Supply Lines to Separate Showers

[DanBoston]

I am adding a new bath to my basement and was wondering what is the best way to make sure that both the water pressure and hot water temperature does not drop while in the shower. I have an existing shower on the first floor of a ranch. When someone turns on the hot water elsewhere in the house, or the heating system kicks on, both the water pressure and hot water temperature drop. What is the best way to reconfigure my plumbing to minimize these effects? I am considering running both separate hot and cold supply lines directly from each (hot and cold) source to each shower. Is that the best way?

Also, when installing a new shower, do I need to have a 3/4" line or is 1/2" OK?

 

[Terry]

You can typically run two plumbing fixtures on a 1/2" line. But there there are "fixture units" that some codes use.
The shower is 2
The tub and washer is 4
1/2" handles 6 to 7 units depending on how short the run is. So if you are running a tub on one side (4) and a shower (2) on the other, then you would have 6 fixture units between the two. That works. That doesn't really stop the up and down temperature though. For that, you will need pressure or thermal balanced valves on both bathrooms.
If you don't switch out the valves, then adding an expansion tank near the water heater will help.
Switching to pressure balanced valves and adding the expansion tank is best.

If you have three plumbing fixtures, then most plumbers bump up to 3/4" pipe.
Your shower valves have 1/2" inlets, so unless you have a multi-head system, or you are filling a very large jetted tub, then 1/2" is all you need.

For a typical bathroom set, I would run 3/4" cold until I pull off a fixture, and 1/2" for the hot.

 

[Sponsor]

 

[DanBoston]

OK Thanks. How many fixture units for sinks and toilets?

In addition to the shower in the basement, we will have a 65-gallon jacuzzi and a toilet.

 

[Terry]

https://terrylove.com/watersize.htm

1.0 for lav
2.0 for shower
2.5 for toilet
4.0 for tub

 

[Basement_Lurker]

If you have an unfinished basement, you should consider upgrading your trunks (and possibly the service), if you are unhappy about pressure/volume drops from simultaneous draws on the system. My guess is that you have 1/2" trunks and a 1/2" supply.

 

[Jadnashua]

I agree, check out what is supplying the home - the type of pipe, its diameter, length, and the size of the water meter. If the house is old, the supply might be galvanized, and even if fairly large, may now only be the equivalent of a much smaller pipe. Even if some other type of pipe, over the years, construction, tree roots, or something else may have crimped it, and is restricting the flow.

The symptom you have of the pressure dropping off is the main reason the government now require anti-scald technology on a shower valve. If the supply was proper, it might not happen, but there's a lot of older units out there and houses that may not be built well where it can and is a problem.