Help Sizing Shower System

[Derek Sutherland]

I am remodeling my master shower (48x34). I would like to put in a rain shower, a main shower, a hand spray (or combined main/hand spray) and 4/6 body sprays. We have the biggest residential tankless hot water system you can buy in the basement and the shower is on the 2nd level. We have a well system with pressure set between 60 and 80 psi. I have quite the water filtration system that includes a contact tank, an iron filter, a charcoal filter and a water softener - all heavy duty but I'm sure this impacts my pressure. The pressure out of our shower head isn't that good - could be the shower head as the pressure out of the garden hose is perfect and that is water after the filtration system.

Anyway that was just a little background. I want to put in the best possible system since I am doing everything myself. I want to maximize the pressure and I'm not worried about amount of water used - although I do understand I am limited to a 2 inch drain. I'm thinking I need to run the entire 3/4 inch pipe (not far away) into my shower and have 3/4 inch piping all they way to the diverter which will then deliver 1/2 to each outlet.

Anyone see any problems with this or have concerns? I am having a hard time finding a complete system with my required specs. I may have to piece meal this together the best I can. Comments/tips welcome.

 

[Jadnashua]

The largest single residential tankless system I'm aware of (you can cascade some of them) has a 199K BTU burner...you will not have enough output to feed that shower unless you like lukewarm to cold showers.

There's also a limit on the volume a 2" shower drain can handle, and while I do not remember the cross-over point...I think if everything you've got planned were running at the same time, it wouldn't pass code with a 2" drain.

The copper institute recommends a maximum of 5fps flow velocity in pipe for hot water. WIth a 3/4" copper line, the equates to 8gpm. Since you would be mixing in some cold (normally, but probably not with a tankless), that will usually exceed the capacity of the supply line recommendations. That doesn't mean it can't flow more, but there can be some issues if you exceed those recommendations.

A thermostatically controlled shower valve can usually flow more than a pressure balanced one, and a 3/4" one, considerably more than a 1/2" one (typically, about double).

If you cannot supply the required volume, the pressure will drop. Ideally, your system would have MORE capacity than the outlets will allow, then, you will get the supply pressure minus any pressure losses from friction along the way (well, elevation changes reduce pressure, too, at 0.43#/foot elevation change). When the pipes can't supply the needed volume, it tries to flow faster, which increases the friction, which also drops the output as well. Up to a point, increasing pressure will overcome some of those losses, but that usually causes noises, and can literally erode the pipes from the inside.

 

[Sponsor]

 

[Derek Sutherland]

The largest single residential tankless system I'm aware of (you can cascade some of them) has a 199K BTU burner...you will not have enough output to feed that shower unless you like lukewarm to cold showers.

There's also a limit on the volume a 2" shower drain can handle, and while I do not remember the cross-over point...I think if everything you've got planned were running at the same time, it wouldn't pass code with a 2" drain.

The copper institute recommends a maximum of 5fps flow velocity in pipe for hot water. WIth a 3/4" copper line, the equates to 8gpm. Since you would be mixing in some cold (normally, but probably not with a tankless), that will usually exceed the capacity of the supply line recommendations. That doesn't mean it can't flow more, but there can be some issues if you exceed those recommendations.

A thermostatically controlled shower valve can usually flow more than a pressure balanced one, and a 3/4" one, considerably more than a 1/2" one (typically, about double).

If you cannot supply the required volume, the pressure will drop. Ideally, your system would have MORE capacity than the outlets will allow, then, you will get the supply pressure minus any pressure losses from friction along the way (well, elevation changes reduce pressure, too, at 0.43#/foot elevation change). When the pipes can't supply the needed volume, it tries to flow faster, which increases the friction, which also drops the output as well. Up to a point, increasing pressure will overcome some of those losses, but that usually causes noises, and can literally erode the pipes from the inside.

Thanks for the information. I'm not sure I understand everything you just said though. I believe the Rinnai water heater I have (yes 199K) can do up to 9.4 GPM in the best case scenario. We would not have everything running at the same time - in fact the diverter I'm looking at I'm not sure would even allow that (if I decide on just the rain head, a hand held and body sprayers I can divert to each one plus 3 2-way diversions for a total of 6 different diversions).

When I installed the tankless, my small town didn't have any codes for me to follow so I'm guessing the same with the shower. The tankless can go up to 140 degrees although that may impact the flow rate. Not sure which is better - heater set at 120 or at 140 with lower flow rate but more cold water.

So unless I have all those outlets running at the same time which is not the desire, I'm probably good with pressure and heat? Even at 6 body sprays at 1.5 each and the handheld that's about 11.5 GPM. To be safe I can go down to 4 body sprayers or separate volume controls for all outlets although I don't really want a bunch of knobs all over the wall.

 

[PorcelainVacation]

It takes 500 btu output to raise one gallon of water by 60 degrees F. (Assuming your supply is 60 degrees and you want 120 degrees out of the water heater, and 8.33 btu raises 1 gallon of water by 1 degree)
Therefore, 10 gallons raised by 60 degrees takes 5000 btu. 10gpm takes 5000btu/m or 300,000 btu/h. I believe your burner is rated by input BTU, so you need to include heat transfer efficiency between the flame and the water.

 

[Jadnashua]

Depending on where you are in Il, in the winter, the incoming water could approach freezing...so, your performance summer/winter can vary radically. You will NOT get max volume with max temperature rise with your typical winter incoming water temperatures. Some tankless systems can maintain a fairly even outlet temp over a significant volume change. They do that partly by varying the burner output to match up with the demand. But, once you exceed the demand capacity of the tankless system, one of two things will happen depending on the design, it will throttle back the volume so it can maintain the outlet temperature, or it will progressively fail to reach the desired output temperature. This gets progressively worse if someone else in the household turns on the hot water for some reason while using the shower. It takes MAJOR burner inputs to raise the temperature of water to a comfortable level.

One thing I'd seriously consider is to use a quality thermostatically controlled shower valve. They not only tend to provide a larger volume than a pressure balanced one but will also hold the temperature better. But, they can't manufacture more hot water.

IMHO, a multi-outlet shower with multiple body sprays will not perform well with a typical tankless in the wintertime. It might work fine in the middle of the summer. Some of the tankless systems allow two or more to be daisy-chained...each one raising the temperature part way stepping it up to your desired level. That allows a much higher volume outlet at higher temperatures, but significantly adds to the cost of the hardware and maintenance.

If there's a particular part of my previous post you had trouble understanding, point it out and I'll try to expound on it. No sense in being disappointed in your project after spending lots of time and money on it!

You need to know at least a couple of things:
- the total volume of hot water you expect to use at one time
- the seasonal incoming water temperatures. Someone in say Hawaii could get by with a very different system than Illinois because of the differences in the incoming water temperature.

 

[Derek Sutherland]

Depending on where you are in Il, in the winter, the incoming water could approach freezing...so, your performance summer/winter can vary radically. You will NOT get max volume with max temperature rise with your typical winter incoming water temperatures. Some tankless systems can maintain a fairly even outlet temp over a significant volume change. They do that partly by varying the burner output to match up with the demand. But, once you exceed the demand capacity of the tankless system, one of two things will happen depending on the design, it will throttle back the volume so it can maintain the outlet temperature, or it will progressively fail to reach the desired output temperature. This gets progressively worse if someone else in the household turns on the hot water for some reason while using the shower. It takes MAJOR burner inputs to raise the temperature of water to a comfortable level.

One thing I'd seriously consider is to use a quality thermostatically controlled shower valve. They not only tend to provide a larger volume than a pressure balanced one but will also hold the temperature better. But, they can't manufacture more hot water.

IMHO, a multi-outlet shower with multiple body sprays will not perform well with a typical tankless in the wintertime. It might work fine in the middle of the summer. Some of the tankless systems allow two or more to be daisy-chained...each one raising the temperature part way stepping it up to your desired level. That allows a much higher volume outlet at higher temperatures, but significantly adds to the cost of the hardware and maintenance.

If there's a particular part of my previous post you had trouble understanding, point it out and I'll try to expound on it. No sense in being disappointed in your project after spending lots of time and money on it!

You need to know at least a couple of things:
- the total volume of hot water you expect to use at one time
- the seasonal incoming water temperatures. Someone in say Hawaii could get by with a very different system than Illinois because of the differences in the incoming water temperature.

All good information indeed. Yes I was aware of the incoming water degree vs. flow rate. The lower the temperature the longer it takes to heat the water and the less flow. I would not be able to measure this, however, I have not noticed any difference between winter and summer after several years of use. That could also be due to the fact I'm not pushing the system. I'll note that most the piping coming in is deep inside my 700 foot well so I believe the water coming in is fairly constant all year round (except perhaps the first several meters of piping). I guess I could run the cold water for awhile then take a temperature check.

What is fairly foggy to me is the information about eroding of the pipes. That seems kind of scary and something I have not considered.

I guess at the end of the day I can deal with not enough hot water by dialing back the volume and/or the amount of outlets running. I can also play with the 120 vs 140 setting. I am more concerned about maximizing my pressure and at the end of the day if running 3/4 lines through my system is the best possible scenario then I can deal with throttling things back in the shower if I'm pulling too much. BTW I believe my tankless system is capable of daisy chaining them if need be.

I will definitely be looking for a thermostatically controlled shower valve now is this at the mixer?

 

[Jadnashua]

Every transition in the pipe, even a joint, but more so on elbows, can cause turbulence. The faster the water flows, the more turbulence. It literally can flow fast enough to eat away at the copper piping. Look up cavitation. Things also get worse if you neglect to straighten the pipe ends after cutting with a tubing cutter (which tends to compress the end, forming a lip). That lip creates more turbulence. Also, the faster you try to run the water in the pipes, they start to create noises. Read this, it will tell you more than you want to know!

https://www.copper.org/publications/pub_list/pdf/copper_tube_handbook.pdf

Straight from the organization that supports the industry. If you follow their guidelines, you'll have a stable, quiet, reliable system. 3/4" pipe...hot water...max flow rate recommended = 5fps, 8gpm max to stay within their guidelines.

I'm on a public water system. During a cold snap, after a 120' run through the ceiling of our condo basements (enclosed and insulated), I've measured just over 32-degrees...really COLD. My townhouse is in the middle of a 220' foot building, so it would have warmed a bit from when it came in below ground. I had considered tankless, and given the choices, it just wouldn't work out. Your results may differ.

To get an idea, most sink faucets are limited to 2gpm. I don't know how many you have in the house, but try turning them all on full and see what temperature you can achieve. That will likely be similar to your shower at full tilt. If you have a couple of tubs, open the tub fillers to max hot and see how well it maintains. That may or may not represent middle of the winter, but if it cools off now, it will only get worse in the winter.