Hydro Hot & Aqua Hot, Air but no Heat at Register

[EngineerMike]

Didn't find this issue in a search, soooo:
Zone 2 (rear, call it Z2) radiators were blowing cold air while fronts, Z1, were blowing hot. Wife not happy (wants full heat in Quartzite next week), so gotta fix it instead of put it on official ignore list.

Set Z2 for 90 degrees on the Dometic thermostat, & removed sheet metal cover on burner side of AquaHot 450 (same unit as the HydroHot 450 in earlier coaches, they just started painting it white & calling Aqua instead of Hydro). There are two 'circulation' pumps that have a clear plastic housing, one for Z1, other for Z2. These push hot boiler fluid thru the zone's radiator loop. Pump for Z2 (call it P2) was spinning but making a buzzing noise. Set Z1 for 90 degrees, P1 spun, no noise to speak of. Tube up from P1 to Z1 in coach was warm to touch immediately, tube from P2 was still cool, so not circulating fluid.
I tapped lightly on the in-line brass check valve just behind P2 (there is another for P1, presumably so you don't drain coach radiators if you replace a circ pump?), and instantly buzzing noise from P2 died (continued spinning), and tube warmed up. Checked inside & good heat to Z2.

Conclusion- check valve stuck, causing back pressure on P2 impeller making it buzz, but only a light rap on valve needed to open it up.
Fix- next to free, happy wife- priceless.
See you in Quarzite

[Orman]

Good thinking Mike
Those check valves do have a tendency to stick at times.
See you in Qsite

Orman Claxton
Aquahot Guy

[TerryM]

Mike and Orman,

I wonder if the Engine Preheat pump, located on the back of the AquaHot Unit also has an inline check valve that could be sticking. I've not had the benefit of the engine preheat for a while....luckily it's not been needed ....just helpful in keeping the neighbors happy with shorter warm up times on those cold mornings before leaving camp. I haven't really had the time to check into it yet.

[EngineerMike]

Could also be the circ pump for that heat loop; we have three- two domestic loops plus the engine pre-heat. Hard on the SE's & Apexes to check that 3rd circ pump on the back side of the HHot unit. If I ever have to replace that pump, I'll relocate it to where I can get at it thereafter.

[Orman]

Terry
The Engine preheat has no Check valve

[EngineerMike]

Terry- The circ pumps are magnetic drive, i.e. no direct coupled impeller, it spins because it is magnetically coupled to the motor. And the motor is a small fraction of a horsepower. I was told sometimes the circ pump impeller can stick in place. If you can hear a buzzing from the engine preheat circ pump, impeller may be stuck, and sometimes a gentle rap will free it. Mine have clear plastic case over the spinning armature, so you don't want to hammer on it. Just a series of gentle raps. If that doesn't do it, probably either the pump or circuit from the motherboard.

Orman- which pin can Terry check for the output to the engine circ pump?

[TerryM]

Mike,

My engine preheat circ pump seems to work at first, but after a few minutes it starts a pretty loud squeal. I don't know if it is still circulating as I shut it off at that point. It's done it the last two times I tried it. I haven't had time to check anything out yet.

[Orman]

Terry

Locate the Electronic Controller's
JP-3 Plug and check for DC
voltage at Pin 7 (+) and Pin 8 (-),
Check for loose wire connections
at Pin 7 (+) and Pin 8 (-) of the JP-
3 Plug. Also, check the wire connections
at the Engine Preheat
Pump. If all connections are in
good condition, listen and feel the
Engine Preheat Pump to ensure
that it is operating,
If the Engine Preheat
Pump is not operating, it must be
replaced.