,
whereby a cold evaporator absorbs heat from the air or a well, and
rejects it to a hot condenser, as this can have a "performance factor"
of 3 or 4, in that the electricity required to drive the compressor is
only 1/3 or 1/4 that which would be required to heat the living space
by electric resistance heaters.  These work best in a more moderate
climate than Jim and his polar bears occupy, and are economical if
switchable between heating and cooling, depending on season.

For the inhabitants of frigid climates who are "cool", and never need
to get cooler, heating-only systems are much cheaper than heat pumps.
Electric heaters allow the heat to be supplied most directly to where
it is required, eg radiant or forced convection heaters in occupied
rooms, and electric blankets in beds, but most people prefer to heat
rooms, even when they are not occupied all the time.  In most places
electricity as a source of heat is more expensive than oil or gas, so
the latter alternatives are usually preferred.

The cheapest of all heating is provided by unflued gas heaters in
individual rooms, with provision for allowing some fresh air inflow.
These are common here, where piped natural gas is supplied to many
houses, and very efficient, as all the heat value of the gas enters
the rooms.  Moving up the scale are flued gas heaters, usually with a
fan to blow the hot air into the rooms.  Some heat is lost via the
flue, but still fairly efficient.

Then comes the remotely located heater or "furnace" more familiar to
people who want to heat large sections of their house.  Using a gas or
oil fire to heat air to a comfortable 45C or so in a gas to gas heat
exchanger results in a flue temperature of 100C or so.  That is the
first place where heat is lost.  That air, blown along ducts, which
should have at least thin insulation, enters the rooms with enough
velocity to ensure that the room air is mixed, and stratification of
hot air at ceiling level is avoided.  Some fresh air is always
required, particularly if there are smokers around, and heat is lost
in the bleed-off air.

The system used for multi-story buildings or where it is not
convenient to use air ducts, is the hot water "boiler" (which,
hopefully, never gets to boil), and the water is pumped around the
building in insulated pipes to "radiators" (which give their heat to
the rooms by convection, not radiation).  Because this water has to be
heated up to some 80C, the temperature of the flue gas has to be of
the order of 130C, which is where the main heat loss occurs.  Some
heat is lost to the environment from the pipes circulating the water,
but that is not significant.  Most of the room heaters I have seen sit
quietly out of the way, and gently warm the air around them.  This
slowly warms the rooms by natural convection, but most of the hot air
remains near the ceiling.  The efficiency of these room heaters can be
greatly improved by adding a, hopefully not too noisy, fan.

(My goodness!  What a lot of writing in response to a 4 word request!)

Looking for a tiny recipe to jam into this space, I found:

MMMMM----- Recipe via Meal-Master (tm) v8.05
 
      Title: UBE YAM JAM
 Categories: Filipino, Asian, Tropical
      Yield: 20 Servings
 
      2 c  Ube, purple yam, boiled and
           - mashed
      1 c  Sugar
      2 c  Evaporated milk (canned)
 
  Combine evaporated milk, ube and sugar.   Cook over a low flame until
  thick.  Keep on stirring to prevent scorching.
  
  from  "Galing Galing"  Philippine Cuisine, by Nora and Mariles Daza
  
  Typed by Glen Jamieson 10/00
 
MMMMM
 

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 * Origin: Braintap BBS Adelaide Australia (3:800/449)