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Buried refrigeration equipment piping for air conditioners & heat pumps:
Is it permitted to route refrigeration tubing underground? If buried refrigeration lines are necessary, how should they be installed.
What problems occur when refrigerant tubing is run underground?
Page top photo: refrigerant pipes routed underground at a Minneapolis home. Note the absence of appropriate protection around these refrigerant lines.
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Underground or Buried HVACR Refrigerant Line Installation
Question: what's wrong with running AC piping underground?
2018/03/13 Kash said:
During the home inspection of the 1-year-old house, the inspector pointing out the AC piping lose, improperly run/buried, find attached the picture please advise if anything wrong with that or not and if it's not right what needed to be done. Thanks in Advance.
Reply: liquid slugging risk with underground / buried refrigerant pipingREF
Thank you for an excellent question that I might re-phrase as "Is it OK to bury air conditioning, heat pump, or other refrigerant lines"
Buried or underground refrigerant piping on residential HVAC systems is not recommended, but at least for some manufacturers and building codes, underground refrigerant distribution piping is not expressly prohibited, but where used it must be properly installed, protected,& insulated.
Refrigerant floodback: The first concern with underground refrigerant piping, particularly the suction line, is that running refrigerant piping underground invites the accumulation of liquid refrigerant in the return or suction line.
Where the refrigerant lines are underground and cooled by that location, the suction line may accumulate liquid refrigerant.
Subsequently at the next compressor on-cycle the return of liquid refrigerant in the suction line can cause liquid slugging at the compressor motor. That in turn destroys the compressor.
Refrigerant line corrosion & leaks: The second concern with un-protected underground refrigerant piping is that of corrosion. Copper refrigerant piping left unprotected underground is exposed to corrosion and leaks.
Johnson Controls Statement on Underground Refrigerant Piping
Here is what Johnson Controls advises where refrigerant piping must be buried:
Use PVC piping as a conduit for all underground installations.
See the illustration below, adapted from Johnson Controls' Application Data Sheet cited also below.
Buried lines must be kept as short as possible to minimize the build up of liquid refrigerant in the vapor line during long periods of shutdown.
Lennox statement on underground refrigerant piping:
Refrigeration lines must not be buried in the ground unless they are insulated and waterproofed. Uninsulated copper lines buried in wet soil or under concrete can cause serious capacity loss and erratic operation as well as early failure due to corrosion.
Systems with buried refrigerant lines can experience significant or total capacity loss if allowed to transmit heat to the
surroundings. In addition, buried lines are susceptible to corrosion which can shorten the life of the system.
For this reason,
buried lines must rest inside a sealed, watertight, thermally insulated conduit. The lines must not contact the soil
for any reason and the conduit must be designed so it cannot collect and retain water.
I will add this and other citations in the article above. We can't know from just your photo how much buried piping is installed at the home in your photo nor exactly where it is routed. It MIGHT be that the only buried piping is vertical and that in this specific instance it's not damaging the compressor - but the possibility is certainly there. Kudos to your home inspector.
The refrigerant piping in your photograph is not following the industry-recommended best-practice and could risk damage to the compressor motor.
Watch out: some HVACR manufacturers explicitly prohibit routing refrigerant distribution piping underground for commercial systems. (Carrier Corporation, McQuay corporation & Trane Corporation). And even where buried refrigerant distribution lines are permitted, that installation may not be a recommended practice.
Mitsubishi Guidelines on Installing Buried Refrigerant Piping
Refrigerant lines must be installed below the frost line. The depth required past the frost
line depends on the amount of foot or vehicle traffic that may pass over the refrigerant line
[Click to enlarge any image]
Include the sum of Lengths A and B (see Figure 1) in the calculation for maximum
vertical lift for the respective unit. See the System Design Section in the Engineering
Manual(s) to obtain the maximum allowable vertical lift.
For Length C, there is no limitation other than the standard piping line length guidelines.
Length D must be a minimum of 20 inches.
Use 45° elbows to simplify covering the refrigerant lines with casing. For refrigerant
piping with outside diameters of up to 3/4″, soft tubing can be used and large sweeping
curves can be bent by hand.
Refrigerant lines must be insulated separately. A minimum of 1/2" thick insulation is required. If possible, install casing as one continuous piece.
If the casing includes joints, contact the casing manufacturer for instructions on how to make them watertight. Flexible watertight drain line is recommended. DO NOT USE flexible perforated drain pipe [to route refrigerant piping] - Mitsubishi guidelines for underground installation of CITY MULTI R2-series and Y-series and S-series refrigerant piping, cited below.
Puron statement on Long Lines, Buried Lines, Refrigerant Migration & Elevation Changes in Refrigerant Lines
Longer line sets require additional refrigerant charge that must be managed throughout the entire range of possible ambient conditions.
Off--cycle refrigerant migration that results in excess refrigerant in the compressor at start up, or condensed liquid refrigerant in the
suction line at start up must be avoided for compressor reliability.
Follow all accessory requirements in this Guideline to control
off--cycle refrigerant migration (see Table 1).
Another concern is proper line set sizing and construction to control oil return to the compressor, and minimize capacity losses. In
residential applications, proper suction line sizing is critical to achieve adequate oil return, and maintain expected system performance.
Oil return in heating mode is different from cooling mode thus, in some cases, heat pumps have additional line set limitations from air
conditioning units. ...
The third concern is refrigerant metering. Elevation changes affect pressure drop in refrigerant lines. These effects must be considered
when sizing liquid lines and orifice--metering devices. - Puron, Residential Piping & Long Line Guideline, cited below
Trane Statements on Underground Refrigerant Lines
Avoid putting refrigerant lines underground. Refrigerant condensation or installation debris inside the line, service access, and abrasion/corrosion can quickly impair reliability.
Any heat that transfers from the surrounding air to the cooler suction lines increases the load on
the condenser (reducing the system’s air-conditioning capacity) and promotes condensate
formation. After operating the system and testing all fittings and joints to verify that the system is
leak-free, insulate suction lines to prevent heat gain and unwanted condensation.
- Trane, "Tube Size and
for RAUC Split Systems (20–120 Tons)" cited below.
It is advisable to avoid running refrigerant lines undergroundw henever possible. If it is absolutely necessary to run refrigerant lines underground, they must be run in 6" P.V.C. conduit.
Use 45° elbows to facilitate pulling the tubing through the conduit. The purpose of the conduit is to keep water away from the refrigerant lines. Careful sealing,
where the lines enter and leave the conduit is critical. Some installers install a drain in the lower parts of the conduit.
Bear in mind, that if the water table rises
above the drain, water may be forced into the conduit. Vapor and liquid lines must be insulated inside the underground chase. - Trane, Refrigerant Piping Manual for Small Split Cooling & Heat Pump Systems - cited below.
Additional Steps to Protect the Compressor from Flood-Back Refrigerant where Lines are Underground
OPINION: even where buried/underground refrigerant lines have been installed following best-practices there are additional practices that can protect the compressor from liquid floodback damage. From my reading it's not clear that these measures are needed if your buried refrigerant piping follows the manufacturer's instructions.
Re-route refrigerant piping above ground where possible
Design the HVACR controls to include a pumpdown sequence cycle at the end of a cooling cycle, removing more refrigerant from the suction side.
Install a suction accumulator in the condenser unit ahead of the compressor motor
Use a hard--shutoff TXV for metering in the cooling mode. This provides adequate refrigerant
migration protection for all cooling applications. [Puron]
Use a liquid-line solenoid to positively shut off refrigerant flow for single state and two-state heat pump systems. [Puron]
CALILFORNIA MECHANICAL CODE, CHAPTER 11, CMC (2010), [PDF] IAPMO, retrieved 2018/03/13 original source: http://www.iapmo.org/California%20Mechanical%20Code/Chapter%2011.pdf Excerpt: 1111.6 Underground Piping. Refrigerant piping
placed underground shall be protected against
Johnson Controls, GENERAL PIPING RECOMMENDATIONS AND REFRIGERANT LINE Length
for Split-System Air Conditioners and Heat Pumps" - Application Data Sheet [PDF], Johnson Controls Unitary Products,
5005 York Drive Norman, OK 73069 USA, Website: http://www.johnsoncontrols.com/, officesd world-wide, retrieved 2018/03/12, original source: http://www.usair-eng.com/pdfs/long-line-piping.pdf
McQuay International delivers engineered, flexible solutions for commercial, industrial and institutional HVAC requirements with reliable products, knowledgeable applications expertise and responsive support.
As part of Daikin Industries, a Fortune 1000 company, McQuay is the second largest air conditioning, heating, ventilating and refrigeration company in the world.
Thanks to reader Anon. for discussing the allowable distance between air conditioner compressor and a building or from the air handler - July 2010
 "Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
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