The disadvantage of on-demand or instant hot water heaters is the upfront cost. Tankless units make up for the upfront cost by twice the longevity and less utility costs. Be careful not to be a sucker for plumbers selling cheap smaller units. They don’t produce enough hot water to serve most households. They’ll only do one faucet at a time. This is a problem if you want to shower while the dishwasher is running. For residential, we install medium-sized units. They will service several showers at a time.