Pump Cycling, Sulfur Odor and a Missing Black Tank Flush Check Valve
A 2021 Class C motorhome comes in for two complaints: the fresh water pump runs for about 10 seconds every 20–30 minutes even with all fixtures closed, and there is a sulfur smell at the kitchen tap. The owner also mentions that a mobile technician removed the check valve from the black tank flush because it was leaking, and the coach now has a sewage odor after flushing. The coach is connected to city water at 45 psi with a regulator, and the city water hose has been left pressurized for three weeks. Inspection finds the vacuum breaker under the bathroom vanity dripping whenever the black tank flush is used, and the water heater has a magnesium anode rod that is about half consumed.
1. Diagnose the pump cycling. List the likely paths and the isolation tests you would run, and explain how the city water connection changes the picture.
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A pump that cycles without an open fixture is losing pressure somewhere or the pump cannot hold its own check valve. Possible paths: the pump internal check valve, the city water inlet check valve, the toilet flush valve, the water heater T&P relief, the outside shower or low-point drains, a leaking ice maker line, or a failed accumulator bladder. A small air pocket can also cause short cycling.
Because the coach is on city water, a leaking city inlet check valve is high on the list. City pressure backfeeds the pump discharge, the pump pressure switch sees the drop when a faucet is used elsewhere, and the pump runs briefly to re-pressurize. The three weeks of continuous city pressure make a tired inlet check valve more likely.
Tests:
- Disconnect city water, open a faucet to bleed pressure, close it and watch the system gauge with the pump off.
- With the pump on, isolate the city inlet, then the water heater, then the toilet, watching for pressure decay.
- Cap the pump outlet and confirm the pump holds pressure; a decaying gauge across a capped pump means the internal check valve.
- Check the accumulator precharge and look for drips at the low-point drains and exterior shower.
Repair the identified path, then confirm the pump holds pressure for at least 15 minutes with no cycling.
2. Explain the hazard created by removing the black tank flush check valve, and describe the correct repair.
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The black tank flush line connects potable city water to a black waste tank. The backflow preventer is the only barrier that stops tank contents from being drawn into the fresh water system if city pressure drops, or from being pushed into the potable system when pressure fluctuates. Removing it creates a direct cross-connection. A siphon event during a campground pressure loss can contaminate the coach's fresh water, the campground supply, or the customer's hose. It also lets flush water overfill the tank if the valve is left on.
The dripping vacuum breaker is a separate failure: the float or seat is not sealing, so the device needs replacement, not deletion.
Correct repair:
- Replace the vacuum breaker with the manufacturer-specified backflow assembly for that flush line, mounted at the required height above the tank and in the original location.
- Use the correct fittings and support the line so the device stays vertical.
- Verify the tank vent is clear and the termination valve is closed during flush.
- Flush with clear water, check for leaks, and confirm no cross-connection by isolating city water and checking that the fresh tank does not rise.
- Sanitize the fresh system if contamination is suspected, following the manufacturer's published ratio.
3. The owner says capping the flush line is cheaper and asks you to do that instead. How do you respond, and what do you check about the sulfur smell?
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I would decline to simply cap it and explain why in customer terms: capping removes the feature but does not remove the cross-connection risk already created, because the line is still tied to the black tank and the missing device is a documented defect. The correct choices are to restore the manufacturer's backflow protection or, if the customer truly does not want the feature, to disconnect and cap the line at the tank side in a way that leaves no possible path between the waste tank and potable water, and document it. A cap on the fresh-water end alone is not acceptable.
On the sulfur smell, separate hot from cold. If it appears only at the hot tap, the magnesium anode is reacting with sulfate in the water and producing hydrogen sulfide; replacement with the correct anode type or a thorough flush and sanitize is the usual path. A half-consumed anode is normal wear, not a defect, but the water chemistry matters. If the smell is at both hot and cold, treat it as a fresh-tank contamination issue and sanitize per the manufacturer's procedure. Also confirm the black tank vent is not plugged and that the toilet seal and AAV are functioning, since a sewage odor after flushing points back to the waste system.