I hope I can explain this properly:
I have a Pex Manifold at my rental property. I have two on-demand, propane water heaters that service this manifold. I also have an outdoor spigot connected to the cold side that I use to refill the hot tub between guest's. Because the water was so cold, especially in the winter, the hot tub couldn't heat the new water before guest's arrived. I decided to run a new line from an open discharge on the hot side and connected it to the spigot cold line with a T connection. Now when I refill the hot tub, my water is much warmer and the heater doesn't need to work so hard to get to the right temperature.
When the hot side valve feeding the hot tub is open (the spigot is not flowing water) I lose hot water to the other faucets that this water heater services. I can open the kitchen and bathroom faucets to hot and the water is luke warm. The water heater is barely running as well. When I close the quarter turn valve on the hot side hot tub line, the water heater gets louder and hot water flows to the faucets. The water heater is not running when I have the hot and cold lines open for the hot tub (not flowing water but lines are open). When water flows through the spigot for the hot tub, the water heater does run.
I thought there could be a leak even if the spigot is off, however, the hot water heater is NOT running when these valves are just open. It doesn't make any sense to me. If I ever forgot to shut the hot side spigot after I refill the hot tub, my guest's would not have hot water on the first floor. This is my main concern.
I am attaching a picture of how I hooked up the new line. I can take a better one when I get to the property this weekend. Any help is appreciated.