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Pre-Charged Line Sets Are For Real, Right?

pre-charged line sets

Pre-charged line sets aren’t exactly a new innovation in the HVAC industry, but they’re not exactly widespread either. When we tell people our MRCOOL DIY line sets do not need to be vacuumed down, plenty of installers out there, professional or otherwise, just don’t believe it. But, hey, don’t take my word for it!

Pre-Charged Line Sets Are the Real Deal

A picture is worth a thousand words, so a YouTube video has to be worth like a novel or something. Anyway, check out the comments in our 2nd-generation DIY install video and you’ll find plenty of folks who don’t quite believe. If you don’t want to take the time, then I’ll sum up the highlights for you.

No vacuum? Are you kidding me?” – St2288

How is that possible?” – E Money

Don’t you have to use a vacuum pump or purge the lines?” – Juan Silva

Wait a sec, no purge of the line-set or vacuuming them down first?” – Butchtropic

Yeah, there are a few skeptics.

So, Are Pre-Charged Line Sets Real or What?

Yes! For the last time, yes!

We here at MRCOOL did not invent pre-charged line sets. They’ve been around for years, but were mainly confined to the modular and mobile home market. That might be one reason why so many people have not heard of them. Don’t worry! Pre-charged line set technology proved itself years ago.  Basically, all we’re doing is applying them in an innovative way.

Innovative How?

One or two manufacturers have used a version of pre-charged line sets on ductless mini-split units before, but we are proud to say that no one has taken it as far as we have. The MRCOOL DIY 2nd-generation Ductless Mini-Split Heat Pump is truly cutting edge technology. Our unique approach makes line sets easy and safe to handle. Best of all, any smart homeowner can put one of our DIY units up in a few hours with no specialized tools.

It’s like they’re comfort made simple or something.

111 thoughts on “Pre-Charged Line Sets Are For Real, Right?”

  1. What size wire is required for the DIY 24k btu? The specs state a minimum circuit ampacity of 15a. That would mean 14 gauge wire. My electrician says 12 or even 10 gauge.
    Also can I use a 30 amp disconnect or it has to be 25amp?

    1. There should be a wiring chart in the installation manual on page 17. We would recommend a 12 gauge. Your disconnect should be 25 amp or more.

  2. How long are the line sets included with the 36k BTU? Can the excess line set be rolled up? I have about a 12 foot run from where the indoor unit is to where I will be installing the outdoor unit.

    1. I believe the line sets on that unit are 25 feet. Yes, the excess can be rolled up to look quite neat. That’s what most people do in fact.

  3. Is there an option to get longer line sets?
    I want to install the air handler in a remodeled attic, so the distance from the top to the ground is closer to 32 feet. Also is there a maximum height distance that you cannot go beyond?

    1. There is not an option right now for longer line sets. We’ve got some things in the works, and that is a product we hope to roll out eventually. Can you give me some more details about how you want to install the unit?

      1. I am in the same boa as Chuck. My run is close to 40 feet. I’d love to buy a 12k unit but the lack of line options is a deal buster. 🙁
        Any ideas on how to find a reasonable HVAC tech to install?

        1. You can always use Google, of course. I know some people have had luck using Craigslist and Angie’s List.

  4. I have some suggestions:

    – Smaller capacity! 12k BTUs is super overkill for a bedroom; something around 3000 is more reasonable. All the better if this raises the SEER and HSPF numbers.
    – Short linesets: if I’m going right through a wall, I don’t need 26 feet.
    – A standard wall plug; no hard-wiring!
    – Get the price under $1k

    If you sold units with those features, I’d buy four, like, yesterday.

    1. Thanks, Nate.

      – Interesting idea.
      – The problem there is about manufacturing cost. We’re working on a way to give customers more precharged lineset options, but it’s still very early in development.
      – That could be possible if we made a smaller capacity unit, but none of the current units can be safely plugged into a standard wall socket.
      – Nate, most dealers sell our Advantage units at around $700.

  5. Regarding the 24k btu, the electrical data on the brochure states “max fuse 25amps”. I read this as saying the disconnect must be fused at 25 amps. However, one of the answers to the questions above says ” preferably non fused”. Can you please clarify the requirements ? Does it require just a 25 amp breaker at the panel or the 25 amp breaker at the panel and the 25 amp fused disconnect?

    1. You do not have to have a fused disconnect. You may have a non-breaker disconnect. At the breaker panel, it has to be 25 amps. The outside disconnect is only for servicing the unit.

    1. The pre-charged linesets only come in 25′. You can get a variable length with an Advantage or Oasis unit. They are not DIY, but they are very good units. A 12k should be sufficient for 500 square feet, but you might want to get an 18k just to be sure.

        1. We have DIY ductless mini-split heat pumps and Packaged DIY units. If it doesn’t say DIY in the title, it’s not DIY. Well, not without some specialized tools and training anyway.

  6. Can u cut the pre charge line set and wiegh the 410a back in I had a homeowner call me today and he said he bought one of these units and the line is to long I am licensed tech

    1. Yes, that would be possible. The refrigerant lines with a DIY unit are pre-charged, but other than that they are basic refrigerant lines.

    1. If the unit is installed and then uninstalled correctly, no, you would not have to charge the lines. If errors occur and refrigerant escapes the system, then it would require charging to the appropriate capacity.

      1. Do you guys make a 3 zone system in the DIY? I have a small 2 bedroom cabin approx 800 SF. I’d really like to have an indoor unit in each bedroom and one in the living room that is open to the kitchen.

        1. We do not currently offer a DIY multi-zone unit. For that, I would recommend the Olympus. It will require a contractor to install, as special equipment is necessary. Alternatively, you could install multiple DIY units.

      1. No I’m asking about the range the inverter throttles down to and up to. I’m guessing 18,000 btu and 24,000 btu is the most it puts out during high load but what’s the lowest it puts out when the load is low like during the shoulder seasons?

          1. Thank you very much for this information that’s exactly what I was looking for. Does the hyper heat units have the same range as well for the 18,000 and 24,000 units.

  7. Thanks for the reply last question,is it more efficient to leave the unit on for longer periods of time or is it better to shut it off if I have to leave the house for an hour or so?

    1. Typically, shutting the air conditioning off for short periods to save money doesn’t really work. The only time I do something like that is when I’m going to be away from home for a few days. I also never turn the unit off completely. Dehumidification is an important part of air conditioning, and keeping interior humidity at a reasonable level can help prevent mold growth. Just set the temperature higher than what you would normally comfortable with, but not so high that the unit never kicks on.

      1. So, should unit be shut down in those temps and auxiliary heat take over? Do you have any units designed for lower ambient?

        1. Yes, that would be recommended. Our Oasis Hyper Heat units are designed to operate more effectively at lower temperatures.

  8. I have the DIY 18000 BTU. I have heard there are alot of problems using the smart phone app. I can’t even get the QR code to read without it saying illegal code! What can I do, and when will the app be taken care of so it works? That’s one of the reasons I purchased a Mrcool!

    1. The ‘illegal code’ error message usually display when you scan the wrong area of the box. Give us a call at 270-366-0457, ask for tech, and they’ll walk you through the process.

      1. Did you put the Indoor unit in AP mode? (by holding the led button on the remote for 4secs) I had the same problem when I bought it last year

  9. Any plans in the works for a multi zone DIY unit?

    We have a single level 1100 sf house (26×43), our Split Ac went out and I am
    looking at all possibilities for replacement. The house is open kitchen, living and dining room followed by a hall with 2 small bedroom and one larger master. Ceiling Mounted return vent in hall for central gas heat and ac.

    Wondering if one 24k unit on living room wall opposite the return vent (hall is off living room center and runs down center of house) would work. When needed turn on furnace blower to circulate air through out the house? Could probably wire furnace blower to come on at a set temp in cool mode (with current ac disconnected of course)?

    The price of your unit beats a $4000-6000 split ac install. Bearing the fact in Boise Idaho we need AC only 3-4 months of the year (wife’s View) 2-3weeks in August (my view)

    1. We have had internal discussions about a multi-zone DIY system. However, if we deploy a product like that, it won’t be anytime soon.

      A 24k in the main living area should be sufficient for a lot of your cooling needs. You could then put a 12k or 18k (at most) in the master bedroom. Together, I think those two would be sufficient for your entire house. You could probably wire the blower to act as an improvised cool air circulator, but I don’t think that would be necessary. Proper mini-split placement should meet your needs without having to worry about jury-rigging your heater.

      1. Thanks for the quick reply. Trying to avoid the cost of 2 systems. I keep pondering If one 24K would work for whole house.
        As to the furnace I think all it would be is; disconnecting the control wire to the current outdoor unit. Setting the thermostat to cool position and temp say to x degrees. This will force the blower to come on till the bedroom area reaches x degrees. Distributing cool air to the bedrooms.

        Our duct work is in a crawl space and not in the attic so it stays pretty cool in the summer.
        Returns are insulated and in the attic though. Right now it is 81 outside left windows open last night and blower running no ac on it is a col 72 inside.

        1. A 24k is going to be good for around 1000 square feet. If you wanted one system for the entire house, you may want to move up to a 36k.

  10. I just installed the 18K DIY unit and love it…was very user friendly..
    I will be moving next year and will be taking the Mini-Split with me to the new residence..

    My ? is ,I know how to pump down a unit ,with the proper gauges and vacuum pump.,but how would this apply to the DIY18K ??
    basically I am asking ,”when I am installing the unit back in the new location,do I follow the same procedures as when it was brand new,or do I need to purge the Lines like a normal non-diy install?
    I have searched for answers on Google but not alot of information as far as DIY pre-charged lines and what to do after disconnecting for move to new location

    1. When you’re taking it apart, you’ll basically want to do the installation backwards, so to speak. Make sure the valves are closed before you disconnect the refrigerant lines, of course. When you’re putting it back together, do it the same way you did it the first time. If ever thing goes as planned, you should not need to vacuum the lines or add any additional refrigerant.

      1. I don’t understand your reply. So, if the refrigerant lines are pre-charged, and then the DIY owner re-seals the outside unit ports (by closing the 5mm allen screws) to the lines, and then disconnects the lines. (I think these are steps you’ve outlined, above). Then, when the lines are disconnected from the outside unit, the lines lose their charge.

        At that point, at re-installation time, the DIY owner is calling a tech to vacuum purge and re-charge the lines.

        I’m attempting to be clear, and demonstrate you’ve apparently left something out of your explanation of re-connection.

        1. They should not lose their charge. The connection couplings should mechanically close once they’re detached.

  11. I found your air conditioners and the reviews are great. They are back ordered or not available every site I find. What is your production schedule? I’m looking to get one delivered to Home Depot in Galveston, if that matters. Looking forward to installing a 36K DIY in my beach house. My window units can’t keep up!



  12. On the 18k diy mr cool one line set is bigger around than the other does it matter which one screws onto the top and bottom of outside unit?

    1. Make sure you connect the line sets properly in accordance with the instructions in the Installation Manual. Yes, one is larger and one is smaller, and it does matter where they are attached.

  13. The manual shows the location of the pass thru hole in the wall is either in the lower left or lower right corner of the indoor unit. The unit as delivered has the tubes bent tightly to cross over to the right side, then the tubes are wound up in the box in what I can only describe as a tight oval. I was going to install the pass thru on the lower right (facing the unit), but the bend in the tube there is pretty tight and I am fearful of bending the tube too tightly or even kinking it, or breaking it if I try to straighten it out. Is there any reason I shouldn’t make the pass thru hole more toward the middle of the unit (just inboard of the wall bracket)? Then I would be bending the tube in a place it has not been prestressed.

    Otherwise, is there a procedure for unbending the tubes to get the pass thru on the right side without risking damage?

    1. I don’t know of a reason why you couldn’t modify the position of the wall hole so long as it can be maneuvered through undamaged. I would recommend you follow-up with our tech guys. Call 270-366-0457 and ask for tech.

  14. Re: DIY 18 for 750 sq ft shop, well-insulated, eastern MA. You’ve stated that lowest outside temp for heat is “low teens”. Is this for making its rated amount of heat, or for making any heat at all? Thoughts on installing compressor in adjoining garage, which even though unheated, never gets really cold (below 15-20°)? Would that benefit the low-temp winter operation? Or should I leave my oil burner functional?

    1. There are a lot of factors to take into account in your example here. Any heat pump, ours or otherwise, takes heat out of the air and puts it inside your house. The colder it gets outside, the harder it is for the unit to gather that heat, so efficiency drops. The AHRI only rates units down to 17 degrees, so that’s the lowest verifiable operating rate we can provide. If you had the condenser in an insulated garage, that could help maintain even efficiency for a longer period of time. However, while the interior air handler would be heating your shop, the condenser in the garage would, in effect, be acting as an air conditioner. The garage location would benefit winter heat pump heating, but at some point, I’m not sure exactly when, the condenser would cool the garage to such an extent that it compromised its own efficiency. Personally, I would try to get as much heat as I could efficiently from the mini-split, then turn the oil burner when it just gets too cold.

      Hyper heat units like our Oasis Hyper Heat and others can handle lower winter temperatures, but, since they’re heat pumps, all of them ‘bottom out’ at some point.

      1. I appreciate your reply, and that makes sense. Gets into the question of how much “heat” can you suck out of “cold” air. As an aside, the shop equipment generates waste heat in the room, and I really don’t need supplemental heat until the outdoor temp is below 30-ish. Bigger issue is winter nights, when the outside temp can be lowest, machinery is off, and I want to maintain that equipment at 60-65. I’m only at the early investigating stage, and again thanks for the reply.

  15. I understand one should switch to other heat sources past -17 degrees, but I would like to know if the unit compromised at colder temperatures (Canadian winters can get below -40)? For example, if I accidentally don’t realize the tempature has a sudden drop to -20, will it simply be ineffective or can it potentially hurt the unit? Also, is it perfectly safe in extreme cold when shut off for the cold winter months?

    1. It will shut down and go into defrost mode. Being exposed to extreme weather should not damage the condenser. It is a good idea to prevent snow and ice from building up around the condenser, but that’s a good rule to follow with any HVAC unit.

      1. AHRI rates heat pump efficiency down to 17 degrees, so that is correct. But where did you get the -17 number from? I’m not aware of any heat pump that would heat efficiently that low.

        1. The user “me” on August 1, 2017 at 11:07 am said “I understand one should switch to other heat sources past -17 degrees” and I wanted to clarify for anyone else who came across the post that it’s 17, not -17.

    1. Everything should be in stock right now except for DIY 12k units. More of those should be back in inventory in 3-4 weeks.

  16. I would recommend MRCOOL to start making pre-charged lines in different sizes like 5′, 10′, 15′, 20′ 25′ and so on. One should be able to order it based on one’s requirement. In my case I want to buy 5 DYI unit but with different sizes of lines to make the installation looks better. What are your thoughts.

    1. This is something we go back and forth on. On the one hand, we want to give people as many options as possible. On the other, we want to minimize the potential for an install going wrong, and different length linesets would necessitate a non-factory installed connection to the air handler. It’s a tough call. We’re going to stick with a single lineset size for now, but that may change in the future.

  17. Can I mount the outdoor compressor unit on the roof about 5 ft. above the indoor unit? Unfortunately I can’t mount the outdoor unit on the ground below the indoor unit and meet the 12 inch clearance criteria behind the unit within the 26 ft. coolant line length. Do I have to worry about sun exposure of the unit on the roof. I live in AZ.

    Thank you

    1. The compressor can be mounted in any relation to the air handler so long as you make sure the drain hose is configured properly. It should still work in direct sunlight. If you can arrange some sort of shade, that would be ideal though.

  18. I just installed a 12 k DIY unit on our 4 season room and it works great but I really wish you could provide different length line sets to make the end job look much neater. I saw an earlier comment that said “I would recommend MRCOOL to start making pre-charged lines in different sizes like 5′, 10′, 15′, 20′ 25′ and so on.” Why would it necessitate a non-factory installed connection to the air handler? I know a production lines like one size fits all but many of us would gladly pay the extra cost to get the line length we need with the correct charge. I currently have a need for another unit with a line set of 6 to eight feet and would love a DYI solution again but with the length issue I’ll go with another product that wont be DYI. I’ve read that coiling extra line set be-hide the condenser is not a good thing and could create oil traps bla, bla, bla. You guys are close to making an even bigger hit than you already have with a few tweaks like different line set lengths.

    1. We would love to be able to do it, and we’ve had a lot of internal discussions to that end. As of right now, we won’t have variable lengths in 2017-18. There is a chance we will develop a variable lineset DIY product by 2019 though.

  19. Thanks for your reply. I understand that gearing up to make a change like variable line sets might take awhile but can you comment on coiling up extra line set and storing it behind the condenser? It it in anyway detrimental to to the compressor? Will it cause an oil trap? The “non-DYIers say it’s bad for compressor life. Besides looking like you know what, is coiling up 20 feet of it ok?

    Thanks again.

    1. Carefully coiling the excess lineset behind the condenser should be fine, but make sure it does not interfere with air flow through the condenser. It should not cause a problem.

  20. Hi. I’m looking to purchase and install a 12k DIY unit for my home office in the basement of my house. The floor above the basement overhangs front and back around 2′ each.

    In the instructions, the drain line is bundled with the rest of the line set. Would it instead be possible, for a neater installation and a smaller hole through my concrete block, to coil the line set in the overhang immediately above the outside unit (and drop the line set from there), and separately have a smaller hole through which the drain line would go?

    This way, the larger hole gets drilled through the wood overhang, line set is concealed (distance from the inside unit to the outside would be all of maybe 6 feet of line), and the drain hole is much smaller.

    I assume the only line where downward angle matters is the drain, right?

    How tight can the line set coil get before kinking? Could it fit between 16 OC joists in that overhang?


    1. You are correct that the only line where downward angle matters is the drain. It can be separated from the other lines. You can really do anything you want with it so long as it will drain properly.

      In terms of how ‘tight’ the line set coil can get before kinking, there is a lot of flexibility in the lines. It should be able to fit between 16″ joists. Just be careful, of course.

  21. I bought the 24k Mr. cool for 240 volt. The wire diagram shows L1 red and L2 black and ground. Where does the neutral (white) go? On the ground screw with the copper wire?

    1. If you have a WHITE conductor in your power supply cable, it should be attached to the ground screw with the bare copper ground, both at the unit and the disconnect. A 220v circuit does not have a neutral.

  22. One of the posts above you said that an installed unit may be uninstalled and moved without recharging if done properly. Can you list the steps necessary to accomplish this as I need to move a 12k unit and replace it with a 24k?

    1. The key area to focus on is the refrigerant piping connection on the condenser. Refer to section 6, page 23 in the installation manual. Basically, you are going to run the piping connection install process backwards – remove the valve covers, close the valves with a 5mm Allen wrench turning clockwise, replace the valve covers, use wrenches to open the connectors, attach the plastic seals (if you still have them) to the connectors. Obviously, the unit should be completely turned off/disconnected from power when you do this.

  23. Having trouble getting these shipped to Hawaii, is it because they are pre charged? Ca these be transported by normal methods such as FEDEX or would they need to ship by boat?

    1. I’m not an expert on shipping to Hawaii. However, we ship them to customers using normal means, so there should not be any special requirements.

  24. I understand that your standard units are rated to produce heat down to 17 degrees, and that for colder climates, you offer the Hyper Heat series. Is there any DIY product that offers Hyper Heat?

          1. I’m sorry to hear that. For what it’s worth (Which I realize isn’t much) I’ve got two floors that I want to put systems in. Each requires dual zone, and I’d like to do hyper heat. The DIY option is what attracted me to MrCool, but if I can’t do that without voiding the warranty by using a non-DIY unit, then I’ll probably go with a local pro installer, and the systems they carry, even though it will likely triple what I end up spending. Thank you for your quick responses.

          2. The number of contractors installing MRCOOL products across the country is growing. Have you tried looking for a MRCOOL dealer in your local area? Alternatively, you may be able to find an installer willing to use equipment you’ve already purchased. Not all installers require their customers buy directly from them.

  25. We live nc mountains in zone 5 but our temps seem to be a little different then most zone 5. Our average high in July is is 71 but only above 80 for about 10 days during the entire summer. Our average low in Jan. is 19.

    Our home is about 12K sq. ft. and I can mount the air handler in the cathedral ceiling wall in the center room of the house. The main reason for wanting a unit is to remove humidity when it hits those 10 days or so during it gets over 80 degrees

    I am thinking a 18K would be better than a 24K to get the humidity out as it would run more. What do you think?

    Also since the unit would be in the middle of the house which unit (Oaisis or DIY)would give better distribution of air flow?

    And one more. Could you explain how the hyper heat is different from the regular Oasis unit and what the benefit would be for my application?


    1. An 18k that runs longer would dehumidify longer. However, you should get the size you need. If you need more dehumidification, we recommend you switch the unit to Dry Operation. This will provide more dehumidification.

      In terms of air flow, there shouldn’t be any significant difference between an Oasis and a DIY.

      The Oasis Hyper Heat performs more efficiently at lower temperatures than a DIY unit. For example a 24k DIY (AHRI 8994007) unit will produce about 15.9k BTUs at 17 degrees. An Oasis Hyper Heat (AHRI 8977714) will produce about 17.2k at the same temperature.

      Also, when you said you have 12k sq. ft. did you mean 1200 sq ft or 12,000? Because if it is 12,000, I think you might need more than one ductless unit to cover that!

      1. Thanks for your quick reply,

        I meant 1200 sq. feet and the room I want to put the air handler in is in the middle of the house. Wooded area not a lot of direct sunlight. Still think I need more than a 18K unit?

        What is dry operation?

        Would 15.9k BTUs at 17 degrees be able to maintain a temperature of 70 degrees in a 1200 sq. ft. house?

        1. Given your summer temperatures are rather mild, you may not need more than an 18k

          Dry operation is when the unit runs on a lower cool setting, so that it runs longer and dehumidifies more effectively. If the interior temperature is where you want it, but it’s still a bit too humid, let the unit run on dry mode for a while.

          As to your question, I’ve sent a message to the tech guys to find out what they think.

          1. So, here’s what one of our tech guys said, “I have no idea what the btu requirement for the specific structure is, however my best guess is probably not.”

  26. Hello I have purchased one of your 24k DIY units but would like to shorten the lines. I know that the Hyper Heat models specify a minimum line length of 10 ft before needing to remove refrigerant however there is not a length specified for this model. Could you give some insight on what this length would be?

    1. We do not recommend modifying the length of the line set at ALL on a DIY unit. If you want a unit in which the line length can be modified, we recommend our Advantage Series. Since the DIY lines are pre-charged, they would have to be vacuumed before modifications were made. Which would also necessitate vacuuming the air handling unit, since the line set and the air handler are preconnected. Without proper licensing, modifying the line length on a DIY unit would void the warranty.

      1. I have an HVAC technician that could vacuum the lines. Isn’t it the unit that is precharged opposed to the lines?
        What would you recommend doing with the excess line?

        1. On the 2nd Generation DIY unit, the line sets, the condenser, and the air handler are all precharged. Customers do different things with the excess line. Some people bundle and leave it propped behind the unit, some build boxes or other housing to store the unit, some have it running along the wall protected with LineGuard, etc. It just varies from customer to customer. Again, if you’re going to have to go to the trouble of having a tech vacuum the lines, I really do recommend the Advantage over the DIY. The DIY is designed to avoid that sort of thing, but the Advantage will work just fine. And it’s cheaper!

          1. Josh, the whole point of the MrCool DIY series is that you don’t need to mess with the lines. There’s no vacuuming needed, there’s no HVAC license needed. There’s no special tools needed. This is all thanks to the fact that they already have refrigerant in the lines, whereas most HVAC eqipment is sold with lines that require all that additional work and expertise.

            The disadvantage to the DIY line is you end up with pre-determined lineset lengths, and may need to coil it up somewhere.

            What the MR COOL rep is trying to tell you is that if you aren’t okay with having that excess lineset laying around, then their non-diy systems would have been a better option. The Non-DIY linesets don’t have refrigerant in them yet, so they’re much more practical for a licensed technician to modify. If you want to modify the lines on a DIY system, you not only negate all of the benefits of getting a DIY one, you also have made more work for the technician vs just getting a non-diy system to begin with.

          2. Would you be able to tell me if the bolt mounting pattern on the outdoor unit of the Advantage 24k unit is the same as the DIY 24k unit? If so I will return the DIY in exchange for the Advantage.

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