1/2 or 3/4 PEX
The inside diameter of PEX tubing is smaller than CPVC or copper. So to keep the same flow rate to the fixtures, wouldn't I need to use one size larger PEX? I'm about to do a complete PEX upgrade and I noticed the manifolds (at Lowes) all seem to have 1/2 inch outlets. I don't have great water pressure or flow rate to begin with and I'm concerned the smaller PEX ID might slow the flow rate even further. So, I'm considering doing a 1 PEX main with 3/4 PEX to the fixtures. Is this up sizing common or necessary (especially to the shower)? Are 1 to 3/4 manifolds commonly available? I haven't located any locally. Thanks Barry The same size pex and cpvc will be compatible. I would run 3/4 throughout the lateral runs and only drop to 1/2 when I turned up to the fixtures. Running 3/4 up to the fixtures will only require you to immediately drop to 3/8 at the fixture, as that is the leader size on toilets and sinks. Thanks for the response Larry. As Larry noted, I would normally run a larger line for lateral runs and then downsize the fixtures runs. In a manifold system, I believe the downsizing occurs at the manifold. Main manifolds typically have 3/4 in and multiple 1/2 out. I already have a borderline flow rate (my opinion) with fairly clean 1/2 copper fixture lines, 3/4 laterals and a 1 main. With the inside diameter of PEX being considerably smaller than copper, I'm concerned about a possible flow rate decrease on long (80-100 feet) 1/2 PEX fixture runs. Thanks for pointing out the 3/8 sink and toilets leaders. There shouldn't be a problem there, but what about showers and baths which have full 1/2 inlets? Should I run 3/4 PEX to those? The ID of 3/4 PEX appears to be similar to 1/2 copper, although I haven't really measured. Also, the 3/4 shower lines would have to be branched off before the main manifold. That could create a flow rate drop to the main manifold when a shower is running. So, I would need a 1 main line up to the shower / manifold branches. A manifold with a 1 inlet and multiple 3/4 outlets (to showers and the main manifold) would solve that problem, but I haven't located one locally. The main question is, Do I need 3/4 PEX runs to showers and baths? Thanks, Barry Main answer is 1/2 pex will deliver enough to the showers and tubs. Good luck with the manifold thing. I just believe any run that is too long in 1/2 will compromise your water supply. I have 3/4 running laterally with the 1/2 where they turn up to the appliances, and you can flush an adjacent toilet and not feel it in the shower. you seem to have a pretty good grasp on the issues. but if you have good copper why are you changing it? shower valves: unless you are putting in a high volume device (like a rain head) 1/2 is fine, heck 3/8 is okay, look into a standard tub/shower valve for the built in restrictions. most of the better grade control valves are stubbed for 3/4. but again they are mostly for controling volume. manifold blocks: check out some of the flow thru blocks by zurn. 3/4 in and out with 1/2 stubs out the sides. check out a local trade quality plumbing supply for options other than lowes. Clean copper (i.e. not clogged with corrosion or goo) doesn't necessarily equate to good copper. This is a complete remodel of an older house and I'm replacing the copper because there's evidence of many minor leaks and past repairs (i.e pinholes). Everyone I have spoken with has mentioned that 1/2 is fine for showers, until I point out the inside diameter of PEX is about one size smaller than copper (i.e 1/2 PEX equates to about 3/8 copper). The primary concern is that in long runs there will be increased friction, a pressure drop and hence, a lower flow rate. Then they stutter a little and have to think about it. If it were a short run, or if I had a normal flow rate, I would probably go with 1/2 PEX. But being it's a fairly long run (100 ft) and I already have a low flow rate (a long, small main line problem), I think I'm going to go with 3/4 PEX to the showers. I checked several local plumbing supply houses and none stock PEX. Professional plumbers here seem to shun it. We're a bit behind the times. I have a few more suppliers to check. Thanks for the lead on Zurn mainfolds. If you plan on installing Delta shower controls, make sure the riser to the shower head or rain head is copper. It is specifically stated in the directions not to use anything else. Originally Posted by CowDoc The inside diameter of PEX tubing is smaller than CPVC or copper. So to keep the same flow rate to the fixtures, wouldn't I need to use one size larger PEX? I'm about to do a complete PEX upgrade and I noticed the manifolds (at Lowes) all seem to have 1/2 inch outlets. I don't have great water pressure or flow rate to begin with and I'm concerned the smaller PEX ID might slow the flow rate even further. So, I'm considering doing a 1 PEX main with 3/4 PEX to the fixtures. Is this up sizing common or necessary (especially to the shower)? Are 1 to 3/4 manifolds commonly available? I haven't located any locally. Thanks Barry If you are running PEX homerun style from a manifold as is the code here you might as well make it all 1/2 inch. Sounds like the case for you too. I recently replaced all the polybutylene plunbing in my house with PEX. Main coming from the meter was 1 inch so I made my manifold of 1 inch Cu. OK, I got off my butt and calculated some real numbers. Type L Copper (Copper Tube Handbook) - 1/2 and 3/4 have 0.545 and 0.785 inside diameters. Frictional losses @ 2 gallons per minute flow are 2.9 psi and 0.5 psi respectively, per 100 feet of pipe. The Hazen-Willaims C factor (i.e. friction factor) is between 130 and 150 for new copper, but can drop as low as 60 due to corrosion. At a C factor of 60, the frictional loss of 1/2 copper is 16 psi per 100 feet and that does not even take into account a reduced diameter. The fixture pressure can drop dramatically over time with copper. No surprises there. CPVC (FlowGuard Gold) - 1/2 and 3/4 have 0.489 and 0.715 IDs. Frictional losses @ 2 GPM are 5.0 psi and 0.8 psi per 100 feet. The C factor is 150 and stays constant (i.e. no corrosion). PEX (Plastic Pipe Institute Guideline) - 1/2 and 3/4 have 0.475 and 0.671 IDs. Frictional losses @ 2 GPM are 6.1 psi and 1.2 psi per 100 feet. The C factor is 150 and stays constant. Bottom line is that new 1/2 copper loses 3 psi and 1/2 Pex loses 6 psi. 3/4 Pex only loses 1.2 psi. For a 40 psi line (like my main) that’s a 7.5%, 15% or 3% pressure loss. I’m sticking with 3/4 PEX to the shower. Larry, I’ve read the copper shower head riser requirement from Delta is due to the smaller ID of other pipe materials. The increased back pressure messes with their control valves. Thanks to all, Barry Hi - I'm new only chiming in because intrested in PEX. I just bought (on line) a bunch of Wirsbo PRO-PEX including the expensive hand expander. I won't mention the name ____supply.moc or something. Great service IMO. I made one connection when I got it and am amazed at the strength. I plan to use the black plastic fittings and small plastic manifolds with 3/4 in/out (4) 1/2 branches. I think I'll run 3/4 to Mstr Bdrm shower even thought I bet 1/2 would be just fine. I have about 45 PSI 60 to 70' run. Sure hope all the bad things people say about PEX don't all come true! Just seems to me that lg. dia. curves fewer fittings can't be bad. Time will tell.... It ain't the pex, it is the badly installed connections that gave it a bad name. If you don't use that expensive crimper properly or fail to have it calibrated, you will be in the same boat. Good luck with it. CowDoc: Don't overlook that routing the hot water in 3/4 instead of 1/2 will more than double your wait for hot water to arrive at the fixture, and double the amount of cold/lukewarm water wasted while you wait, every time you bathe/shave/need hot water for anything/etc. To get a true comparison of flowrate at the fixture between pex and copper, you need to add in the losses from all the elbows and tees you'll need for the copper installation. With the pex homerun system you don't need all those fittings, and the pex wall stubouts for the sink, toilet, etc. are long sweep elbows which cut down on head loss. A straight run pipe comparison doesn't tell the whole story. Again, thanks to everyone for all the thoughts and tips. Anvilt, I recognized I had to allow for losses in elbows, etc to get the entire picture. However, in this case, there would only be two elbows and no tees. Those head losses are minimal compared to the long run. So I just disregarded them. Nice catch though. Aq_guy, the 3/4 inch run only goes to the shower/bath. I don't mind waiting a few seconds (or gallons) for the water to warm up there. With a bath, the cold water in the pipe doesn't get wasted. It's just added to the mix. With a shower, well, there is some water waste. 100 feet of 3/4 inch PEX holds about 1.3 gallons. For 1/2 inch PEX it's about 0.7 gallons. I can live with the additional 0.6 gallon loss as a trade off for an extra 5 psi of pressure. I have a small undersink water heater that provides hot water to a side tap at the bathroom sink. It's only a quart or so, but it's plenty to wash up with. So we rarely use the primary hot water at the bathroom sink. The heater is fairly energy efficient, eliminates wasting the cold water in the pipes and saves heating about a gallon of water every time we wash. Good for making instant coffee before you stumble into the morning shower too.
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