Stupid Question About Water Pump

All the systems that I have dealt with are as described above. The pump runs through a check vavle to a tee and the city water runs through a check valve to the other side of the tee. Both of these check valves run toward the tee, which supplys all the domestic water from either source. Running the pump while having the city water on will do nothing as long as you have water in your water tank. Otherwise the pump will just keep running since it has no water to build up pressure with. If it has water it will build up pressure and stop running unless the city pressure is less than the pump set point. If that is the case, you will be using your tank water.

In the rigs I have had the city water goes in past the water pump. There is a tee near the pump and a check valve that keeps the city water from going back to the pump. If my water tank is empty and the pump has no water turning on the city water will not put water in the pump so even with city water on it can just run dry. But if there is water in the tank and pump then the pump is left on it will come on only if city water pressure falls below pump pressure.

We always use one of the screw on regs when hooked up to city water. Some are set very near the pressure the pump is set at so pump might come on. On the Regs we buy.. They are a good idea to use. I have seen water pressure in some camping locations up over 200 lbs. Our rigs will not handle that.

In case you have not had one fail yet. The Water Pressure Regs fail in Closed Position. So if one fails the water is off and does not over pressure your lines.. If you do not have a spare you can always just fill your water tank and use your pump. Most of us that have been at this for years carry a spare.

I have to wonder if the OP had water in the water tank and pump or was just hooked to city water. But eather way I see no reason the pump would be damaged. I also wonder if the fact there is little noise with out water is why they may think its not working. Or if it had water it would not run.

I really understand how people new to all the RV stuff can not really understand how it all works. Just went thru this with a friend that just could not understand why the water system in an RV is really two systems. City and tank/pump with check valves between.

There is a tee in the plumbing that has city water connecting into the cold water piping. The same tee has cold water coming from the pump into the system. From there city water or water from the pump flow into the system. If everything works like it should the pump pressure usually is lower than the city water pressure. When the water pressure in the system reaches the pump set pressure the pump cuts off. If the pump is turned on at the same time you are connected to city water the pump will not run because the city water flows through the system and back through the tee increasing pressure against the pump pressure switch and check valve. The pump senses the pressure being higher than the set pressure and the pump will not run. If the pump is running and pumping water from the tank and you connect to city water the pump will shut off since the pressure will rise above the pump set pressure. The pump will only run when one, you are not connected to city water or two, the city water pressure drops below the pump set pressure while you are running water.

I don't understand the "pump won't run if there is city water pressure"? My pump is in line from my fresh water tank there is no city water anywhere near it. If the fresh water tank is empty the pump will just run.

SailingOn wrote:
Left our water pump on for a week once while the trailer was in storage, fresh water tank empty. On our return, the pump was fine.
Battery was dead though.

There you have it....running the pump kills the batteries! I killed a propane tank, and the batteries by running the heater in storage. NEVER double check...Triple check, as we hate to blame ourselves!

Michael

After all these years, now I learn that the pump won't get damaged if it runs dry. More than once, while pumping out the water tank, I've been distacted by other thing and the pump continued to run without any water flow and I've always thought I was just "lucky"! Thanks for the info, everyone!

A lot of water pumps are diaphragm models, not impellers... Diaphragms don't care one whit when the valves are opening/closing, while an impeller gets cooled by the liquid.

I wouldn't hesitate to run my water pump dry; my FloJet macerator? I use a clear section of sewer extension to flip it off the second the water is gone.

Left our water pump on for a week once while the trailer was in storage, fresh water tank empty. On our return, the pump was fine.
Battery was dead though.

snowdance wrote:
If your water tank was empty.. And you turned the pump and it ran dry the whole time.. It still will not hurt the pump.

Been there done that. Aunt has been staying in the TT while recovering from a broken knee. Water tank ran dry, but since the pump is on an isolation block it is very quiet, and the light in the switch burned out. One evening I go out to check something in the TT, hear a faint noise, and realize the pump is on and water is gone. I asked her about it, she said "Oh, the faucet stopped flowing last night but I did not want to bother you" She didn't realize the pump continued to run after the water ran out. Pump ran dry for over 24hrs continuously, but I have had no trouble with it since.

I also try to blame wife with burning stuff up. So far no luck on getting her to budge on new RV.

Out of habit I usually flip the water pump switch to "on" when opening bathroom door, and usually forget to turn off. No big deal.