Tim, as David notes, the engine has an extra line that routes it’s coolant through the hydronic unit. When the engine is warm that coolant’s heat acts like a 3rd source (besides the diesel and the 110v electric), moved by the engine’s water pump. It is exchanged inside the hydronic unit to the floor heat glycol tubes that run to and from your heat registers, and to your hot water, and then the resulting cooler coolant loops back to the engine. It isn’t intended as a stand-alone source though, but rather as an augmentor, so for complete house heat or hot water your hydronic unit must be on. As long as the engine is running you may as well use that heat energy to reduce the use of diesel by the hydronic burner.
I’ve not heard of one of our coaches not having that feature unless perhaps they don’t have hydronic. But I understand that some rigs may not have the reverse, an engine preheat pump as part of their hydronic unit. If you do, there will be somewhere inside the living area an Engine Preheat switch that activates a separate pump to move the hydronic-warmed engine coolant back through the loop to the engine. On particularly cold mornings that is handy for getting the engine temp closer to 100 degrees prior to breaking camp. Once the engine is started you can turn off the Engine Preheat switch/circulating pump.
Now don’t confuse that switch with another commonly found by the driver’s seat that may labeled “Block Heater”. The Block Heater is an entirely different animal used in freezing climates, and involves a 110v heating circuit to warm the engine oil well in advance of anticipated engine startup. Warmer, less viscous oil is easier for the engine to move at startup. You must plug it into an extension cord from a 20 amp park post or other source. The dash switch merely activates the outlet in the engine compartment that the plug is wired into... at least that’s my take on it since I’ve never had to use it on our coach. Most people don’t as few use their coach in subzero weather deep enough that the hydronic Engine Preheat feature isn’t adequate alone.
Unless you live in Alaska or Canada year-round or have an unusual circumstance requiring the block oil to be heated, you can probably ignore that switch. You should look for an Engine Preheat one however. Ours is handy on the same galley switch plate as the diesel and electric Hydrohot switches, but I know on other coaches the Preheat switch can be mounted elsewhere, or you may not have that feature at all, as I alluded to in paragraph two - it may have been an option and not included. Someone else here with a 2000 Patriot may provide input to help you locate it if it exists.
Joel
Addendum: I see by the 2000 Patriot brochure that a Hurricane hydronic system is standard, not Aguahot. I see no available Preheat upgrade listed, so unless a previous owner changed to an Aquahot along the way, one with a Preheat pump, it may very well be you won’t find a Preheat switch.
That said, our engines will start at nominal temps sans preheating, but it may take longer letting them warm up before pulling out, to better-handle highway rpm’s.
