Plumber Problem
January 29, 2004 8:55 AM   Subscribe

Do you understand the physics of water pressure and/or plumbing? If so, I would love your opinion on this dispute I'm having with a plumber. (more inside)

I had the plumber build me a shower with two sprays -- one on the front wall, the standard location where the shower typically is. The other is overhead, so we could have the total immersion, perfect-storm effect. First question: Was the onus on us to explain that we wanted water to come out of these sprays simultaneously? Because he installed a diverter -- switch it one way, the water comes out the overhead spray, switch it the other way, the wall spray. My point is that, if one wanted to alternate between the spray coming down on top of him, and the spray coming down at an angle, wouldn't one just step forward and step back, under the one standard wall spray? What else would the overhead spray be for, if not simultaneous rainforest action? A plumber should understand certain things without the client having to spell it out. Am I wrong about this? Do I have to start telling plumbers "lefty-loosey, righty-tighty", also? (If that's even true with plumbing, who knows)

OK, now let's get hydrophysical. This diverter is graduated, meaning that if set at the halfway point, water comes out of both sprays at once. Given that shower is finished and tiled, etc., I anticipate the plumber will try to sell this to me as being the setting I'm looking for. However, as you might imagine, you get 50% pressure out of each spray when you do this. Not good. My question is, if the water did not go through a diverter, but just branched two ways off the main supply to the bathroom (one going to the wall, one going overhead), wouldn't I get much better pressure? It's all 1/2 inch pipes, if that matters. My home's water pressure is excellent in general.
posted by stupidsexyFlanders to Home & Garden (12 answers total)
In answer to your first question, since you are looking for a "reasonable man" sort of answer.... me personally, I would think you wanted a diverter so it would be like the standard shower/tub dichotomy. Would I have also asked you, to make damned sure, since you're the one with the money and it's your custom gig? Sure I would have. But, I don't think your plumber is 100% nutso for thinking you wanted a diverter. I cannot answer your physics question, I have never had good enough water pressure where you could even flush the toilet when someone else is taking a shower.
posted by jessamyn at 9:05 AM on January 29, 2004

Dunno about the hydrophysics of it all, but generally speaking, things that are obvious to one are not obvious to others. Seems to me that the onus to communicate your concept lies with you, not the plumber.
posted by Irontom at 9:09 AM on January 29, 2004

I agree that the onus was with you, stupidsexy. IANAP, but I would've assumed if you wanted a "perfect storm" effect, you would want all of the water coming out the top. It's not like you can have it coming full out of one faucet and just suddenly generate twice as much hot water. You'd best back down on this as gracefully as you can.
posted by soyjoy at 9:16 AM on January 29, 2004

ok, i was never any good at fluid dynamics (and it's horribly difficult), but here's my best guess.

first, static pressure (the pressure at the shower head you'd need to apply to stop the water squirting out) should be the same at 50/50 as at one full-on. that's the easy bit.

your problem is that when the water is flowing the "pressure budget" is being used up somewhere along the flow path. it could be one of three things, i think. 1 the diverter is "getting in the way" of the flow (which seems pretty likely); 2 your flow rate is limited by pipes way back in the plumbing, so when you open the second head no extra water can get there (possible - you could test this by seeing whether the flow drops significantly if you also open the bath tap (assuming it's separate)); 3 the extra piping on 50/50 is significant (i don't think this is likely).

however, this is hopelessly academic. you can always argue that extra pressure would solve things.

practically, the solution you've got sounds like a reasonable job to me, since the 50/50 setting is there (what you're asking for would have meant two shower controllers, as far as i can tell - that would be a pretty unusual request).

don't most people who want a really squirty shower install an extra pump these days? we had one in edinburgh. if i were you, that's what i'd do - with a bit of luck it can be added to the piping under the floor without messing up your new tiling.
posted by andrew cooke at 9:21 AM on January 29, 2004

Honestly, I can't see any difference between a graduated splitter set at 50% and a simple fork. If the openings on the graduated splitter are less than 1/2" then you will have a temporary pressure increase but it will be gone when it goes back to the 1/2" pipe.
posted by smackfu at 12:09 PM on January 29, 2004

So you've got hot and cold 1/2" supply pipes entering the shower body, which has either two valves or one to mix them into a 1/2" shower pipe which hits the diverter and is split into the front and top shower heads?

Well, then, yeah, whatever pressure you have past the valves will be split between the two heads. And since the plumber knew that and took it for granted that you would too, it was reasonable for him to assume that you'd only be using one head at a time - otherwise you'd have said something about needing double the pressure out of the body and he'd have run 3/4" from the valves to the diverter.

Try this: turn on only the cold water, and turn the diverter to the front head only. Is the pressure sufficient? If so, then with both the hot and cold on you should have enough pressure for both heads. So, ask the plumber if it would be possible to switch the 1/2" length that connects the shower body to the diverter for a 3/4", and replace the 1/2"-to-1/2" diverter with a 3/4"-to-1/2" part.

You may have to replace the shower body too. That'll get expensive but maybe the plumber will buy the old parts back from you...

Good luck!

Oh - here's an "Ask the Builder" column on water pressure and pipe diameter.
posted by nicwolff at 12:37 PM on January 29, 2004

smackfu: the splitter is more complicated than the fork and probably offers the passing water more resistance. Remember, pressure itself isn't what you want in the shower - you're going to spray this water out into the air, so it won't be under any pressure then - you want volume, or flow rate. Just as in electricity, where current in amps = voltage in volts/resistance in ohms, flow rate = pressure/resistance. We have some initial pressure coming from the combined hot and cold supply lines, and we're trying to minimize the resistance between them and the shower heads (or, actually, your body, since the heads themselves offer a lot of resistance).

Now, 1/2" pipe has less than half the area in cross-section of 3/4" pipe (pie are squared, you know). So you can feed the combined hot and cold into the 3/4" pipe with no increase in resistance, and then break them back into two 1/2" pipes, but each fixture in the way does add resistance and reduce the flow rate.

(IANAP, but I did have my apartment's shower rebuilt with 3/4" pipe when I moved in and it is fucking sweet.)
posted by nicwolff at 12:58 PM on January 29, 2004

Y really NAP? Wow. OK, I'm going to take nic's advice and have the plumber run 3/4" between the valves and the diverter...Or do you run the 3/4" between the supply lines, into the showerbody, and out of there into the diverter?

And you're saying that going back down to 1/2" for the journey from the diverter to the showerheads isn't going to hurt me volume-wise...Am I understanding you correctly on that?

Also, wondering if the extra flow will cause any problems with the showerheads if I happen to move the diverter over to 100% flow coming out of one or the other?
posted by stupidsexyFlanders at 1:24 PM on January 29, 2004

Having any 3/4" pipe installed is not necessarily going to change anything. The line supplying your entire bath is not likely to be more than 1/2" -- Nicwolf touches on this best -- the flow rate for the entire system is going top be the controlling factor.

Try this test: when you are running the shower and then turn on the sink, is there a pressure drop at the shower? If not, 1/2" pipe is all you need. If so, no 3/4" pipe is going to solve that problem: the weak link is in the overall supply to that part of the house and possibly the entire house.

Try this with three items and then four: you may or may not reach a practical limit to your test. If you are the only person using the plumbing at any one time, no problems will present themselves at your shower if two or three fixtures open made no impact on the single valve. Recognize too that this test is an approximation -- different fixtures call for different flow rates.

What can be supplied in total is a mystery known only by investigating all the pipes and turns between the supply from the street (which could even be a hopelessly corroded lead -- *yes, they are still out there* -- or galvanized pipe) which might have the equivalent diameter of a 1/4" supply line from calcification etc.

On plumbers, (and any other building professional) the very best at their craft may think to ask you what your intention is. The average will not offer much beyond expecting the average and the common solution in this situation is a two-way (a.k.a. three-port) diverter. The second most common solution, which does not sound like an option now (although you might be surprised) would be separate valves as suggested above.

What might work best is to change to a four-port diverter (or five-port, I;m not really sure what's available and who is counting supply lines v. output) where the selections can allow for one head on, the other head on, both heads on and of course, both heads off.
posted by Dick Paris at 2:10 PM on January 29, 2004

I don't know about America, but here in Korea you can buy these monster multiple-head, wall-encompassing shower-hydra assemblies that do pretty much what ssF is talking about - water from every direction but behind you. Too late, I know, but I've always wanted one....
posted by stavrosthewonderchicken at 3:06 PM on January 29, 2004

the five-port diverter sounds like the answer. I like this one -- two in, three out -- giving you ceiling on, wall off; wall on, ceiling off; both on. Perfect.

I feel so much smarter about this now. You all are great.
posted by stupidsexyFlanders at 3:20 PM on January 29, 2004

Well, my thinking is that you have two 1/2" supply pipes coming in to the shower body: one hot and one cold. So coming out of the shower body you've potentially got twice the flow a 1/2" pipe can handle, being choked down into the one pipe to the diverter - which is probably OK if it's going to one shower head and that's what you're used to, but if you're splitting it out to two 1/2" pipes to two heads then each of those pipes will only get half the pressure.

(Yes, you could run 3/4" right out to the shower heads, and then potentially have more pressure when you're running one head than you do now, but you say the one-head pressure is OK so that's a luxury.)

So the feed to the bathroom doesn't really matter: if you can get adequate shower pressure for one shower head from the cold supply alone and from the hot supply alone, then you know that coming in to the valves you've got enough pressure for two showers - the problem is just to get it to the shower heads.

However, all this assumes that the shower temperature you want is reached at 50%/50% hot/cold - if, as is more likely, your preferred shower is all-hot-with-a-little-cold then we're back near square one and, as Dick Paris points out, you'll have to see if you can run 3/4" back to the water heater or whatever.

So, take all this with the requisite grain of salt, and see what your plumber thinks - or ask another plumber if you no longer like this one.
posted by nicwolff at 6:38 PM on January 29, 2004

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