Drainback - Unusual Behavior

Martin Romero

Greetings,



When my drainback combi-system kicked on today (SDHW portion), I noticed that the sound of the water returning to the drainback tank was unusual at the ball valve and tee right before the tank.  Normally, it just sounds like a muffled cascade into the tank, but today it had almost like a crackling or rattling sound at the mentioned valve/tee. 



Later in the evening, after the system had shut down and drained back, I looked at the sight tube of the tank and noticed that the water level was down about 1/4 inch from normal.  I also noticed some small brownish or reddish particles in the sight tube that I had never noticed before; usually the water in the tube is crystal clear.



Does this sound problematic to anyone?  If so, please drop a line.





Thanks,



Martín Romero

hot_rod

sounds like cavitation

is it a pressurized drainback? Can you attach some pictures or drawings?



hr

Martin Romero

Drainback System Sketch

Hot Rod and all others,



I've been in denial about a light crackling in my circulator since system startup.  Recent reading has me thinking that the tee 4 inches from the circulator inlet might be the culprit.  Otherwise, I believe I followed Lane's guidelines for circulator installation pretty closely.



I've attached a sketch.





Thanks,



Martín

Martin Romero

Drainback detail

Hot Rod,



I forgot to answer your question: the drainback system is un-pressurized.





Martín

hot_rod

Is that a

3 way ZV on the left of the DB tank? What type of HX for the radiant. You may have some high pressure drop components in the loop before the circ.



Cavitation is not just a pump impeller induced issue, it can happen at restrictive valves also, google cavitation for some other data.



Is it possible to pressurize the system? Adding pressure raises the boiling point of the fluid and can solve some of the vaporization issues.



hr

Martin Romero

Drainback Component Specs

Hot Rod,



Yes: three-way zone valve just outside tank (caleffi). 



The SDHW HX is the quad-rod 36" from AAA Solar in Albuquerque:  http://www.aaasolar.com/ProdLit/Semco/QuadRodBrochure.pdf.  



The Radiant Floor HX is the Carlson 3.5 from W-cubed:  http://www.aaasolar.com/ProdLit/Carlson/CarlsonHESpecifications.pdf



The Circulator is Taco 0011.



I'm not sure if the tank is made to operate under pressure.  I looked at the tanks specs and it doesn't say; it does have a pressure relief valve, but something tells me I read something about the TPR being for system testing.  I'll have to look further.



I'm considering draining the system and looking at the pump cartridge.





Thanks for your help,



Martín

Martin Romero

Pulled pump cartridge

Greetings,



I removed the pump cartridge and examined the impeller.  I'm no expert - it's my first time to do such a thing - but I saw no signs of damage.  I researched cavitation online and saw examples of damaged impellers, and mine doesn't look anything like those.



I'm now wondering howthe small particles were introduced into the closed loop (maybe some debris inside the piping came loose) and what the strange sound was at the valve/tee at the inlet of the drain back tank.



Please pass along any ideas or experiences that might be helpful.





Gracias,



Martín

hot_rod

do you need that much pump?

what is your lift head? That pump is about 30- 31' head. Are you running the pump with a variable speed control so it ramps down when the siphon is formed? You really only need the high head to get the flow up to the high point, once it has filled the system and starts flowing back down you only need enough "pump" to provide the required gpm. With four large collectors, maybe 1.5 gpm per collector or 6gpm.



The variable speed drainback controls will watch the delta T from the tank to the collector and adjust the speed accordingly. I'd try a variable speed controller, and pressurize the system a few, maybe 15 psi if possible



Here is how a pump, or pumps "behave" in a DB system. A page from Caleffi I-dronics 6 on DB design.



I built a drainback system including the tank coils with clear tube. Sure enough the pump flow in the flowmeter drops considerable from first start until the water reaches the top. As the fluid returns down the GPM jumps back up to the operating point.



There is a bit of science involved with DB design and implemetation that cannot be denied



hr

Martin Romero

Too much pump

Hot Rod,



I did the numbers manually, and the Taco 009 was right on the border with my system components and lift head (about 14 ft).  I think a 009 would have made it to get the lift, and once the siphon is established would certainly handle it.



I don't have the variable control, just the simple Goldline model.  However, I sure am getting a good education with all of this conversation and the recommended readings.  I'll know much more for the next system, and I'll consider some modifications to my present system when/if some dough becomes available.





Gracias,



Martín

Martin Romero

Drainback saga continues

Solar Friends,



My education continues: after draining my system and removing the pump for inspection, I re-filled and re-fired - see the first entry in this thread.  The following day, I came home to a half-empty drainback tank.  The only thing I could figure was that a pressure buildup had occurred due to my failure to fully close the fill valve (my own invented physics), and the P&T valve had opened.  I re-filled, and there appeared to be a negligible water loss, but the system seemed to function OK. 



Today, I came back to the half-empty tank again (thanks be to the inspector for insisting on a drain pan), but this time I noticed that due to a programming error with my thermostats, there never was a call for space heat, so all four 4x10 collectors were feeding my 50 gallon DHW storage tank with external HX.  The temperature at the top of the tank was well over 200 degrees.  The drainback tank has a leak and a bulge, and I figure it was damaged by the high temperatures.  Any thoughts?

hot_rod

not enough expansion room?

how much fluid does the system hold, what size drainback tank? Most of the drainback control have high temperature limits. You should be able to shut 'er down and drainback at 140, 168, 180, or whatever you chose. Some tank manufacturers have a 180F max operating temperature.



With 80 square feet you want at least 10 gallons of tank capacity.



Remember those Goldline controls fail "on" If a senser goes bad the pump will run. this could cause a freeze up condition. Best to use a drainback specific control with all the good features, like overheat protection and variable speed pump operation based on the delta T.



Also with DB, it's best to get the sensor inside the collector, against the absorber. I cut a small hole in the back with a holesaw to get the sensor in, then a plastic plug.



The new Caleffi collectors have sensor wells built into the collectors in two locations, shameless plug.



hr

Mark Eatherton

P&T valve....

On the solar drain back loop? Really?



You would normally only want a pressure relief valve on the DB loop, not temperature and pressure.



Whatcha got?



Also, with 160 square feet of array, you should have at least 240 gallons of storage, not 50...



That WILL Drive the loop temp high enough that a temperature and pressure relief valve wold relieve on temperature alone.



ME

Martin Romero

15 gallon storage/Pressure valve

I checked the drainback tank, and its pressure relief valve (no temp relief, it appears) is rated for 50psi. The tank holds 15 gallons; the collectors hold 5 gallons total when full, and the rest of the piping and components another couple of gallons. This gives about a 7 gallon or so cushion.  I run the system without pressure.



I'm not sure why the tank sprung a leak.  I'm pretty sure now that the pressure valve did not blow and that the water loss was through the tank leak.  I'm also sure that the high-limit setting on the Goldline-GL-30 DHW controller  was too high.  I had it set at around 140, but when all four collectors are on the DHW, the upper level temp (water side out of HX) can get way ahead of the lower level temp (water side inlet to HX), and by the time the sensor reads 140 degrees on the lower level, things might be boiling up top.  I don't really understand how that relates to pressure buildup yet, but I'm ready to learn.



Either way, I'll have to replace the tank.  I'll consider it part of my education, although I'm not solid yet on the tank leak.  It doesn't seem likely that it was due to pressure overload, since it does have the relief valve.  How likely is it that the valve is not working?



I'm grateful for all of your help.  This ongoing conversation has been a real gift.   I wish all of you a joyful and peaceful thanksgiving and upcoming holiday/Christmas season.





Martín

Mark Eatherton

Got steam???

Sounds to me like your system was kicking some steam to cause that kind of damage.



Steam flash happens so fast that hydronic pressure relief valves can't respond quick enough to save the equipment form the temperature and pressure stress associated with its production.



Remember, absorber temperatures of 350 degrees F during stagnation, combined with low water flow on start up, COULD kick some serious steam back down the dry downcomer.



The problem is, it's occurring when you're not at home watching what's going on. You need a data logger to strap all over the system so you can "see" what is going on ALL the time...



And what is the answer to the less than usual storage/array ratio? I can see what it is that you are trying to do (charging floor mass), but what happens in the summer, when you CAN'T charge the floor mass? If your tank maxes out by 11:00 AM, and then stagnates to 350 degrees F, and then you start using hot water, and the system starts back up again at noon, you CAN expect it to sound like a Boeing 747 is landing on your roof....



ME

Larry Weingarten

hot startup

Hello:  One old trick is to put a snap switch in line with the solar sensor to tell/fool the solar control into thinking the collectors are cold when the switch snaps at whatever you decide on.  If you install a 180 degree snap switch, the collectors won't have fluid running through them again until the temp is back down around 180.  This will make a Cessna out of a 747.



Yours,  Larry

Martin Romero

Snap Switch/ lowest 'high limit' setting

Solar friends and experts,



So what about a normally-closed snap switch that opens at 180 degrees or so and works on a 10-20 degree differential?  What would be wrong with this approach?



The radiant floor side of things (you're right, Mark, floor mass as storage) works on a normally- open snap switch that closes at 110 degrees and operates off of a separate controller.  I handle overheating in the summer by covering three of the collectors; the system worked fine all summer, and I don't consider the one hour to place or remove the covers too much of a burden.  I knew since the start that I would have to cover them, but it's only now that I REALLY understand why.



I'm also wondering if adjusting the controller high limit to the lowest (or a lower) setting could also help prevent some of the problems.



I'll have to read up more on data loggers and consider this as a system diagnostics tool.  I believe some of the new controllers already come with some data logging features.

Mark Eatherton

Larry's old trick..

Works GREAT with an old style of controller. I use to use it ALL the time. N.C. snap switch, wired in series with the storage tank sensor, and if temperature is reached (usually bond it to the solar return line), then disc opens up, sending a message to the controller that the storage tank is SUPER cold, thereby keeping the collector loop flowing.



A word of CAUTION: The storage tank can get as hot as 200 + degrees F. Make darned sure you have an anti scald mixing valve on the outlet of the DHW pre heat storage tank, and one on the outlet of the aux. storage tank for that matter.



As the day turns to evening, the collectors will re-radiate energy back out the glazing, thereby lowering the system temperature, eventually closing the snap disc with a tank full of HOT water.



Some people use to just turn the control to the ON/RUN position if they were going on vacation, and when they got back, the tank was completely cold. THis method (snap discs) avoids that scenario, and avoids system melt down (closed loop or drain back)



The problem with trying this "trick" on the new controllers is that they can tell the difference between a real cold tank, and an open sensor, and in most cases (I think...) if they see an open sensor, they shut down EVERYTHING.



Now, what a mechanically/electrically skilled person COULD do would be to place a DPDT relay in the storage tank sensor circuit, and if the snap switch activates, then it flip flops a relay, and "shows" the control a resistor that has the same ohmage as a 40 degree F storage tank.



I am not familiar enough (yet...) with the new controllers to be able to say for certain, but am sure HR or someone with more recent experience with the new class of controllers will step in and say something.



I don't think turning down your tank high limit is going to do you much good Martin.



ME

hot_rod

all of those functions

are in the current batch of controllers. With a drainback specific controller you get these adjustable parameters

Initialization, filling time (pump at 100% speed), stabilization, pump speed control, operating hour counter, delta T regulation, booster pump run (for two pump systems)

Max. tank temperature, BTU metering (with additional sensor, collector emergency temperature shut down (185- 284F adjustable) hand mode, and a thermostat function for the second relay output.



Some brands have data loggers with a small SD card that slips into the control.



Our next generation will have an anti legionella function and some other neat tricks



hr

Mark Eatherton

A year from now....

I could have answered my own questions :-)



Thanks for the update HR. Got any controls you want to put out on Beta test at 8,000' above sea level? (What, you don't have to derate electricity for altitude? That's not what the inspector told me....) ;-)



ME

Martin Romero

New Controllers....

are a must on my study list.  Thanks for all the information and support.  I'll keep up the study and work toward getting my system up and running again, implementing the ideas we've discussed.  I'll keep you posted



MR