Solar Drainback system not circulating at high temperatures?!?!?

[jmccannjr]

Installed a solar drainback system to heat hot water. System consists of 3 4x8 flat panels set at 52 degrees latitude (I am at 42 degrees latitude) due south. Piped to a 12 gallon converted electric hot water heater used as the drainback tank. I have fitted this tank with a sight glass to see the level of water in the drainback tank when operating. This feeds the bottom coil of an HTC 120 gallon dual heat exchanger hot water tank. This is fed to a Taco model 9 electric pump designed to pump over 36 feet of "head". A Caleffi Solar model iSolar4 controller set to drainback mode is the brains.

Problem: I have the tank max shutoff set for 160. The iSolar4 unit automatically sets the panel shutdown temp to 200 when in "drainback mode". I recently came home to find the unit shut down due to excessive panel temperature. ( This is done because distilled water is used as the heat transfer fluid and will boil at 210. ) So the assumption is made that the panel temp reached 200 and the controller shut it down. A blinking code on the panel of the controller tells me so. Checking the current temp of the panels at 180, I reset the system and it promptly restarted. Here is the problem: The pump started and began to feed the water to the panels. The water level stayed 1 1/2 inches above normal operating level in the sight glass. I also did not hear the water trickle back down from the panels into the tank indicating that the water did not make it up to the top of the panels. This caused the panels to continue to climb in temp until it again reached 200 degrees and the controller shut it down.

What could be causing the water to be "blocked from circulating through the panels? It should be noted that the water in the hot water tank was at 150 degrees. Plenty of hot water you say. My concern is that the pump may burn out. Any thoughts would be appreciated!

[inetdog]

Installed a solar drainback system to heat hot water. System consists of 3 4x8 flat panels set at 52 degrees latitude (I am at 42 degrees latitude) due south. Piped to a 12 gallon converted electric hot water heater used as the drainback tank. I have fitted this tank with a sight glass to see the level of water in the drainback tank when operating. This feeds the bottom coil of an HTC 120 gallon dual heat exchanger hot water tank. This is fed to a Taco model 9 electric pump designed to pump over 36 feet of "head". A Caleffi Solar model iSolar4 controller set to drainback mode is the brains.

Problem: I have the tank max shutoff set for 160. The iSolar4 unit automatically sets the panel shutdown temp to 200 when in "drainback mode". I recently came home to find the unit shut down due to excessive panel temperature. ( This is done because distilled water is used as the heat transfer fluid and will boil at 210. ) So the assumption is made that the panel temp reached 200 and the controller shut it down. A blinking code on the panel of the controller tells me so. Checking the current temp of the panels at 180, I reset the system and it promptly restarted. Here is the problem: The pump started and began to feed the water to the panels. The water level stayed 1 1/2 inches above normal operating level in the sight glass. I also did not hear the water trickle back down from the panels into the tank indicating that the water did not make it up to the top of the panels. This caused the panels to continue to climb in temp until it again reached 200 degrees and the controller shut it down.

What could be causing the water to be "blocked from circulating through the panels? It should be noted that the water in the hot water tank was at 150 degrees. Plenty of hot water you say. My concern is that the pump may burn out. Any thoughts would be appreciated!

Where is the overtemperature sensor located?
If it is attached to the manifold of a solar PV panel, it might easily reach 200+ with the system drained of water.
Unless there is some sort of thermostatic valve in the panel piping, my next guess is that you have a kink in a plastic pipe or hose that gets pinched off when the panel temperature reaches a critical level. Possibly from thermal expansion or from softening of the pipe.

[Naptown]

Make and model of the pump please

[Naptown]

You should never use PEX in this type of system IMHO

[LucMan]

Sounds like your Taco 009 is not pumping to maximum capacity, or there is a restriction in your piping to or from the panels. Do you have a inline flow meter installed after the pump? If so it should read at least 6 gpm . Are all your lines copper or is there some pex piping installed.
Is it a bronze or a cast iron body pump? It should be bronze or stainless to prevent rust formation that could clog the impeller.
Something could be stuck in the impeller, IE solder, or some other foreign object remove the cartridge and inspect the impeller & the inside of the body of the pump for any restriction.

[inetdog]

Sounds like your Taco 009 is not pumping to maximum capacity, or there is a restriction in your piping to or from the panels. Do you have a inline flow meter installed after the pump? If so it should read at least 6 gpm . Are all your lines copper or is there some pex piping installed.
Is it a bronze or a cast iron body pump? It should be bronze or stainless to prevent rust formation that could clog the impeller.
Something could be stuck in the impeller, IE solder, or some other foreign object remove the cartridge and inspect the impeller & the inside of the body of the pump for any restriction.

One piece of data that is not clear in the OP's post is whether the circulation resumes (with sound of water trickling back, etc.) once the panels cool. That will determine how likely it is that there is a problem with the pump or elsewhere in the system.

[Robert1234]

I don't know much about your system, but your description certainly SOUNDS like a pumping problem (not a controller issue). Any chance you have a pressure gauge on the discharge side of the pump? Would be very useful to help in identify if you have a restriction in the inlet or outlet piping.

Without any other information, I kind of lean towards the pump possibly caveating due to a suction side problem. Typically total pump flow failures are due to failed motor (you say you hear it start so that shouldn't be it), broken couplings between motor and pump (normally not present in consumer pumps) or cavitation due to suction side issues. Even broken vanes in the pump normally only reduce water flow. Therefore I lean towards an air leak or restriction in the pump inlet piping.

[jmccannjr]

Make and model of the pump please

Taco 09

[jmccannjr]

You should never use PEX in this type of system IMHO

No Pex is used anywhere in the system.

[jmccannjr]

cool pumping

Thanks to all for the posts.

Difficult to monitor system during the day due to my work schedule. Need a sunny day on a weekend to check out. I do NOT have an in line flow meter after the pump (probably will install one now).

As for the behavior of the pump and system. It seems the controller is shutting down because the panels ARE reaching 200 degrees because the water volume isn't getting through system. It is not the controller!

When the system cools down it does NOT return to full circulation mode, so it seems that it is unrelated to the overheating. I have a spare Taco 09 pump that I plan to swap out this weekend and see if I can get back to full circulation. Was considering installing boiler drain valves to force potable water up through the panels and measure the output. I have a hard time believing that their is a blockage in the panels, although, I purchased them used for $600.00 for the 3 of them. They are 30 plus years old. Had to replace one of the headers that had been damaged from an evident freeze. They do NOT leak as the same volume of water returns to the drainback tank every time since they were put into operation in May. I also pressure tested them before installing.

I guess I'm having a hard time believing a brand new Taco 09 would fail already! Thanks again for the thoughts! I will keep you posted.

[jmccannjr]

Pump material

Sounds like your Taco 009 is not pumping to maximum capacity, or there is a restriction in your piping to or from the panels. Do you have a inline flow meter installed after the pump? If so it should read at least 6 gpm . Are all your lines copper or is there some pex piping installed.
Is it a bronze or a cast iron body pump? It should be bronze or stainless to prevent rust formation that could clog the impeller.
Something could be stuck in the impeller, IE solder, or some other foreign object remove the cartridge and inspect the impeller & the inside of the body of the pump for any restriction.

The Taco 09 pump is cast iron. Because I am not using oxygenated water (the heat transfer fluid is distilled water) I don't need to use bronze or stainless. It is a closed loop, non-pressurized system, therefore no oxygenated water is entering the pump to cause rust or mineral build up. Thank you for your thoughts.

[ChrisJ]

Hi fellow Rhode Islander.
I am just a homeowner planning to do a DIY solar thermal project. My questions may be dumb but I am just trying to learn.

"It is a closed loop, non-pressurized system"

How does the water drainback if no air gets in the pipes?

Is it the air in the top of the 12 gal tank?

I thought outside air needed to be introduced into the system to get it to drainback.

ChrisJ

[Naptown]

Yes it is the air in the tank

[jmccannjr]

Solution

Hi All,

Just replaced the brand new Taco 09 pump with the 30 plus year old Taco 09 pump and voila!!! All fixed!!!

Thank you to all for comments and suggestions on the problem. Now to try and get my a warranty covered replacement!

Jim

[solarusmc]

Possible solution to drainback system 'not' circulating..

Hi Jim,

I updated this post after seeing Jim's post.

Jim my original post is below however I see that you replaced the pump. Hopefully you won't go thru another one.

It sounds like your system is developing pressure as it gets hotter water which will 'stop' the water from circulating.

If you haven't done so yet, you'll need to get 'air' into the 'return pipe' near or on your drainback tank.

Just before my return line enters my drainback tank I inserted a T fitting / small piece of copper pipe standing up straight and stuck a ball cock shut off valve on that little pipe.

My return from the collectors is all 3/4 copper however the T fitting I installed drops the T to 1/2 inch copper pipe.

What I have to do is leave the ball cock valve cracked OPEN which allows air to enter the piping thus 'breaking the vacum' inside the return pipe.

I also installed a 'vacum valve' at the highest point on the collector piping outside which allows 'air' to be sucked in to the piping when the pump shuts off or the system is running normally, either way.. if you do not allow air to enter the return pipe you will have issues trying to get it to drain back and circulate once it gets real hot.

Hope this helps you out some.
If so, please let me know if this solved your problem.

By the way.. I too use a small 6 gallon used hot water heater for my drainback tank.

I can send you a pdf file showing my tanks and collector build if you email me directly ok?

PS: I just realized you are from RI? Heck you can come on over and check out my system if ya want.

Take care,
Pat.