Low flow pumps - Is Air in Solar system ok?

[tomfrh]

Hi,

I have a pool solar system like this: (modified image from InTheSwim Pool Blog | Blog for swimming pool owners, Care s Guides and Pool Fun information)

View attachment 72403

I changed the main pump to a low flow (3 speed Davey on low speed) and solar pumps to 0.5HP single speed. It works, however the slow flow never flushes the air from the return solar line, which remains largely filled with air, causing some air bubbling into the pool. If I turn up the flow rate the water level in the solar return pipe rises, and the bubbles dissappear.

Questions:

1. Any tricks to solve stop the air getting in whilst maintaining very low RPM? I was thinking I could install a simple P trap under the T junction to stop the air getting back into the main line. Could splicing in a check valve or 2 way valve (Kept mostly closed) work? Many solar diagrams show an "optional" check valve or two way valve above the T.

2. Aside from air bubbling, is it a problem having an aim chamber in return leg of the solar system? Am I losing a lot of energy not having a closed siphonic solar system? Is having air there not a done thing?

Any other suggestions would be appreciated too.

 

[needsajet]

Re: Low flow pumps - Is Air in Solar system ok?

Welcome to TFP! Good to have you here

Add a valve on the down leg and partially close that valve (throttle it) so the pressure at the vacuum relief valve is high enough to keep it closed when the system is running. Or do something to increase pressure and flow (e.g. larger pump). A check valve on the return side is sometimes used to stop the system from pressurizing when it's shut off, but that would not help for your challenge.

You want the system full so you get the most heat possible. With the system full, your pump will move more water. By allowing the air in, you're losing the siphon effect and adding to the head your pump must overcome. Once the system is full, the only energy needed is to overcome friction loss of water moving through the pipes. It needs to be a closed loop because otherwise the pump will be consuming energy to lift water, which won't be offset by energy gained from the water pulling down.

To get the most from your heating system, push as much water as you can through it, without exceeding its design pressure of course. If you can get the temperature gain down to around 1 or 2 degrees C, that's usually pretty good.

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

Thanks for the comment.

I'll add the valve and adjust it to suit and see how I go.


How do I measure the temperature gain?




To clarify, the solar pump is actually a 0.5hp pump, not a low flow pump. The filter pump is low flow.

 

[mas985]

Re: Low flow pumps - Is Air in Solar system ok?

I used to have a 1/2 HP pump and it would prime the panels just fine so there may be something else going on.

What is the make and model pump?

What is the filter pressure with and without running solar?

Initially you mentioned running on low RPM. Is this a two speed pump and are you running on low speed?

Also, you mention a solar pump. Is this pump just used for solar or is it really your main circulation pump?

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

The main pump is a Davey three speed eco, which I run on the lowest setting.

The solar pump is a 0.5HP single speed pump.

It's as per the diagram.

The solar pump primes the solar system, the issue (I think) is that it doesn't purge all the air from the return line due to the free flowing return pipe.

 

[mas985]

Re: Low flow pumps - Is Air in Solar system ok?

The main pump is a Davey three speed eco, which I run on the lowest setting.

That is most likely the problem. Running the main pump on low speed will starve the solar pump of water. It needs a good supply of water to build up the pressure necessary to prime the panels. Increase the speed of the main pump until the panels prime properly. The filter pressure is very important. It needs to be high enough to keep the VRV closed. How high are the panels installed and what is the filter pressure in solar mode?

Also, with that main pump, you really should not need a solar booster unless the panels are installed more than 3 stories high. In fact, the system will be much more energy efficient without it.

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

Yes I can solve the problem by increasing the main pump speed, but I don’t want to do that, as it substantially increases power consumption, even if I omitted the solar pump altogether.

Plus this solar system is setup with a controller that drives a separate solar pump.

I’m going to add the throttle valve to add back pressure to the solar return line, to hopefully prevent the air chamber.

if that fails I’ll try the P trap idea.

 

[mas985]

Re: Low flow pumps - Is Air in Solar system ok?

I’m going to add the throttle valve to add back pressure to the solar return line, to hopefully prevent the air chamber.

if that fails I’ll try the P trap idea.

Solar panel priming requires a minimum flow rate. Usually around 40 GPM for a two story roof. By adding a throttling valve you are reducing flow rate of the pump not increasing it so that will make the problem even worse. A P-Trap will do nothing because the VRV allows the air into the panels so they can drain after the pump is shut off and P-Trap will not prevent that. A better solution is to remove the VRV and have the panels primed all the time so they will not need to be primed every time the pump shuts off.

Another option is that some controllers support priming at higher speeds and then dropping to lower speeds, my controller does this. However, you would still need to lower the VRV as well so it remains closed under the lower pressure of lower speeds.

Either way, you really should ditch the solar pump. That plus the main pump is costing you in extra energy. Probably as much as 750 watts extra.

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

The solar system is priming, the issue is that it’s not fully completely flushing the air and bubbles are being caught and dragged into the return line. I think what’s happening is the pump isn’t pumping the return water faster than the water can fall under gravity, and hence a waterfall and air chamber exists.

ive read some more and adding the brake valve above the return T appears to be a standard solution. presumably it prevents the return hot water from draining down under gravity faster than it’s being being pumped.

That’s my guess at this stage.


removing the solar pump, as well as requiring a lot of work, will use more power in my case.

The lowest speed of the main pump has insufficient head. I would need at least medium speed. But medium speed (~720W) uses more power than low speed (~180W) and solar pump (~370W) combined.

 

[mas985]

Re: Low flow pumps - Is Air in Solar system ok?

The solar system is priming, the issue is that it’s not fully completely flushing the air and bubbles are being caught and dragged into the return line.

Which means that solar is actually NOT priming. The definition of prime is when all the air is removed. Again, the reason there is air left in the solar plumbing is because the flow rate is too low.

ive read some more and adding the brake valve appears to be a standard solution. presumably it prevents the return hot water from draining back down faster than it’s being being pumped up.

You need the pump at an RPM high enough to lift the water up to the panels and to provide a flow rate fast enough to push the air out of the panels. Adding a check valve will not help. In might even hurt since they add head loss to the system. You are fighting physics.

The lowest speed of the main pump has insufficient head. I would need at least medium speed. But medium speed (~720W) uses more power than low speed (~180W) and solar pump (~370W) combined.

But it is not working at that is the point I am trying to make. Have you tried medium speed without the Solar Pump inline?

Adding the solar pump will NOT increase the head unless the flow rate is fast enough. It is flow rate that creates pressure in the plumbing. Higher flow rate results in higher pressure. The main pump will restrict flow rate at lower RPM so it doesn't matter that you have the solar pump. Do you have the head curve of the solar pump? Is it high enough to lift the water to the panels?

So if you are willing, I can help you optimize your system for the lowest energy use possible while still operating properly.

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

The pump is filling the collectors, however the water seems to be draining too fast down the hot water return line, which leaves air in there.

adding a valve above the T makes sense to me, as it will cause the falling water to back up, like traffic congestion, which will close the loop.


Using the main pump on higher speed for everything will use more power, and the system is built around two pumps anyway.

 

[mas985]

Re: Low flow pumps - Is Air in Solar system ok?

The pump is filling the collectors, however the water seems to be draining too fast down the hot water return line, which leaves air in there.

Again, this is because air is entering the return pipe from the VRV. This will not stop unless you either remove the VRV OR have enough pressure at the top of the panels to shut the VRV. However, that will only occur if the pump is large enough to produce that pressure. So again, do you have a head curve for the solar pump and can you post it on the forum?

Also, how high are the panels installed above the pump? This is important to size the pump and RPM needed.

Where is the VRV installed? Moving it might help as well.

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

I’m going to try the valve idea first. I think it’ll work by stopping the solar water draining too quick...

 

[mas985]

Re: Low flow pumps - Is Air in Solar system ok?

The installers should of installed a return side check valve anyway. That prevents the panels from filling when the solar valve is off.

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

This system doesn’t have a solar valve.

some diagrams I’ve looked up shown a check valve on the solar return line, but it’s generally listed as optional. When is/isn’t it required?

 

[mas985]

Re: Low flow pumps - Is Air in Solar system ok?

I think it always should be include as should a solar valve. Otherwise, there is no way to turn off solar without turning off the pump. Yours is a bit different because of the solar pump which is normally not needed.

 

[needsajet]

Re: Low flow pumps - Is Air in Solar system ok?

I would not remove the VRV entirely, depending on the collectors, because the suction will deform things when they're hot and empty. That's part of the reason for draining, and especially important here where the system runs very little in summer. It gets well over 80C inside the empty system when it's off.

Putting the VRV at a lower elevation is an option. You'll get more gurgling when the system shuts down, but that may not be important to you.

The throttling valve (brake valve) on the solar return side is a common component as you mentioned.

In case this is helpful, here's the manual for the OKU panels I used. Included is a plumbing diagram for the solar booster pump setup. Your configuration is shown on page 42.
http://www.ecoonline.com.au/content/manuals/oku_solar_pool_heating_install_manual.pdf

It's an important point that you would not want the solar booster pump to create enough suction to zero out the pressure at the filter. If that's going to occur, and I think there's a good chance it won't, it will happen after the solar system is full. If it does, you may need to tweak your low speed up from the default setting just a bit, or throttle a bit more.

Be sure that your main pump timer always overlaps solar system run times. Let us know how it goes.

To measure temperature, I bought a fairly cheap IR thermometer from Jaycar. It's pocket size. Mini Non-Contact IR IP67 Thermometer | Jaycar Electronics I point it at the inlet and outlet side of the solar after it's been running a while, and it gives me the temp difference. Just be sure you're pointed at two spots that are equally shaded or sunny, same colour, etc. I've also slid my pool thermometer into my solar outlet (my solar is on a dedicated circuit), and read the difference that way, and it's the same temp difference as the little pocket IR thermometer. I can't vouch for the accuracy, but it's the delta you're after anyway. I use that little thermometer a lot now that I have it (even checked the temp of coldies in my outdoor vs. indoor fridge, haha)

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

Thanks for that link. It helps a lot.

Looking at it I realised I can change to config 3 if need be, as I have a spare suction line from a defunct pressure side pool cleaner. that would eliminate any potential issue of solar pump zeroing the low flow main pump.

my VRV is at the bottom of the collectors. My collectors are black rubber tubes.



another question - the system also bubbles when the solar system is off, due to air being captured from the solar return line. This problem goes away at higher flow, because the water rises sufficiently inside the solar return coliumn.


At low flow this would obviously still occur even with a brake valve. So maybe I could also put in a U trap below the brake valve and T in from the underside? That way the air can’t find its way back into the main return even at low flow.




One final question - how do people confirm there are no air chambers in a solar system? The guides say to tune the brake valve until bubbling stops - but that occurs prior to full purging of air doesn’t it?

 

[needsajet]

Re: Low flow pumps - Is Air in Solar system ok?

You have to let it run for a period of time when tuning, maybe 10 minutes or so. And you can observe as time goes along as well, because flows vary depending on how dirty the filter is. You won't know right away.

The way to prevent air entrapment is to orient the collectors properly (outlet corner highest) and keep the inlet plumbing and outlet plumbing equal length and size, to the greatest extent possible. I don't know if there's a way to know if there are air pockets. Going up and listening for gurgling would be easiest I'm guessing.

I have no experience with a trap connected to the underside but I can't think of a reason why it wouldn't work. I would make it long for starters because there would be some venturi force, but I have zero idea how much!

Be cautious about a single suction inlet dedicated to a pump. At the very least, it should be within 300mm of the surface and have a proper wide drain cover on it. Even then, it would not be compliant most places. My solar suction is a pair of inlets 1.5 m apart, so that it can't get a hold of body parts. The risk is entrapment which can lead to drowning or very serious bodily injury. Even 1/2 HP can exert 100's of pounds of force holding a body or body part against it, far more force than a child can generate to get away, and even more than several adults can muster because of the awkwardness of a lift or pull in water. I strongly recommend not doing that when there's an alternative.

Here's an example of a drain cover that fits 40mm pipe. Note that it requires a screwdriver for removal which stops it from falling out or kids playing with it.
pool suction cover - Google

 

[tomfrh]

Re: Low flow pumps - Is Air in Solar system ok?

Yes I’ll keep it in mind about the entrapment risk if I go down that route. I’ll split it or put a special cover on it or similar.

Or maybe I could connect the solar pump suction line before the main pump and draw from the existing suction lines (one 50mm and one 40mm)?