[Richard]

Bryan,

That is a great question and I hope others chime in with what they know. On the 95 coach I owned there was a hot water circulation pump back in the engine bay. When you turned the heat on max it turned on and helped circulate water from the engine to both the dash heater and the aquahot. When driving in cold weather I had plenty of heat at the dash, and the Aquahot display would run in the 170s.

The 99 I have now does NOT have that circulation pump. I have lukewarm heat at the dash and the AH display runs 120 or so in cold weather. If I turn on the preheat pump which is part of the aquahot system, it does improve the dash heat and raised the AH temp by 20 degrees or so depending on how cold it is.

So my way of thinking is the preheat pump is trying to do what the old circulation pump used to do.

If I still lived in cold weather I would be installing such a pump. They are commonly used on school buses.

I get the preheat light anytime I turn on the preheat pump.