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Old 06-06-13, 05:06 PM   #8
jeff5may
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Zennlasurfer,

If you need the unit to operate down to 0 degF, that's a tall order for a first project. Unless you are going with a ground loop (not affected much by outdoor temp swings), you will need to devise a defrost control scheme to heat below about 40 degF. Otherwise, the evaporator will freeze up and the unit will quit heating until it thaws out.

With small units, you have 3 options:

1. Electric resistance melter (not efficient)
2. Hot gas defrost (suction accumulator and defrost valve needed)
3. Reverse cycle defrost (reversing valve needed)

By far the easiest to accomplish with a first project is option 1. But it is the least energy efficient. Good for a proof of concept that your unit will operate in heating mode, but not a long-term solution.

Both options 2 and 3 require you to break into the refrigerant plumbing and add new components. Option 2 is somewhat simpler, but the unit will only operate in heating or cooling mode (not both). Option 3 will allow the unit to work in heating and cooling mode, since defrost is just reverse-cycle heating. So you can use it year-round if need be.

Another thing you want to consider is this: As the outdoor temps drop below freezing, your heat pump output will drop in direct proportion. Unfortunately, this is the time you will want the most heat. Also, the smaller units I hacked all had undersized evaporators, which compounds this problem. At a certain outdoor temperature, the unit ceases to be useful. The unit will run constantly to maintain an indoor temp it cannot sustain. Below this temperature, you will need a backup heat source.

With my hacked 12k btu window a/c unit, I found that it would provide adequate useful heat down to about 25 degF. Below 20, it uses more energy than it heats, mainly due to the evaporator not having enough surface area. I believe if the evaporator was wider and/or taller and not so thick (2 layers of tube instead of 4), it would hold its own at 20 degF. But I doubt it would do much good at zero degrees. For my dirt cheap, recycled, beat down old window shaker project, working like it should for 350 days of the year is a better outcome than I had expected.

If you study the really high efficiency or sub-zero heat pumps, this is exactly how they are built. The outdoor unit has a heat exchanger like the back of a vintage refrigerator: lots of surface area and not very thick. The cube-shaped split units are twice as tall as the less efficient, older units of comparable capacity. The two ton mini-split units (24k btu capacity) have a heat exchanger in them the size and shape of a 42 inch flat screen tv! The indoor unit, in contrast, resembles a large space heater. The major manufacturers have figured this one out for us already.

Hint: automobile a/c condensers are cheap at salvage yards everywhere...

Last edited by jeff5may; 06-06-13 at 07:09 PM.. Reason: insight
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