I'm interested in comparing two designs of water-water heat exchanger,
where one side is pressurized and one is not (gravity flow). The
pressurized flow rate will be 50-75% of the unpressurized flow rate.
Design #1: Unpressurized flow through 2" copper pipe, pressurized flow
through 1/2" copper pipe wound helically around the 2" copper pipe. I
guess the helical 1/2" coil should be soldered to the 2" pipe for
improved heat transfer?
Design #2: Unpressurized flow through 1.5" copper pipe, which is
sleeved in a 2" copper pipe. The space between the pipes is
pressurized; water enters one end and exits at the other through the
creative use of reducing tee connectors.
So which design is better? If Design #1 used 1.5" pipe instead of 2"
pipe, and Design #2 used a helical baffle between the two pipes to
direct the flow, I think there is no question that Design #2 is
better. But absent those changes, does Design #1 win? By alot?
Maybe I am missing something, How do you get the pressurized side to go
slower than the non pressured side?
Not knowing more about the delta t it is impossible to guess which would
work. Discounting efficiency completely.
That might help. The GFX is just tightly wrapped, enough so that the outer
pipe becomes oval, which distorts the inner pipe wall into a spiral (I think),
which spins the the drainwater and makes it hug the walls of the inner pipe.
Ream out the Ts. That might be easier to build, and illegal according to
plumbing codes, with a single wall, but who cares? :-) Many codes are
designed by committees who want to sell stuff and keep people employed.
The bottleneck in both designs may be that the drainwater doesn't
completely cover the inside of the inner pipe.
It sounds like the pressurized water is the cold water supply to the shower
and the unpressurized water is the drainwater, a mixture of hot and cold
water with greater flow. This can be more efficient if the hot water temp
is the same as the shower temp, or you might change the plumbing to run
both the cold water supply to the shower AND the cold water supply to the
water heater through the pressurized side, so the flows are equal. That's
the novel part of the GFX patent.
Impossible for some people :-) The shower might be 105 F and the drain might
be 100 and the cold might be 60, but the heat exchanger efficiency does not
depend on these temperatures. Gary Reysa and I have been working on this with
a different and maybe more efficient approach, trying to make a counterflow
heat exchanger with stored stratified drainwater in a drum with PE pipe at
More likely, the pressurized water is the cold water supply to the water heater.
The other way, the shower temp would
change as the pipes warmed up, etc, and special plumbing to the shower would be
I'm intending to just run the cold water supply to the shower through
the heat exchanger. This is easier than running the entire cold water
supply to the hot water heater through the heat exchanger. I don't
really want to have the pressure drop for all the hot water outlets,
As to the shower temperature increasing as the heat exchanger heats
up, that is true. Of course, it is already the case that the shower
temp slowly increases as the hot water pipes heat up and the stale hot
water is drained out. If it becomes a problem, I will either switch
to a thermostatic shower valve, or I'll put a mixing valve into the
cold water supply to the shower, mixing preheated cold water with hot
water to, say, 90 degrees.
How does a thermostatic mixing valve behave if the set temperature is
above the temperature of both inlets, or below the temperature of both
On 2005-10-28, email@example.com wrote:
Indeed, particularly as I am going to be installing this horizontally
rather than vertically. I could only fit maybe a 2' vertical unit in
my crawlspace. I'm assuming an 8' horiztonal unit will be more
efficient than a 2' vertical unit.
I guess my real question is this: design #2 is easier to build, so is
design #1 enough better to be worth building? Of course, my plumbing
inspector may not allow the single-walled design #2 and require the
double-walled design #1. But I'm hoping he will allow design #2 since
the positive pressurization of the potable water should mean any leaks
in the single wall won't contaminate the potable water.
Hmm, thinking about this some more, if there is a small leak in the
single wall, and water is being drawn through the outer pressurized
region, could this suck water in from the unpressurized drain area? A
sort of Venturi effect? Or is the 35 psi (minimum) of domestic water
pressure avoid this?
I'll just about guarantee that #2 will not be acceptable. A
small leak between the two would not be visable to you - it
will just go down the drain. Then, when the city shuts off
the water for repairs, it will suck your sewage into the
city water. #1 would leak outside where you will see it, and
not offer a siphon path to the city water.
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