Heat transfer requires time, which is why I've always assumed that such a system would have to use coils somewhere. In the system under discussion, all the hea

Heat transfer requires time, which is why I've always assumed that such a system would have to use coils somewhere. In the system under discussion, all the heat-transferring parts had better be metal, and it won't be worth a damn except when handling a hot shower or a draining bathtub.