OK, it was chilly last night, went down to 47F.
House stayed above 72 with no heat.
Temps will be up in the 80's today.
I normally keep the windows and doors closed due to allergies.
Is it less expensive to run the AC now when the house is 72F and the
outside temp is still below 60? (taking into account I don't mind the
house being down to 65 for the first half of the day) Or is it better
to wait until the house gets above 72F and the outside temp is 80F?
Whoa! Gotta turn off the ac now, it went down to 64 quite fast!
It will be slightly more efficient running w/ lower exhaust temperatures
but it'll not be enough to be noticeable until the outside temp's are
100F or thereabouts. For "close enough", it'll simply be the amount of
time the unit runs that controls cost.
Actually I think this is incorrect. The colder the ambiant air around the
condensor the more heat that the condensor will shed upping the subcooling
of the liquid leaving the condensor and reduce the flash gas in the
Also if the nighttime temp is 47F the humidity will be low when the heat of
the day peeks at 80F.
Also if the max temp outside is 80F the house is not going to gain much heat
as the heat transfer is greater when the temp difference is higher.
Pehaps a better approach would be to open windows in the early morning
letting the cool morning air into the house, and then shut the drapes and
windows on the south side of the house in the morning as the sun hit them.
By doing this you can avoid any AC use.
About the time I had mastered getting the toothpaste back in the tube, then
I install a lot of units in businesses where AC has to run year round.
The OP mentioned allergies and an aversion to outside air. For units
meant to run in cold weather, I install head pressure controls on the
condensing units, it's hard to get a lot of HVAC techs to go to all
that trouble in order to make a unit work better in cold weather. A
while back I started seeing dual compressor units for homes that would
drop out one compressor during low ambient outdoor temps and reduce
the capacity of the system. Now I'm seeing inverter controls for the
compressor motors to vary the capacity of the system. I haven't had
an opportunity to install one of those yet but it is interesting.
I use pressure switches and electronic temperature sensors
for turning on/off the condenser fan for head pressure control
on AC units. Of course a crankcase heater is a given if it's
not installed at the factory.
Electronic head pressure control:
I've used this type for refrigeration systems:
The least expensive is a fan switch:
If you have an intelligent learning thermostat(programmable digital),
it'll help you quite a bit. How is your humidity? Our blower is going at
lowest speed setting most of time. When heat ir collis called for it
witches to it's proper speed. I think this way it gives best comfort level.
I recently had my HVAC guy change out my
Honeywell thermostat to a different
model which has a built in humidity
sensor. I've only had one or 2 days
experience, but it seems great. Here in
the western mountains of NC the
temperature usually doesn't get real
high during the day. But, the humidity
can. This thermostat will (over) run
the AC to remove moisture. I have it
set for 76 degrees and 45%. On the 2
days where is ran, it did exactly as
advertised. It will let the temp go up
to 3 degrees low to control humidity.
However, in my case it held the 76. It
probably also helped that the AC unit is
a 2 stage and of course it was running
on only the 1st stage. Now, I suspect
if you live in the real high humidity
areas like Florida, the results might
not be as good. BTW, you can also hook
up a whole house dehumidifying system to
this thermostat if it is needed. I
don't think I'll need that here. I'll
check it out during this coming summer
and report back. So far, I'm impressed.
Climatouch makes an even better multistage tstat which controls humidity inside
a 4 RH
(+2 and -2) variance.
I use it on a dual compressor heatpump in deepest part of S.Florida and it does
change the inside temperature when running a humidity control cycle.
For example, I set RH to 50% and system comes on at 52% and runs until RH is
it runs due to humidity it engages stage1 compressor and runs variable speed (DC
motor) blower at ultralow speeds allowing the coil to collect all moisture very
No other tstat comes close to this ability.
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