HVAC Pro Blog

Every once in a while, I come across some basic knowledge that seems to have fallen by the wayside with the old timers. A few weeks ago, I was asked about which flare nuts to use on a refrigerant circuit for a ductless heat pump. For years I have been informing technicians to cut off flare nuts shipped with line sets as they were not “frost rated”, and to use the ones supplied by every reputable ductless manufacturer. Of course, I mentioned to use a frost proof flare nut when making this union. Then it occurred to me, there probably are not that many technicians out there that have ever seen a frost proof flare nut, or if they have didn’t even realize it and just thought it looked different. Or worse, how about the guy picking the order at the distributor…but certainly not his fault, when was the last time you asked for one?


There are several types of Refrigeration Flare nuts in use today, all of which have a 45 degree flare angle, and can work in the system. The problem typically happens during defrost cycles on heat pumps. Here is the scenario:


Contractor uses a “Long Flare Nut” to make the connections for the ductless heat pump at the condenser (or worse, the less accessible connections at the evaporator).


1. System operates just fine all Summer, but the first sign of a cold New England winter creates a call back - system lost the refrigerant charge!


2. During the defrost cycle, condensation can accumulate under the long flare nut, but not becoming an issue until the off-cycle.


3. If the system cycles off for enough time during freezing conditions, the ice will crack the brass flare nut easily.


To combat this, most manufacturers ship their equipment with the short, forged type flare nuts. This leaves little-to-no room for condensation to build, but still a possibility. The best flare nuts to use are the “frost-proof”, forged versions that have small pin holes to drain any condensation that could build during defrost and off cycles.


Just an FYI - most installation manuals mention that flared connections should be sealed with heat insulation (aka armaflex or pipe insulation), with the end sealed to avoid condensation from drawing back to the flared connections.


Frost Proof Flare Nuts Required!


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Manual D:

Residential Duct Design

Manual D:

Residential Duct Design

Manual D:

Residential Duct Design

Manual D:

Residential Duct Design

Manual D:

Residential Duct Design

Manual D:

Residential Duct Design

Manual D:

Residential Duct Design

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