Variable speed pump and heat pump, how low can you go!

[shingo]

Hi everyone, I recently bought a inground pool with heat pump and a 3 speed pump (starrite supermax VS)

In my heat pump manual, it says it require a minimum flow of 15GPH and a maximum flow of 45GPH. It also state that a flow under 15GPH can cause damage on the unit.

By default the RPM for the 3 speed are 1400, 2200, 3000.

How can I know if the minimal speed (1400RPM) is sufficient (over 15GPH) ? I can also modify the default value with 10RPM increment.

Also, would it make sense to run the 1400 speed at night, there's less chance to break anything because it's colder and the heat pump will transfer less heat, does it make sense to do that?

So my main concern, is there any chance that I can be under 15GPH with this pump and my setup at 1400RPM? keep in mind, everything is brand new, it was a straight forward installation.

Thank you

-pool 12X24
-1-1/2" plumbing, with 2 return, 1 skimer and 1 pool drain
-the pump is 25 feet away from the pool
-55000 BTU heat pump
-Hayward sand filter
-No elevation for the pool equipment, sit on level ground

 

[Agent99]

450 RPM is the lowest my Pentair IntelliFlo VarSpd can go, FWIW, and it can do 5 rpm resolution.

I suggest you get a flow meter so you can measure things.

Your pump should come with some pump curves that show flow vs. head loss. You can use those to estimate your flow rate.

You could also disconnect your filter and run the pump at its lowest rpm and see how fast you fill a 5-gallon bucket. That will also give you an idea of your flow rate. I have a feeling that at your lowest rpm, you will fill that bucket very quickly and thus meet the 15 GPM your heat pump requires. BTW, you do mean gallons per minute and not gallons per hour, right?

 

[boston62]

I just kept lowering my rpm on the vsp, until my heat pump kicked out for low flow, then increased it a little more...
Don't know if that is the correct way, but it works..

 

[pooldv]

That is what I did with my VS pump and my SWCG, at 900 rpm I got intermittent low flow errors so I upped it to 1100 rpm and never get a low flow error. It only draws 150 watts at 1100 rpm. I would think with the heater you would want to go several hundred rpm above a low flow error.

 

[chem geek]

Just keep in mind why there are low flow limits with heaters in the first place. At a lower flow rate there is more temperature buildup and the higher temperature can wear on interior heater components faster. This is particularly an issue in gas heaters, but I would expect similar issues in large heat pumps as well. So while you are saving money with lower flow rates, you may be wearing out your heater faster.

 

[shingo]

Just keep in mind why there are low flow limits with heaters in the first place. At a lower flow rate there is more temperature buildup and the higher temperature can wear on interior heater components faster. This is particularly an issue in gas heaters, but I would expect similar issues in large heat pumps as well. So while you are saving money with lower flow rates, you may be wearing out your heater faster.

I will test how low I need to go on the RPM to trigger the low flow, I think 1400RPM is probably keeping it safe, it's only 55000 BTU.

Also, the schedule is like this :

8:00-10:00 3000RPM
10:00-12:00 2200RPM
12:00-23:00 1400RPM

Between 23:00 and 8:00 the system is down, so when it restart, it will have to heat the pool a bit but it will be running at high RPM until 12:00, which it will be back at it's set temperature.

From 12:00 to 23:00 the heat pump will almost never be running because the sun is heating the pool quite a bit.

 

[pooldv]

That looks like a good start. I run my pump at 1950 rpm for solar heating, so you might be able to run at lower RPM in the morning.

Also, please add your pool info to your signature as shown here, Pool School - Getting Started

 

[yetis]

From what I have read, isn't it advised that you want to run your pump at a high RPM when the heater is on and then drop down when its off? This protects the elements, but also is more efficient?

 

[pooldv]

Yes, depending how many hp the pump is. The 3hp intelliflo pumps way too much for my set up. I never run it above 2500 rpm.

 

[chem geek]

From what I have read, isn't it advised that you want to run your pump at a high RPM when the heater is on and then drop down when its off? This protects the elements, but also is more efficient?

There are diminishing returns so while it is true that running at the highest speed would be the most efficient and have the lowest wear on the heater, it would also be very expensive. However, it's not so simple because too fast a flow rate and the temperature inside the heater can get too low and cause more condensation that could rust components. Basically the pump cost varies roughly as the cube of the speed or flow rate while the efficiency varies with greatly diminishing returns at higher flow rates (much less than linear). So overall a lower flow rate saves a lot on overall cost so one just needs to trade that off against the higher temperature in the heater from that lower flow rate in terms of both wear and efficiency. The manufacturer has already set a minimum flow rate so that wear and efficiency aren't horrible.