Running HVAC linesets underground

[tmoritz]

I would like to relocate our A/C compressor/condensor unit approximately 25' from its current location. This will allow me to construct a patio as well as reduce the amount of fan noise transferred into the house. I would like to run the liquid and vapor lines under ground in a PVC conduit. When I had a local HVAC contractor look at the work he recommended we NOT run the lines underground. He said there is an increased chance of the compressor ingesting liquid refrigerant.

Can somebody speak on the validity of this statement? If this is a real problem can I engineer around it with a liquid trap or sufficient incline of the vapor line? I'm not ready to give up on this idea yet.

Thanks in advance,

Tom Moritz

 

[chilled]

As long as the added piping follows proper proceedures for sizing, insulation and possible pitch, you shouldn't have a problem. I'd give up on the contractor before I'd give up on the idea!!

 

[tmoritz]

With a little bit of network research I found suction accumulators which should solve my problem. Now the question is should I go with a suction accumulator with an integrated liquid to vapor heat exchanger or should I just use a straight suction accumulator and add a desuperheater in the future.

Tom Moritz

 

[chilled]

Just have the piping added, it will be fine. The cheapest solution to prevent liquid migration to the suction line in the off cycle would be to add a recyclable pumpdown; liquid line solenoid before the indoor coil with the compressor cycling on and off via a pressure control. Hard start kit possible required too. Personally, I don't think any of this is needed.

Let's assume 1/2 the refrigerant charge condenses in the suction line during the off cycle (Say 3-4 lbs R-22??) The system starts; will the compressor draw this liquid up the suction line, into the compressor, accross the motor, into the valves?? I doubt it.

More likely the refrigerant will simply boil off in the suction line as the compressor reduces pressure.

 

[DrRTU]

As a contractor and engineer I would not bury the DX lines. Yes, you can install systems in attempt to minimize the problem such as pump down sequence (minimize charge migration during off cycle), suction accumulator (remove liquid at start) and heat trace (drive out liquid). A PVC pipe sleeve also a good idea to check for leaks and protect the lines. Trane, Carrier and McQuay piping diagrams prohibit below grade DX lines on commercial condensing units. As a contractor, I would not guarantee the piping would not harm the compressor. I have seen the addition of a barrel and converting the ACCU into a small chiller and running CW lines below grade to solve the problem.

 

[tmoritz]

DrRTU, I understand that Trane, Carrier, and McQua piping diagrams prohibit below grade DX lines, but why? If the issue is slugging the compressor then won't a suction line accumulator address that problem? What other problems could below grad DX lines introduce?

If I understand the problem of underground DX lines correctly it's an issue of lower conductive heat flux in soil vs. convetive air. This could result in chilling the air in the PVC sleeve and surrounding soil sufficiently to prevent vaporization of any refrigerant that migrates through the expansion coil or prevent condensation of refrigerant in the suction line as the pressure rises while the compressor is off. The larger the system the more this is a problem. We have a 4-ton AC system, not very big.

Thanks for the feedback.

Tom Moritz

 

[Golestan]

Tom,

I don't think that conductivity of the sorrounding material is too detrimental to the DX lines. Pressure drop and corrosion issues must be considered. Suction accum should be installed. Is foundation shifting/soil movement the real reason why the big guys are not recommending this approach?

regards,

 

[NCDesign]

I know that Trane does not prohibit DX lines underground (they won't void your warranty if you install in that manner), they just strongly advise against it. In my opinion the biggest issues are being missed, access and serviceability. Usually the lines go not only underground, but they go under concrete or asphalt (see the OP). This creates installation problems and later on can create huge headaches if you have any issues with the refrigerant pipe.

From Trane Engineers Newsletter — Vol. 27, No. 4

Don’t install suction lines underground. The likelihood of corrosion, vibration, condensation of water outside—and refrigerant inside—the line, combined with inaccessibility and difficulty in maintaining cleanliness, make this practice unwise. If underground installation is unavoidable, make provisions to insulate, waterproof and encase the lines in a hard sleeve.

You can read the entire thing at Split systems and interconnecting refrigerant lines: As equipment evolves, so must piping practices by Paul Solberg, 1998, volume 27-4 (revised Oct 2001).

To the OP I agree more with chilled on the technical aspects for your unit size. Just keep in mind that you have the very real possibility of having to tear your entire patio up to fix a problem with the refrigerant lines. Overall, I would be with DrRTU (& Trane) and avoid them at all costs.

 

[DrRTU]

Tmoritz, you are correct, the main issue is liquid slugging and the proper action is a suction accumulator. Standard has a very good unit

http://www.stanref.com/sa/sa.htm#

I your case, a 25’ of run under an inaccessible area is asking for a compressor reliability issue. The line run sounds like a large percentage of the line length.

 

[BronYrAur]

Carrier Bulletin number 111-84-46 discusses buried lines. I have an old hard copy but I'm sure it can be found on-line. The gist is that during the off cycle, refrigerant will migrate to the coldest part of the system (most likely the buried piping) and condense there. At startup, the liquid will slug the compressor.