On Sat, 01 Mar 2014 06:45:14 -0500, Stormin Mormon
wrote:
On 3/1/2014 12:22 AM, wrote:
Closing a cold air return in an unheated room won't impact the combustion
air intake.
No, but it MAY cause the furnace to "short cycle" because the plenum
temperature will rize too high due to restricted air flow. This causes
the high limit switch to shut off the burner untill the blower sucks
the heat out of the heat exchanger, when it will relight. This makes
the efficiency of the furnace drop SIGNIFICANTLY.
Heat can go into one of a couple places. Into the
house, into the cellar, up the chimney. When
efficiency drops SIGNIFIGANTLY, the heat is lost
some where.
Up the stack. When a furnace short cycles it is running in the
in-efficient zone for more of the time. The heat goes up the stack
instead of into the house.
SM: So, the air handler blower is running, the gas is off, and the heat
is going up the stack, you say? I'd think the heat is going into the
building.
So the burner shuts off, and the blower still runs.
How does this make the furnace less EFFICIENT? I
don't picture it. Shutting off the gas lowers
the efficiency? Really?
At church we have converted oil furnaces, which
now run NG. They have a high limit sensor in the
discharge air, when it gets to 110 or 120 it turns
off the flame. The three blowers keep running.
So, turning off the gas now and again makes the
efficiency of the furnace drop SIGNIFICANTLY?
How's that, again?
see:
http://www.ehow.com/info_12081271_sh...-problems.html
SM: Says decresed efficiency, but doesn't explain
the process, and where the heat goes.
http://www.ask.com/question/why-does...ing-on-and-off
http://books.google.ca/books?id=Vdxw...ciency&f=false
SM: This page is not available, or you have exceeded
your limit for the day (my limit seems to be zero).
SM: I still don't see it, sorry.
OK Stormey, I'll try to explain it in simple terms.
You start the cold furnace, and it has to warm up the stack to get a
good draft going (unless it is a high efficiency burner - where you
get a forced draft, but it still needs to warm up) When it gets up to
operating temp, it is running at top effieniency. It warms up the heat
exchanger and plenum, and at a certain plenum temperature the blower
comes on to extract the heat from the heat exchanger and move it
through the house. The Delta T between the return air and the heat
exchanger temperature affects the efficiency of heat transfer - the
goal is to keep the delta T (temperature difference) between the
return air and the heat exchanger in the "sweet spot" where the
maximum heat is extracted from the exchanger and delivered to the
house. Now, if the airflow cannot extract enough heat, the heat
exchanger/plenum gets too hot, and for safety reasons the furnace
shuts down (short cycles). Now the firebox/burner/stack go into
cooldown mode along with the heat exchanger. When the heat exchanger
drops down to the "reset" temperature, the furnace refires and starts
the cycle all over again. Much of this time the delta T between the
exchanger and the return air is sub-optimal - dropping the heat
exchanger efficiency. - and the furnace starts up in it's lower
efficiency range untill it warms up again. Low airflow causes the heat
exchanger/plenum temp to go above limits again, and the (short) cycle
repeats itself. During this whole cycle, the efficiency is less than
optimal. When the airflow is not removing the heat from the exchanger
efficiently, heat goes up the stack.
In a properly operating system, when the furnace gets the plenum up to
temperature and the blower comes on, the correct amount of air passes
the exchanger to transfer enough heat from the exchanger to keep the
delta T in the "sweet spot" where maximum heat energy is removed from
the heat exchanger, and the minimum required heat escapes up the
stack. On variable speed blowers, the system is set up for a specific
temperature size across the heat exchanger. On my furnace it is spec'd
at between 30-60 degree F temperature rize across the heat exchanger
with 28000-40000btu output
The bigger model of this furnace is 45-75F at 89-72000 btu output.
My blower is set at about 650 CFM