Aqua Hot Register Intermittently Blows Cold then Hot on 2005 Patriot Thunder

[Dominic . Sue Ambrosino]

I think i saw this problem listed before but can't find it again. here's my issue:
2005 Beaver MH with Aqua Hot model AHE-100-03S.
I have 3 zones in my coach.  2 out of the 3 zones work perfectly. One zone heats intermittently. That being-it starts out heating and then after a while just blows cold. This happens on either Diesel or Electric. The other 2 zones work fine and are not affected. I can usually get the register to blow hot again by turning the thermostat off and then on for that zone. Again, it will blow hot for a while (could be as long as hours) and then turn cold again.
I don't think it's the register because the hoses going into the register are cold when it's not providing heat.
the green lights are on-no red lights
The circulating pump seems to be running even if it's blowing cold.
I'm stumped....
any help would be greatly appreciated.
thank you

[David T. Richelderfer]

Our coach has the AHE-100-02S Aquahot... so, it's an older model.  Our Aquahot has four zone-pumps, one for each of the three heating zones and a fourth pump for the engine preheat function.  From our 02S manual I copied and pasted:

"For interior heating, it is the room thermostats that trigger the Aqua-Hot’s interior heating system. When a thermostat recognizes that heat is required in a particular area, it sends a signal to the Aqua-Hot’s corresponding zone relay. The Zone Relay responds by activating the circulation pump for that zone, which sends the heated antifreeze and water heating solution through the Heating Loop corresponding to the zone requesting heat."

I suspect the zone pump corresponding to the zone losing heat is running intermittently because the pump is "sticking" and stopping or the zone relay controlling that pump is not staying engaged.  If possible, open the Aquahot and turn on the heat, then identify the pump that is not running.  Tap the pump with the handle of a screwdriver or something similar to see if the pump will turn on.  The pump may simply be sticking internally.  If that doesn't make the pump run, then the zone relay may be intermittently operating the pump and require maintenance.

[Fred Brooks]

   David,
Another thing to check is the check valve just above the respective zone pump. The check valve is gravity controlled to be in the closed position with the pump off. As the coolant ages it gets a little sticky and prevents the check valve from opening. To prove if that is your issue, turn on the zone in question and let the Aqua hot warm up. The tubing between the pump and the check valve will be hot and the tubing above the check valve will be cold. With the pump running tap on the check valve with a small hammer and the valve will open. Please advise, Fred

[David T. Richelderfer]

Again... from my 02S manual regarding the check valves.  The check valves are located immediately above the zone pumps.

"Check Valve:

"Check valves are installed into the zone outlet ports for each heating loop to ensure that the antifreeze and water heating
solution only flows in one direction. If the heating solution attempts to backflow into the boiler tank, the check valve
closes to prevent that from happening.  Troubleshoot the check valves if the following condition has occurred:
       There is a lack of interior heat in a particular zone.

"Troubleshooting:

"1. Verify that the heating zone is operating properly by checking the following:
A. Verify that the room thermostat inside the interior of the motor home is on and calling for heat.
B. Check the circulation pump for operation by feeling the pump for a vibration, indicating the pump is running.

"2. With the circulation pump operating, tap on the check valve and wait five minutes to evaluate if interior heat is now present.
If interior heat is present after tapping the check valve, the check valve was stuck closed and no further action is necessary.

"3. Check the Aqua-Hot’s antifreeze and water heating solution’s ratio of water to antifreeze. The mixture ratio should be approximately 50/50. If the solution is comprised fully of antifreeze (100%), the check valves will continue to stick."

[Dominic . Sue Ambrosino]

Thanks to all for your help. we tried all that you suggested. the problem hasn't been resolved.
it seems the pump is running to the zone, but it appears to be running a bit slower than the other pumps. also, it may be running counter-clockwise while the other pumps run clockwise (not sure on this as it's hard to tell).

does anyone have insight on how difficult it is to replace the pump? I'm not even sure aqua hot still has any.
thanks again