One can install HRVs and ERVs with no centralized system. We have, oh, three heat sources and half an AC in our house (old house) and can install a freestanding HRV or ERV simply as an air exchange (bringing in fresh air and letting out stale air). As we live in a cold climate, we're working out what we need in terms of HRV vs ERV. It seems like a slightly better solution than "leave the upstairs window cracked" which is what we do now.