City water connection backing up into freshwater tank

wrad23 · 11-01-2017, 05:25 PM

I feel sure the freshwater pump has a check valve that I can not see. Anyone had to fix this? My first thought is to install another check valve into the tubing.

ThePair · 11-02-2017, 09:00 AM

There is a check valve that allows the pressurized side to remain pressurized, separate from the fresh water tank which is unpressurized. That valve, if I remember correctly, lives just past the water pump. The city port connects directly to the pressurized side, which is why the fresh tank shouldn't fill if you use the city port. If, as you have surmised, the check valve is leaking, one would expect backflow through the valve, the pump (which will allow backflow) and ultimately into the fresh tank.

I don't know physically where that valve might be on your set up, but it should be on the discharge side of the pump, before any split (hot/cold) occurs. It may in fact be attached directly to the pump outflow, with PEX then coming out, which might be why it's not obvious to your inspection.

I'm sorry I can't be more specific, I've never had to remove or disassemble my pump so I'm not familiar with the actuals, just the theory. Plus, it's model dependent (Slide vs front bed) as to where the fresh tank and pump are, and how they arrange the piping. Plus, my TM is in winter storage right now, so I can't open it up to look for you!

I know people here have replaced pumps before, and I'm sure someone will chime in with more specific information.

Shane826 · 11-02-2017, 01:22 PM

If the check valve is after the pump and the pump allows back flow, wouldn’t the pump continuously cycle on and off as the water between the check valve and the pump flows back through the pump, causing the pump to see a drop in pressure? I could be totally wrong...

**Bill** · 11-02-2017, 03:25 PM

A diaphragm pump (such as the Shur Flow in our TMs) has ball-on-port valves built in as part of its operation. They are not separate valves, so they are neither ahead of, nor behind, the pump mechanism, but are part of it. There is a good explanation, with a very understandable diagram, in wikipedia at
https://en.wikipedia.org/wiki/Diaphragm_pump.

Bottom line - a diaphragm pump will not permit reverse flow.

Bill

ThePair · 11-03-2017, 07:09 AM

Quote:

Originally Posted by Bill

A diaphragm pump (such as the Shur Flow in our TMs) has ball-on-port valves built in as part of its operation. They are not separate valves, so they are neither ahead of, nor behind, the pump mechanism, but are part of it. There is a good explanation, with a very understandable diagram, in wikipedia at
https://en.wikipedia.org/wiki/Diaphragm_pump.

Bottom line - a diaphragm pump will not permit reverse flow.

Bill

Ah, my misunderstanding, then! Thanks for the correction!

**Bill** · 11-03-2017, 08:10 AM

As I follow up on this, I'm not sure that I told the complete story in my post above. My TM is closed up, so I can't confirm the model number, but I think we all have Shur-Flo pumps. In years past, the model number was 2088. This was a 3-diaphragm pump, and is now obsolete. The literature for this whole family of pumps says that they have a built-in check valve (as we all surmised above), so under normal conditions, reverse flow is not possible. I don't know when TM stopped using the 2088 pump.

More modern pumps are model number 4008. This is a 4-diaphragm pump, and they also have a built-in check valve.

However, unlike the 2088, these more modern pumps also have an "internal bypass" which permits water flow from output to input if the reverse pressure gets too high. Furthermore, the pressure at which this valve opens is adjustable, and there is a big "WARNING: To be adjusted only by a professional technician with appropriate gauges and equipment!" If mis-adjusted, this valve can open at too low a pressure, and presumably there will be reverse flow.

So wrad23, which pump do you have, and has anyone adjusted it?

Having said all that, the usual mode of failure in these pumps is a ruptured diaphragm, and an easy-to-install repair kit is readily available. An outboard check valve would prevent reverse flow through the pump, but if the pump has a problem, it won't operate well when/if you need it. Rather than adding an outboard check valve, why not get the diaphragm repair kit?

Bill