View Full Version : Head Pressure affect RO/DI production?

02/07/2017, 10:08 PM
I recently made some changes to my setup. I have two separate units, a 4 stage RO unit, and a MaxCap (dual stage) DI. It used to be set up inline, the two units next to each other, with the water storage container located below the DI "out" water line. Due to space constraints with the change made to other areas of the fish room, I mounted the the units above/below each other- the RO unit is now below the DI unit, maybe 12-15". And now, the DI "out" water line feeds into a container that is about 6" higher that the DI unit.
I've noticed that it seems my water production has dropped off considerably, probably a third or better. The production usually drops some in the winter (our tap water is in the 40's), but I don't recall it being this much of a drop.
So it got me to thinking.... Would the head pressure affect the water production? I assumed it wouldn't. It's maybe a couple of feet up, and I have really good water pressure (90+psi) But now I'm not so sure.
Any help would be appreciated.

02/08/2017, 08:12 AM
That tiny difference in height is not significant.

40 deg water makes 1/4 as much as 77 deg water.

If you don't think it's due to temperature, then the membrane might be plugging up and needs to be replaced.

Scott in Support at SpectraPure

02/09/2017, 05:11 AM
Scott, thanks for your time.
I wouldn't think so either. But that being said, the water that is pushed through the membrane cannot just "drain away" like it does in most RO/DI setups. It has to push through the membrane and then push up another couple of feet.
I'm getting a little over 20 gallons in a 10 hour time span. So around 50 GPD.
When I redid the system, I also took the opportunity to plumb in a HM Digital FM-1 Water Filter Monitor that I had purchased quite a while back. I plumbed it in between the two carbon block filters. I was getting some crazy numbers from the unit, something along the lines of 180 gallons of water through the meter to make 20 gallons of water- 8:1 or more. I figured the meter was no good so I pulled the batteries out of it, but haven't removed it from the system yet.
As far as the drop in production, at the start of every run, I turn on the system, open the bypass valves (one for the system and one for the DI) and allow the water to run for a minute or so to flush out the water laying in the line that's room temperature. Once I'm into the water coming directly from outside the house, I close off the system bypass and set the 4:1 water production rate. I don't use a flow restrictor in the RO line as most , I have a needle valve plumbed into the system which allows me to adjust water production based on water temp and, to a lesser extent, (in the summer when the it seems the entire neighborhood is running their sprinkler systems all at the same time), pressure. I adjust the flow to get the 4:1 ratio. The lines where the water is measured from are located below the RO unit. Once I get the 4:1 ratio and the TDS from the unit have settled down and dropped to an acceptable level, I close off the DI bypass. So, I have the 4:1 rate when bypassing the DI (and allowing the water to "drain" like a traditional setup), but according the flow meter, the waste is more than double when being forced up the couple of feet through the DI.
As far as the membrane, it is around a year old I think. Our water, while it has a high ph and chloramines, has reasonable level of TDS (180ish, give or take). I don't think the membrane is the issue.
I have not timed the water production rate when bypassing the DI unit. I guess I'll do that on the next water run.
I hope you are correct about there being little to no effect of the head pressure. It will be a major PITA if I have to figure a way to get the RO unit above the storage barrel. The barrel top is almost to the floor joists in the basement...

02/09/2017, 08:46 AM
7 feet of head costs about 1 psi. It's not pressure - you have 90psi.

Simple test: try holding your thumb over the waste line OR the product line.

You will find so much pressure that you can't stop it.

Here's the math for de-rating a membrane:

90 GPD at 77deg / 60 psi.

Temperature is 40 - that's a factor of .25.
Pressure is 90 - that's a factor of 90/60 = 1.5

90 x .25 x 1.5 = 33.75 GPD. You are making 50 GPD.

I don't see the problem, you are doing better than expected.

Scott at SpectraPure