A modular heating and cooling unit comprising an independent set of headers for each of the heating and cooling loads and the source. A bank of these modular units provides a system that is capable of incremental simultaneous heating and cooling and redundancy. Valves in the internal piping of the unit eliminate the need for valves in the headers between units. This substantially reduces the overall footprint of the unit. Because of the parallel flow between the heat exchangers and the heating and cooling load, the modules can be operated in cooling mode and heating mode in any order.