Should I install a flow meter on a solar heater?

[haz]

I am planning on installing 450 ft2 of solar heating panels (9 panels of Heliocol HC-50, 4 x 12.5 ft. each) using 2" pipes. I currently have a Jandy Zodiac FloPro FHPM 2.0 2-speed pump. I will need to run the pump in high-speed mode when solar is on, and low-speed mode otherwise.

After installation, I might try to change (likely downgrade) the impeller to target the ideal flow rate to the solar panels in high-speed. Some people here have suggested adding a flow meter like FloVis:
http://tftestkits.net/FloVis-Flow-Meter-Full-Meter-p74.html

on the feed side going to the solar panels. Since this is something that I will only have to do once and the flow meter will forever forward generate some back pressure and increase the work of the pump, does this make sense to do this? I was looking at some of the approximate flow calculation spreadsheets some people here have created, and none I found mentions my pump. Any input would be much appreciated!

 

[skeeter_ca]

A flow meter can be a helpful tool in fine tuning a solar system and determining what kind of efficiency you are getting out of your solar panels. However it really isn't needed unless you just want to know what kind of BTU'S your panels are generating. Usually you use temperature difference to obtain the maximum temperature gain in your system possible. Once the system is setup the flowmeter would be useless except to maybe monitor pump performance. They do have cheaper fairly accruate flow meters ($45) that you drill a hole in the pipe and then clamp on to check flow and then can be removed after everything is setup and a seal is put over the hole it was in. The flow meter could then be resold to recoup your expense or saved for later testing. I'm sure others will have there own opinions on the matter.

skeeter

 

[jblizzle]

You do not NEED a flow meter, but could be useful if you are trying to dial in a system.

There is no point to a check valve on the solar supply side, but you do need one on the solar return side, so put the FloVis there if you get it.

I just run my full flow through the solar, so did not adjust anything.

 

[mas985]

I agree with Jason, it is pointless to put a check valve on the solar supply line as it doesn't do anything and in fact can prevent the panels from draining depending on the location.

But as I mentioned in your first thread, you will need a check valve between the filter and the solar valve. However, if you put the flow meter there, it will only measure total flow rate and not the panel flow rate should you bypass water. If the flow meter is installed on the solar return side, then it will measure the panel only flow rate.

Also, your pump is in the spreadsheets that I use. It is named "FHPM 2.0-2 Jandy". However, the spreadsheet will not be able to determine your bypass level, only the total flow rate from the pump. Much like putting the flow meter at the filter position.

 

[Isaac-1]

Like the others have said a flow meter can be nice to have, but certainly is not a requirement, you are right it is mostly for initial tuning, but can also be nice to have to confirm you have not developed excessive back pressure in your filter, etc..

 

[jblizzle]

You do in fact need 2 check valves anyway, so making one of them the flow meter would not really add unnecessary head loss and give you the benefit of extra information ... if the cost is not an issue.

 

[dschlic1]

On my setup, I have a three-way valve controlling flow to the solar panels. When off, full flow from the pump is sent directly to the returns. When the solar panels are on, the valve diverts some of the flow through the solar panels. I set the flow via the pressure gauge on the filter. According to the specifications of my solar panels, the pressure drop through them at recommended flow is about 2 psi. I adjust the stop on the valve that when diverting, the filter pressure increases by 2 psi. Seems to work.

 

[mas985]

That specification is the pressure drop for just the panels but you will also have pressure drop from the plumbing to and from the panels so you really should at least double that number (4 PSI filter pressure rise) so that flow rate is sufficient in the panels. With only a 2 PSI filter pressure rise, the pressure drop across your panels will be much less than 2 PSI so your panels are probably not operating a peak efficiency.

 

[haz]

Thanks for all the info. So it sounds like the flow meter can serve instead of a check valve between the filter and the solar valve. I really want to just be able to downgrade my pump impellor to get the optimal efficiency to my solar panels. It sounds like there's only need to divert water around the solar if the pump gives too much output/GPM for what the solar panels need. But instead of diverting, downgrading the impellor to the point where the entire GPM from the pump is at the solar panels' optimal GPM would provide the most efficient system. And in order to do this, I would need a flow valve, at least initially.

And it seem like I could more precisely downgrade the pump knowing more exactly the flow rate, rather than a less-precise one that gives me a reading of e.g. 40-60 GPM.

 

[mas985]

For 450 sq-ft of panels, the pump only needs to deliver 45 GPM which nearly any pump can do. I think you can easily go down to a 3/4 HP without any problem at all. Even my 1/2 HP pump delivers more than that through my solar (~56 GPM). So even with a 3/4 HP impeller, you will need to bypass at least some of the water.

 

[haz]

For 450 sq-ft of panels, the pump only needs to deliver 45 GPM which nearly any pump can do. I think you can easily go down to a 3/4 HP without any problem at all. Even my 1/2 HP pump delivers more than that through my solar (~56 GPM). So even with a 3/4 HP impeller, you will need to bypass at least some of the water.

My roof is 22' above the pool level, which is more than most people's, and based on my recollection of physics, the increased work required to pump the water up against gravity to that height will increase the force needed to get water through the solar heater. So with the same HP pump, my GPM would be lower than e.g. someone with a regular 1-story roof.

So let's say I don't install a flow meter and just downgrade my impeller to a 3/4 HP one. How would I know if I were getting the required 45 GPM? I know that if you feel heat at the end of the solar paneling, it's too slow and not absorbing the heat, but my roof is very high and not easily accessible.

 

[mas985]

My roof is 25' high and yet a 1/2 HP pump still works.

The FHPM 0.75 has a similar head curve to my pump so if my pump works with solar on a 25' roof, yours should easily work on a 22' roof. And as I pointed out before, I can bypass some of my water and still get high efficiency out of my panels. So I am very confident that impeller will work for you.

The extra lift to the roof is only required during the priming of the panels which is also when dynamic head is reduced since not all the pipe is filled with water yet. Once all the pipe is primed, the weight of the water on the return side of the plumbing offsets the required lift of the water on the supply side of the plumbing so there is no longer a requirement to "lift" the water and the excess static head is eliminated. So the flow rate in the elevated panels will be the same as an identical system with the panels on the ground.

Also, since you have such a large area of panels for the volume of pool water, you are going to find that you will not need to run solar for very long to maintain water temperature. I have 33% less panel area per water volume and I can easily get up to 10 degrees per day if needed. But on most days with a solar cover, I don't need to run solar more than 1-2 hours to get the pool back to 88 degrees. I think you could actually get away with 6 panels. Why do you have so many panels? I wouldn't think south Florida would require much in the way of solar.

So let's say I don't install a flow meter and just downgrade my impeller to a 3/4 HP one. How would I know if I were getting the required 45 GPM? I know that if you feel heat at the end of the solar paneling, it's too slow and not absorbing the heat, but my roof is very high and not easily accessible.

As I pointed out before, I don't think that will be an issue. The FHPM 0.75 will deliver much more than 45 GPM on even the most restrictive plumbing. But if you would feel better with a flow meter, then by all means, get one.

 

[haz]

My roof is 25' high and yet a 1/2 HP pump still works.

Thanks again, mas985. Actually my roof is also 25' like yours - I misstated 22", which is the height of the roof above the pool level, which is elevated, but the pump base is 25' below the roof.

The crash course on pool hydraulics you wrote:
http://www.troublefreepool.com/hydraulics-101-have-you-lost-your-head-t915.html

is excellent - I should have read through it before to understand some of the things you were saying better.

The FHPM 0.75 has a similar head curve to my pump so if my pump works with solar on a 25' roof, yours should easily work on a 22' roof. And as I pointed out before, I can bypass some of my water and still get high efficiency out of my panels. So I am very confident that impeller will work for you.

I was looking at the pump's head curve, and on high-speed at 0 GPM the 0.75 HP FloPro FHPM has around 57 ft of water head, which at a roof height of 25'+ is at least twice my height, so that looks perfect. I assume a FloPro FHPM 2.0 HP with a downgraded impeller to 0.75 functions like the native FloPro FHPM 0.75 HP, which I figure is what they refer to in the chart. And even though their 2-speed models start with 1.0 HP and up, I assume if I change the impeller with the replacement for the FloPro FHPM 0.75 HP 1-speed:
http://www.amazon.com/gp/product/B00564PVNG/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

it will function in a 2-speed capacity in my pump (FloPro FHPM 2.0-2). In terms of the flow rate using a 0.75 HP pump at low-speed, would that generally be sufficient in case I wanted to use the gas heater, which requires a flow rate of 30-125 GPM?

In case it helps, the plans for the pools state:
system flow rate: 75 GPM
max flow rate from pump curve: 130 GPM
total dynamic head: 27.99

Also, is changing an impeller relatively straightforward for someone who's never opened a pool pump before but is generally good mechanically, if I follow the directions? Or better to get someone to do it if there's some tricky part that could mess the whole thing up.

Also, since you have such a large area of panels for the volume of pool water, you are going to find that you will not need to run solar for very long to maintain water temperature. I have 33% less panel area per water volume and I can easily get up to 10 degrees per day if needed. But on most days with a solar cover, I don't need to run solar more than 1-2 hours to get the pool back to 88 degrees. I think you could actually get away with 6 panels. Why do you have so many panels? I wouldn't think south Florida would require much in the way of solar.

I spoke to a Heliocol installer, who originally recommended 8 50-ft2 panels = 400 ft2. An Aquatherm installer rec'd 9 4’x12’ EcoSun panels = 432 ft2. When I mentioned to them that I was going to install a solar enclosure, which the basic screen blocks around 10-15% of heat, and on the east/northeast side and on most of the top I was going to install 80% UV/heat-blocking screen, and we'll probably add some degree of landscaping that blocks even more sun. So I specifically asked if they thought I needed more panels, they offered options of more: Heliocol to 9 = 450 ft2, and Aquatherm 12 or 14 4x10' = 480 or 560 ft2. So maybe they were all overselling to begin with and saw a chance to over oversell based on my question? Looking around this forum, I saw different opinions on the right amount to install, but it seemed that some got notably longer swim seasons w/ more panels and felt the more the merrier. Do you think I'm getting past the point of diminished marginal returns with 9 panels?

 

[mas985]

it will function in a 2-speed capacity in my pump (FloPro FHPM 2.0-2). In terms of the flow rate using a 0.75 HP pump at low-speed, would that generally be sufficient in case I wanted to use the gas heater, which requires a flow rate of 30-125 GPM?

Definitely. Depending on the rest of your plumbing, that pump/impeller will deliver somewhere between 50 and 60 GPM.

Also, is changing an impeller relatively straightforward for someone who's never opened a pool pump before but is generally good mechanically, if I follow the directions? Or better to get someone to do it if there's some tricky part that could mess the whole thing up.

This video shows how to take a pump apart and to remove the impeller and seal. It shows several different pump models and as you can see, most pumps are fairly similar in their construction.

[youtube:24bqvm97]YQl0W5doYSI[/youtube:24bqvm97]

Looking around this forum, I saw different opinions on the right amount to install, but it seemed that some got notably longer swim seasons w/ more panels and felt the more the merrier. Do you think I'm getting past the point of diminished marginal returns with 9 panels?

Given that you have several things working against you, (i.e. exposure and shading) then it might be good to oversize just in case.

 

[haz]

Great, thanks for the video link. The 9th panel is $300 more, so I figured if it makes the pool a degree or so warmer in the coldest months that we swim, probably worth it in the long run. I've ordered the 3/4 HP impeller - I think better to install before the solar guys install their system, that way they can see if it's working. I'll do without the flow meter. Thanks to all for all your suggestions and help.