Running New Copper Lines Inside Old Lines
ianrking
| Posted in Mechanicals on
Thinking about downsizing a mini-split head. The new lower-capacity head requires a 3/8” suction line vs 1/2” that exists now. In theory a 3/8” copper pipe will fit inside the inside diameter of the existing 1/2”. Would it be possible to slip the new 3/8” line into the existing 1/2” line to avoid opening up all of my interior walls and fudging with the penetration to the exterior? Anyone try this before?
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Type L ID 3/8 has a ½ OD so there will be no room to fit it inside and the first bend will reduce the diameter.
If this is a multi head system my feeling is you can get by with the larger lines with a small adjustment in the charge.
Walta
I don't think the lines are type L copper tubes. They are flexible refrigerant linesets so I think the dimensions are a bit different than what you're thinking. OD of a 3/8" pipe is 0.375" and ID of a 1/2" is 0.430".
Being able to keep the 1/2" line in any case would be great because it would mean not having to open up more of the walls and messing with the penetration. The system is a Mitsubishi multi-split with an MXZ-2C20NA2 outdoor unit and two heads. Have you seen this done before? The lineset is about 10’ long and has about 6’ of rise from the outdoor unit to the indoor unit.
There's no need to replace the 1/2" line and it's likely if it's only 10' long it won't make sense to even adjust the charge. Any contractor should be able to get a flare coupling that'll accept a 1/2" line on one side and a 3/8" line on the other at their local plumbing supplier.
All flare connections leak a small amount, I would only use adapters as a last resort. Adding them in to every line on both sides doubles the number of flares in the system, chances of the new setup lasting are pretty slim. If you must reduce, braze. Make sure the tech back purges the line set while brazing to avoid issues with contamination.
Also keep in mind that the oil used in these is sensitive to moisture, if the system had a leak and has set for a while, reusing the line set might create issues with the new install.
Adding to that, if it's only 10 feet, the idea of pushing a new line through the old is at least worth a try. I'd be tempted to use electrical conduit pulling lube to help it slide through, but you wouldn't want that to get inside the new line, so only try that if there's a good way to cap it first and clean it off before removing the cap.
Don't bother trying to snake a new line inside the old one. With a 0.375" OD for the new line, and a 0.430" ID for the old one, you have 0.0275" of clearance between the two all around. That's less than 1/32 of an inch. The line will never go in straight, and as a result it will bind up after a foot or two, and you'll never get it around any bends. For a little perspective, the National Electric Code specifies a maximum conduit fill for a single wire of 53% by area, so the wire (similar to your "new" pipe here), isn't supposed to be larger than 53% of the area of the conduit (similar to your "old" pipe here). The reason is that if you get too tight, it gets impossible to slide the wire into the conduit because of friction with the inside walls of the conduit.
Neat idea, but it won't work in practice.
I don't think you'll have much issue with the larger lineset though, just use what you have. You'll need slightly more refrigerant when charging the system is all. The contractor that charges the system for you will likely be able to braze any fittings that may be required (or install new flare fittings), so that's not a problem. Do mention to the contractor before they come out that that might need to braze a line or two -- residential HVAC contractors don't always bring their torches with them.
Bill
Thank you for all your replies. This seems like great news that I can keep the existing 1/2” line. How would one determine how much if any refrigerant needs to be added?
Usually this is done by checking the pressures in the system and knowing the temperatures of the coils. This is known as the "superheat" or "subcool" method, there are differences between the two methods based on the type of equipment being installed. In either case, I would have your installer purge the old lineset with dry nitrogen and pressure test it, then pull vacuum. I like to have the installers leave the vacuum pump running over lunch. Once that's all done, the system can be charged with new refrigerant.
There is another way to charge a system using an ultrasonic listening device, but I don't trust it and have had problems with that method in the past.
Bill
Bill, I'm going to disagree with you on this one. If it was a conventional system you'd be correct. However, because it's a mini-split the charge should always be weighed-in.
If the 1/2" line set is reused and the refrigerant is properly evacuated before the head is swapped out, then the refrigerant charge should be the same as it was before. If you want to check the tech, the installation manual will give the factory charge value for the outdoor unit and detailed calcs for adding/subtracting refrigerant based on line set size and length.
Thanks, both of you. This is actually a question I’ve been wondering.
I’ve looked in the manual about additional charge necessary and it mentions how much must be added based on length of the line but not size. Thus my initial trepidation with using the original oversized line in the first place.
Id love to ask Mitsubishi directly but their engineering division is very difficult to get on the phone or email.
Mike, I agree with you -- I'm just going on the assumption that the length of the old line set is not known, so weighing the refrigerant would still leave a variable.
After thinking about that a bit though, I thought "why not poke a fish tape down the old lineset after the refrigerant has been reclaimed, and measure it that way". I wouldn't trust weighing the reclaimed refrigerant on an old system since there may have been leaks, so you don't know if the reclaimed volume of refrigerant was "right" for whatever the installed system should have had.
For the OP: This stuff really isn't black magic, you just have to have accurate info. Charging by refrigernant weight is the most accurate way to go IF you know excactly how much charge each part of the system -- including the lineset -- needs to have. If you have the length of the lineset, that can be worked out, so you just need a way to measure the length of the lineset. The install guide for your equipment probably assumes you are using the recommended size lineset, so if you have larger tubing you'll need more refrigerant (obviously :-). You could work out how much more by the ratio of enclosed areas between the recommended lineset and the lineset you have. If the bigger lineset is 33% bigger by area, then you need 33% more refrigerant than you would with the recommended size lineset.
Bill
This all makes perfect sense.
What doesn’t is that this outdoor unit comes precharged with a certain amount of refrigerant and the instructions do not require the addition of any refrigerant if the lines are less than approx 132 feet. So somehow is the unit able to compensate for linesets up to 132’ long.
My linesets are only 10 feet long. The area of a 1/2” pipe is approx 2x the area of a 3/8” pipe so the volume of a 10 foot line of 1/2” is approx equal to 20 feet of a 3/8”. Still below the total allowable volume of a 132’ line of 3/8”. So is this okay with no additional charge?
Mini splits are able to come factory charged with enough refrigerant for substantial line set length because they typically have a large accumulator in the refrigerant circuit. The accumulator does the job of, you guessed it, accumulating the refrigerant so that it does not slug the compressor as a liquid upon return.
If your total line set for both zones is well below the 132' allowable for the factory charge, then the correct amount of refrigerant for your system is the factory charge with no adjustment necessary. This is listed in the installation manual or typically on outdoor unit name plate.
I'm hoping you'll be hiring an HVAC tech to do the head replacement and recovery/recharge of your system?
Understood and thank you. So the diameter of the lineset really isn’t all that important? For a larger line, I’d think there must be some pressure drop relative to a smaller line but perhaps for the reasons you’re saying it is essentially inconsequential with such a short line. (Thinking P*V/T)
I will definitely be hiring a contractor to do this work.
Right now I am trying to verify whether it would be okay to operate this system “out of spec” by replacing the existing head with a new one with a lower capacity but keeping the line. Sounds like it would be okay in practice. But possibly void the warranty.
Does anyone have any contacts at Mitsubishi HVAC that could be consulted on this? The customer service people are just regurgitating what the installation manual says. But they are not able to consider whether it would be acceptable in this circumstance given the specific criteria.