Final Inspection & TDH

[BoilerSim]

My builder was out yesterday to prepare the pool for final inspection with the county. I was at work, and the only reason I noticed was that I happened to open the ScreenLogic app and saw a max pressure warning from the pump because it was running at near full speed. I called him and he let me know that he was trying to get the PSI to the level it needed to be for final inspection. Something about the Total Dynamic Head pressure needing to match the plans? I’ll admit that’s way above my level of understanding.

Anyway, he let me know he wasn’t able to get the pressure high enough (mentioned he needed it to get to 25 PSI) and will have to come back next week to try again and may need to add flow restrictors to a return or two. Is that something I should be concerned with? It seems to me that lower pressure is better from an efficiency standpoint, so I’m not quite following why PSI would intentionally be increased.

 

[JamesW]

We need a better explanation from the builder about what they're trying to do, because it doesn't really make sense.

Maybe they're trying to prove the efficiency of the system by creating a specific flow rate and measuring the pressure.

If that's the case, you would want to show that the pressure (TDH) is below a specific amount, not above a specific amount.

Maybe they're doing a pressure test to show that nothing leaks?

 

[jblizzle]

I agree. The builder explanation as you described is completely illogical.

 

[BoilerSim]

Thanks, @JamesW. He’s coming back early next week, so I’ll see if he can explain things a little better.

Should I be pushing back against the idea of putting restrictors in returns? I really don’t want to make the system less efficient intentionally when things seem to be working just fine.

 

[JamesW]

There's no reason to intentionally restrict the flow to increase TDH.

Something doesn't make sense.

 

[BoilerSim]

There's no reason to intentionally restrict the flow to increase TDH.

Something doesn't make sense.

So I've yet to get a clear explanation from them, but I had the chance to dig into the permit documents and do some pump speed/PSI tests this morning. There is a gauge on the pump measuring the vacuum (I believe its required for inspection and will be replaced with the drain plug after passing) and I've added that info as well. Anything look off to any of the experts? I have noticed that it seems like my system may be rather restrictive based upon some other numbers I've seen on here. I've added pics of the equipment pad and permit docs below. A few other notes:

• Return piping is 2.5"
• Suction piping is 2.5"
• Piping between equipment (from pump output through IC40) is 2"
• Valves set to return through pool only and suction 50/50 from main drain and skimmer
• Filter was cleaned this AM

Pump RPM	GPM	Pump PSI	Filter PSI	Pump Vacuum	Watts
750	0	1	1	0	38
1000	7	3	3	<1	62
1250	12	5	5	<1	106
1500	16	7	7	<1	177
1750	26	10	9	1	300
2000	35	12	12	2	464
2250	43	16	14	2	671
2500	50	19	16	4	925
2750	56	23	19	5	1234
3000	62	28	22	6	1595
3250	68	32	26	7	2020
3450	73	36	29	8	2404

I was able to find that the county requires the TDH of the completed system to match the plans. Based upon my permit docs it looks to be 73.36, but I have no idea how you'd confirm that on a completed system.

Any ideas? Thanks!!

 

[JamesW]

What are the units for pump vacuum?

 

[BoilerSim]

What are the units for pump vacuum?

inHg

 

[BoilerSim]

I was able to find a TDH calculation on INYO's website of (vacuum inHg x 1.13) + (PSI x 2.31). It looks like it is very different at the 36 GPM flow rate than what it should be per the permit docs. Using the 35 GPM readings above, we'd be at 29.98, which is about double the estimated 15.49. Could that be the issue? Also, the printout shows that at a max flow rate of 112 GPM, the head loss would be 73.36. Our system is higher than that at 73 GPM. It looks like perhaps the system is far more restrictive than designed and permitted?

 

[JamesW]

TDH gets complicated.

Every component has a component curve that contributes to the system curve.

To generate a theoretical system curve, you need to look at the head loss of each component at each flow rate and add them up.

So, you would look for the published curve for every piece of equipment, pipe and fitting.

The filter should have a curve in the manual or brochure or otherwise available from the manufacturer. Same for the heater etc.

Plumbing data can be found in charts or by using a head loss calculator. Plumbing is based on total equivalent length.

Actual total dynamic head can be measured by gauges on the suction and return. Convert inhg to psi by multiplying by 0.4912. Add return and suction and multiply by 2.31.

At 35 gpm, the head loss (TDH) is
(12 + (2 x 0.4912)) x 2.31 = 30 feet.

At full speed, the head loss is
(29 + (8 x 0.4912)) x 2.31 = 76

If the theoretical TDH calculations at design flow rate and total flow rate don't match the actual TDH, then the inspector and the builder need to decide if it's too far off or if corrective action is required.

Actual TDH should be less than or equal to theoretical TDH.

 

[BoilerSim]

Thanks, @JamesW! Looks like our system is about 2x more restrictive than the design based upon the above. I would assume this won't pass inspection and the builder will be forced to address it somehow, but who knows. Either way, I'm certainly not happy as it would seem my pump is having to work far harder than it should, and therefore is costing me more money.

 

[JamesW]

One sheet shows the component head loss at 36 gpm. Another sheet shows head loss of the plumbing at 36 gpm. You have to add all components plus the plumbing to get TDH.

You need the theoretical head loss of the plumbing and each component at full speed as well.

I don't see the component head loss for full speed.

Also, does the inspector require that the full speed match if the system will never run at full speed?

 

[BoilerSim]

One sheet shows the component head loss at 36 gpm. Another sheet shows head loss of the plumbing at 36 gpm. You have to add all components plus the plumbing to get TDH.

You need the theoretical head loss of the plumbing and each component at full speed as well.

I don't see the component head loss for full speed.

Also, does the inspector require that the full speed match if the system will never run at full speed?

It looks like the Pentair TDH Calculator printout I attached shows 15.49 ft of TDH at 36 GPM (0.33 ft inlet piping + 0.33 ft discharge piping + 14.83 ft from components & piping elbows/tees). Is there something missing there? It doesn't show it all full speed, and I assume the Pentair calculator accounts for that but doesn't print it. I'm not sure about the full speed question relative to the inspection, but I do know when I spoke with the builder when he was here the other day he was running it all full speed to get measurements, so maybe?

Assuming the 15.49 ft. at 36 GPM is correct, is it safe to say that the actual TDH of ~30 at 35 GPM shows there's a material issue? If so, anything in particular you recommend I discuss with them or look at? The plumbing on the equipment pad seems to be very complex and looks like it has to be restrictive, especially with three 90 degree elbows between the pump and filter alone.

 

[JamesW]

You're going to have to ask the builder to clarify the details as we're not exactly sure how they did each calculation.

In any case, I don't understand why they would want to add restrictions because the idea is to minimize the TDH.

 

[BoilerSim]

At 36 gpm, the plumbing theoretically contributes 15.49 feet and the components add 14.41 feet for a total of 29.9, which seems to match the actual TDH.

At full speed, the plumbing adds 73 feet, but you would need to add the contribution of the individual components to calculate TDH.

Isn't the 15.49 inclusive of the components? That's how I read the calculator, but perhaps I'm off base... Total Piping of 0.66 + Components of 13.83 + Piping Elbows/Tees of 1 = 15.49 TDH

 

[JamesW]

You're going to have to ask the builder to clarify the details as we're not exactly sure how they did each calculation.

In any case, I don't understand why they would want to add restrictions because the idea is to minimize the TDH.

 

[albizz8]

BoilerSim, what happened in the end?