To those that have the Furion on demand water heaters. Are you getting a cold water shot after you turn the water off them back on in the shower when you’re trying to conserve water while boondocking? The Furrion has a device on the back of the heater that is supposed to eliminate this problem.
The mixing tank usually does a great job of dampening the temporary fluctuations in temperature. However it doesn't do as well at dampening the "cold water sandwich" as it's called in tankless circles. If you turn off the flow of water, it turns off the tankless and puts out the flame, then it has to purge the exhaust gases from the flue. When you turn it back on, it may have to complete the purge cycle, if it hasn't already, before starting up again from scratch.
During start up, it has to determine the incoming cold water temp to calculate how many BTUs to produce to make up the difference between set-point and inlet water temp at the rate of speed it is traveling through your pipes.
While it's figuring that out, it also has to "prove flame" to make sure it isn't passing unlit gas into the flue. Once flame is proved, propane is allowed to flow and the flame is dialed in to produce just the right amount of heat to bring the cold water to your set-point at whatever rate of flow the water is traveling.
In the meantime, cold water is passing through the tankless, unheated, on its way to your back!! In front of that slug is the water that was heated on your last rinse and is still sitting in the pipes (albeit a little cooler, but not COLD!). Warm in the front, cold in the middle, hot at the end. Cold Water Sandwich.
The onboard computer chip of your tankless is WAY FAST at calculating all that stuff, it ain't fast enough to eliminate that WOO-HOO cold shot that got past the equipment.
In short, your tankless is working as it should. Also, the advice of those here who change the set-point to a hot-water-only temperature are right on the money for insuring comfort, but even their advice won't prevent the cold water slug from happening if you decide to turn off the faucet to conserve water. No flow? No flame and no heat.
Step aside for that one little second to let the cold slug pass, then enjoy precise and endless hot water until shut it off again.
If you have a ready source of incoming water and an outside hot/cold water fixture (and current outdoor temps are acceptable to you), fashion yourself an outdoor shower enclosure and you can forget about having to dump at all.
Hope that helps.