View Full Version : 1/2" or 3/4" supply for bathroom in far end of house?

10-23-2009, 05:35 AM
Hi all-
We are renovating our master bathroom. It is in a first-floor addition, probably about 45 feet away from the hot water heater and water supply inlet (whatever you call it- where it comes from the street). The bath was originally plumbed in 1990 with copper supply tubing running in the exterior walls (!) which froze last winter. We are re-routing with PEX installed under the floor and above the rigid insulation we are installing (crawl space below).

We will have two sinks, toilet, 5' soaking tub, and shower with regular showerhead plus handheld.

I have read the pros and cons of using 1/2" or 3/4". Given the distance from the hot water heater, I'm inclined to lean towards 1/2" to lessen the amount of cold water that needs to be flushed from the hot tap each time. ]

Also given our usage, it is rare to have both lav sinks on at once. However maybe the shower and toilet flush may happen at the same time.

I believe the existing supply lines are 1/2". It we stay with 1/2" we can simply switch to PEX where they exit the full basement, saving more work and materials. (If we go with 3/4" we will need to replace all the way back to the main supply.)

What would you do?


10-23-2009, 05:48 AM
I would run 3/4" to the bathroom and recirc the hot side.
I personally would not run PEX, I don't trust it yet.

Gary Swart
10-23-2009, 09:13 AM
Use copper and install a recirculation pump. You'll have instant hot water.

10-23-2009, 10:23 AM
Ooh, recirculating pump sounds expensive and complicated! This area is on an inaccessible crawl space for the last 30' of the run. (It's gutted now, but after it's closed up it is inaccessible: only 3-4" between the floor joists and the dirt.)

This area is an addition to the main house, which has a full basement. it was built in 1990 with NO clearance under the floor joists. The bathroom is at the far end.

The bathroom floor is ripped up for the last 15' so we can do whatever we want at that point. But my husband had wanted to use PEX because for 15' (between where the supply plumbing exits the old full basement and where the floor is torn up) it will have to remain IN THE EXTERIOR WALL. Ripping up the hardwood floor in that area is not an option. Next spring, we're going to be residing the house, so his plan was to leave the copper supply pipes in the walls AS IS this winter (and maybe have to run the faucets when it dips super low temps like we did last winter) and then next spring when we take the exterior siding off we can access that area from the outside, replace the copper with PEX (since it is less prone to failure when frozen) and wrap it well with foam insulation before closing it back up.

At the end of the house where the bathroom is, where the floor is already ripped up, he was also going to run PEX (easier) but do under the floor (above new insulation) rather than in the exterior walls as it is now.

Bad plan??

10-23-2009, 11:09 AM
What is above the ceiling? Any chance you could run your lines there?

One thing to keep in mind is that 1/2" PEX is not the same as 1/2" copper (pex has a smaller inside diameter).

I don't think you would want to supply a full bath (shower, 2 sinks, toilet) with a single 1/2" pex line.

10-23-2009, 12:12 PM
Recirc systems, there is one which attaches to the hot and cold at the furthest fixture (I don't have any experience with it but antoher member does and likes it) or the "standard" which is a pump, timer aquastat and check valve at the water heater and a 1/2" line from the hot line as close to the last fixture as possible. We design for the later type all the time.

10-23-2009, 04:33 PM
There are three generic ways to do hot water recirculation:

Gravity...you must run your pipes properly for this to work.
Recirculation with the pump at the furthest point (I use this at my condo)...hassle is you need an outlet and it takes up room under the sink. Easy to install, takes all of about 10-minutes if you have power to plug it in. I added an outlet in the vanity when I did the remodel, so it was easy.
Recirculation with the pump near the WH and thermostatic valves at at least one and possibly multiple locations. The small pump doesn't take much power on either of these, and when the water reaches the set temperature, the valve closes.
You should run a dedicated return line to the water heater if you can, otherwise, it pumps the hot water back along the cold water line. The thermostatic control limits how much, but you can end up with warm or even hot water in the cold line as well if you don't use a dedicated return line. The one I have has a user adjustable knob that sets the temperature. I've got it so it is warm so the cold line doesn't get really hot. The takeoff for the shower is further back, so it gets hot in about 5-seconds, whereas before I added this, it took almost a minute to get hot in the winter. A flush of the toilet usually clears it so I can then get some cold at the sink if I want, so it isn't a big deal.

I've read that some places are mandating recirculation systems to reduce water waste while people run the hot waiting for it to actually get warm.

I'd probably use 3/4" pex, then branch to 1/2" for the fixtures. Note, if you have a big tub or a multi-head shower, you may want to run 3/4" to it, but the other things will live fine with 1/2" to it (and the valves are less expensive than a 3/4" one).

10-24-2009, 07:46 AM
Recirc systems, there is one which attaches to the hot and cold at the furthest fixture (I don't have any experience with it but antoher member does and likes it) or the "standard" which is a pump, timer aquastat and check valve at the water heater and a 1/2" line from the hot line as close to the last fixture as possible. We design for the later type all the time.

My sister's place uses the cold line as the recirc return. A couple times, I've used the shower at the end of the line and been unable to regulate the temperature because both hot and cold were delivering scalding water :eek: Not a pleasant experience :mad:

10-24-2009, 08:29 AM
Regarding the various recirculating systems: the vanity we've chosen does not have any space for a pump there, so it sounds like the only way this would work is to put the pump near the HW heater, right? My husband is NOT going to be happy with me for suggesting he has to run a whole extra length of pipe! And, in fact we would not be able to do this now since we're not opening up the exterior wall until we re-do the siding next year, so we've no access at all to about 30' of the line. (unless we rerouted everything into the attic as suggested above.)

And back to the 1/2" vs. 3/4" dilemma... both pipe size and valve size...
I was looking at thermostatic valves and indeed there's a huge savings in sticking with the 1/2" valves. The web sites I was looking at were noncommital on which was better to use if your supply lines are 1/2". What do you think? If my shower has a standard showerhead and a handheld shower, can I get away with the 1/2" valve?

DH says the supply is definitely 1/2" currently. So we'll be coming off 1/2" unless we rip it all out all the way back to the inlet and heater, which is a LOT of extra work and materials that I think DH will balk at (as well as the access issues noted above)..........

Go ahead and offer arguments why this is really preferable and I will show them to him, if you guys feel strongly about it. :)

Otherwise, assuming we leave the 1/2" in place up to where we're starting the new runs, and assuming DH insists on using PEX, should we go to 3/4" PEX?

REgarding the attic space... yes it is accessible. But again it's unheated. Isn't that sort of 6-of-one-half-dozen-of-the-other compared to running the lines under the floor? And, wouldn't we lose some of the water pressure by forcing it up? I'd love to hear more about this if it's a viable option...

10-24-2009, 11:03 AM
A 1/2" line at nominal household pressure can flow around 5-6 gpm safely. So, since a normal showerhead is limited to a max of 2.5gpm, even two of them should probably be okay on a 1/2" line (note that the ID of pex is smaller than copper, so 3/4" pex is bigger than 1/2" copper ID, but not by all that much). If you were going t ouse this to fill a big soaking or whirlpool tub or have more heads running at once, you'd like a larger supply and valve. A 3/4" valve is overkill if you don't have 3/4" supply lines.

If you can run the lines in the attic UNDER the insulation, next to the ceiling, the added heat up there in the summer or cold in the winter won't be much worse than the normal room temperature at the ceiling.

The better recirculation systems use a valve to stop the recirculation once you have hot water at the sensor, so the return line (cold if not dedicated return) shouldn't get too hot if everything's working properly. As noted, mine sits entirely underneath the vanity, and I have it set to only run until I have warm at the sink...this means hot is available a few feet away, which is closer to the shower supply line branch. All I typically need at the sink is warm to wash my hands, and hot isn't that far away if it want it hotter, but I limit the amount of hot that gets pushed into the cold line. On the type I have, the pump turns on and off based on the thermostat. On the other types, the pump runs all the time and the water circulates based on the sensing valves. The advantage of this system is there isn't as much under the sink, and you can use more than one valve to keep multiple branchs warm that might be bypassed with the system I have. In theory, if the vanity backed up onto a closet, you could run the lines there, rather than having the thing sit under the vanity.

10-24-2009, 12:36 PM
This is very interesting. Thank you all so much for your thoughts! My husband and I were just discussing this. He thinks running the supply lines in the attic is a great idea. We REALLY want to get them out of the exterior walls. But we wondered if adding the additional lift and turns and stuff would reduce the water pressure? And about the summer temps, but it sounds like if we keep it under the insulation we'll be OK, huh?

Water pressure is a concern, this is a 1956 house and although we are on city water it does not seem to have great water pressure... not sure why exactly.

So it sounds like the thing to do would be to transition from the existing 1/2" copper supply lines to 3/4" PEX for most of the run, only branching down to 1/2" where fixtures tee off in the bathroom. Do you agree? We were wondering if that might cause some loss of water pressure though... we always though that you were not supposed to go from a smaller I.D. to a larger one because you'd lose water pressure. Is that not the case? We measured and the I.D. of the 3/4" PEX looks to be about 1/8" larger than the copper (~5/8" for the PEX).


10-24-2009, 09:43 PM
The pressure drop due to the "additional lift" is negligible in the attic, and zero at your tap -- whatever pressure you lose pushing the water into the attic will be regained when it drops back to the tap.

As for the low pressure overall - rent, borrow or buy a water pressure gauge that connects to a hose bib. Check the pressure at the bib at your service entrance, and again at a faucet at the other end of the house. Normal pressure is 50-100psi. If the pressure at the entrance is good but the other end of the house is low, you've got issues with your internal plumbing. If both are low, check that the shut off valves at the meter and at the service entrance are fully open. If you've got an old gate valve at the service entrance, they often fail to fully open and/or close after years of use. Replace it with a ball valve. If you have a pressure regulator at the service entrance it could certainly be your problem. If you've eliminated everything on your side of the meter as a possible cause, contact your water company.

10-25-2009, 05:01 AM
Thank you! We actually have a pressure gauge, which DH used when he was installing the new baseboard radiators.

Pressure at the service entrance is: 55#

We can't check pressure in the furthest point (master bath we're discussing here) because all fixtures are removed and capped off. But at the furthest one we can get to, it's: 55#

So this seems okay then??? I definitely notice a big drop if I am using garden hoses to water the lawn on two spigots at the same time. Perhaps that is just normal though?

10-25-2009, 06:57 AM
Your pressure may be fine but the volume of water may be an issue. Test the pressure again with one or two other taps open.

Do you know the size of your service line. A lot of older houses have undersized (by today's standards) service lines, also some older houses have galvanized piping which will corrode internally and restrict water volume.

10-25-2009, 08:14 AM
Your pressure may be fine but the volume of water may be an issue. Test the pressure again with one or two other taps open.

Orange beat me to it -- next question is water flow. Take a stop watch and a 5-gallon bucket. With the tap wide open, time how long it takes to fill the bucket from the faucet at the service entrance. Repeat at the faucet at the far end of the house. Finally, using two buckets, repeat filling both at the same time.

The back faucet *should* fill in 1:00 to 1:15 (4-5gpm, assuming it's supplied by a 1/2" line). The bucket at the service entrance should fill in a minute or less.

If the front bucket time is good, but the back is significantly slower, you've got restrictions in your in-house plumbing (test at other faucets to see if it's all over, or specific lines).

If the front bucket takes significantly more than a minute, or the two-bucket test significantly increases the time, you've got low volume into the house. Fist thing I'd check/replace is an old shutoff valve that's not fully opening, and verify that you've got at least a 3/4" line into the house (you could have good pressure/volume at the meter, but have an undersized line from the meter into the house).

10-25-2009, 12:09 PM
When all of the valves are closed, you'll have the same pressure everywhere in the house. Doesn't matter if that is through a soda straw or a fire hose. But, when you are using water, if you have soda straws, you won't get much water out. Old galvanized water lines can be equivalent to a soda straw, regardless of their outside diameter. This is where a larger line comes in handy - you'll get the volume you desire. You can't push a huge volume through small pipes without risking failures (erosion) and noise issues. The more volume you want, the larger the pipe needs to be. Keep in mind there are friction losses WHILE THE WATER IS FLOWING, too. A nice thing with pex made with home runs is that there aren't a bunch of fittings in-between to get it around corners...it's one smooth piece of pipe so the friction losses are lower.

10-27-2009, 09:16 AM
Thanks so much for all your input. We ran out of time to test the flow this weekend, but it's on the list for next Saturday!