New Construction Started - Willis, TX (north of Houston)

Duffatola said:

Assume the distance (5x pipe i.d.) is based on an industry standard/best practices?

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That's the most important one.

The flowmeter also requires a minimum distance as referenced.

Mostly, it's to make sure that you have laminar flow into the pump impeller or to the flowmeter.

In my opinion, the suction requirement is not critical.

If you use a 4" sweep as the final elbow, the distance requirement is even less important, but I would try to do it anyway to keep Pentair happy.

Duffatola said:

I envision any 4" pipe will reduce to 3" at the pad in order to connect directly to the pump(s).

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Correct, but I would put the reducer at the horizontal output of the the 4" 90 instead of reducing to 3" and using a 3" 90 to transition from vertical to horizontal.

You can put the valve on the vertical or horizontal.

I would put the valve on the horizontal.

Duffatola said:

Regarding the Harwil Q-8 flow switch would a 44-65 GPM adjustment range be sufficient? I'm ready to order.

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That should be fine.

The heater requires 40 GPM minimum, so setting it at about 44 to 48 will give you enough flow with a small margin.


No check valve needed at the indicated location.






The purpose of the spring check valve is to bypass excess flow around the heater while the flow is going to the heater.

The purpose of the bypass with an Intellivalve is to divert water around the heater when the heater is not being used.

When the IntelliCenter wants to turn on the heater, it first turns the Intellivalve to send water to the heater.

Then, it waits to get a good pressure/flow signal and then it starts the heater.

When the "Call for Heat" is over, the heater will turn off and wait for a few seconds and then turn the intellivalve to bypass the heater.

Duffatola said:

The operation of the check valve is dependent on the flow which could be effected by dirty filter and pump speed

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That's how it is designed to work.

It only opens when the flow is over about 58 GPM with a 5 lb check valve.

GPM...........Pressure differential

30...................0.2167 psi

40...................2.0 psi

48...................3.5 psi

50...................3.7 psi

58...................5.0 psi

60...................5.2 psi

70...................6.3 psi

80...................6.8 psi.

Duffatola said:

The check valve operation could be affected by chemical breakdown

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Everything eventually breaks down.

Duffatola said:

What serviceability service will be performed on the valve during its lifespan

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JamesW said:

I would put unions on both ends of the check valve and have a replacement with the same unions just in case the original ever fails.

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JamesW said:

For all Jandy check valves, I would get one spare for each size so that you can have a replacement ready to go if one fails, which will probably happen.

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Duffatola said:

When it fails how will it tell you

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It's clear and you can see it working.

Duffatola said:

The heater already has a pressure switch in it so unless the spa/pool is more than 4” above or below or on a commercial installation a external flow switch isn’t needed

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Completely wrong.

The pressure switch can be activated with no flow when the bypass is being used because pressure can be trapped in the heater.

Using a simple pressure switch on the heater inlet is foolish because it does absolutely nothing to ensure that there is any actual flow.

The only way that a pressure switch would be reliable would be if it was a differential pressure switch that measured the difference in pressure between the inlet and the outlet.

Duffatola said:

Unless larger than 2” pipe is used on the system the max flow through the system at 7FPS is rated at 72GPM using sch 40 PVC pipe.

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I don't understand what this is asking.

The plumbing to the heater is 2" and the bypass is 2", so the flow will begin diverting around the heater at about 58 to 60 gpm.

So, you will have 60 gpm going to the heater and 60 gpm going through the bypass at 120 gpm total.

Even if you didn't have a bypass, you can send 120 gpm through a 2" line and through the heater.

For suction, you want to keep the water velocity below 6 ft/sec. For returns, you want to keep the water velocity below 8 ft/sec.

Size.......6 ft/sec......8 ft/sec.
1.5"...........38...............51 gpm
2"..............63...............84 gpm
2.5............90.............119 gpm
3.0".........138............184 gpm
4.0"........238.............317 gpm

Without a bypass, you could use 2.5" plumbing to the heater which is 8 feet per second.

Inside the heater, there is a spring bypass and all of the water goes through the bypass when the heater is not heating.

A thermal regulator blocks the flow from exiting the heat exchanger until the water warms up.

The internal manifold bypass is about 2" in diameter.

Below, you can see the manifold bypass and the thermal regulator.

When the heater is on, part of the flow goes through the manifold bypass and part of the flow goes through the heat exchanger.

Putting 120 GPM through the manifold bypass is about 12.3 feet per second and it puts a lot of stress on the plunger and it can break off as shown in the below video.










Below is a picture of an old and a new heat exchanger.

The old one developed a leak and had to be replaced.

The heat exchanger can develop a leak due to corrosion caused by chemical damage or by excessive flow causing mechanical erosion.

The heat exchanger is a two pass model with 6 tubes.

The tubes are about 1" in diameter.

If you have 60 GPM going through (3) 1" tubes, that's 20 gpm per tube and that is about 8.8 feet per second, which is more than the 8 feet per second recommended by Pentair.





Note: To avoid erosion of copper pipe, velocities should not exceed eight feet per second.




The two-pass design is a bad design because it is less efficient than a single pass design.

People think that giving the water a second pass allows the water to pick up more heat, which it does for that water, but the hotter the water gets the less heat is transferred because there is a lower temperature differential between the water and the copper tubing.

The most efficient design is one where the water temperature rise is as low as possible, which is counterintuitive.

The lower the water temperature rise, the more heat is transferred from the hot copper heat exchanger to the water.

The water temperature exiting the heater is irrelevant.

All that matters is how much heat (BTUs) is transferred to the water.

For a 400,000 btu/hr at 84% efficiency, the amount of heat transferred to the water should be about 336,000 btu/hr.

Duffatola said:

The operation of the check valve is dependent on the flow which could be effected by dirty filter and pump speed

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The manual describes adding a manual bypass, but that is dependent on flow because it will only work correctly with a specific amount of flow.

The 5 lb. check valve bypass automatically adjusts to the flow because it is pressure activated.

Below is the Jandy JXI, which is a similar design to the Mastertemp, which shows a double pass heat exchanger with flow going through 4 tubes one way and then two tubes going the other way.

If the Mastertemp is the same, then the flow going through the two bottom tubes is 30 gpm at 1" copper pipe, which is about 12.3 feet per second, which is 1.54 X more than the Pentair recommended limit of 8 feet per second, which means that at 60 gpm through the heat exchanger, the mechanical erosion is much higher than desirable.

However, if you limit the flow to the heater to 60 GPM using an external check valve bypass, the flow will be divided between the manifold bypass and the heat exchanger.

If you have 30 GPM going through the exchanger and 30 gpm going through the manifold bypass, then the flow going through each lower tube is 15 GPM, which is 6.13 feet per second.

7.7 Temperature Rise

A temperature rise test confirms proper water flow through the heater.

The temperature rise is the difference between the temperature of the pool or spa water before and after heating, as measured in the header.

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This is from the Jandy book.

A temperature rise of 30 degrees Fahrenheit implies a flow through the exchanger of about 22.38 gpm.

A temperature rise of 39 degrees Fahrenheit implies a flow through the exchanger of about 17.2 gpm.

400,000 btu/hr (0.84) ÷ 30 = 11,200 lbs water per hour= 1,343 gallons per hour = 22.38 gpm.

400,000 btu/hr (0.84) ÷ 39 = 8,615 lbs water per hour = 1,033 gallons per hour = 17.2 gpm.

So, at 40 GPM, 17.2 GPM to 22.38 GPM are going through the heat exchanger and 17.62 to 22.8 GPM are going through the bypass.

The pumps come with unions, which is nice.

The unions have screws where you can remove the internals and replace them.

Duffatola said:

Would having unions on each side of the sheers' filters make sense?

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That can make sense if we assume that one will eventually fail.

Having an extra with the same unions makes replacement quick and easy.

I don't know how likely they are to fail.

I always like to have various spares for fast and easy replacements to avoid long downtimes.

I keep checking in on the progress of this pool build, but when I walk in, I seemed to have actually walked into an engineering class, and slowly walk out. Interesting stuff!