Oh LCHIEN, help with heat from a resistor

[jessrice]

wasn't sure where to post this,

Anyway, i will accept anyone's help, but i am guessing that LCHIEN will know the answer

I have a wine barrel room that is being cooled with a kenmore window ac, that can only be set to 60 degrees. i want to go to 55 degrees.

The AC temp sensor is currently mounted in front of the return air vent. The wires are about 20 inches long. Anyway i want to place the sensor into a PVC electric box with a small resistor to trick the thing into thinking it is actually warmer.

i cant claim this idea as my own, so that is why I need to inquire about the heat/energy output

So if this is up your alley, so to speak, does this look right to you?

254 ohm, 5 watt resistor, connected to 120volt to 9volt, 31 VA should produce about 1 watt of heat?

I'll tape the resistor to the temp sensor, seal the pvc box and be done!

Another simpler method many have done is to drill a hole in the wall and put the sensor outside, but i think they have trouble with the dialy change in outside temp. heating the sensor slightly should give a more constant temp for me to reference the thermostat from.

thanks

Jesse

[LCHIEN]

wasn't sure where to post this,

Anyway, i will accept anyone's help, but i am guessing that LCHIEN will know the answer

I have a wine barrel room that is being cooled with a kenmore window ac, that can only be set to 60 degrees. i want to go to 55 degrees.

The AC temp sensor is currently mounted in front of the return air vent. The wires are about 20 inches long. Anyway i want to place the sensor into a PVC electric box with a small resistor to trick the thing into thinking it is actually warmer.

i cant claim this idea as my own, so that is why I need to inquire about the heat/energy output

So if this is up your alley, so to speak, does this look right to you?

254 ohm, 5 watt resistor, connected to 120volt to 9volt, 31 VA should produce about 1 watt of heat?

I'll tape the resistor to the temp sensor, seal the pvc box and be done!

Another simpler method many have done is to drill a hole in the wall and put the sensor outside, but i think they have trouble with the dialy change in outside temp. heating the sensor slightly should give a more constant temp for me to reference the thermostat from.

thanks

Jesse

Not going to work.
For one your power calculation is way off.
Two, tying a hot resistor to the thermostat will just result in the AC being on all the time. The end temperature will be a balance of heat/cold input vs heat/cold loss. Certainly not the most efficient way, electricity wise.
Any means of trying to heat or warm the sensor, and moving it outside, or insulating it from, the feedback loop would be subject to thermal balance problems that would be very difficult to control.

The correct approach would just be to get a proper thermostat. They should exist but a 30 second search did not turn one up.

[TheRic]

First I'm not sure a window AC will get a room that cool. That 60 degree setting might be there for a reason such as safety, efficiency, operating parameters, etc.

If I wanted to be cheap I would mount the sensor away from the unit, then get a light bulb and mount it near the sensor. You could move the senor closer / further away until you find the spot. You could also change light bulb sizes. I have no clue how well this will work, it would be something easy to try.

If I wanted to do it right, I would find a thermostat that will allow it to go down to what you want.

If a thermo stat attached to the unit will not work then you could turn the Window AC off and on my a high voltage thermostat (120v or 240v not sure what type you have). The thermostat would have to be bypassed to always be on. This method would be hard on the AC, also you would have to have a non digital AC. When you lose electric on the digital ones they don't come back on. The ones with dials will come back on and run at what it was set for.

Personally I would look at finding an AC that will do what you want, or finding a totally different way to keep the room cooled. Maybe your local AC person has an old walkin cooler unit that works just laying around.

[jessrice]

LCHIEN and Ric
thanks for replying,

Ric, i was basically trying to do the same thing as the ligthbulb heat trick, but with a resistor instead.

I found this originally posted on a wine board, about several people using it, to do what i had asked about, but really wasn't sure about the idea, which is why i counted on LCHIEN knowledge.

The kenmore units use a thermocouple type thermostat instead, so i don't think i can replace it. I know of several others that just stick the thermocouple outside and then use the external plug in type controller, but since the new models offer the energy saving fan feature, it defeats there purpose.

there are manufactures of specific wine room coolers, however in my 100 degree climate, the models start out at about 3200.00, so the Kenmore was and cheap way to go at 1/8th the cost.

Here is the original post, just so you guys don't think i am crazy!

"When I initially installed the Kenmore, I discovered it was designed to only be programmed down to 60 degrees F. The temp sensor probe mounted on the air intake was pulled out of the unit. I then attached the probe to a 254 ohm, 5 watt resistor which was in turn connected to a 120 volt, 31 VA, wall plug-in convertor.This setup outputs about one watt of power which slightly heated the resistor (and thus the attached probe). The AC is fooled into thinking the cellar temp is warmer than it actually is. I now set the AC for 64 degrees and my cellar is cooled to 54 degrees. Since it cycles so infrequently my humidity maintains at 65%. I mounted the probe/resistor to a wall out of the flow of air."

Thanks again for evaluting the idea!

Jesse

[LCHIEN]

jesse,
upon reconsideration there is a way this could work.
What originally put me off is the suggestions of isolating the sensor in box or putting it outside or whatever.
I am assuming you want to control the temperature at 55 and you want it fairly close (other wise 60 degrees would do).

Because the room temerature is influenced greatly by the outside temperature and whatever insulation you have then you need a feedback loop to adjust how much the AC should run to keep it cool.

Within the room the AC works when the local temperature is below some absolute point set by the thermostat. This forms a feedback mechanism.

as i said isolating the thermostat breaks the feedback loop and ther'll be no control over what the end room temp will be.

The way to make the feedback loop work is to keep the thermostat in the loop sensing room temperature Plus some fixed offset temperature.

Now heat sources run at some temperature which is above ambient and the exact rise is a function of the thermal resistance of the package. So to get a consistent thermal offset you need
1) thermal source (e.g the resistor)
2) thermal resistance (non-varying)
3) good thermal contact between the heat source, thermal resistor and the sensor.

You can quantify the exact thermal resistance /wattage, etc but its not necessary

if you want to make this work then you need
a power resistor with mounting tabs for controlled heat dissipation
A heat sink to mount the resistor and the thermostat-probably a metal plate will do.
and a way to mount the thermostat which is designed to measure air but which now needs to meaure the heat sink temp. This requires some heat sink (thermal ) adhesive. The whole thing could be placed in the intake air path but that would change the thermal resistance (bad) so it should be off to the side where it feels the room air temperatre but is not subjected to a high velocity air flow. An IR contactless thermometer would be a great help to set this up.
The heat sink temperature will be the Ambient temperature + the Power dissipation x the thermal resistance.
Thus the offset will be pretty closely the thermal resistance x the power.
You can change the heat sink size (thermal resistance) the resistor value and the voltage on the resistor (power).
All three are easy to change but probably not that easy for a home DIYer unless he's an electrical engineer.