Solar Pool heat introduces bubbles to my return

[wayner92]

I live in Toronto, Ontario and a couple of years ago I installed an evacuated tube system on my roof to supplement my Natural Gas pool heater. The system just uses my existing pool pump and the water is pumped up to the roof of my house where there are 6 evac tube panels that look like the panels on this site . The water flows up to my roof which is about 16-18 ft above the pool level as I have a walk-out basement so the water needs to go up to the roof on top of the first floor. Four of the panels are on a flat roof and two more are on a slanted roof.

The installer has made some adjustments through the years but I continue to have one problem - when I direct water through the solar panels it introduces air into the return in the form of bubbles. This causes a bit of noise and I also don't think it is good for the pool. Apparently this is due to the water rushing down the return pipes from the roof at a fast rate. Does anyone have any ideas on how to fix this.

p.s. - There is also one minor problem which is that every winter there seems to be some water that gets trapped and freezes which shatters tubes. This year three tubes got broken. The installer said that over the fall (my pool is closed in the first week of October) the water left in the tubes should boil off. But this obviously is not happening.

FYI - here is a picture of my house from Google Earth:
home goog earth.jpg

[J.P.M.]

How long does the air entrainment continue after you redirect the flow ? I expect the bubbles show up at the pool ? If a continuous stream of bubbles, you have a leak most likely at or downstream of the collectors. What's the flow rate and what's the return line size ? Atmospheric air may be getting sucked into the line from collectors to pool by the venturi effect or elevation changes as manifested by velocity head changes with some dissolved air being released. Restrict the return line at the pool outlet and see if the bubbles decrease or disappear. I they do (disappear), it's probably caused by a leak downstream or at the collectors, or at the site of one or more of the failed tubes. Deadheading the flow completely at the pool outlet may also help to find any leak. Just be prepared to mop up the leakage or look for a flooded collector.

Any vac. breakers in the line ? Hope so. Have they been checked ? They are a common source of entrapped air.

Vac. collectors can produce high temps. and should have pressure relieving devices for cases of system stagnation or other malfunction. Have relief valves been checked/maintained ?

As for freezing, it's often difficult to get all liquid out of a line and near impossible in horizontal panels. Ensuring system slopes for drainage and understanding that H2O/fluids can remain in almost any system and designing accordingly is part of good design for any piping system, solar or anything else. Assuming trapped or remaining water will "Boil off" is a poor to terrible idea and may be unsafe. First because is doesn' t go "off" as much as it vaporizes, raising system pressure (some) and, if the the resulting vapor pressure is not high enough to cause the pressure relief valve(s) to lift, the H2O stays in the system and condenses when things cool down (and later maybe refreezes). Second, depending on temp. attained, overpressured vapor systems can possibly explode if system pressure is grossly exceeded and relieving devices fail, which can happen if the relieving devices are not maintained. Besides, H2O in a rigid piping system only needs to freeze once to do damage, with that damage possibly being local to some wee corner or pocket. Third, the pressure cycling that is induced won't do the collectors or the rest of the system any good. Fourth, if the pressure relieving devices operate 1X/awhile due to such overpressure, they may "weep" and may accumulate some corrosion and not close tightly, allowing some air into a system when things cool down and mostly contract. Lots of other stuff can happen. I only hit a few things.

As a general statement, vac. systems are good for mid temp. applications and not very good for DHW production, and poor for pool heating.

Not to rain on your parade, but perhaps giving others reading this who are considering similar applications to yours something to ponder as they see it: Why did you use vacuum tube collectors ? You could have accomplished the same amount of pool heating for a boatload less C$ and hassle w/much simpler pool heaters made for the purpose. Not a plug, but see an outfit called Fafco. Flexible type pool heaters are also quite freeze tolerant. What you have now works, but it's sort of like using a Rolls Royce for pizza delivery. Vac tube heaters are more suited to applications that require high temp. fluids close to 100 C. Pool heating is a very low temp. application. Pool heaters made for that purpose are usually more efficient than vac. tube units in such applications. Use the right tool for the situation. Heating pool water with vac. tube collectors is like cutting butter with a chain saw.

[wayner92]

The reason that I used this type of system is that I looked for a solar pool heating system and this is what I found on the net. The installer said that vac tubes are better for climates where there is a large difference between the air and the desired pool water as is the case in Canada - at least in the shoulder parts of the season. He has tried to fix the bubbles but he says that it is due to the large height drop that we have on my house.

The bubbles are there continuously. The system does have some pressure relief valves so that is where I believe the air is entering the system. I believe the rationale for this is to prevent a vacuum from occurring. Therefore when you get the water rushing down the pipes it pulls in the air. There are valves on the lines that go both to and from this solar heating system. I have tried restricting the flow on the return from these panels to stop the bubbles but I haven't been successful doing so.

In terms of the overall effectiveness - yes I do wish that I would have gone with a more simple solar heating method. The evacuated tube system seems too fragile for this application as you can also crack the tubes by having the system shut off for a while during the day and then start back up, sending coolish water through very hot tubes and shattering them.

My system is saving me money as I don't need to use nearly as much natural gas to keep the pool at 85F as I used to but it is hard to say exactly how much I have saved as every summer is a little different in terms of temperature, sunlight, etc.

This same company also installed my PV solar system and that is doing much better and works perfectly.

[J.P.M.]

The reason that I used this type of system is that I looked for a solar pool heating system and this is what I found on the net. The installer said that vac tubes are better for climates where there is a large difference between the air and the desired pool water as is the case in Canada - at least in the shoulder parts of the season. He has tried to fix the bubbles but he says that it is due to the large height drop that we have on my house.

The bubbles are there continuously. The system does have some pressure relief valves so that is where I believe the air is entering the system. I believe the rationale for this is to prevent a vacuum from occurring. Therefore when you get the water rushing down the pipes it pulls in the air. There are valves on the lines that go both to and from this solar heating system. I have tried restricting the flow on the return from these panels to stop the bubbles but I haven't been successful doing so.

In terms of the overall effectiveness - yes I do wish that I would have gone with a more simple solar heating method. The evacuated tube system seems too fragile for this application as you can also crack the tubes by having the system shut off for a while during the day and then start back up, sending coolish water through very hot tubes and shattering them.

My system is saving me money as I don't need to use nearly as much natural gas to keep the pool at 85F as I used to but it is hard to say exactly how much I have saved as every summer is a little different in terms of temperature, sunlight, etc.

This same company also installed my PV solar system and that is doing much better and works perfectly.

Enjoy your system.

[LucMan]

The air problem can be remedied by adding a slight pressure to the system by installing a valve on the outlet line going back into the pool. Close the valve slightly until you have a positive pressure. Be sure to use a pvc valve as the high chlorine concentration in pool water and brass are not compatible.
Your panels seem to be the U tube type, if they are freezing in winter you may have to slope the problem tubes back towards the header slightly for the water to drain, and or hook an air compressor to the lines and blow them out, again using the valve to pressurize all the tubes.
Maintaining a neutral PH is very important when using copper in a pool system (all the u tubes and headers) to prevent premature failure.