On-Demand Hot Water Circulation

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Kabra

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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.

I think we're finally on to something here that might explain things. There seems to be a startup ramp that hasn't been part of the equation. Water doesn't go from 0 to 8gpm instantaneously and I'll describe what I see in this application.

Regarding expected pump performance you came up with an estimate of 8gpm (based on 6.75ft of head). I come up with about 6.5gpm (based on actual differential pressure for a single pump of 4psi * 2.31). Both of these flow points of course are on the "Max" curve. So we're pretty close. But here's the rub... I don't think I'm getting anywhere near that kind of flow and I think that startup ramp is why. It seems to take a lot longer that we realize.

With a single pump that longest loop of about 150' takes about 40 seconds to get hot water to the faucet. With about two gallons of water in 150' of 3/4 PEX you would think that your 8gpm or even my 6.5gpm would deliver hot water much faster than that. And we also know it's really just the hot water supply side of the circuit that needs purged plus whatever extra to make up for whatever is lost from cold pipes (basement is always in the low to mid 60s). In this sense the pump is seriously underperforming against expectations and again, I think it's because of that startup ramp.

When I flip the switch and observe the system I notice a few things that indicate this rather long startup ramp. First, based on the lights and sounds from the pump it seems to "think" a bit before it applies full power. These pumps have an ECM motor and maybe that's part of it. It takes a good 7 seconds before the light goes solid blue and it starts to sound like it's spinning full speed. Second, watching the pressures of the two gauges, they start to stabilize at about the 12-15 second mark. But then they continue to drift very slowly after that for another 15 to 20 seconds. Third, that velocity noise I hear in the PEX when two pumps are installed also took a while to ramp up all the way. I never timed it but my recollection is maybe around 10-15 seconds after hitting the switch.

You mentioned this is a very unusual application that you pro's just don't come across and I think this gets to the essence of it. Most application are long-distance runners where this application is a sprinter. Pump curves don't show the startup part of the cycle and plumbers certainly don't think in those terms.

I also think this explains why the extra head from the second pump improves hot water delivery performance so much. That additional head helps in a big way to get that water moving faster and sooner.

Almost from the start of this thread there's been a huge gap in what you've been telling me and what I actually experience with how long it takes to get hot water, and how much that second pump helps. Again, I think this startup ramp notion explains that too.

If I'm on the right track with this startup ramp notion there's just no practical way to determine what the right pump is for this application. It seems it becomes more of an art than a science and that changes everything including my getting the answers I was hoping for. So I think at this point I'll wait for any responses to see if we're on the same page or not. I feel kind of bad I dragged you all through this, especially John who put so much time and thought into it.
 

Kabra

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I spoke with with a Taco tech support rep this morning about my application and went through all the numbers. He confirmed that there is a startup ramp but it's just a few seconds and I'm over estimating it quite a bit.

He also confirmed all the math was correct and the differential pressure fully corroborates the pressure drop calculations. Differential pressure of both pumps is 8psi (19' of head) and that equates to 5gpm and 4.4fps in 3/4 PEX. 5gpm also correlates very well with the performance I'm seeing with two pumps which is about 20 seconds for fully hot water coming out the tap of the longest loop. About 15 seconds for the others.

He said there absolutely nothing wrong with putting those two pumps in series and it won't cause cavitation or any other problems. The noise I hear with both pumps running he confirms would be related to velocity but he's not surprised and not concerned about it. He feels it's perfectly reasonable and safe to use the two pumps in series and the velocity won't cause any erosion especially the way the system is used so infrequently.

I suspect not everyone agrees with the Taco Tech's assessment but he was extremely knowledgeable and it all makes perfect sense in my mind. So that's what I'm going forward with and I've already put the second pump back in place and enjoying the fast hot water.

I really appreciate all the input from everyone, especially John who I think I pushed close to his limits at times.

Thank you!
 

Fitter30

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Solenoids being non potable water work differently and can have different elastomers that change the taste of the water. Here's a valve it doesn't have a direct lift.
 

John Gayewski

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Did he say why you're not getting 8gpm (or near it) with one pump?

The taco guys are pretty good. I think you would be happier long term with a different system. Only time will really tell.


Did you set up a gauge between the pumps?

I estimate pump size semi regularly. I can't imagine why their curve would say one thing and in real life could be so much different, and the tech support guy says "oh yeah that's normal".
 
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Kabra

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Did he say why you're not getting 8gpm (or near it) with one pump?

The taco guys are pretty good. I think you would be happier long term with a different system. Only time will really tell.


Did you set up a gauge between the pumps?

I estimate pump size semi regularly. I can't imagine why their curve would say one thing and in real life could be so much different, and the tech support guy says "oh yeah that's normal".

Did he say why you're not getting 8gpm (or near it) with one pump?
I estimate pump size semi regularly. I can't imagine why their curve would say one thing and in real life could be so much different, and the tech support guy says "oh yeah that's normal".
I think I see why you're asking that. He seemed to be referring to pressure drop tables rather than the pump chart which I think you are. According the chart using the 6.75' of head that you estimate definitely shows around 8gpm. But the pressure drop table says 42' of head for 8gpm in 150' of 3/4 PEX. And it really does make intuitive sense there's no way that pump will push 8gpm through 3/4" PEX. There must be an assumption or something in those charts that's not stated and is misleading.

I think you would be happier long term with a different system. Only time will really tell.
Remember, I've lived with this for four years with a single pump (for each loop). I am thoroughly happy with it except it could be faster, and two pumps solves that. So as long as I'm not destroying my plumbing it's the perfect system for me.

Did you set up a gauge between the pumps?
I've wondered about the pressure between them as well. I have not done it only because I'm afraid of wearing out my push to connect fittings by taking them on and off so much. I'd like to understand this issue with the pump charts first and then maybe I'll reconsider putting a gauge between the pumps depending on what the outcome is.
 

Kabra

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What was wrong with the original four pump system?
Nothing was wrong, it worked great. I just always wished for faster hot water delivery. I realized I could reconfigure the system to work with a common pump(s) and get rid of all the check valves since I had added the solenoid valves as a modification. And because I had three extra pumps I decided why not try two of them together. So trying that second pump was the real motivation reconfigure the system and it paid off.
 

Kabra

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Solenoids being non potable water work differently and can have different elastomers that change the taste of the water. Here's a valve it doesn't have a direct lift.
Thanks for posting that. The point about the bacteria is something I hadn't thought about. Not sure it matters for hot water but I'll definitely think about it. Looks like the proper valves are around $100 each or a little more.
 

Fitter30

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Thanks for posting that. The point about the bacteria is something I hadn't thought about. Not sure it matters for hot water but I'll definitely think about it. Looks like the proper valves are around $100 each or a little more.
There is big difference between China and others.Go to their web site they have both portable and not it might be stainless.
 

John Gayewski

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I think I see why you're asking that. He seemed to be referring to pressure drop tables rather than the pump chart which I think you are. According the chart using the 6.75' of head that you estimate definitely shows around 8gpm. But the pressure drop table says 42' of head for 8gpm in 150' of 3/4 PEX. And it really does make intuitive sense there's no way that pump will push 8gpm through 3/4" PEX. There must be an assumption or something in those charts that's not stated and is misleading.


Remember, I've lived with this for four years with a single pump (for each loop). I am thoroughly happy with it except it could be faster, and two pumps solves that. So as long as I'm not destroying my plumbing it's the perfect system for me.


I've wondered about the pressure between them as well. I have not done it only because I'm afraid of wearing out my push to connect fittings by taking them on and off so much. I'd like to understand this issue with the pump charts first and then maybe I'll reconsider putting a gauge between the pumps depending on what the outcome is.
I'll have to look into this. It's not making much sense to me. Getting 8gpm through 3/4" pipe is pretty common. Not exactly sure where 42' of head is coming from.
 

John Gayewski

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Ironically I'm using Taco teaching tools for thy pump selection and numbers I find. So I think I'll call them to see where the disconnect is coming from.
 

Kabra

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Ironically I'm using Taco teaching tools for thy pump selection and numbers I find. So I think I'll call them to see where the disconnect is coming from.
I see what it is and it's just a simple oversight. It's not the chart but using an invalid head number. You need to know what GPM to target to determine approximate head using pressure drop tables, and then use that head with the chart. So for 8gpm in 150' of 3/4 PEX head is 42' which is of course way off the charts. The 6.75' of head that you were using pertains to just under 3gpm in 3/4 PEX.
 

John Gayewski

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I see what it is and it's just a simple oversight. It's not the chart but using an invalid head number. You need to know what GPM to target to determine approximate head using pressure drop tables, and then use that head with the chart. So for 8gpm in 150' of 3/4 PEX head is 42' which is of course way off the charts. The 6.75' of head that you were using pertains to just under 3gpm in 3/4 PEX.
Tacos pump sizing training gives step by step instructions on sizing pumps and using the multiplier I gave earlier for finding the head. It's what our instructor printed off in a packet of other info for sizing systems. I'm not around to snap a picture of it with taco's letterhead next to "step 1. Find the head multiply footage by 0.04 (.03l for pex).

Yes we target flow rate for heating depending on heat loss (in the case of hydronics). Then we find the head with the correct multiplier. Then find the pumps we need.

I can't think of a reason the head loss could be so much different for heating were still theoretically moving 8 (or whatever) gpm through 150' ft (or whatever) of pipe. I do understand why he used the pressure loss tables and I looked at them myself. I just dont understand how it's feasible to use two different methods for pump sizing if they are so far apart. This isn't just taco's training that specifies this method. It is many many other texts that use this method.
 

Kabra

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That factor of .03 assumes a gpm of 2.5gpm. You did make an adjustment for fittings but not for the higher gpm.

I got very tripped up with this too and that 2.5gpm for circulation pumps I found is overly engrained as an assumption. Taco has a pretty simple sizing calculator called “DHWR” that does the pressure drop calculation based on gpm and pipe type and length. It then suggests applicable pumps. I just found out about it from the Taco rep. In my case for 5gpm it calculated 21’ of head including fittings and valves and which I used the default values for. Here’s a link to that:

http://apps.tacocomfort.com/wizard_dhw.html
 

Kabra

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That factor of .03 assumes a gpm of 2.5gpm. You did make an adjustment for fittings but not for the higher gpm.

I got very tripped up with this too and that 2.5gpm for circulation pumps I found is overly engrained as an assumption. Taco has a pretty simple sizing calculator called “DHWR” that does the pressure drop calculation based on gpm and pipe type and length. It then suggests applicable pumps. I just found out about it from the Taco rep. In my case for 5gpm it calculated 21’ of head including fittings and valves and which I used the default values for. Here’s a link to that:

http://apps.tacocomfort.com/wizard_dhw.html
I made a mistake describing the results from the sizing calculator. I didn’t include fittings and valves. It’s 34’ of total head including that.

1707324580472.png
 
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John Gayewski

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That factor of .03 assumes a gpm of 2.5gpm. You did make an adjustment for fittings but not for the higher gpm.

I got very tripped up with this too and that 2.5gpm for circulation pumps I found is overly engrained as an assumption. Taco has a pretty simple sizing calculator called “DHWR” that does the pressure drop calculation based on gpm and pipe type and length. It then suggests applicable pumps. I just found out about it from the Taco rep. In my case for 5gpm it calculated 21’ of head including fittings and valves and which I used the default values for. Here’s a link to that:

http://apps.tacocomfort.com/wizard_dhw.html
Sorry that makes no sense. If I need to heat 70kbtu I need 7gpm (depending on other factors) you can't assume only 2.5 gpm when 7 is needed to heat. I'm gonna have to make some calls to some reps.
 
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Kabra

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Sorry that makes no sense. If I need to heat 70kbtu I need 7gpm (depending on other factors) you can't assume only 2.5 gpm when 7 is needed to heat. I'm gonna have to make some calls to some reps.

1" pipe or whatever gets to 7gpm. I was talking specifically about 3/4 PEX nd neglected to say that. But the point is the charts assume something about the pipe diameter. So for your .03 or .04 per foot of pipe, for that to apply you need the right size pipe.
 

Kabra

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The 4 loops 3/4" pex what are the lengths of each loop? Sol valves have 1 lb pressure drop @ 5 gpm 60 lbs
1 x 150 is the longest. 2 x 100 give or take, and 1 x about 50'. But each loop operates individually and more than one open at a time effectively never happens.
 
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