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Mini-Split Question


lurk6969's Avatar
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Join Date: Dec 2008
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12-12-08, 11:18 PM   #1  
Mini-Split Question

Greetings all! I have a cabin that I would like to install a mini-split heat pump into for cooling (gets hot during the summer) but I would like to use the waste heat from the pump to heat my hot tub. The cabin has two zones I would like to cool, does anyone have any knowledge/experience with this type of application? Do they make a mini split to do what I am after, and if so can anyone point me in the right direction? Thank you in advance.

 
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Bill190's Avatar
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12-14-08, 03:30 PM   #2  
Interesting idea!

Most of these types of things I have done for myself around my home I have had to engineer myself.

And in the past electric rates were inexpensive, so no need to do this type of thing. But these days...

Anyway what you would need is a heat exchanger instead of the outside A/C unit's "condenser coil". The condenser coil is shown in the following diagram...
http://static.howstuffworks.com/gif/...g-system-1.jpg

I searched all over the internet for what temperature the condenser coil would reach while the unit is running and could not find this. The service manuals talk all about how cold the inside coil should be, but not how hot the outside coil gets.

I know that air conditioners work in Arizona and it gets to be 120 F. degrees in Arizona. And the outside air would need to "cool" the outside condenser coil, so it must reach temperatures greater than 120 degrees F.

I also know that warm feeling water is about 102 degrees F. Hot would be warmer than that.

So the first question is if the outside condenser coil would reach a high enough temperature to heat the hot tub water.

Then next you would need a heat exchanger instead of the condenser coil. The refrigerant from your air conditioner would flow through one side of this and circulating water from your hot tub would flow through the other side of this.

Here is how a heat exchanger works...
Heat exchanger - Wikipedia, the free encyclopedia

Then next is when the hot tub water approached the output temperature of the refrigerant, it would no longer cool the refrigerant and your air conditioner would no longer work. This is like in a car on a very hot day when the air conditioner does not work very well. The air outside is too hot to "cool" the condenser coils.

So when the hot tub water reached a certain temperature, you would need to switch the water flowing through the heat exchanger to some other source of water which would cool the refrigerant. Or switch to another "regular" air conditioner.

And I suppose it is possible that an air conditioner would use less energy if the outside air was cooler? I don't know???

Would a solar water heater be more energy efficient if this is the case?


Last edited by Bill190; 12-14-08 at 03:59 PM.
 
GregH's Avatar
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12-15-08, 04:11 AM   #3  
Unless you find something specifically made to heat water there would be too many complications trying to do this with a mini split.

The refrigerant charge and the temperature that the condenser operates at in this type of air conditioner is critical.
You would need several specialized valves and a refrigeration mechanic to try to get it to work.

Something simple like a wood fired boiler or solar as Bill suggested would be better to heat water with.


GregH.........HVAC/R Tech

 
lurk6969's Avatar
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12-15-08, 11:13 PM   #4  
Hey guys thank you for the responses. Yeah the problem that I'm seeming to have is that the mini split is made to heat/cool with focus in one direction only. (ie inside a cabin) but it would seems so very efficient to have the heat pulled out of a building in the summer to go to a hot water source rather than dump to atmosphere. In today's "waste not want not" attitude concerning energy etc. I was wondering if someone had invented what I was after yet
One of the ideas I had was to somehow use a heat pump water heater (heat pump sits on top of the hot water tank) and to cool the building through a thermostat controlled damper - allowing cool air to enter the cabin but when temp is reached draw the heat from an outside source. I will have to consult an HVAC/Refridgeration expert i think to get it to work properly though.
Again thank you so much for your excellent responses.

 
GregH's Avatar
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12-16-08, 03:51 AM   #5  
You can not practically capture waste heat from a mini split but can from a central a/c system.


GregH.........HVAC/R Tech

 
XRing's Avatar
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01-14-09, 03:27 PM   #6  
Solar

I'll bet 2 or 3 solar hotwater panels would work.
Or, you could get a smaller version of a pool warmer.

Solar Pool Heaters for Above Ground Pools - Solar swimming pool heaters

The mini-split sounds like a good idea, and if you get one that does heating too, you could use the cabin in the winter too.
The new inverter models can pump in a lot of heat, even when it's down below 20 degrees F outdoors.

 
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