Optimal location of coolant pipes for heat pump (inside/outside)

14 years ago

For my new project, I am going to have a long run of coolant pipes, and am trying to figure out if it would be better to run the pipes outside or inside. I have the option of running the pipes either just under the peak of the roof (which is inside conditioned space as there is to be spray foam insulation in the rafters) or along the soffit (which is outside.)

(NOTE: I will use the cycle independent terminology of internal & external heat exchanger rather than evaporator and compressor.)

As I understand it, the heat pump coolant line system is set up as such:

- for cooling:

-- compressor -> reversing value -> external heat exchanger -> external expansion value (bypassed) -> (long run b/w heat exchangers) -> internal expansion valve -> internal heat exchanger -> (long run to/from compressor) -> reversing value -> compressor

- for heating: simply reverse the direction and bypass the internal expansion value instead of the external one

So with this in place, it seems that the relative benefits of inside vs. outside coolants lines are:

- cooling:

-- For the long run b/w heat exchangers, it would be much better to run outside as the coolant would be hot coming from the external heat exchanger, allowing for even more heat to be released, rather than having that heat being released on the inside

-- For the long run to the compressor, it would be marginally better to run inside as the coolant line would still have a little cooling left that has not totally been wrung out at the internal heat exchanger

- heating:

- For the long run b/w heat exchangers, it would be marginally better to run inside as the coolant line would still have a little heat left that has not been wrung out at the internal heat exchanger

- For the long run to the compressor, it would be much better to run the inside as the line would have be hot and heat would be released inside instead of being lost outside

So a particular location for a line would be much better or worse for one heat direction and marginally worse or better, respectively, in the other heat direction. Thus, the best course would be to use the location in which it would be much better for one of the directions, and just accept the marginal loss in the other.

- line b/w heat exchangers: outside

- line to/from the compressor: inside

I'm curious if this is done in practice for folks with long runs.

Any comments?

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Comments (5)

swampwiz
Original Author
14 years ago
Something I didn't think about is that for cooling, the line going to the compressor may cause condensation. This alone may make it not worth doing.
creek_side
14 years ago
Define long.

You may be trying to design stuff that is in the province of the HVAC company. There are limitations on how long a lineset can be. Unless you know exactly what combination of equipment is to be installed, you can't know if your "long" lineset it too long.

Some long linesets can require that a system be derated. If the lineset is too long, the system will not work properly and the manufacter won't stand behind it.
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refrigman1
14 years ago
The line that you are calling "the line to the compressor" (which would be the larger of the two lines) is ALWAYS insulated with a good quality tubing insulation to conservre the leftover heating/cooling. This line is the line that carries the cooling in the summer and the heating in the winter.
The other line in the system (the smaller of the two) is a constat temp. (approx 95 deg F).
I would suggest running the lines where ever they will be accessable to repair or replace them in the future. Also, if the lines will be covered with spray foam if you run them inside, I would definitly not recommend that
swampwiz
Original Author
14 years ago
The line to/from the compressor would be the line between the internal heat exchanger and the compressor (or reversing valve.)

refrigman1, I don't understand what you mean by saying that this same line carries the cooling and heating. For cooling, the line to/from the compressor does not carry cooling - it is the return line to the compressor; for heating, it indeed carries the heating as the coolant flows from the compressor to the internal heat exchanger. This line would be at low pressure for cooling (as the coolant would have been throttled right before the internal heat exchanger), but high pressure for heating (as the coolant would be coming directly from the compressor.) Perhaps your terminology is not quite accurate here.

I can see, however, why it would be desired for this line to not have any heat exchange (i.e., it should be insulated) - especially for heating (as that would be heat lost before the exchanger) - but also for cooling as the additional heat gained before the compressor would cause the back pressure to rise, which would result in a higher temperature for the evaporation process.

I understand how the line between the 2 heat exchangers is at the constant temperature of 95F as for both cycles, the state of the coolant would be the high pressure saturated liquid (and would thus be thin because as a liquid, it does not need much volume.) It would always be a good idea for this line to give off heat as it would make the job of the following heat exchanger (i.e., the one in the role of the evaporator) a little easier. It would not be a good idea for it to gain any heat, but at 95F, it would not be gaining much even during the hottest of summers.

Since this line between the heat exchangers (i.e., the one that does not go to/from the compressor) is at 95F, it would be very bad for it to be inside for cooling, but would be good for heating.

So it seems that both of these lines should go outside as much as possible. Running along the soffit space, they would be fairly easily accessible, and not in the spray foam space.

creek_side, the length will be a total of about 75' between the internal and external units. Whatever must be done will be done to support this. The internal unit must be located at one place, and the external in another, and unfortunately on the other side of a long house. As I understand it, at the very worst, I would need wider lines and perhaps some sort of step up pump unit, which I could locate at about 50' from the external unit, or in a pinch at about 20' away (although I really don't want to do that.)
energy_rater_la
14 years ago
have you spoken to a contractor who has seen the design of your home?

Optimal location of coolant pipes for heat pump (inside/outside)

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Optimal location of coolant pipes for heat pump (inside/outside)

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creek_side

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