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By Kenneth in Lees Summit, MO on 3/20/2007
Without giving you all of the answers, let me lead you through the process I used to get to what was right for me (and as I used a manifold PEX system, this is somewhat relevant to you).
I like the idea of using a home run for every fixture directly back to the manifold. This way, if you are taking a shower, and your spouse flushes the toilet in the same bathroom, you shouldn’t have any pressure drop at the shower. Sure you have anti-scald valves now, but I learned years ago that pressure drop in the shower was not a good thing, and you learn to move pretty quickly. Old habits die hard, and even though you have anti-scald plumbing fixtures this is still my way of thinking.
What you need to do is figure out how much water you need to have at the bathroom to adequately run the fixtures you will double up using. Will you use the shower at the same time you are running your roman tub faucet, probably not. But you might use a toilet, perhaps one lavatory sink, and the shower at the same time, depending on your schedule. If you sell, the prospective purchaser might, so you might want to consider future too. From the water flow, you then size your supply pipe to each bathroom so you have adequate flow and pressure. If this sounds complex, you can do it the easy way and home run each fixture back to manifold and use either 3/8” or 1/2” depending on fixture size. Another reason I like the home run system is that there is only two joints in the line, at the fixture and at the manifold, and when I am DIY simplicity is a good thing (not to mention less potential joints for leaks). Start putting T’s in the line, and even with the crimpers and the go/no-go gauge, you still run the risk of leaks. Perhaps for a plumber, not a big risk, but for a person with little plumbing background simplicity is a good thing (let me clarify, I hired a plumber and still felt that less joints was good). Every crimp adds potential for leaks, however small.
I used two manifolds. My house is not nearly as large as yours, but my water uses are somewhat separated. I have normal bath, master bath (2 lavatories, roman tub, two shower, water closet), and associated hose bibbs at one end of the house, at the other end I have laundry room with washer and sink, more hose bibs, and kitchen. So I had two choices, one manifold and a whole lot of 1/2” PEX getting water from the manifold to the other end of the house, or two manifolds each with nice short runs off the manifold. I needed a hot water loop between the manifolds, so I have three 3/4” lines connecting the two manifolds (cold water supply, hot water supply, hot water return loop). Figuring my own materials, it was cheaper for me to buy 300’ of 3/4” PEX and a 18-port manifold than it was to buy the 1/2” PEX I would have needed to home run all of these fixtures back to my main utility room. To date, I have not hooked up my hot water return, but the lines are there if I ever desire to do this (and running the lines later was not an easy or cheap option). Is this the right answer for your house, you have to figure that one out yourself.
And don’t forget that what you can get from your plumbing supplier is also a factor. I could have used a 36-port manifold and been done, however I didn’t realize until I went to actually go buy the manifolds that the 36-port was special order at my plumbing supply house. However they stocked 14-port, 18-port, and 24-port manifolds. As luck would have it, they were out of 14-port manifolds so I was stuck. It is only after I ran the numbers re-figuring my solution that I realized two manifolds was actually a cost savings, so bonus for me. I also used all 1/2” PEX instead of using some 3/8”. The reason was that the manifolds are set up for all 1/2” PEX (at least Vanguards are), so less parts to buy is always good. Also you need separate crimpers for 1/2” than you do for 3/8” (and for the 3/4” since I only needed a couple I used compression fittings and not crimp fittings, perfectly allowable in my locale), you get to buy one less tool that you would only use one time and has no other use in the future. And lastly, by upgrading to all 1/2” PEX I was able to buy in a larger quantity that my price reduced per linear foot of PEX, so using all 1/2” PEX home runs from the fixture to the manifold saved me money three ways; less material price, less fittings to buy, and less tools to buy. Nice, and I figured this out before I went to the supply house.
That should answer some of your questions. However before you give me any credit, let me clarify that I am not a pro. A pro already has the necessary crimp tools and buys in sufficient quantity that the decision to use all 1/2” isn’t a factor (buy the 1,000’ coil of both 1/2” and 3/8”, leave the excess on your truck for the next job).
Let me edit this to clarify a couple of items. I used a professional plumber for my water line install, I did my finish plumbing myself. However, I also bought all of my own materials and required my subcontractors to separate material bids from my installation bids. Since I was buying materials, it worked out cheaper to have the plumber install all 1/2" PEX, if I had a future use in the short term for the excess material I would have used 3/8" for some uses (lavatory, water closet, ice maker, low flow uses). As to the tools, my plumber was experienced with Wirsbo PEX, which uses a different style connector than the Vanguard PEX, and hence a different style tool. I selected Vanguard because it is somewhat local (made in McPherson, Kansas, if I can support the local economy that would always be my choice) and it is also not aimed exclusively at professional plumbers so if I need to make a change in the future (eg. hooking up my hot water recirculation loop) I don't automatically void all warranties and I can get materials from multiple sources. However since I was buying my own materials, my plumber also indicated that specialized tools based on this material selection were also my responsibility - fair enough in my book and I now have a 1/2" crimp tool that I have absolutely no use for but not really worth the trouble of listing it on eBay either.
There are several other discussion topics on PEX that might help you out, the search tool is your friend ;-). And one more thing, when you are counting your fixtures, don't forget your hose bibbs - you probably want these on all four sides of your house. And hot water to the outside is also a nice bonus most people don't think about...