Turning a window unit into a mini split?

[F357]

I have an office I am trying to build an approximately 10k BTU heat pump for. I've been wanting to do a simple backwards air conditioner, but I'm running into more problems than I would like in my test. I'll need to break open the refrigerant circuit to make it do what I really want...

So, I've been thinking homemade mini split. It would be nice if I could mount the compressor outside of my office, I notice the back side of the air conditioner makes a lot more noise than the front!

I'm thinking I could take a 10K BTU window unit, and plumb both evaporator and condenser in parallel. Then run a line set into my office where I would build an indoor unit with an evaporator and a fan. Hopefully it would be more efficient due to having a larger outside coil. The original window unit fan would blow on both coils.

Is this something that might work? How far would I be able to extend the lines between inside/outside units, and what size would I use? The "outdoor" unit is actually going inside a shop area, so it doesn't need to be weather proof.

[theoldwizard1]

Originally Posted by F357

I have an office I am trying to build an approximately 10k BTU heat pump for. I've been wanting to do a simple backwards air conditioner, ...

Do you want a true heat pump that both heats and cool ? There are a lot of additional parts required. Study this diagram.



Even if you ONLY want heat from your homemade mini-split, 2 fans are likely going to be required.

[jeff5may]

IT CAN BE DONE. I am considering doing the same thing with a prefab window unit. Run the two heat exchangers in SERIES (not parallel) by swapping the cap tube out for a straight pipe. Pipe the condenser end of the series connected assembly into the reversing valve, and install a txv and check valve (in parallel) on the evaporator end. The arrow on the check valve should point away from the evaporator. This way, the condenser will not burp liquid or fog into the compressor in heating mode. The inlet to the TXV and the outlet of the check valve should merge, then that pipe would be the liquid line running indoors. The gas line of the set connects to the other open pipe on the reversing valve outside.

ECOMODDED has started a thread recently, asking about the same basic idea, on a smaller scale. He is planning on doing this with a dehumidifier as an indoor unit. For outdoors, he plans on using an automobile condenser.

You are correct in assuming that running both heat exchangers together will be more efficient. The more surface area and airflow you have, the more heat you can harvest from outdoors. How much more efficient? No one knows yet.

Indoors, a much larger heat exchanger takes longer to heat up (or cool down) as the cost of higher efficiency. Some like it, but most don't. The "warm" air the unit is spewing out the first few minutes at super-high efficiency doesn't warm the soul like a buck stove. I understand the first two minutes (while the liquid is building up in the hx) and the last two minutes (while the fan cools the hx after the pot stops spinning) are when the unit saves me the most money. To mom and the kids, that air just isn't warm enough.

[F357]

Originally Posted by theoldwizard1

Do you want a true heat pump that both heats and cool ?

Yeah, I will definitely try to put a reversing valve in the mix.

Originally Posted by jeff5may

Indoors, a much larger heat exchanger takes longer to heat up (or cool down) as the cost of higher efficiency. Some like it, but most don't.

My goal here will be to keep my office at roughly the same temp all the time, so I don't have to swing it up and down all the time. Can't do that with my electric heater now, takes too much to run. Need some more insulation also.

I'm thinking the easiest way to build an inside unit would be to just gut another window unit. Take the compressor out of this one and use both the evaporator and condenser again.

Do you think I could get away with 30-40 feet of tubing between the units, if I go a size larger than what they come with? I'm thinking I could put the "outdoor" unit in an attic area, which stays a few degrees warmer in the winter.

I'm glad I'm not the only one who lays awake in bed at night thinking about air conditioners. In November.

[TechShop]

Originally Posted by jeff5may

IT CAN BE DONE. I am considering doing the same thing with a prefab window unit. Run the two heat exchangers in SERIES (not parallel) by swapping the cap tube out for a straight pipe. Pipe the condenser end into the reversing valve, and install a txv and check valve on the evaporator end. This way, the condenser will not burp liquid or fog into the compressor.

...

You are correct in assuming that running both heat exchangers together will be more efficient. The more surface area and airflow you have, the more heat you can harvest from outdoors. How much more efficient? No one knows yet.

...

I like it and this brings something to my mind:

I have a dehumidifier I made from a $10 second-hand window-unit that I gutted. I simply installed a drip hose into the drip-pan and placed it on a 5 gallon bucket. I used packing tape to seal up all of the side vents on the sheet metal shroud. and threw out the foam blocks that separate the hot side from the cold side.
Next I removed the cold-side fan blade.

Now, the unit is like a little piece of duct, bringing in air through the cold side and out the hot side, powered by the axial blades of the condenser fan.

The reason I bring this up, is that if you did a similar configuration (sandwich the compressor between the the series heat exchangers) and use the plumbing scheme you've described [Jeff], you would recover a majority of the heat from the compressor during heating mode. This configuration might also keep the unit from icing up on the colder days for two reasons:

1) You have a larger evaporator surface for the given heat load with the dual HX.

2) You have the compressor in there giving you the electric resistance heat from the electric motors (compressor & fan).

[jeff5may]

Originally Posted by TechShop

I like it and this brings something to my mind:

I have a dehumidifier I made from a $10 second-hand window-unit that I gutted. I simply installed a drip hose into the drip-pan and placed it on a 5 gallon bucket. I used packing tape to seal up all of the side vents on the sheet metal shroud. and threw out the foam blocks that separate the hot side from the cold side.
Next I removed the cold-side fan blade.

Now, the unit is like a little piece of duct, bringing in air through the cold side and out the hot side, powered by the axial blades of the condenser fan.

The reason I bring this up, is that if you did a similar configuration (sandwich the compressor between the the series heat exchangers) and use the plumbing scheme you've described [Jeff], you would recover a majority of the heat from the compressor during heating mode. This configuration might also keep the unit from icing up on the colder days for two reasons:

1) You have a larger evaporator surface for the given heat load with the dual HX.

2) You have the compressor in there giving you the electric resistance heat from the electric motors (compressor & fan).

The newer GE Zoneline PTAC units do this with the indoor air stream using heat pipes. The indoor HX is sandwiched in between an upstream and a downstream HX that are connected. When the unit starts in cooling mode, the downstream HX is cooled by the exiting airflow. This causes a pre-cooling and dehumidifying effect in the upstream HX. Because of this arrangement, the indoor HX settles at a lower balance point temperature than it would without the extra help. The unit removes 25% more moisture from the indoor air, and the indoor coil doesn't frost up as quickly. It sounds like a gimmick, but it really helps the unit to provide much more comfort in humid environments.

[jeff5may]

Sounds like it would work to me. The easiest way would be to use a small unit indoors and a larger unit outdoors. Stick a txv and a check valve in each unit and plumb them to each other. If the indoor unit is too loud, a speed control could be rigged up to quiet it down.

[Daox]

Any updates on this project?

Unless you're HVAC certified and have the equipment I personally wouldn't be fooling with air conditioning units that involve working with connecting high pressure tubes, not like that... The pressures going inside these units frequently exceed 100 psi and the stuff really is dangerous.

That having been said...

I believe you can purchase a unit such as those used in a motel room already made, for under a thousand dollars. They're called PTAC's and doing a search on Google reveals several between $500 and $900, might even get a little break in price if you can find reconditioned / refurbished PTAC units.

They're quite efficient, with EER ratings of around 11+
Granted that's not much to compete with newer heat pumps but for single rooms and as a supplement to older HVAC systems it could be quite the money saver over the years.

[AC_Hacker]

Originally Posted by 8307c4

Unless you're HVAC certified and have the equipment I personally wouldn't be fooling with air conditioning units that involve working with connecting high pressure tubes, not like that... The pressures going inside these units frequently exceed 100 psi and the stuff really is dangerous...

8307c4,

Hate to burst your tepid bubble, but this forum is rich with examples of hearty hackers, from all over the world, who have gleefully and successfully violated every part of your warning.

You might want to check out THIS_LINK.

-AC_Hacker