Mixing Valve with RO System and Tap Water?

by **Dogshot** on Mon Feb 28, 2011 5:43 pm

I am going to plumb my next machine in, and I need some advice about the wisdom and potential mistakes involved in using a mixing valve with tap water and an RO setup.

My tap water tests at about 80-100mg/l (5-6 grains) hardness. I have a decent RO system (sediment filter, carbon filter, RO cartridge, carbon filter, 10 gallon tank, 3/8 inch line) that has a single small faucet at the kitchen sink. The RO water consistently tests at 0 for hardness.

I've been thinking that I can save some space under the sink by using a mixing valve to blend the tap and RO water down to a hardness level suitable for the espresso.

Is this a sound idea? Would I be able to get adequate pressure and adjustment (mixing) from this? One problem I see in this is that I have a single 3/8 line running from the RO system to the RO tap. Can this be cut (its John Guest 3/8 230psi plastic tubing) and a splitter be inserted (so that water can flow both to the RO tap and to the mixing valve)? I would obviously not use the RO tap while the espresso machine is drawing water.

To implement, I plan to get a plumber to install a splitter with a shut-off valve on the cold water line. Then run 3/8 line from the cold water line and connect it and the RO line to the mixing valve. From there I would have another shut-off valve (so that I could test the water, or insert a bucket with descaler), a pressure regulator, and then connect to the espresso machine intake line.

I'd really appreciate some thoughts from those who have been there, or done that.

Mark

by **shadowfax** on Mon Feb 28, 2011 6:08 pm

I have a friend, pjones, who's been there and done that. I believe there will be a significant struggle with getting a consistent mixing rate if you are not extremely careful to regulate the inlet pressures of each component (RO, Tap—which you should take care to carbon filter). The RO supply, if I understand right, is the most difficult one to hold steady because it will decline as you use it up and flush the holding tank faster than the system can replenish.

There are some commercial blending options available that would be considerably easier to use, but certainly expensive. I understand that Everpure's MRS-100CC has a blending feature on it, and costs in the range of $900-1200 on the street. Note, you will need a storage tank to go with it; I don't believe it's included. I've also heard/read about Cirqua planning on releasing a similar consumer-grade blended RO system that would cost even less than Everpure's solution. If you're looking at RO, I suspect options like that, while more expensive, will probably turn out to be a lot more reliable than setting up your own mixing valve. However, if you are looking to build your own setup, you should talk to your local Swagelok dealer about the right mixing valve!

by **Dogshot** on Mon Feb 28, 2011 9:53 pm

Thanks for the comments. I'm not sure how mixing valves work - are they compensatory? What I mean is, if the flow from the RO dwindles, would the mixing valve reduce flow from the cold water tap? I would not want to risk a situation where the pump would not be receiving the amount of water it is calling for. Other than that, I am not all that concerned about maintaing a rock steady and precise hardness level.

Something else I do not follow is the suggestion of using a carbon filter. My water comes from Lake Ontario, so there is not much sediment. Are you concerned about sediment from the municipal water pipes?

Overall, it seems like it might ultimately be easier to get a satisfactory solution using water from the tap and putting it through a cation and then carbon filter, and forgetting about incorporating the RO. Would you say that's a fair assessment?

Mark

by **shadowfax** on Mon Feb 28, 2011 10:48 pm

Yes, a cation softener with a carbon filter is a satisfactory solution that's much cheaper and easier than RO with a mixing valve.

A mixing valve at its cheapest and simplest is just a tee with a knob that adjusts the proportional orifice size of the 2 incoming tubes as they mix into the third (outlet) tube. There are many ways to skin that cat, however, and I can't claim to understand or even be aware of even a small proportion of such designs. In general, if you're dealing with mechanical valves that aren't controlled electronically in some fashion, such valves will not provide a fixed TDS or proportion of RO to tap water if the inlet pressures to the mixing valve aren't held constant or in constant proportion to one another.

I am far from an expert on this topic, though; Eric Svendson is no doubt the man to ask on HB, and I'd think your local Swagelok dealer would also be very helpful in explaining the issues at stake when mixing two water sources with non-constant pressure.

As for why I suggested a carbon filter on the tap side, carbon filters are more than just sediment filters. They also are effective at removing chlorine and organic compounds. In my experience, they quite simply make water taste better, which is one of the main reasons why I use them.

by **Dogshot** on Mon Feb 28, 2011 10:55 pm

Thanks for the help, Nicholas. Based on your advice, I think I will move my attention toward the simpler solution and hook up the softener and carbon filter. Chris Coffee sells a kit that makes the whole thing very straightforward.

Mark


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