Recirculation pump sizing

[dstutz]

I am planning on replacing the polybutylene pipe in my house with PEX-A basically using Uponor's "Structured Plumbing" recommendations. I'd like to add an on-demand recirculation system while re-piping. I have a ranch house with everything on one level, the pipes run in the crawlspace under the joists (hot and cold are fully insulated and I plan to continue that). The main hot water trunk and the return will be 3/4" using flow-through manifolds. I was planning on using a Taco 006-ST4 pump triggered via timer-relays (ie run for X seconds after switch is pressed then turn off, where X is the time it takes to get hot water to the T/manifold serving the fixture). I'm on a well, the pressure switch runs between 30 and 60psi.

006-ST4 pump specs:
HP: 1/40
Flow Range: 0 – 11 GPM
Head Range: 0 – 9.5 Feet

I've calculated very roughly/generously 75' of pipe to get to my farthest fixture (master bath, which of course is allllll the way on the other end of the house from the WH). Once the line goes down into the crawlspace I plan on only using 90 bend supports and no elbows in basically the whole hot supply/return loop (Actually, I lied. I think I have to use one T (to house a vacuum relief valve) which will function as an elbow for the water flow back into the WH), all fixtures will be T'd off the main 3/4" line. The return line should be ~60-70'.

I'm a little out of my element on calculating "head", pressure loss, friction loss and whether this pump is "too much"or "not enough" for my situation. I've looked at the Uponor pressure loss tables and Hydronic friction loss tables and I'm just scratching my head.
In the Uponor pipe sizing document for domestic hot water piping it recommends:
Maximum velocity of 12 ft./sec. through pipe
Recommended velocity of 8 ft./sec. through pipe

Another document details for the 3/4" fittings:
6.9gpm @ 8ft/sec for brass (6.8gpm plastic)
10.4gpm @ 12ft/sec for brass (10.2gpm plastic)

Since the pump can do 11gpm is this a problem or no, because the pump won't actually move water that fast due to the ~140' of pipe in the whole loop and friction losses? On the contrary, is this pump powerful enough? Should I go bigger (would a 007-SF5-IFC be too much)?
I recently came upon the D'MAND Kontrols® Systems by ACT C3-100 which uses a 0-18GPM pump using the same 3/4" connections....so am I overthinking this/worrying about nothing?

Thanks,
Dave

 

[hj]

For a recirculation system the pump ONLY has to move the water fast enough to keep the pipe warm. DO NOT use a "large" pump since that will create excessive velocity through the piping.

 

[Sponsor]

 

[Reach4]

I'd like to add an on-demand recirculation system while re-piping.

For on-demand, I think you want to move recirc water quickly.

 

[dstutz]

Yes, this is an on-demand system. I have 2 bathrooms at the back of the house and the kitchen sink. I'd probably wire both bathrooms to one timer (they're side by side) with a switch in each and the kitchen to a shorter timer since it's closer to the WH. I figured the pump would only have for run for <10 secs at a time a few times a day.

 

[Jadnashua]

To move the water 70' and stay below the maximum flow rate for the tubing involved, it WILL take longer than a few seconds. Plus, depending on how long the pipes have had to cool off, you have to warm the pipe up as well. I think you'll find that a system on a timer with a small pump will be much more convenient than remembering to push a button to start things up. Make sure to insulate both the hot supply and return lines. High velocity will start to become noisy and the power used to push it that fast will also go up. You could use an occupant sensor to turn the thing on as soon as you enter a room, but you'd need multiple ones to cover each area and the wiring could get a bit involved. That would eliminate the need to remember to push a switch to activate it.

Because the water is flowing around a loop, the water falling back helps to 'pull' the water forward, so the pump does not need to be very large. You only need a trickle to keep the water hot. The system I use turns the pump on/off and uses an aquastat to determine when the water is sufficient (it's adjustable on this model). The first time it comes on in the morning with the timer, it might run 2-minutes. After that, it might run 30-seconds or less about every 15-minutes to introduces some hot into the lines that have cooled off enough to trigger the aquastat. There are engineered systems out there you can use as examples if you want to roll your own...they've already done the engineering for you. They all use a small pump made either out of bronze or stainless steel which is required on potable water verses one designed for hydronic heating purposes.

 

[Reach4]

I think at about 7 gpm, you will hit your 10 second estimate or better.
At 5 gpm, maybe 14 seconds... which would be better for a 1/2 inch pex return line.

https://www.pexuniverse.com/pex-tubing-technical-specs has some info you might find useful including Volume in gal/100ft. You probably already had that data.

 

[dstutz]

I think you'll find that a system on a timer with a small pump will be much more convenient than remembering to push a button to start things up. Make sure to insulate both the hot supply and return lines. High velocity will start to become noisy and the power used to push it that fast will also go up.

I'm more concerned with efficiency than forgetting to press a button. A demand-based system is more efficient than one running ~12-14hrs a day. All of my hot and cold lines are already insulated (all running in crawlspace) and after the new work is done they will all be insulated including the return line. I feel the power used to heat the water for the system running the slow pump 12-14hrs a day would outweigh the power used to heat a lesser amount a few times a day and like 1-2 minutes run-time of a more powerful pump. Is this assumption incorrect?

At 5 gpm, maybe 14 seconds... which would be better for a 1/2 inch pex return line.

I am planning a full 3/4" recirc loop. 1.86gal/100' of tubing, so 75' should be 1.4gal to get to the farthest fixture. 1.4gal @10gpm = 8.4 seconds.

I guess the simpler question to answer ignoring all my math is:

Is it OK to run a powerful pump very occasionally through these lines?

I guess answering my own question is the existence of these systems:
https://www.plumbingsupply.com/images/act-dmand-kontrols-system-c3-100-specs.png <---this thing is 0-18gpm and looks to be 1/6HP to be hooked up to 3/4" lines and says "For pipe runs up to 120 lineal ft." (Does that include the return line?)

 

[Reach4]

Is it OK to run a powerful pump very occasionally through these lines?

Yes. It will be really nice.

I don't know what that graph you pointed to means. If it says 100 lineal feet, I would think that would mean 50 out and 50 back. But maybe you can find different literature or ask the maker.

 

[dstutz]

Awesome...Thank you.

 

[Jadnashua]

A small pump, running periodically, may end up more economical. A large pump, running fast can result in water flow noise you might find objectionable, especially if it happened in the middle of the night. The Copper Institute calls for a maximum flow rate on hot copper of 5 fps.

Many the systems do not have flow constantly if the pipes are well insulated. They do it either by a temperature controlled valve, or a switch to turn the pump off. There's a reason why all of the packaged systems use a small pump.

 

[dstutz]

@jadnashua, I'm not trying to argue...trying to understand. These systems don't seem to use small pumps: https://www.plumbingsupply.com/on-demand-recirculating-pumps.html
They're 3 speed pumps and the max power of the smallest is 1/8-1/9 HP, it can do 18 gpm.

I'm actually considering getting a Grundfos ALPHA2 15-55SF pump now. It's variable speed so I can slow it down or speed it up (or let it figure it out).

 

[Jadnashua]

The actual gpm of a pump will depend on how long the line is, it's diameter, and how many elbows there are, and thus, the effective head. FWIW, on a 3/4" copper line, at a maximum velocity recommended by the industry institute that represents the copper tubing manufacturers is 5fps. That equates to 8gpm on a 3/4" line. Yes, you can exceed that, but there are ramifications, some constant, some long term. The constant one is the potential for flow noise. The long-term one is literal internal wear on the pipes. I think you said you had 70', so at 5fps, that's 14-seconds, but you'll need a little longer at the furthest point to warm up the pipes. If I remember properly, a HP=768W, mine uses less than 20Whr if it ran constantly. Given that mine runs all of about 4-minutes/hour, that's less than 2W/hour. How much energy you lose in the pipes being hot will depend on how well insulated they are and how hot your WH is, plus how warm the space is the pipes run through.

A constant flow system may only need a 1/10th gpm or maybe even less, and can use a very small pump. The one that I have that works for me is a 1/40HP motor. If your pipes are run to best advantage, you may get the whole thing to work entirely via convection, and not need a pump at all. Mine runs all of about 4-minutes/hour and I have it on a timer so it only runs when I'm likely to need water that's hot. While I can force it on anytime, generally, that's not an issue. Without the pump on, it takes over a minute (closer to two) to get hot water at the furthest sink. With it, you just have to purge the supply hose, or almost instantaneous. On the system I have (it's been in for nearly 12-years now), it has an adjustable water sensor temperature control. I have mine set so the furthest sink is warm, but hot is not far away. The rest of the fixtures in the house are closer to the WH, and are hot, less their local branch, which purges quickly. You could turn the pump on with an occupancy sensor, or a timed relay control, depending on how quickly you want the hot water, it could take some time. But say, in a bathroom where you're using the toilet, you'd probably have plenty of time before you wanted hot/warm water to wash your hands, that would be okay. An occupancy sensor in say the kitchen may not be the best since you may be just cooking, and not need hot water instantly.

You don't need a lot of power to move water in a closed loop.

http://readytemp.com/Specs.pdf

 

[Jadziedzic]

Look into Bell & Gossett ecocirc series recirculating pumps - they can be configured with thermostat and/or timer, speed control (I think), and use very little electricity. If you've insulated all the pipes the cost to run is very modest.