I think they can recover up to 60-70% of the heat in the drain water. Not bad for totally passive. The installs I have seen are on the main drain for the whole

I think they can recover up to 60-70% of the heat in the drain water. Not bad for totally passive. The installs I have seen are on the main drain for the whole house and located right next to the hot water heater. For it to really work you need a hot water tank, but I have seen some small tanks combined with tankless.