Question 1.) TLDR; is my design, overall, ok? (Pictured at bottom)

I was originally going to do a dedicated "race-track" style hot water recirc line, then I came across a this YT video (https://www.youtube.com/watch?v=s1UEdHNSBpw&t=281s) with the idea to make the track dip down ~5' to the fixture before coming back up to the main race track.

Question 2.) Will these 5' dips totaling 10' in length cause any issues with my pressure since they counter each other or with air trapping?

Question 3.) With my highest fixture being below the race track, will opening all the hot/cold valves/fixtures remove the air or will I have to add some sort of hook that T's up off the race-track and comes down to remove the air properly?

I plan on using PEX-B, a Taco 006e3 recirc pump on the lowest setting, and shark-bite pro-max fittings into the copper stub outs to the W/H and where I t-off to the 1/2" at the fixtures as not to interfere with the internal diameter that a traditional crimp fitting would propose. Sharkbite says these fittings are rated for behind the wall use*

Question 4.) It's my understanding that the pump is designed, when set correctly, to not go over the GPM that the pipe/length is rated for but that's if I use 3/4 through and through. Will a 1/2" return cause any issues with my pump? Do I stand to gain anything from the last 66' of my return line being 1/2"? I'm not looking to penny pinch. I'm looking for efficiency and will pay for it.

The aerial view is to scale, the side view below it is not and is only to illustrate the dips in the race-track to bring the hot water closer to the fixture. Below the side view are exact measurements from one fixture to the next at the point where I t-off to the actual fixture "colored circle". The red lines are 3/4" PEX-B and the orange lines are 1/2" PEX-B

For clarity the original plumbing is under our "lvp > gypcrete > subfloor" in the floor joists and is not practical to get to and luckily we have space/access to run the plumbing above "in areas where the pipe can be insulated/secured, and then dropped down to each fixture...


1 Answer 1


I thought about hot water recirculation but concluded it didn't make sense in most new builds. There are better cost efficient and environmental efficient options.

A faucet is 1.5 GPM = 0.025gps. 1/2" pex volume is 0.00961 gallons per foot. 3/8 pex volume is 0.00489 gallons per foot.

If your faucet is home run plumbed to your water tank and the wt is 100' away then it will take 38.4 seconds for hot from the tank water to reach your faucet. With 3/8" pex it will take 19 seconds. You'll have a little additional time as the heat from the tank water warms up the pipe on the way to your faucet but generally it isn't much. In general your hwt should be closer than 100' from your farthest faucet. 50' is probably the most you'd see in a 4000 sq ft 3 level house.

If you traditionally plumb the house and use tree/branch/twig then a recirculation pump can make sense as the volume in the line before getting to your faucet can be ridiculous.

If you have the ability to run pipe fresh then you centrally locate the hwt to minimize the pipe lengths and use home runs to minimize the volume of water needed to be dumped. On a 50' 1/2" pex run at 19 seconds of purge you dump half a gallon of water. The average cost of water is .15 cents a gallon so this will cost you .076 cents. If you purge 100 times it will cost you 76 cents on your water bill. I'd argue that dump half a gallon of water is exponentially more environmentally friendly than having a recirculation pump dumping heat (probably produced via fossil fuels at some point) into your walls which may also add to your cooling load in a cooling climate.

The heat loss cost to circulate hot water around your house even if you insulate your pipe is going to quickly exceed this. That ignores the cost to buy and run the pump. Keep in mind this is the worst case at 50' most of your pipe lengths should be shorter than this.

The downside to home run plumbing is that you are pulling a lot more pipe but pex pipe is fairly cheap and easy to run these days.

I wouldn't worry about using sharkbite max to avoid the loss in pipe diameters the bore holes in modern shower valves and faucets are already going to be smaller than the ID of the pex fittings. I like crimpex for water supply. It is cheap, reliable and easy to install.

I don't home run all my lines. I will do a 1/2" supply to the kitchen sink and then tee off that to the dishwasher. Some dishwashers will heat the water they use others won't. In the tee scenario you can run the hot for 19 seconds and then the dishwasher will only have the small amount of pipe between it and the kitchen sink before getting hot water. For a bathroom you can do the sink and shower off the one hot line since for the most part people aren't using the sink and shower at the same time.

I also insulate the 1/2" lines all the way up to the fixture rough in so once these lines charge the heat loss is minimal while using a shower or fixture and if someone happens to use the fixture after the first person then the water is likely still at or close to design temperature already.

Even in the case of a bathtub it doesn't make sense as you just turn the water to maximum hot and the water you'd normally purge just serves to mix with the piping hot water once that arrives such that your bath is the correct temperature when you have achieved the correct volume.

I suppose you could be in an area where water is 100x more expensive or you rely on a rain water cistern or well water and in that scenario a recirculation pump might make sense.

As far as the pump question goes, you should probably stick to the 3/4" pipe, decreasing the size of the pipe leads to greater friction losses and greater head pressures. Head pressure is one of the key constraints around pump design.

I like Risinger but waiting 19 seconds for hot water just doesn't seem like a huge deal when the heat loss cost of recirculating is so high. You can put timers on your recirculation pump or sensors so that it is only running while you are home and that might rejig the equation a little but the insulation around your pipe isn't going to be as good as your tank insulation and the surface area of your hot water supply pipes is going to be quite large.

  • I've read about using 3/8" home runs and I keep seeing conflicting information on that. I'm also not sure if TFU converts to GPM. Or how to convert GPM to GPF (Gallons/ft) and back. My home run lines would be as follows WH > BATH/SHOWER (4 TFU) 25' -- WH > BATHROOM LAV (1 TFU) 25' -- WH > KITCHEN SINK/DISHWASHER (1.5 TFU) 40' From what I've read in Zurn PDF install guides I'd lose about 10psi on the 40' run for 2GPM requirements. I've also read that 3/8" wouldn't provide enough "flow" for the tub fill.
    – Larry Lupe
    Mar 7 at 12:43
  • According to TACO, per the 006e3 installation manual, there are three different settings: low, medium, and high. They say "low" is for 200' and under and when I called said the low setting would be fine for 100' and that I could turn it closer if not all the way counter clockwise if I saw fit. Their manual states under "setting selection" their recommendation is based on 3/4" supply and 1/2" return. What I'm trying to do is reduce to 1/2" before the return. 3/4" 20' to TUB/SHOWER combo > 3/4" 20' to BATHROOM LAV > 1/2" 26' to KITCHEN SINK/DISHWASHER > THEN 1/2" 40' RETURN
    – Larry Lupe
    Mar 7 at 12:53
  • I know 1/2" is more than enough to provide flow to kitchen sink/dishwasher but was curious if you could reduce before the return. In dreamland would like to run 3/8 to everything. That would be awesome.
    – Larry Lupe
    Mar 7 at 12:55
  • You increase friction losses and head when you decrease the size of the pipe. I expect you'd be fine to run 1/2" returns instead of the 3/4". I've seen the recirc for a 3 story building with 50+ units running a 1/2" copper line for the recirc. I wouldn't do 3/8 for tub fill - I run 3/4" line up to the tub. TFU is total fixture unit isn't it? You can get the chart of volume per foot of the various pex which gives you volume per foot. A 1/2 line will give ~14GPM at residential pressures. You typically don't have to worry about pressure or GPM in normal scenarios. Mar 7 at 17:11
  • resources.hy-techroof.com/blog/… If you are starting at 14GPM with a 1/2" pipe then you don't really have to worry if you'll have the 1.5 GPM ( actually .75GPM since typically mixed with 2x 1/2" lines ) at the fixture. Mar 7 at 17:16

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