We have two Willis heaters plumbed parallel for our UFH. Generally only 1 is required to maintain the desired temperature in the 'off peak' window when the UFH is on. The idea was that if one failed we could turn the other one on for a seamless transition and keep the heat flowing.   There was no way to isolate the Willis heater that was 'off' so I essentially had a mixer where half of the UFH return water would go through the 'on' Willis heater and be heated, the other half would go t