Temperature problem

[KasperBrew]

Hi I'm new here and have registered here only because I'm having a problem with my fermentation chamber that I have built from a fridge. I have birdink temp controller and I'm using a full body size fridge because I wanted to be able to ferment two carboys at the same time. I have placed a 40w greenhouse heather on the bottom and still can't get 20C with heather running nonstop. Have to mention that I'm in Norway and it can get very cold at night and I have the fridge outside in a shead. Anyone can suggest what I can do to make the fridge hold a heat better?

 

[passedpawn]

Hmm. The fridge insulation must be bad/leaky, or the fridge is running at the same time as the heater. I have a normal (not for beer) fridge in my garage here in FL. If the freezer can stay cold when it's 100F in my garage, you're fridge should be able to stay warm.

I'm no expert. Maybe I'm looking at it the wrong way.

 

[passedpawn]

BTW, where is the temperature sensor located in relation to the heater and refrigerator. Maybe you have it too close to the heater output.

 

[dyqik]

Hmm. The fridge insulation must be bad/leaky, or the fridge is running at the same time as the heater. I have a normal (not for beer) fridge in my garage here in FL. If the freezer can stay cold when it's 100F in my garage, you're fridge should be able to stay warm.

A fridge typically has a cooling capacity in the one hundred to few hundred watts range. For it to get cold, it has to have a heat leak that's smaller than that at a temperature difference of say 65F/35C* (to cool to 35F/3C in a 100F/38C garage). The power required to achieve a particular temperature difference is proportional to the temperature difference (assuming that the insulation properties don't change in the temperature range in question).

To heat a fridge by say 65F/35C (from 0F/-17C to 65F/18C) will need about the same amount of power from the heater. Quite possibly 40W won't be enough in a full size fridge. I use a 40W seed heating mat in a under-counter fridge with a wooden collar in my 50F/10C basement which can heat the fridge to about 90F/35C, but not much more. I doubt it could even heat that much in a full-size fridge (greater surface area for heat loss).


For the OP - you can very carefully check the fridge for heat leaks - e.g. that the door seals and the like. It's quite likely though that you just need more power to maintain the large temperature difference you need.

*Temperature conversion not exact, but numbers are given in both units for illustrative purposes in Norway and Florida

 

[passedpawn]

A fridge typically has a cooling capacity in the one hundred to few hundred watts range. For it to get cold, it has to have a heat leak that's smaller than that at a temperature difference of say 65F/35C* (to cool to 35F/3C in a 100F/38C garage). The power required to achieve a particular temperature difference is proportional to the temperature difference (assuming that the insulation properties don't change in the temperature range in question).

To heat a fridge by say 65F/35C (from 0F/-17C to 65F/18C) will need about the same amount of power from the heater. Quite possibly 40W won't be enough in a full size fridge. I use a 40W seed heating mat in a under-counter fridge with a wooden collar in my 50F/10C basement which can heat the fridge to about 90F/35C, but not much more. I doubt it could even heat that much in a full-size fridge (greater surface area for heat loss).


For the OP - you can very carefully check the fridge for heat leaks - e.g. that the door seals and the like. It's quite likely though that you just need more power to maintain the large temperature difference you need.

*Temperature conversion not exact, but numbers are given in both units for illustrative purposes in Norway and Florida

I'd guess )) that a resistive heater is more efficient than a compressor. Might not be a lot though.

Also, I think the implication here is that the heater is running 100% of the time (because the controller never reaches the set point). A fridge compressor is not likely to run continuously (though I'm not really sure about the one in my garage). So, the instantaneous power draw from the compressor is much higher than the heater, but that might not compare to the effect of the heater running 100% of the time.

Not arguing, just riffing while eating my lunch here.

 

[dyqik]

I'd guess )) that a resistive heater is more efficient than a compressor. Might not be a lot though.

Also, I think the implication here is that the heater is running 100% of the time (because the controller never reaches the set point). A fridge compressor is not likely to run continuously (though I'm not really sure about the one in my garage). So, the instantaneous power draw from the compressor is much higher than the heater, but that might not compare to the effect of the heater running 100% of the time.

Not arguing, just riffing while eating my lunch here.

The ability to remove heat for a household refrigerator is typically somewhat higher than the electrical power input - the coefficient of performance. The theoretical maximum for a Carnot cycle fridge is given by Tcold/(Thot-Tcold). For a 20C difference with Thot at 25C/298K, the heat lift from a fridge has a theoretical maximum of 13.9 - that is nearly 14 times as much heat energy can be pumped out as is put in by the electrical supply.

A typical household fridge has a coefficient of performance more like 2.5 - so a 100W electrical draw means 250W removed from the inside of the fridge. That's why you won't get the same temperature difference in the opposite direction with a 100W heater in the fridge as you would for a 100W of electricity going into cooling the fridge.

TL;DR version - heat pumps are typically more efficient than resistive heaters.

 

[passedpawn]

ha, that makes sense. I didn't consider that a compressor is moving the heat energy (through physical state change), not creating it. Big difference.