Whole house Heat recovery work pretty well.
The only potential inconvenient with the technology are the following :
a) For your shower, you might prefer to get a "Thermostatic compensation valve" to kept the temperature consistent... these are generally a little more expensive starting at about 250$ for the cheapest model. The often seen Pressure compensation valve will fail to maintain a consistent temperature. In Home depot, you might find 1 or 2 "Thermostatic Compensation valve" for 100 of Pressure compensation.
b) Sometime, you might notice that your cold water is lukewarm at some of your faucet (Ex: Kitchen). However, this is seldom really noticed or a real problem. The water is really not that hot.
c) With most GFX technologies, their is very little recovery for batch "Usage" such as bath, home dishwasher, cloth washer, etc...
d) So, at the end, unless you have several teenager in the house, the return on investment for a commercial installation is very long. (How much hot water should you safe to offset a 800$ to 1000$ GFX installation).
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Horizontal vs Vertical for the sewer pipe: You can do heat recovery with both type. However, Vertical generally win by a very wide margin.
The reason explaining the large "advantage" of the vertical pipe is in the turbulence of the water. The more turbulence, the better heat transfer you get. See "Reynold Number" (
Heat transfer coefficient - Wikipedia, the free encyclopedia)
When the pipe is vertical, the water stick to the surface of the pipe because of the movement of the air. So, vertical drop make a thin film of water all around the copper sewer pipe -- and that water is more turbulent because of the acceleration made possible by the gravity..