Installing a permeate pump on a manifold-style RO system

Question

I've got a Whirlpool WHER25 RO system. Recently I added a hot water dispenser to our kitchen sink as well and would like to route the RO product to it instead of running it off of unfiltered water. The hot water dispenser requires 30psi to operate properly. My RO product water is 42psi when the tank is full, but the pressure drops rapidly into the 20s when the tap is turned on. Currently the stock system looks like this:

To ensure I get the necessary pressure to the hot water dispenser, I'd like to install a permeate pump to bring up the tank pressure close to the 68psi going into the system. I've found out that's difficult with a WHER25 since it is a manifold system and I can't add tees willy nilly anywhere I'd like. I'd also still like to be able to use the post-filter as intended - when the product water is dispensed and not before going into the tank. This limits my options since most people with manifold systems relocate the tank to after the post filter.

I think I've come up with a solution using another automatic shutoff valve in a different place in the system. Does this look like it will work or am I going to end up breaking something? Not sure if I'm correct, but I think this should only allow water out of the tank when there's a significant drop in pressure on the product out line, like when the tap is opened or the hot water tank is filling.

Additionally, does anyone know where to get an automatic shutoff valve with 3/8" push fit or JACO connections for both the high and low side or am I going to be stuck with a bunch of reducers? The only one I can find is the Payne PWS-792Q, but I can't find a distributor offering it. If I am stuck with reducers will they ultimately end up limiting my pressure or throughput?

Answer 1

I'm not sure if I understand your drawing, but I think you're saying you want to add a pump on the product side to keep pressure above 42 psi. I don't believe this is going to work due to the shut off valves. I'm pretty sure that if you attempt this your RO will never turn on.

Here is how the ASV (automatic shut off valve) works. The feed water is connected to the ASV on the indicated port, which goes to the membrane element. Some are in and out in the same side, some are straight thru.

The product water somewhere after the check valve is hooked up to the other side. Inside is a diaphragm valve. When your tank is empty, this valve is forced open by the incoming pressure (for simplicity let's say 60psi). The ASV pressure is specified in ratios but a good one is 2/3. So this means that when the product water reached 40psi the feed water will be shut off to the membrane. And of course the product water doesn't flow backwards due to the check valve. The physics of how this is done might be explained later. But that's how those work. It should make sense why you can't just put a pump on the product water. You also just can't add air to the tank for a higher faucet pressure either. The limiting factor is that 3 in our ASV 2/3 ratio.

But is there a solution: raise the feed pressure.

Membranes operate better on high pressure. They tend to last longer but more importantly its rejection rate is much higher than when run at line pressure. But we have to get rid of our ASV, and substitute a couple of pressure switches; one high pressure switch for the product side and a low pressure switch for the feed side. The HP switch will turn the RO pump on and thus make water until it reaches its set pressure then turns the pump off and then stops making water. The low pressure switch protects your pump in case your feed water pressure drops or if it gets no water at all.

What's the risk? Let's say your feed pressure is greatly lowered. Maybe a fire in the neighborhood and the fire trucks are using all the water. So your RO kicked off. But no big deal right. Well after they removed their back flow preventer a bunch of contaminate was stirred up and plugged your pressure regulator. No water or low flow condition.

You don't notice and go to use your insta hot. It works fine, but all of a sudden you tank is empty and the heater burns up. This is a crazy example but you have to keep in mind that if something happens to the pump then your insta hot is in trouble.

What do I recommend? Leave the insta hot on the soft water. And just use the RO for the faucet and icemaker.

Adding a pump is not all that uncommon the benefits are staggering but to retrofit one to a unit especially under counter had some major drawbacks: the pump will be noisy, and high pressure across old plastic spells flood.

The higher pressure switch had a range let's say 30-60psi but our tank pressure needs to be 28psi, so that the RO doesn't need to run very long to keep that switch closed.

Hope some of this helped.

Answer 2

The second drawing placing a pump on the drain would only pull more water through the RO system filter not increasing your output.

There is a limit on input pressure to an RO system so one can not boost and exceed that limit.

The blue line in drawings is the output. To increase this pressure and its duration has two methods.

One, to start with, is add a second storage tank. Tee the blue line to the first tank and run the teed line to the second added tank. Or purchase a larger tank that will last longer. These both allow pressure to stay higher longer and increase the volume of water available for use.

In addition you could place your pressure pump in the line that your diagram shows going to hot water tank. placed in the blue line and feed it to the hot water heater. The pump will engage when pressure is needed.

Answer 3

Here's how I solved this. I plugged the yellow line going to the storage tank. I took the blue output line and reduced it to 1/4" and fed it to the permeate pump. From the pump output, I went to a tee the runs to the Tank and to the faucet. Minor downside is the water from the tank is no longer filtered, but it is filtered post-RO before going into the tank.
This change greatly increased the output pressure and allows me to store more (I added a 10 Gal tank).