Need help with low flow through radiant floor loops

softieroberto

Hello, I had a question for the group. I'm getting low flow rates through my Uponor EP manifold, and I'm trying to understand why. Here's my setup from boiler through the system with 1" copper throughout:

Cast iron boiler > caleffi air/dirt separator > Uponor thermostatic valve > Grundfos Magna3 pump > Uponor EP manifold > 9 floor loops > high-flow check valve > back to boiler.

There's also a loop for my cast iron radiators, but that's not in use at the moment and I've closed the valves.

Floor loops are mostly 3/8" Uponor pex and around 200 feet. I have some 1/2" pex as well and shorter 3/8" pex loops as well that I balance at the manifold. There's about 50 feet of 1" copper distribution pipe with about ten long-turn elbows.

My grundfos pump can put out 32 ft of head on "constant pressure" mode, or 42 ft of head in max mode.

In the 200 ft 3/8" loop, I'm getting only about 0.5 gpm. That's only about 9 feet of head in that loop at 110 degrees of 100% water, according to Uponor's manual (200 x 0.04648). On "max" mode at 42 ft of head I only get like .1 GPM more.

Can it possibly be right that there's 23 feet of head elsewhere in the system? Just eye-balling it, do these numbers seem right? If not, what can I do to troubleshoot?

I've tried purging the loops multiple times — no change. I've tried closing all valves at the manifold except for a short 100 foot 1/2" loop, and the manifold reads 1 gpm flow (it doesn't go higher), but the pump tells me the flow is too low to measure.

Any ideas welcome!

Jamie Hall

Keep in mind that those head figures for the pump are what are called the shutoff head — that is the pressure the pump will create at no flow. As soon as flow starts, the head will drop off rather sharply. You need to look at the actual flow vs. head curve for the pumps to find out where the pump is actually operating

GroundUp

What are the actual model numbers of the circulator and "thermostatic valve"? How are you calculating head loss? The flow rates you've provided are about what can be expected from what you have- what gives you the impression that they are low? Thermostatic mixing valves tend to have a pretty high resistance to flow so depending on the cV rating of whatever you have and the actual performance curve of your circulator, this does not seem "low" at all. Also, whatever flow numbers you're seeing on the circulator display should be disregarded. They are a semi-educated guess at best (based on pressure differential through the volute) and are almost never accurate. The flowmeters will be much more accurate.

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808425#Comment_1808425

Thanks for your comment! The Cv of the thermostatic valve is 4.0. Here's the link to the product.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Uponor-A5402112-Product-Overview.pdf

The pump is the Grundfos Alpha 3 32-120 [edit Magna3]. The reason I think the flow rate of .5 is low is explained in the calculations above. I'd think with 32 ft of head that pump would be able to pump through at a higher flow rate (measured at the manifold).

Jamie Hall

If it developed 32 feet of head at that flow rate, it could, But as I said above, at 32 feet of head it isn't pumping any flow at all. That's called shutoff. You need to have both the actual head developed and the flow to find out how the pump is operating — or, in specifying, you need to have the required head and flow and pick a pump which can do that.

GroundUp

Is it a 32-120 or is it an Alpha? They don't make an Alpha3 32-120 from what I can see. Just the 200ft loop of 3/8" PEX along with the 4.0 cV of the valve at .5 GPM is approximately 16ft of head (not counting any other fittings or manifold restrictions) so even the UP or Magna 32-120 (max head of 24ft- not 32- at 0 flow) would be right on par. Half a GPM in that loop is 1.67 FPS velocity already, which is completely normal. Is it heating the space adequately? What is the temperature delta?

HomerJSmith

I don't use thermostatic mixing valves in your situation, even tho they do work. I prefer using a Taco I-series set point mixing valve with sensor to regulate temp, which is more costly of course. It has a much, much higher CV.

If you aren't getting flow, you have too much resistance (restrictions) for the pump to overcome. Your thermostatic mixing valve may not providing the flow and creating high resistance because the temp differences or pressure differences between the cold and hot ports (just guessing).

Also, even tho the temp output isn't much different between 3/8" and 1/2" pex, the pressure loss between the two is considerable. High resistance = less flow all thing being equal.

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808438#Comment_1808438

I see what you mean. Based on the pump curves, it should be able to generate 32 ft of head at 10+ gpm. https://product-selection.grundfos.com/us/products/magna/magna3/magna3-32-120-f-97924259?pumpsystemid=2414387493&tab=variant-curves

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808447#Comment_1808447

They do make an Alpha 3 32-120 [edit Magna3], which can generate 32 ft of head at 10+ gpm in constant pressure mode, or 40 ft of head at 10+ gpm in max mode. It's hard for me to imagine everything else other than the 3/8" pex and thermostatic valve generating 16ft of head in resistance.

You're right to ask about the practical effects. I'm seeing about 12-15 degree deltaT, which isn't bad. And I do think it can heat the space adequately. So it's somewhat theoretical, but I do want to get to the bottom of it.

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808449#Comment_1808449

Yeah, agree it seems like I have more resistance that I was expecting. If I'm adding things up, still seems like lots of room to pump at a higher GPM.

200 ft 3/8" pex at 0.5gpm and 110 degree water = 9.3 ft

Manifold valves at Cv 1.35 and .5 gpm = 0.3 ft.

Thermostatic valve at cv 4 and 6 gpm = 5.2 ft

100 ft of 1"copper = 1.6 ft (this is way more than what I have, even including fittings)

Cast iron boiler = ?? assume 5?

Even with all this, I'm only getting to about 20 feet of head. I should have about 12 ft of head left. Where's the rest of it going?

softieroberto

In case it's relevant, if I put the pump on "max" mode, it generate 40 ft of head, but total GPM across all 9 loops increases by only about 1 gpm.

hot_rod

basically the pump within seconds finds hydraulic equilibrium. The pump will move the flow that the head of the circuit allows.

You can calculate that, develop a system curve, then lay it on the pump curve and define the operating point.

Or measure the pressure difference across the pump and define actual flow.

Don’t bet the farm on the readouts on ECM circulators, they are usually calculated based on algorithms Some Magnas do have flow sensors however.

Is that Alpha 3 a Euro model ? 230V 50 cycle by chance?

Info on how to do that here

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_16_NA_Circulation%20in%20hydronic%20systems.pdf

Any chance there is a kink or obstruction in the loop?

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808488#Comment_1808488

Thanks Hot Rod. I was very careful with installing the loops, and I have several loops around 200 ft, all with about 0.5 gpm, so it's unlikely they all have a kink. I don't believe it's a Euro model. It works with 120 volts.

Any other thoughts? Given the amount of head this pump can generate, the operating point should be much higher than what I'm seeing. And I'm not just relying on the pump reading — I'm measuring this mainly based on the flow rate sensors on the Uponor manifold.

I'm going to switch out the QS-style compression fittings on the manifold connection, which are pretty narrow, with propex style fittings, which have a slightly bigger inner diameter. I know this probably won't move the needle much, but I'm going to try it.

Any other suggestions welcome. Maybe the check valves I installed, which I'm now realizing weren't necessary. They're very high flow, though, at 30 Cv.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Caleffi-NA51369-Product%20Overview.pdf.

I suppose it could also be some obstruction in my 100 year old cast iron boiler. I'm going to switch that out in a year or so for a Chilltrix.

Jamie Hall

As far as I can make out, you still haven't compared the flow rate and pressure gain through the pump to the pump's operating curve. You're still thinking about the pump's shutoff head, which isn't relevant. I'm going to bet that when you do you'll find that the pump is doing exactly what it is meant to do.

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808510#Comment_1808510

Thanks for sticking with me this far! I think I have, but it's entirely possible I'm misunderstanding. You see my estimate of head loss based on the current flow rate above — about 20 feet of head.

This is a beefy pump. Based on the curve below, which is the max setting, at 20 ft of head I'm well below the curve. The "constant pressure" mode that I have it in sets the head at a constant 32 ft, but even then I'm well below the curve. When I put it in max mode and head pressure is at 40 ft of head, I get about 1 more GPM — so about 7 GPM total — across all the loops.

I recognize that I haven't plotted my duty curve because I don't have the software to do that, but I think I have enough information to tell me that something is off. But let me know if I'm missing something.

https://product-selection.grundfos.com/us/products/magna/magna3/magna3-32-120-f-97924259?pumpsystemid=2428174604&tab=variant-curves

hot_rod

.3 gpm is what the RadPad calculator shows for 200’ 3/8” loops. You shouldn’t need higher flows unless you want a tighter delta or have a high load area? As you increase flow you also increase the velocity, stay within the tube manufacturers recommended fos , feet per second.

The free calculator at PPI, plastic pipe institute is handy for calculating pressure drop and velocities

Section 4&5 of that Idronics 16 show you how to develop a system curve and use it, no software required

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hot_rod

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softieroberto

https://forum.heatinghelp.com/discussion/comment/1808532#Comment_1808532

Makes sense, thank you again. Agree that the existing flow is probably sufficient for my purposes, but it's still odd to me that I'm so far below the pump's curve and can't increase flow rate at all. The operating point should have a much higher GPM. So there's some significant head loss in my system that's not accounted for, and I have no idea what the source of it is.

GroundUp

So you keep saying it's an Alpha, yet are linking the curve to a Magna (which has 24ft shutoff head, according to Grundfos specs). Which is it? Also, the cV rating is in PSI not ft/hd so you need to multiply your cV numbers by 2.31.

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808573#Comment_1808573

Damn, sorry, I meant Magna3. Doesn't the curve show up to 40ft of head? I see rated head is 24ft, but the display on my pump in constant pressure mode says 32ft, and if I turn on "max" it goes to 40ft. If rated head = max head, then why does the pump curve go much higher than 24ft? Thanks in advance of explaining!

https://product-selection.grundfos.com/us/products/magna/magna3/magna3-32-120-f-97924259?pumpsystemid=2414387493&tab=variant-curves

In terms of Cv, I included the 2.31 multiplier in my calculations already.

DCContrarian

If you're not measuring, you're guessing.

Do you have hose bibs where you could attach a pressure gauge? That might tell you what's actually going on.

Jamie Hall

How many times do I have to say it? A centrifugal pump — which is what you have there — has a characteristic pump curve. If the pump is operating at no flow, it will develop its maximum pressure — which is what you are quoting. In principle, if it is operating against zero pressure, it will develop its maximum flow. Everu other condition will have a characteristic flow and pressure relationship.

You are not operating at zero flow — shutoff condition. You are operating that pump at a considerable flow rate, and thus the pressure developed by the pump will be considerably less.

Now that is one big pump. As noted in the pump curve, it is rated at (reaches optimum efficiency at) about 50 gpm at 24 feet of head. At the maximum head, just under 40 feet, it is still capable of 20 gpm.(the maximum head, in this case, is controlled by the motor controller, not the characteristic curve of the pump; that pump is computer controlled), so either something in you piping has so much restriction that pushing the 7 or so gpm you seen to be actually flowing requires 40 feet of head.

An interesting and sometimes frustrating characteristic of this type of computer controlled pump is that the pump responds to the system characteristic curve, so what you are actually seeing is that the total head loss through your system at 7 pm is 40 feet.

Unfortunately you can't beat hydraulics with beef. You need to completely reevaluate the head loss at various flow rates through your system. Somewhere in there — or somewheres — there is a significant restriction.

I'd very much suggest laying out — or tabulating if you like — your complete system, listing all the components — every elbow, every other fitting, all the pipe sizes, everything — and figuring out where the head is being lost.

I might add that if you can get the system head loss down to something reasonable, you can probably use a much smaller pump — and save a surprising amount of electricity.

hot_rod

the next Idronics will go into methods and tools to determine actual flow and heat output in existing systems

With this information you can better make decisions on equipment upgrades, changing heat sources, etc

There will be Coffee with Caleffi webinars also around this topic.

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808581#Comment_1808581

I appreciate the thoughtful and lengthy response. Agree there must be a significant restriction. That was the whole point of this thread — I can't seem to find the restriction in measuring the normal head loss in the components themselves, so I'm curious what else I should be troubleshooting.

GroundUp

This is something that has befuddled professionals for a long time. Radiant seems to have a finite flow rate, regardless of the circulator used to push it. By calculating out the head loss of a standard 300ft loop of 1/2" PEX, your typical Grundfos 15-58 type circ "should" move upward of 1.3 GPM through that loop. The reality is that this never happens. You can drop a 26-150 on that same loop and still not even get that 1.3 GPM, despite that curve suggesting 2.2 GPM. That loop will always flow less than 1 GPM with any residential circ- either the curve or the head loss calc is a liar. You're welcome to continue beating your head against the wall and we'll all be grateful if you ever figure it out, but the rest of us have just learned to accept reality and adjust our calcs as such.

HomerJSmith

Here's a thought, why don't you check your mixing valve and make sure it is piped hot side to hot side and cold side to cold side and not reversed.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1808590#Comment_1808590

Or take the guts out of the mix valve and see if that changes flow.

If it is a ASSE 1070 or dual rated valve, it has checks and usually screens at H &C.

Jamie Hall

One of the things which is hard to visualize is that the head loss in a system of piping varies in proportion to the square (well, actually the 1.853 power) of the flow. In practical terms, this means that doubling the flow will require four times as much pressure. In more complex systems the arithmetic can get really messy (there are times that I don't miss slide rules), but any system which has some shared piping or shared valves or other restrictions — and most heating systems do — you simply won't get twice as much flow through the boiler or main pump if you run two loops than you had running just one. It doesn't work that way. The only way to avoid that sort of gotcha is a pump which is controlled by flow rate, not pressure, and thus varies the pressure as needed to maintain the flow.

Oddly, delta T operation, though slow in response, will do that better than delta P.

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808585#Comment_1808585

This is very helpful to know — thank you! It honestly makes me feel a bit better. I'll just let it go since it's heating the space just fine.

softieroberto

Just checked it — it's piped correctly. Good idea though. Thanks!

softieroberto

https://forum.heatinghelp.com/discussion/comment/1808600#Comment_1808600

Thanks!