Heating with a variable speed pump

[Wicked]

I hope this is in the right forum area but I have a question on heating a pool with a variable speed pump.

Assuming there is enough flow for the heater on a low speed - lets say 1500RPM

Is there benefit to heating a pool on a lower speed? Some pro's I can think of are:

1) lower electricity on pump (obvious)
2) Water sits on the exchanger longer and thus heats the water more before exiting to pool

The con's I can think of are:

1) slow pool turnover rate so heating water could potentially take longer?
2) The efficiency is not as good with potential heat loss in the PVC return to the pool


What are your thoughts and what speed do you heat your pool at?

 

[mas985]

1) lower electricity on pump (obvious)
2) Water sits on the exchanger longer and thus heats the water more before exiting to pool

I agree with #1 but #2 is actually a con. This is because heat transfer slows downs as the water heats up. Plus heat loss increases as water temperature rises (see below). Basically, the heater becomes less efficient at higher water temperatures. Although probably not much different.

The con's I can think of are:
1) slow pool turnover rate so heating water could potentially take longer?
2) The efficiency is not as good with potential heat loss in the PVC return to the pool
What are your thoughts and what speed do you heat your pool at?

#1 - The heat rise within the heat exchanger is proportional to the flow rate so 1/2 the flow rate results in the twice the temperature gain so if heat loss were the same, it would take just as long to heat the pool on low vs high. But as you point out in #2, heat loss is going to be higher on low speed because of the higher water temp. This is also true of the heater efficiency a I mentioned above. Higher waters temps result in lower efficiency. This is true of a solar heater as well. Heat transfer efficiency is higher with higher flow rates.

However, you have to look at total efficiency and cost. It may be worth the extra heat loss to run the pump at lower speeds.

 

[JasonLion]

Lower speeds use less electricity, which is good. However the water sitting in the heat exchanger longer and reaching a higher temperature is bad/less efficient. In most situations, it makes more of a difference to the pump than it does to the heater, so total energy usage is lowest near the minimum flow rate for the heater (which may well be noticeably above 1500 RPM on a larger heater).

Slower pool turnovers have no effect. The pool remains well mixed even at low speeds, so no issues come up there.

 

[Wicked]

Thanks guys. Currently it's at 2250 when the heater is on (I think this is by default for heater mode)

That being said from testing I can run it slower and the heater is happy with the flow rate.

This is why my question came up so I figure I would see where others run their RPM. I agree with the logic and points that we all made.

Anybody able to share what their VSP is set for when the pool is heating?

Thanks again!

 

[Cajun]

I have never done a true comparison. I usually circulate my water at 45% but I set the heater flow to 90% just because I figured it would heat the pool faster. Once the heater turns off the pumps slows back to 45%. When using the spa I have it set to 100%.

I guess you could run a series of test. But for it to be accurate the outside and starting temp would need to be the same. I am curious now and may try my own test.

 

[Wicked]

I wonder if the heater manufactures advertise the most efficient flow rate for best heat production. Although with older units they may not since VSP were not out yet.

 

[JasonLion]

No, they all say that the flow rate should be between X and Y, where X and Y mostly depend on the number of BTUs of the heater. Though different brands use different words and split things up differently, it all boils down to the same thing.

 

[swimcmp]

From an engineer in Pentair Aquatics heater development. Heaters are designed for best efficiency at 60 GPM. Water flow below that will only lower heater efficiency by about 1%.

 

[Wicked]

From an engineer in Pentair Aquatics heater development. Heaters are designed for best efficiency at 60 GPM. Water flow below that will only lower heater efficiency by about 1%.

This is great info. Question - how is best to determine GPM on a variable speed pump? Do manufactures post charts? Measurement tools? other? I have a Jandy 1.5 ePump

 

[mas985]

Short of a flow meter, I have some spreadsheets on this web site that can be used to determine flow rates.

However, since you have an SWG with a temperature sensor, there is a much easier way to determine flow rates. A 400k BTU heater will produce about 320k BTUs (80% efficiency) to the water so if you measure the exit temperature of the heater (via the swg sensor) and the water temperature, pre-heater, the flow rate is approximately:

Flow rate (GPM) = 320000 / (Tr * 60 * 8.34)

So for 60 GPM, the temperature rise (Tr) will be about 11 degrees.

Keep in mind though that the lower the flow rate, the larger the temperature rise of the water so if the flow rate is too low, then the exit water temperature of the spa could get close to scalding temperatures so for the spa, I wouldn't go much less than 60 GPM. But for the pool, you could drop the RPM more if desired.

 

[Wicked]

Short of a flow meter, I have some spreadsheets on this web site that can be used to determine flow rates.

However, since you have an SWG with a temperature sensor, there is a much easier way to determine flow rates. A 400k BTU heater will produce about 320k BTUs (80% efficiency) to the water so if you measure the exit temperature of the heater (via the swg sensor) and the water temperature, pre-heater, the flow rate is approximately:

Flow rate (GPM) = 320000 / (Tr * 60 * 8.34)

So for 60 GPM, the temperature rise (Tr) will be about 11 degrees.

Keep in mind though that the lower the flow rate, the larger the temperature rise of the water so if the flow rate is too low, then the exit water temperature of the spa could get close to scalding temperatures so for the spa, I wouldn't go much less than 60 GPM. But for the pool, you could drop the RPM more if desired.

excellent information! I will test next time I have the heater on and report back!

 

[Cajun]

So I did some testing today when heating the pool. I decided to chart my results as seen below but before I get to that here are a few of my observations based on my own tracking of today’s results and my own gas and electric cost.

My cost for electrical is 8.5 cents per KW and my price for Natural Gas is $6.14 per MFC. These are both loaded cost (taxes, supply, surcharges, because we can fees, ect) I did take out the customer charge to reflect the true cost.

Before today I always ran my pump at 90% or 3100 RPMs when heating the pool. Today I heated with the pump at 60% or 2075 RPMs. Naturally the heater uses the same amount of gas regardless of the flow rate. Here is a breakdown of cost over a given hour.

Pump at 90% or 3100 RPMs - 1600 watts - $.14 per hour
Pump at 60% or 2075 RPMs - 474 watts - $.04 per hour

Heater running for an hour - .3628 MCF - $2.23 per hour

So as you can see based on my equipment, electrical, and gas any cost savings from the pump is irrelevant and the only thing that matters is running the heater less. That being said I don't know if this was any slower or faster than any other time I heated the pool as I never really tracked it. It seemed to take longer but who knows as the outside and ground temperature probably plays a large role in all of this. Also the fact that I kept checking on it may have made it seem longer.

Here is my chart.

The break in the chart is when the family decided to heat the spa up. I have the spa set to run at 100% and heat to 97 deg. This took almost no time to rise to this temperature due to the limited water volume. Once we were out of the spa I put it back in pool mode and started tracking again till it got to 85 degrees.

So what was the cost you may ask? Well normally my pool pump would have run at 40% 1375 RPMs using 152 watts and consuming 1 cent per hour. I estimate this cost me roughly 33 cents in electricity and $30 in gas. The gas usage was the actual entire house gas usage for the day but aside from the pool heater the only gas usage was the bone-in-ribeyes I grilled up for dinner.

I'm also thinking I have probably been very foolish in the past to allow the heater to cycle throughout the night as needed because I imagine it would run quite a bit of the night due to the overnight air temperature drop. I have turned the heater off for the evening and plan to repeat some test tomorrow morning running the pump at 90%, I know it won't be exact since the air temp and starting pool temp will not be the same. If anyone has any thoughts/comments or have done similar test please share. Thanks

 

[Cajun]

ok I did some additional test on Saturday running the pump at 90% - 3100 rpms. Here were the result.

I then tried my best to correlate the results as best I could. The next two graphs I offset the time of day based on a common starting temperature the first graph using air temp (53 deg in my case) and the second using water temp (77 deg). I then applied this offset to all data from that point forward so I could get some kind of commonality. I would like to get thoughts from others on these results.

common starting air temp

common starting water temp

I'm not sure what to make out of these results as the rise in pool temps seem fairly consistent over time although the second graph it seems the 90% speed was raising the pool temp at a faster rate but them again so was the air temp.

 

[Wicked]

Great information. I agree with you that the results seem fairly linear comparing temp increase @ 60% with 90%

 

[Cajun]

Great information. I agree with you that the results seem fairly linear comparing temp increase @ 60% with 90%

Wicked. I added another graph using the same method but starting with a common water temp and not air temp.

 

[linen]

Was the pool covered during the test? If it wasn't, do you recall if the RH was similar both days?

Seems to me in the short term of your testing, the air temp will have little impact, so I think your last graph is a better comparison. In addition, if the pool was covered, then the air temp would be even less significant. The ground temperature probably has more impact then air temp (and is likely similar in both cases being just a day apart).

Although probably not statistically significant from the data, I think your last graph at least suggests that it is more efficient to move more water with a smaller delta T.

 

[Cajun]

The pool was not covered. Based on my zip code and the weather.com past stats the humidity between the two days were similar but fluctuated between 70-35% throughout each day. I would agree that the ground temp was probably very similar between the two days.

 

[mas985]

The heat transfer rate is going to be a little higher when the water is colder (and heat loss a little lower) so for a fair comparison, you really need start at the same temperature for both as is the last chart. That is a result that one might expect but I would repeat it several times to make sure there are not other factors involved.

But why not use the spa as a proxy? It will heat much faster so there is less time for other outside factors to play a roll. By replacing the spa water with pool water, you can reset fairly quickly and have nearly identical conditions. Plus you could do the test at night and remove the sun factor.

 

[poolneophyte]

No, they all say that the flow rate should be between X and Y, where X and Y mostly depend on the number of BTUs of the heater. Though different brands use different words and split things up differently, it all boils down to the same thing.

I have a Jandy heat pump and the owners manual lists three flow values: Maximum, Optimum and Minimum. Optimum happens to be at 60 GPM which I can get only when running my variable speed pump at full speed.

 

[jhalpinjr]

No, they all say that the flow rate should be between X and Y, where X and Y mostly depend on the number of BTUs of the heater. Though different brands use different words and split things up differently, it all boils down to the same thing.

I have a Jandy heat pump and the owners manual lists three flow values: Maximum, Optimum and Minimum. Optimum happens to be at 60 GPM which I can get only when running my variable speed pump at full speed.

Speaking with Gulfstream direct, their 125K BTU heat pump unit is the most efficient at 45GPM...I would definitely say that all heaters are going to be different.