Schuco system - erratic panel temps at controller and glycol charging

[ChuckD]

Hi,
I have a question or 4 about my 6 year old Shuco system and I've been abandoned by my installer.

I've been seeing inaccurate readings from the in-panel sensor (a thermistor) that are telling the controller it's not warm enough to do anything. I know it's wrong because I can easily monitor the actual temperature of the glycol. For example, today the fluid temp was pushing 120F while this sensor was only reading around 50, although it would cycle from a low in the mid 40's, slowly warm up to around the low 60's, turn on the pump and slowly drop down again. Outside temps were in the 20's.
The sensor is mounted in a rubber grommet set into the side of the panel frame, upper corner.
My first thought was to push the wire into the grommet deeper thinking there was too much exposure to the ambient. That didn't seem to do much to the reading at the controller.
Then I pulled the wire and grommet out to actually see what was what. It was what I expected, a chrome metal looking tip which I then thought might actually be touching framework inside the panel, so I pulled it back a bit so the tip would not be contacting anything but air (inside the frame). This also didn't seem to change much but I started to lose sun anyway, so here I am with my humble question(s):

Am I right that the sensor tip should be suspended in air inside the frame (seems like a no-brainer)? If that's the case I'm concerned about exposure on the outside of the grommet and the wire itself to the elements. Would that conduct cold to the sensor and throw it off and should it be insulated?

Also, can these sensors go bad and I'm seeing a dying one?

And a separate question: can I handle checking and adding glycol if needed? It hasn't been serviced in a couple years. I have the service manuals which discuss it but they make it out to be something requiring expensive equipment and a minimum 2 year degree.

Thanks in advance.

Schuco Solar Station PS 1.3
Schuco "Advance" Collectors CTE 215 CH (x3)
Schuco SDC 204 system control unit

[peakbagger]

I am not familiar with that system but normally the controller looks at the temperature of the glycol at the top of the collector and compares it to the temperature of the domestic hot water tank top. If the temperature of the glycol is higher than the DHW tank the glycol pump turns on and runs until the temperature in the DHW tank is equal to the temperature at the top of the panel. Note there are some dead bands typically incorporated in the controller so that the system doesn't short cycle. I have no idea why there would be RTD for measuring the interior air temperature of the panel. It really isn't that relevant. On mine and most other systems, the RTD at the top of the panel is either mounted in a thermowell in the piping or just attached to the exterior of the pipe under the insulation at the top of the collector.

If you have an expansion tank on the glycol loop and there are no leaks the system will run for quite awhile without a need for "servicing". The only real service requirement is to check the pH of the glycol especially if the glycol has been overheated. Its more of an issue with evacuated tube type collectors but if a circulator pump fails and the glycol is left in the loop it can get overheated. When glycol gets overheated it turns acidic and can corrode the piping and collectors. Various additives can also wear out but that is pretty rare.

If your system pressure is low or you have a leak there are specialized tools you would need to buy to get the air out. They are not outrageous to buy if you are "handy". Schuco is long since out of the solar business and they could have gotten cute with connectors so that could be issues specific to the system. Generally you need a working pressure gauge on the system and a temporary glycol pump which is capable of putting out enough pressure, I looked around and found this one which works well http://www.northerntool.com/shop/too...9074_200249074 a friend in another state was in the same situation and bought one and he was happy with it. You do need to convert the pipe thread connectors to something more useful like hose thread. I used washing machine hoses on my set up. The final tools are a large bucket and large funnel and three washing machine hoses. I use a drywall bucket but its small for the job, ideally if I had bucket with a hose connection on the bottom it would be easier. The linked pump also needs a power cord and plug. I would recommend that it be plugged into a GFI receptacle as its highly likely you will be getting wet.

If the pH is good and you just need to re-pressurize a system due to a leak, you will need a couple of gallons of compatible glycol. Unless the installer went cheap the glycol should be polypropylene glycol (PEG) rather than ethylene glycol. PEG is non toxic, ethylene is toxic and difficult to get rid of legally. They are compatible so if you don't know just buy PEG if you are just adding fluid.

The temp pump is hooked up on either side of the permanent glycol pump usually through a set of tees with valves on them where the temporary hoses connect. There has to be a valve on the main line either immediately upstream or downstream of the permanent pump. The set up is, the discharge of the temporary pump connects up to the Tee on the downstream of the permanent glycol pump, the inlet of the temporary pump is hooked onto a hose that is run to the bottom of the glycol bucket. The last hose connects to the Tee upstream of the permanent glycol pump and the other end is hung in the bucket. If you buy the linked pump, it needs a flooded suction, that means you need to fill the inlet hose before you connect it up with the loop, that's where the funnel comes in. The pump should be located below the bucket. Now fill up the bucket with about a gallon of the new PEG and stick the inlet hose of the temporary pump so its below the glycol level. The valve between the tees on the permanent pump has to be closed.

The next part requires a bit of coordination. The valve on the tee to the temporary return line is left closed, the valve on the tee from the temporary pump is opened and the temporary pump is turned on and then the valve is opened on the return hose which is routed to the bucket. If you do it right you are now circulating fluid up through the system and then returning it to the bucket. If you stick the return line below the glycol level in the bucket you will see bubbles in the fluid, you need to let the system circulate for several minutes until the bubbles disappear completely. Once the bubbles are gone, then you slowly crank down on the valve on the return line and read the system pressure gauge. It should go up. Keep cranking down the return valve until you hit the pressure you want. Note the pump I linked has built in pressure cut off, it may be set too low so you may need to have screwdriver handy to turn the pressure setting up if its set lower than the desired glycol pressure. Once the system pressure is set, make sure the return line valve is closed, then close the valve on the temporary pump discharge and open the valve on the permanent pump piping. If you did it correctly, the pressure in the loop shouldn't drop.

If you want or need to change the fluid in the system due to overheating situation, instead of returning to the bucket you need to have a pile of jugs or one large container to store the entire volume of the system plus some extra volume. What you need to do is drain the system out via the two tees as much as possible. You then do the same setup as I described previously except that you need to add water to the bucket and run the return hose to the larger container that you use to collect dirty glycol. You now need to flush the system out by running water through the system until the return water looks clear. Ideally once its running clear if the system is really crapped up inside, trisodium phosphate (TSP) mixed into the water can be used to clean out the interior of the pipes after you have flushed with water. If you needed to do this you then need to flush the system again with water. Finally you then drain the system and use fresh glycol to charge it up again. You will end up with lot of contaminated water, if you are on municipal system most folks dump it in their sewer. If you are on a private system you really should find someone with city sewer. PEG and TSP are not toxic (PEG is food additive) and TSP is effectively industrial strength soap so it shouldn't raise havoc with a septic system if diluted and added slowly.

The biggest failure point with these systems is usually the check valves on the discharge of the pump and the expansion tank bladder failing (mine lasted about 8 year). If the check valve fails, the system can thermosyphon in cold weather and cause a lot of problems as below freezing water from the roof can naturally circulate down through the system without the pump running and freeze the potable water in the heat exchanger. This is usually why older systems stop getting used and I have seen a fair share of frozen heat exchanger bundles over the years.

Most service techs have charge pump system that has all the part on one cart, when they know they need to work on SHW they throw the cart on the truck. These cost a lot more than the homebrew system I described and most folks would not justify by buying a commercial one since its used rarely. The norm for SHW systems are that they are orphaned quickly by the manufacturer and he installer they eventually fail unless the owner takes over. I expect there are quite a few SWH systems out there that are still installed and out of service due to lack of someone understanding how to work on them.

[J.P.M.]

FWIW, +1 on what Peakbagger wrote. Any chance of getting a new service provider ?

[ChuckD]

FWIW, +1 on what Peakbagger wrote. Any chance of getting a new service provider ?

Yeah, you got that right. Working on it. So far I've had one strike-out (after the guys who built it), local guy who did not follow through and seemed uninterested anyway.

Peakbagger, epic post sir, thanks. That's one for the notes here and reinforces that this isn't something I should attempt.

The solar station has three gauges. 2 for temps to and from the collectors and one showing pressure (around 29psi right now) and there is an expansion tank. I believe those two temperature readings because I can confirm them with a probe via some ports. But sadly the control unit isn't using that to turn the system on and off. The control unit has a graphical display which you can scroll through to show different readings including one with an icon depicting the collectors and a temperature. There are also minimum and maximum readings. It's this reading that seems to trigger the pump and which right now fluctuates between 44 and 63 (tank temperature is around 61). The pump turns on and the temperature starts to slowly drop. The pump turns off when it reaches around 59 and continues to drop to around 44. Then it slowly starts to rise again. It's called "logic". [sarcasm off]

[LucMan]

Did you check your controller settings to make sure the temp readout is set to Fahrenheit not celcious?
You can check the thermister by disconnecting it at the controller, using an ohmmeter record the the ohm reading and compare it to a 10k ohm thermister chart. Usually 10, 000 ohms at 77degrees .
Rarely thermisters are are 1000 ohms rated so if you have one of those then a 1000 ohm chart is required.
Purchase a pack of Cryotec test strips if you are concerned with glycol quality.
Drain a few drops onto the test strip to check for PH, and freeze protection and compare to the colors on the container. If it is brown the fluid has been overheated and highly acidic.
If the heat transfer liguid is out of spec. The system will need to be flushed and refilled with a purge pump available for about $125.

[ChuckD]

Thanks for that LucMan. As it happens 2 guys from another company just left. They swapped out the probe (it's actually 2 wire, I just saw the single jacket) telling me they do go bad. Not much sun at the moment so I have no feedback for if it worked yet.
There is a sightglass in the pump where we can see the glycol is clear. We'll be getting the charge boosted to 35psi later this week.
I'll be sure to update this when we can see it perform.
Thanks again.

[ChuckD]

Welp, got a new sensor installed and it made no difference.
Observations:
I've checked the resistance of the sensor at the control unit (unhook the wires and test on those) and it appears to be working within spec compared with a chart I have in the manual. I noticed that when I hooked up the ohm meter it started at around 1100 and gradually rose to 1250 over 3-4 minutes or so where it stabilized. This compares with the glycol temps at around 80-90F.

After I unplug the unit and then plug back in, the temp reading rises quickly to what seems like normal (90F-120F today) and then gradually drops ultimately to 50F or so when the pump clicks off. Then it starts to rise again to around 63F when it starts the pump back up, and the temp starts to drop again.

Pulled the fuse and tested it for continuity.

This is leading me to think there's a fault in the control unit although the start up self-diagnostic doesn't report any.

[LucMan]

Sounds like the controller is doing what its supposed to do, cycle the pump according to panel and water temps.Insufficient production. What are your DHW temps at the end of the day? I would check more into panel performance than looking at the controller. Is it possible that the collector tubes have delaminated from the sheet?

[ChuckD]

The collectors are heating the glycol to 120. The control unit is only seeing temps at 63 or so and cuts out the pump. So the DHW is only getting to around 70.
I've had this system for 5 years now and this is not normal. The collector temps should be close to the glycol temp.

[J.P.M.]

The collectors are heating the glycol to 120. The control unit is only seeing temps at 63 or so and cuts out the pump. So the DHW is only getting to around 70.
I've had this system for 5 years now and this is not normal. The collector temps should be close to the glycol temp.

Three things come to mind as I read this:

1.) Following LucMan's question about "delamination", actually, deterioration of the tube to absorber bond and the nature of that bond - clamped/glued/soldered/ etc., has there been any point or times when the collector may have stagnated - that is, the system pumps being off while the sun was shining for some hours ? Such conditions, either a few large temp. excursions, or smaller but cyclic ones, can lead to such failures over time.

2.) It looks like the Schuco collectors have a tubing arrangement that is serpentine (one unbroken tube snaking through the entire collector) rather than the more common parallel risers/header arrangement. Depending on the absorber design - usually either one continuous sheet or individual ~ 6"-8" strips attached to each parallel section of tube(s), the serpentine arrangement can incur more thermally induced bond stresses depending on the actual operating conditions encountered, particularly if the absorber is one continuous sheet. I could find no info on the absorber configuration.

2.) The glycol (collector) loop transfers heat to the potable water loop through a heat exchanger of some sort. From the advert. blurbs, it looks like an in tank type. Heat exchangers, regardless of design particulars, foul up and /or need service over time. Depending on that heat exchanger's design and particulars, it may be at least contributing to or perhaps largely responsible for a lot or all of the problems either through blocked/reduced flow or substantially decreased rates of heat transfer due to unavoidable and normal fouling of the heat transfer surfaces over time.

[ChuckD]

First, thank you folks for hearing this out. It takes some of the bitterness away of being deserted by a guy who sat in my living room, looked me in the eye and promised they would be around to support the system. I hope I can return the favor.

We have a 115 gallon Schuco storage tank that the heat is being dumped to. That's essentially a preheater for a standard electric 50 gallon water heater. Since we're on a 235' deep well solar thermal provides a significant boost. The storage tank water often can get to 125 to 135F for 5 months of the year.

There's a another component to this system I haven't mentioned since I don't feel it plays into this problem, but I will anyway. This solar thermal system interfaces with a geothermal ground loop via a heat exchanger. When this happens is controlled by this Schuco solar control unit. Simply, when there isn't enough energy being collected to heat the DHW, it switches over to the ground loop exchanger adding a degree and a half or so to the return until it senses the sun's bright enough again, or until the sun goes down. While I'm somewhat underwhelmed by what it contributes to the ground loop, it has worked flawlessly otherwise these past 5 years and still does as far as I can tell. Right now when I've said the pump "clicks off", it's often switching over to the ground, then switching back. But it's also dumping 120F glycol to the ground exchanger whereas it would normally only be 60F or so since it would only do this on cloudy days.

That said, I must be missing something in concerns about the collector's performance. As far as I can see, through 2 different temperature readings separate from the control unit's, the collectors are performing well. It seems pretty clear the processor of the unit is getting the wrong signal as to the temperature. Garbage in, garbage out, etc.
Or am I missing something?

[LucMan]

But it's also dumping 120F glycol to the ground exchanger whereas it would normally only be 60F or so since it would only do this on cloudy days.


There is the answer to your problem. Your dumping into the ground all your absorbed heat. Manually disable the geo dump and see what happens.

[J.P.M.]

Or am I missing something?

Sounds like a lot of the collected heat may be going into the earth.

[ChuckD]

Thanks, but I think you're overlooking the misreading of the temperature at the controller that it's getting from the collector. As I've said, I have 2 other ways of confirming the reading there is not what the glycol is, by a long shot. The controller is either diverting to the ground loop or just shutting off the circulator because it "thinks" the glycol is fluctuating between 45 and 63. I have changed the set point for this diversion to summer values (when we don't want to add heat to the ground) and makes no difference.