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Community and Q&A

Lineset losses?

Hobbit _ | Posted in Mechanicals on

Has anyone tried to quantify heat loss through the outdoor parts
of typical heat pump lineset installations? Does it matter much?
It occurs to me that some token amount of insulation is applied
there, but is it sufficient? Especially on the vapor line, which
runs pretty warm in heating mode and the farther the lineset goes
before coming indoors, the more it’s exposed to a high delta.

Trying to ballpark it, assuming typical pipe insulation with about
a 1.5″ outside diameter and half an inch thick, what is that stuff
usually, about R-3? Assume a ten-foot length running outdoors to
reach the compressor, and a discharge gas temp of 140F [which is
on the low side from what I’ve observed]. At an average radius
of say 1/2 inch at the middle of the pipe foam, the ten-foot run is
2.6 square feet of surface … already doesn’t seem like a lot, but
going ahead and figuring U * A * dT at zero F outside gives potential
conductive loss of 121 btu/hr if I’ve done this right. Is that
worth worrying about? In addition, the liquid lines in a lot of
installations aren’t even insulated at all, although they are [or
are recommended to be] in the better modern split-system setups …
but still run on the warmish side as they exit the building.

I’ve got a really short lineset hop outdoors and added a blanket
of fiberglass and wrapped it all in that silvery outer jacket stuff
from some flex-duct, to not only meta-insulate but also maybe give
a little better UV protection, and it still feels mildly warm
when the system is running on a cold day. That stops at the back
of the compressor cabinet and there’s a lot of piping inside there
that isn’t insulated at all, so maybe I’m just kidding myself.

As central/ducted split systems start gaining popularity and
presence, I’m nonetheless starting to wonder why we aren’t building
systems for heating-dominated climates that have the compressor
and its relevant support located *indoors*, with just the coil and
a couple of sensors left outside to fend for itself in the weather.
Ground-source systems have their compressor units inside, and nobody
complains about that. Modern inverter-drive compressors probably run
quieter than a lot of combustion systems, and at least the annoying
2 kwh/day minimum I’m throwing away outside right now for crankcase
heat would be helping warm the house instead…


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  1. Nick Welch | | #1

    Mitsubishi manuals (like for the FE18NA) have a table showing capacity de-ratings for longer lengths of lineset.

    See comment #7 here:

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