Being on a well (or any system where supply press. to the home might not be all that great), pay careful attention to pressure drop in the device at anticipated maximum numbers of hot water consuming fixtures running simultaneously. No, not everything turned on at once, but say two showers running at the same time and possibly a sink faucet, maybe the clothes or dishwasher as well. In that case the well supply pressure might be a bit low too and then there is the pressure drop to the house. I'm on city water and have to use a PRV to keep the pressure down, but I would still size the system to not drop the hot water tap pressure too much with all three of our showers running simultaneously (which we do once in awhile.)
Same basic issue for me. 2 of 3 showers in our home would be served by this, serving 3 of the 4 occupants and over 75% of the showering.The main shower in the house presents and ideal installation as its drain immediately drops one story to the main drain below grade. Because the concept works best when drain flow matches inlet flow I wouldn't bother re-designing the main stack to accommodate one of these as the shower is the only load in my house that matches the basic criteria.
Yes, you can. That is the preferred way efficiency wise because it will increase the overall heat extraction because the delta T at the outlet will be greater (even with greater heat recovery) than it would be at a lower cold water flow rate. Q = U*A*LMTD Where Q = dury, U = overall heat transfer coefficient, A = area, LMTD = log mean temperature difference.The only question in my mind is can I use the drain waste heat recovery unit to warm the shower's cold water supply or with this present a problem for the mixing valve?
For simplicity sake just think of LMTD as the average delta T at both ends of the heat exchanger. (The formula is slightly more complex but this is close enough for understanding.) When you increase the cold water flow at a given duty the hot water outlet temperature wouldn't change, but the cold water temperature rise would be less than before. In reality the duty is not fixed, but area is, as is the drainwater supply temp. (U will increase slightly because of the flow increase, but since the tube side coefficient is probably not the limiting side, it won't have as much impact.) The LMTD increases, so the duty also increases.
The one negative of having the shower cold water leg go through the heat recovery system is that as the system warms up, your "cold" water supply to the shower will be warming as well, so you will likely want to adjust the mix of hot/cold manually during the shower. Other hot water users cycling on/off during the shower will impact this some too. So some sort of thermostatic control would be desirable for this (particularly for children...or a cranky spouse.)