On-Demand Hot Water Circulation

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Chucky_ott

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I agree with John that in a closed loop system, pumps in series will not have the effect you think. As he explains, the water has nowhere to go except back to the first pump. The system will get to equilibrium pretty fast.

That said, as soon as you open a faucet, it is no longer a closed loop. In this case, you might see a benefit from pumps in series as you have observed.
 

Fitter30

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Pumps in series double the head gpm stays the same.
Solenoid valves brand, model and pipe size pipe?
 

John Gayewski

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Pumps in series double the head gpm stays the same.
Solenoid valves brand, model and pipe size pipe?
You gotta keep reading. Effeciancy goes down as move away from the middle of the curve. Which means the second pump is doing nothing.

In world where such simple statements where actually true then one could bolt thirty tiny little circulators together and pump anything.

The only time I've ever seen pumps in series in a functioning professionally designed system is when the run of piping is moving around a very very large footprint (like a school). Once you've traveled 3 or 5 hundred feet then another circulator is added to travel the next 500 ft.

That's becuse that is where the pump has moved far enough away from the effeciancy standard then another is added.
 

John Gayewski

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I'm not doing a good job of explaining this.

My house is cold when it gets cold out. My heater isn't keeping up. Should I light my house on fire? It's very inefficient to do that.

My pump can olny pump 7gpm at 5ft of head. Should I add more head to the pump?

No.

Why? I can add two pumps together and get more head.

Because the secund pump is only getting what the first can give it. If you pump faster than that (that pump's max at the system's head) your adding friction, heat, head, velocity, electricity. Your only moving so many marbles they don't stretch.

You can pump more head and get more flow but not for double the electricity. Probably more like 3 times the electricity and suet wear.

Add a pump that is sized for the sytem and unbolt the pumps. If you can't be happy with the system's max then run longer with the correct pump. Your system won't burn down.

I'm sorry I'm not doing a good job of mapping this out.
 

Fitter30

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Mechanical engineer that works in the hvac could figure this out. Never have I've seen solenoid valves and check valves together. Every device that is piped in a system is a pressure drop including 90°s and the size of the pipe. You have spent more on pumps, check valves and solenoid valves than what could be your payback. Sorry i don't understand reinventing the wheel with a return line. Most people want hot water at certain times 5 am to 7 am and 5 pm to 10pm, 7 hours a day. Hot button going to use a sink or shower turn on a light switch on ,pump comes on, use the toilet need the shower ,sink leave light goes off pump turns off. Why aren't the return lines insulated if your looking for being energy efficient?
 

Kabra

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I agree with John that in a closed loop system, pumps in series will not have the effect you think. As he explains, the water has nowhere to go except back to the first pump. The system will get to equilibrium pretty fast.

That said, as soon as you open a faucet, it is no longer a closed loop. In this case, you might see a benefit from pumps in series as you have observed.
I am definitely not qualified to debate this with you and you and John could well be correct that I'm destroying the pumps. Two points here.

1. Somehow or another I am getting the desired effect from the two pumps in series! Hot water on demand in 15 - 20 seconds. That is indisputable because I experience it numerous times per day and it's darn near twice as fast as it's been the last four years since I built the system.

2. I really don't care a lick if I am ruining the pumps and I think this thread has gotten far too hung up on that. I'll get whatever pump makes sense to achieve the performance I am getting now if I can do it without ruining the plumbing. And that's been the objective of this thread from the beginning.

So my sole concern and what I've been trying to get an answer to is if I can get the performance I have now and not ruin my plumbing? It seems the minute I mention recirculation and 3/4" pex up pops a red flag that says thou shalt not have flow rates higher than 2.5GPM. But for this application where the circulation system runs for just a few minutes a day I can violate that limitation but I don't want to be stupid about it and ruin my plumbing.

So what I would really like help with for starters is determining what the flow rate is with this current two pumps in series setup. With that I would know what I'm shooting for and whether it's "off the charts" or not which seems to have been the assumption from the get go but nobody but me has done the math and I don't know if I did it right.

What I came up with if it's correct is about 5gpm based on differential pressure of 8psi and the pressure drop of 150' of 3/4" PEX. Pump issue aside, is 5gpm really going to ruin my plumbing when running for a few minutes per day?
 

Kabra

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Pumps in series double the head gpm stays the same.
Solenoid valves brand, model and pipe size pipe?
U.S. Solid 3/4' 110v Normally Closed. Pex is 3/4"

https://www.amazon.com/gp/product/B007N0J98E/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
 

John Gayewski

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If 5gpm is right (which I don't think it is) you would be within range at about 4.4 ft per second.

But your pump curve would say your wrong.

Which setting are these pumps on?

We're out of range for normal here so asking a group who doesn't do this kind of stuff (yet still has happy customers) means you might not get the answer you want.

You could have on demand hot water circulation with an occupancy sensor and half of the components and none of the maintenence.

No one would do this with such a small amount of head. Your only overcoming 150' of resistance.
 
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Kabra

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Mechanical engineer that works in the hvac could figure this out. Never have I've seen solenoid valves and check valves together. Every device that is piped in a system is a pressure drop including 90°s and the size of the pipe. You have spent more on pumps, check valves and solenoid valves than what could be your payback. Sorry i don't understand reinventing the wheel with a return line. Most people want hot water at certain times 5 am to 7 am and 5 pm to 10pm, 7 hours a day. Hot button going to use a sink or shower turn on a light switch on ,pump comes on, use the toilet need the shower ,sink leave light goes off pump turns off. Why aren't the return lines insulated if your looking for being energy efficient?
Good catch. Note the pictures are the old design and the current revised design where the check valves are gone. The solenoid valves came after the system was built because of a sporadic problem with water temperature. So yes I could have removed the check valves then but I didn't. What lead to the redesign is that I realized with the solenoid valves I could use a common motor rather than one for each zone. Then I thought why not try two motors and while making that change I removed the check valves.

Your assumption that I'm most people is not a good one. I live by myself most of the time and I'm retired and my schedule is whatever the hell I want it to be. It's a great life but schedules don't apply. I just want hot water fast when I want it. In fact the most used hot water faucet in the house is the kitchen faucet and that one is definitely not on a schedule. The overriding objective as I said in my other post is fast hot water without running it 24/7 which made no sense $$ wise and I'd rather not put that load on my water heater. FYI the pumps were around $200 each, solenoid valves $30, check valves $15.

Why in the world would I insulate the return when I use it on demand? This is really more of a hot water line purging application than it is hot water recirculation. So the return lines see very little "hot" water. I do have the hot water lines insulated.
 

Kabra

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If 5gpm is right (which I don't think it is) you would be within range at about 4.4 ft per second.

But your pump curve would say your wrong.

Which setting are these pumps on?

We're out of range for normal here so asking a group who doesn't do this kind of stuff (yet still has happy customers) means you might not get the answer you want.

You could have on demand hot water circulation with an occupancy sensor and half of the components and none of the maintenence.

No one would do this with such a small amount of head. Your only overcoming 150' of resistance.
John, here's some new differential pressure information below. But to answer your questions, Pumps are on MAX setting. Your comment about pro's not doing this sort of thing was why I did it myself. And one pump or two pumps I've VERY HAPPY that I did.

You say nobody would do this but as I've said numerous times is that it works and it works well! Am I ruining my pumps? But if I am I'll get the right one so I won't ruin it. Am I ruining my plumbing? You just answered that about the velocity in 3/4 for 5gpm is not off the charts. Is my 5gpm calculation correct? I have no idea and wish someone could help with a more confidant estimate.

Here's new information on differential pressure with one pump vs. two. I think this supports the notion that two pumps in series doubles the head and as I say it nearly doubles the performance.

IMG_1546.jpeg
IMG_1545.jpeg
 

John Gayewski

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Let's think of this a different way. 150' of 3/4" pex only has 2.6 gal of water in it.

One pump on high (turned all the way up) with 5ft of head can move 9gpm.

9 gallon in sixty seconds is 1.5 gallons in ten seconds. That should get you to your sink.

So if your right your torching your piping and your getting way more than 5 gpm more like 18 gpm. Piping is getting hammered. Pumps hammered. Noise is velocity noise.

So you say I'm only using it a little bit. Does it hurt to run sandpaper through your piping? Even if you only do it five times a day? Yes.
 

John Gayewski

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The head goes up as you add speed so the 0.03 number I gave you only applies at normal velocities. That is a design number used for head at speeds below what is normal design.

You would max the speed for pex with one pump turned all the way up.
 

John Gayewski

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We insulate the lines. All of them to minimize heat loss. That way you could use an aqua stat and get hot water any time you want it without losing all of the heat. Your pump would cycle less and not be ruined. Or you could just leave it on all the time and get the longest life possible out of it. And if it were insulated correctly you would not lose that much heat. Or since you live in a climate that gets cold and has a long heating season the heat is only wasted when it's above 80 outside.
 

Kabra

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Those solenoid valves are not for potable water and most likely don't meet the lead free standards.
Thanks and I'm well aware of that. The hot water which is also softened doesn't get consumed in this house. I have a pot filler faucet supplied with city water for that.
 

Kabra

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Let's think of this a different way. 150' of 3/4" pex only has 2.6 gal of water in it.

One pump on high (turned all the way up) with 5ft of head can move 9gpm.

9 gallon in sixty seconds is 1.5 gallons in ten seconds. That should get you to your sink.

So if your right your torching your piping and your getting way more than 5 gpm more like 18 gpm. Piping is getting hammered. Pumps hammered. Noise is velocity noise.

So you say I'm only using it a little bit. Does it hurt to run sandpaper through your piping? Even if you only do it five times a day? Yes.
Where do you come up with 5' of head? Doesn't the differential pressures (4psi with one pump and 8psi with two pumps) say otherwise? My estimates based on these differential pressures are about 9' of head and 20' of head for one and two pumps. Based on that I estimate about 3.5GPM for one pump and a little over 5gpm for two pumps.

If my estimates are wrong could you please point out why?

Also as I mentioned earlier, the performance I'm seeing with two pumps correlates reasonably with 5gpm but definitely not 9gpm.
 

Kabra

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The head goes up as you add speed so the 0.03 number I gave you only applies at normal velocities. That is a design number used for head at speeds below what is normal design.

You would max the speed for pex with one pump turned all the way up.
Yes, I figured out a while ago the .03 made no sense. My current estimate is .0525 with the two pumps.

If I were maxing the speed for pex with one pump turned all the way up I would have hot water faster than I dreamed. But that's not happening so I'm thinking you're making an incorrect assumption somewhere. I'm guessing from your other post that your 5" of head is the culprit. Doesn't the differential pressure clearly show 5' of head isn't realistic?
 

John Gayewski

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Yes, I figured out a while ago the .03 made no sense. My current estimate is .0525 with the two pumps.

If I were maxing the speed for pex with one pump turned all the way up I would have hot water faster than I dreamed. But that's not happening so I'm thinking you're making an incorrect assumptio
Screenshot_20240204-193025_Drive.jpg
n somewhere. I'm guessing from your other post that your 5" of head is the culprit. Doesn't the differential pressure clearly show 5' of head isn't realistic?
You have a lot of components on your system. Even with a safe multiplier of 1.5 your still only at 6.75 ft of head.

If this is your pump just follow the line over where 6ft (or 7) of head intersects with the high setting.

Unless the components on your system are causing a whole lot more resistance than normal design accommodations would account for your should be gulping a lot of water with one pump. Makes me think the second pump is causing harm and creating more head than can be estimated with normal operations.

Head is how much resistance your system provides. 150' of pex should be minimal. Since you have all of those components I recently added a 1.5 multiplier. 150x 1.5x 0.03 (based on pex's resistance per 100',copper we use 0.04 iron pipe 0.06)

That means your system is causing 6.75 ft of head. The designers of your pump say it's designed to do about 8gpm in your system.
 
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