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larry

wrote:
larry wrote:
wrote:
Uncle Monster wrote:

... The heat is just enough to trick the thermostat, not to warm the
refrigerator compartment.
How would that accomplish the goal?
The slight heat produced by the heater is enough to raise the temperature
around the thermostat to trick it into running the compressor for a longer
period of time. It's not necessary to increase the temperature of the whole
refrigerated box, just the temperature around the thermostat.
That doesn't make sense to me...
It's like the timer with a 10K resistor clipped under a
regular wall thermostat to do an over-night "setback".
Thermostat thinks it's getting warmer and turns off the
furnace when the room isn't warmer.

But that makes the room colder at night. How do we keep the fridge
a constant 40 F and the freezer frozen in a 40 F room?

If the freezer stays 0 F in a 60 F room while the 40 F fridge box
is gaining Q Btu/h from the room, cooling the room to 50 will reduce
the fridge gain to Q/2, so the compressor will run half the time.

But the required freezer run time will only go down by 50/60, so
we need to add enough heat to the fridge box (vs the thermostat box)
to raise the run time back up to 50/60 of the 60 F room run time, no?

The fridge box won't absorb any heat at all from a 40 F room, but
the freezer still needs 40/60 of the 60 F room run time to stay 0 F.

Nick

The way the closed loop control works on modern frig's is
working against you. You need a little redesign.

Given:
1. the freezer, goal=0F, always needs cool in your ambient
temp range, 40F to 80?F.
2. the coldbox (frig - i hate these terms), goal=36F, needs
cool on hot days and maybe none on cold days.

Plan:
1. freezer, meet the demand first, put the thermostat for
the freezer IN THE FREEZER, where the coils are to begin
with. Closed the loop on the freezer, it will take care of
itself.

2. Coldbox. Block the existing air path between the freezer
and the coldbox below. Control that airflow with a moving
vane and and fan connected to a thermostat in the coldbox to
cool the coldbox with cold air from the freezer to the
target temp 40F.

Cheap way of doing the coldbox control is a bimetalic strip
in the coldbox that pushes the vane open as the temperature
exceeds 40F. A small microswitch could switch on the fan as
long as it detects the vane is open.

Just about everything you need is already in the frig. Take
up the baseplate in the freezer, you will see the cooling
coils, the air path, with vane, to the coldbox, and a fan
near the back, that circulates cool in the freezer and
coldbox. Plus a big heater to thaw the coils. There is a
schematic in an envelop near the compressor or defrost tray.

I don't know how this will affect defrost cycle.

second thought, close off the kitchen and roll the frig to
the living room... You can't be married... ;-)

-- larry/dallas

btw, the resistor/timer will work if you invert the
function, resistor on for less heating, resistor off for
less cooling.