cheap window reverse cycle ac build

[jeff5may]

Spring update:

The unit has been slaving away all winter, saving me money. There are a few issues that I have decided to live with until the weather warms up. We are in the "swing" season now, where the unit sometimes provides heating and sometimes cooling. Regardless of operating mode, the unit consumes 1100 Watts when running. With a compressor rated at 15000 btu, it should run around a COP of 4, not bad for a hacked unit running on bernz0matic propane. Unfortunately, I do not have a definitive way to measure raw btu output.

The only exception to this operation is during winter operation at low outdoor temps (near or just below freezing). In the range of 25 - 35 degF outdoor temp, the unit frosts up like mad and takes extra time to defrost. Below 25 degF, the unit continues to defrost, but unless it is actually snowing, the unit gets through its defrost cycle in under a minute.

As the outdoor temperature plummets below 25 degF, the unit moves less heat indoors. However, it consumes less power in the process. I imagine the COP of the unit drops faster than the power usage, but the main issue is maintaining "warm enough" air temperature to effectively transfer heat indoors. To combat this problem, I block off a portion of the blower exhaust with a piece of cardboard. The cardboard trick chokes the indoor heat exchanger airflow and raises its output airflow temperature enough to have "warm enough" airflow down to about 5 degF outdoors.

When it gets really cold outdoors, I just shut the unit off. If the temp drops into this range late at night, I don't do it until I wake up the next morning. The unit has become the way I tell whether or not to go out and start my car early on those questionable cold days.

Since last time it was shut off, the kill-a-watt says 331 run hrs and 312 KWH. This was during mostly cold weather, so the overall power usage is running below "normal" temperature range. When temperatures rise, power consumption and btu transfer will tend to follow.

The main issue I have with this unit is that I have to mount the thermostat remotely. In heating mode, the indoor coil transfers heat into the frame of the unit. This heat throws off the thermostat, causing short cycling. Strangely, cooling mode doesn't affect the thermostat. So I just moved the thermostat to the wall a few inches away from the unit. Now, the unit provides much better temperature control.

The other issue is with the whole cardboard method described above. If it had separate fans indoors and out, I could match fan speeds to temperature conditions. Unfortunately, both fans are tied to opposite sides of the same motor. I can live with this for the time being (maybe forever). I don't really see an easy way around this issue to make it automatic.

Besides the few bugs early on, and a few refinements along the way, I am very happy with this unit. It has obliterated my gas bill during the winter, while not adding a noticeable amount to my electric bill.

[AC_Hacker]

Originally Posted by jeff5may

...The other issue is with the whole cardboard method described above. If it had separate fans indoors and out, I could match fan speeds to temperature conditions. Unfortunately, both fans are tied to opposite sides of the same motor. I can live with this for the time being (maybe forever). I don't really see an easy way around this issue to make it automatic...

I'm amazed at how much use and value you have gotten out of your hack!

Now, I think that one of those portable heat pumps would be an interesting hack.

I have a 12,000 BTU one I got for free from Craig's List, just needed a starting cap.

The way it's built, the upper unit, with its condenser/evaporator and fan, could be fairly easily separated from the lower unit which has all that stuff, too. you'd have to hacksaw through some angle iron, clip some wires and solder in extensions, and make up your own line-set.

What you'd end up with could be a DIY minisplit. My unit even has a remote control. Can't beat that for fanciness!

You could run your line set through a narrow window opening, so you wouldn't need to drill through a wall.

Having the compressor and large blower outside, not acoustically coupled to the house is such a milestone.

Best,

-AC

[jeff5may]

Cousin It is lurking in the barn:


Unfortunately, he isn't a heat pump. Straight piped A/C only. Howls like a werewolf, due to the large condenser impeller... the top half is nice and quiet, though.

[AC_Hacker]

Originally Posted by jeff5may

...Unfortunately, he isn't a heat pump. Straight piped A/C only. Howls like a werewolf, due to the large condenser impeller... the top half is nice and quiet, though.

But you can see what I mean, that the top half could be easily separated from the bottom half, and a DIY mini-split could be created.

It is odd that the condenser impeller would be so noisy. But if it was outside on its own pad, it wouldn't be such an issue. With its case off, mine makes quite a noise, but I'm not getting any werewolf sounds.

Radial Fan

I am working on a HRV project and I have selected some fans like the one above. They use PM motors that are integral to the unit. They are very quiet, efficient and controllable with a controlled voltage or PWM. Unfortunately they can be expensive, too. But they get scrapped out with larger computers, so you just might find one. They would make a good, quiet, controllable blower for your indoor portion, if you were to pursue the mini-split configuration.

A potential "gotcha" is that you need to supply a 'shroud' that is placed within 1/8", around of the front opening, to separate the in-coming from the out-going air streams.

But back to the unit in your photo, re-routing the cap tube would give you a heat pump for next winter. But if you were going to go to all the trouble, a TXV has plenty of advantages.

I think that in the interest of portability, units like that one featured coils that are thick with several layers, rather than flat with fewer layers. I think that the flatter coils favor efficiency... at least that is what I see in the really efficient mini-splits.

I have some coils I scavenged from larger 1-Ton units that are larger and flatter, that I'd like to experiment with in the future.

Best,

-AC

[jeff5may]

I have been toying with the idea of using the top portion as a water-to-air exchanger. It's whisper quiet, and would be easy to box in and make look pretty. Just run a couple small, insulated flex lines to the outdoor unit...

[AC_Hacker]

Originally Posted by jeff5may

I have been toying with the idea of using the top portion as a water-to-air exchanger. It's whisper quiet, and would be easy to box in and make look pretty. Just run a couple small, insulated flex lines to the outdoor unit...

Sounds interesting.

Any more details?

-AC

[jeff5may]

Well, I'm not sure what I'm gonna do yet. I'm torn between a couple different options. Let me explain.

This top portion, if I built it, would be for my son's bedroom in the basement. He's one of those people who has to have it 75 in the winter and 65 in the summer. The bedroom is approx 250 ft^2 and is long and skinny. It is bordered on 2 sides by the uninsulated poured wall, so the top 2-3 feet of those 2 walls floats with the outdoor temperature. This winter, the temp on that portion of the drywall got down into the high 40's as the outdoor temp approached zero. Even with mt dirt cheap heat pump running in the unfinished portion, keeping it in the low 70's, he felt he HAD TO run a toaster-style space heater in his room.

I'm not sure how the space does during the cooling season yet, since we moved into this place in September. If it heats up as much as it cools off, I'll probably need the thing to heat and cool. Not a major problem, it just complicates the part most people don't look inside.

I am also considering making this new machine provide hot water as well. If it does, it would have one heat exchanger for dhw, one for room comfort, and one outdoors. I don't want the room blower to run off house water pressure due to the possibilities of contamination and/or having a massive leak trying to flood the basement. I can pretty well isolate the dhw loop from mayhem, but the room heater mixed with kids will need to be dummy-proofed.

I may end up making two separate units, though. It would sure be a whole lot easier to design. But if I had one unit doing it all, I could heat water and cool the room at the same time. Due to the uninsulated basement issue, I don't want to cool the basement during the winter while heating water.

[Daox]

Any updates on this?