Coordinating VSP Filter Pump with Heat Pump Heater

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burgerchef

Active member
May 31, 2022
32
east central pa
Hello, in July we had a replacement filter pump and a heat pump installed. Pentair Superflo VST and Nirvana FC120. I do not have an automation system or a flow meter or a nightime cover. I winged it initially with the programming/settings and all went very well and we're pleased as punch. So, that being totally unacceptable, I put on my Hydraulic Engineering cap and started wondering about how what was happening related to specs and best performance stuff. Nirvana talks about GPM, Pentair is RPM. Nirvana says GPM should be between 40 and 80, and that 60 to 80 is ideal. I wanted to know what the filter pump RPMs I was using in the programs I set up were delivering in GPM. The VSP's RPM max is 3,400 or 3,450. Initially I was using speeds around 2,000, 2,400, 2,800. The only way I could think to test GPM was to select an RPM, hook up a few feet of old Kreepy Krauly pool vacuum hose to a return and time a bucket fill. I did this very carefully for each of the two returns and repeated the test for comparison. I'm confident in how I did the test but don't know if its a proper test. So I measured how much water was coming out of both returns. I did one return immediately after the other hoping that is equavalent to doing both at once. 35 GPM at 2,800 RPM, and 44 GPM at 3,400 RPM. Comparing that to the heat pump efficiency recommendation (40 to 80) I am barely in the range with my highest VSP RPM setting. While I was at it, I measured the temp of the water coming out of the returns. It was 77 degrees when heat pump showed 74 for the water going through it. I am wondering if you folks see any problems in my test methods or red flags in the results, and if you have any input on my desire to coordinate the specific pump and heater I have. Thanks!
 
With your test method, you are altering the head loss, adding lift and additional pipe to the plumbing which will reduce the flow rate out of the measurement return and force more water into the other returns. So it may not be very accurate.

As a secondary check, I have several spreadsheets that can estimate flow rate depending on the information known.

The most accurate would be a suction and pressure measurement at the pump drain plugs but I can also estimate it based upon just the following information:

Filter pressure at full speed 3450 RPM

Height of filter gauge relative to water level

Length, diameter & number of each suction pipe pool to pump

Picture of pad plumbing will help with the rest


But in reality, as long as the heater does not alarm, the flow rate doesn't matter that much.

An alternate way of measuring flow rate is to measure the temperature difference between the inlet and the outlet of the heater.

GPM = Heater BTUh / (dTemp*60*8.34)

For heaters, this is actually a better way to do it as it is a direct measurement of heat gain.
 
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Note that a heat pump does not have a single output like a gas heater.

A heat pump will have a variable heat output based on the ambient conditions.

So, you should calculate the heat output based on the manufacturer's specifications.

Below are examples of the specifications given by the manufacturer.

You can contact the manufacturer for more detailed information.

Noted according to AHRI 1160: Ambient temperature (oF) / Relative humidity (%) / Water temperature (oF).

With a water flow of 0.45 GPM per 1,000 BTU per AHRI condition 80/63/80.

(80/80/80 – AHRI) =120,000 btu/hr. 54 gpm.

(80/63/80 – AHRI) = 111,000 btu/hr. 50 gpm

(50/63/80 – AHRI) = 77,000 btu/hr. 35 gpm.

nirvanahp.com

Pool Heat Pump - FC Series : The Highest Efficiency

The FC is a Pool Heat Pump offering unbeatable performance | Best warranty on the market | 🏊‍♀️Learn more
nirvanahp.com nirvanahp.com
 
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James- thanks for that, I believe I saw that info at some point. I did not grasp it at first glance, but I can now see that it is valuable and applicable info. I will study it and prob have questions.

Mark- here is the info and pics you requested:

Filter pressure at full speed 3450 RPM is 16.5 PSI - I checked some baselines two weeks ago after backwashing, 3,400 was 15 PSI

Height of filter gauge relative to water level - gauge is 43" above water surface - water is currently at lowest point due to weather

Length, diameter & number of each suction pipe pool to pump:
Near skimmer - 30'
Bottom Drain - 40' (depth factored in)
Far skimmer - 88'
All 1-1/2"
lines underground are flexible pipe
photo shows paths measured


pool.jpg
 
So based upon that information and assuming the filter gauge is correct, which sometimes they are not, I get an operating point of 80 GPM @ 51' of head. The flow rate as a function of RPM would be:

GPM = 0.0232 * RPM

This is quite a bit higher than I think what you found, so a few follow up questions:

Does the filter gauge go to zero when the pump shuts off?

I also assumed this pump was running on 230v rather than 115v. Is that correct?


[EDIT] - Updated to remove MPV that I inadvertently left in. Filter pressure seems too low for a HP in line.
 
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burgerchef said:
Nirvana says GPM should be between 40 and 80, and that 60 to 80 is ideal.
Click to expand...
In my opinion, there is really no significant benefit to going over 40 GPM.

The difference in water temperature rise is insignificant.

The heat transfer depends on the water temperature and the heat exchanger temperature.

The heat exchanger temp is pretty high, so you have a large delta between the water and heat exchanger.

The water temp is only 3 degrees lower at 80 GPM compared to 40 GPM.

The cost of power, however, is very significant.

The power will cost about 6 to 7 times as much at 80 GPM vs. 40 GPM.

I would target 40 GPM.

Also, the manufacturer specifies 0.45 GPM per 1,000 BTU, which indicates flow between 35 GPM and 54 GPM depending on the weather conditions, which affects the production of heat.

With a water flow of 0.45 GPM per 1,000 BTU per AHRI condition 80/63/80.

(80/80/80 – AHRI) =120,000 btu/hr. 54 gpm.

(80/63/80 – AHRI) = 111,000 btu/hr. 50 gpm

(50/63/80 – AHRI) = 77,000 btu/hr. 35 gpm.

Ambient temperature (oF) / Relative humidity (%) / Water temperature (oF).

1726083865647.png

plot Y = 240/X, y from 2.5 to 20 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com
 
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The tiny amount of performance increase at flow over 40 GPM is more than offset by the higher cost to run the pump at higher speed.

In my opinion, 40 GPM gives the best performance/cost ratio.
 
(50/63/80 – AHRI) = 77,000 btu/hr. 35 gpm.

Ambient temperature (oF) / Relative humidity (%) / Water temperature (oF).

Assuming a heat output of 77,000 btu/hr, then you will have about 52 gpm at a 3 degree temperature difference.


1726087564776.png

plot Y = 154/X, y from 2 to 20 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com
 
mas985 said:
So based upon that information and assuming the filter gauge is correct, which sometimes they are not, I get an operating point of 80 GPM @ 51' of head. The flow rate as a function of RPM would be:

GPM = 0.0232 * RPM
Click to expand...

I think I will stick with the above estimate. The plumbing curve for that operating point is

Head (ft) = 0.008 * GPM^2

The return side of the plumbing curve is 0.007 which is reasonable for both the filter and the heat pump together plus the parallel return lines.

So I think you are getting much higher flow rates than you think you are. Unless the heater alarms for low flow rate, I would not get too concerned about it.

For warranty reasons, you might want to stick with the manufactures minimum recommendation of 54 GPM which would be around 2300 RPM. However, there is no way they would know what RPM/GPM you are running at unless they ask or come out and look at the pump display.
 

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