Troubleshooting of recurring overheating events

zxcv_1984

Good day, I have a 2 X 30 vacuum tubes set for a DHW application.
Heat transfer is through a glycol circuit.
Lately I've had several shutdowns of the system due to overheating.
The glycol mix is about 50%, pH is good.
I've re-charged the circuit. I had less shutdowns for a while, but within 4-6 weeks it's now as bad as it used to be.
I observe heat transfer in the DHW tank (DHW tank infeed temp reads within 10 deg C of collector temp, DHW outfeed reads close to DHW lower tank temp).

The system used to overheat less frequently in the last few years.

What would you guys look for at this point?
I have my ideas, but I don't have much experience and I could really use your educated opinions.

Thanks for your help!

109A_5

Hello @zxcv_1984,

What I would want to know;
Is it actually overheating or is the controller just thinking the system it is overheating ?
When it trips out on overheat are all the sensors reading an elevated temperature or just one or maybe none ?
Does it log any history data ? On a per sensor basis ?

Depending on what is actually going on helps determine your troubleshooting strategy.

Like a sensor that is intermittent or a poor connection to a sensor, directly causing the alarm.

Or the controller is keeping the pump running too long. Again maybe a sensor issue, the controller thinks the system is too cool so it runs the pump and then the same intermittent sensor or another sensor catches the overheat issue or a controller issue that just keeps the pump running for too long.

In general check, inspect all the sensor connections for their integrity.

zxcv_1984

Good day!

Thanks for your input!
I can confirm that the single collector sensor is good, as it's been freshly replaced and sends reading that are consistent with the DHW tank infeed mechanical temp sensor reading.
The DHW tank sensor seems to work well, and sends readings consistent with the in tank mechanical temp sensor and the DHW tank outfeed sensor.

Unfortunately, the system does not log the sensor readings.
I've sat a few time to observe the system slowly go into overheating.
It's a bit of a mystery to me, because the system has now been working just fine for 4 days in a row, with bright sunshine.

109A_5

Hello @zxcv_1984,
Intermittent problems can be the worst. A lot of time can be spent trying to catch them in the act. Sometimes it helps if whatever stimulus is causing the problem can de determined. Is it Weather related, water ingress, amount of use, vibration, loose connections, chafed wires, etc.

If the sensors and their electrical connections are all fine, wouldn't the controller shut the pump off correctly to limit the storage tank temperature ?

If the relay that controls the pump was sticking, for example, how would the controller shut the pump down for an overheating event, it would need a secondary means.

Is the normal operating high limit temperature setting too close to the overheating limit temperature setting ?

hot_rod

The main cause of over heat is you have no load for the collectors. As such they go into stagnation and can quickly reach 400f! This will destroy the glycol quickly also. The pump can also airlock if the fluid flashes to steam.

Does the system have a dump zone? Apricus evac tubes had a small aluminum radiator that’s mounted outdoors to dump excess production. There would be a 3 way zone valve near the pump to divert to dump. 

what brand of controller do you have, snap a pic and post it, a pic if the piping near the solar tank also

Most of the electronics controllers gave an evac tube mode as and several over heat functions that can help considerably

Other options are a second or larger tank to put excessive energy into. Cover some of the tubes in summer months

There were companies that made covers, similar to boat covers to cover some of the array

The motto for solar thermal is use it or lose it🥴

zxcv_1984

Good afternoon,

My apologies for the delay in my return.
@109A_5 I agree that intermittent issues are the worst. And just to tell, I hadn't had any other event whatsoever since my last post. I fear my system is growing a conscience and feared I would replace it (just kidding).

@hot_rod my DHW circuit is built to dump excess heat in the heating water storage tank (about 1500L). Not as straightforward as the Apricus setup, but it worked just fine in the past.
Controller is a good old DeltaSol BS Plus. I know it under that name, but I've seen the same unit branded with many other name. Pic to follow.

Again, sensors and electronic seem to work fine. What I observed when the system went haywire was that collector temperature would start to increase FAST (like one degree a minute) and wouldn't find a new steady state as the pump speed increases.

...

hot_rod

A fast temperature rise is usually an air lock, or air bubbles, or the pump has not ramped up.

The collector sensors take a beating, no harm in replacing that one occasionally. It or the splice to the control wire are a common failure.

Looks like you have a pump cart to give it a good power purge.

Do you know how to get into the menu and force the pump on, the "hand" mode? Should be a flowmeter under the insulation to assure you are actually flowing.

The Euro installers tend to run pressure around 4 bar, 60 psi range. You have a large enough expansion tank, crank up the pre charge and pump up to a higher pressure.
Higher system pressure increases the boiling point of the glycol in case of a power outage or pump mis-fire. 60 psi takes you up around 300F boiling point. That is why those pump stations have 90 psi relief valves.

That is a double check valve on top of the expansion tank to allow you to remove the tank without a drain down. One side keeps the fluid in the system, the other side checks the pressure in the tank, so you don't get a face full. Split it between the two brass nuts. Then you can boost the pre charge.

Have you checked the glycol Ph? That is a good indicator of it going into stagnation.

Those Wilo pumps are getting hard to come by, if you see a spare grab it. We may still have some Grundfos Solar pumps, yellow label that fit into those union connections inside.

zxcv_1984

@hot_rod Thanks for the insight, I appreciate!

I'll definitely see to find a spare pump while they're still somewhat available.
Collector sensor is freshly replaced. It failed recently. Just as you said, they have a hard life.

My flow meter is out of order. I'll replace it. My hypothesis for the intermittent trouble I've experience is bad circulation, and having a flow meter would confirm whether circulation is a problem or not.
When the system was overheating I would observe high temperature at the DHW tank intake, so I know there's SOME flow, but maybe circulation was bad.

Pump looked like it was running, I removed the purge screw and checked that the impeller was running. Air lock or air bubbles is likely, but I still can't get my head around why the problem resolved on its own. I guess I'm better not obsess with it. Better be prepared, though, I'll fix that flow meter.

Yes I know about the pump manual mode on the controller, thanks for asking!

As for the glycol pH, I'm afraid it's not where it should be. the glycol's only a year and a half old, and it's already growing a bit more acidic. I've pushed fresh glycol when I recharged the circuit.

It's not "Solar Rated" glycol. As far as I know it's a decent product, but it may not be designed to withstand the abuses of a solar collector circuit. This may contribute to the premature aging.

Thanks again for you help, I'm grateful for the opportunity to learn with this forum.

hot_rod

There is a small air purger inside those pump stations. If the collectors stagnate, and flash to steam, the bubbles will eventually come out of the air purger. As long as the pump can move any flow at all.

The new Grundfos Alphas have a air purge mode. They sense a no flow condition and take the pump to full speed to flush the trapped air. That would be a nice feature on solar pumps. Some of the Resol built controls had an output to run ECM pumps. An ECM will not vary its speed via the "wave chopper" technology in that control, that pulses the PSC type circulators.

Every now and then you see a good deal on Resol data loggers. They run 7- 800 bucks new! Maybe make an offer on this one? Its about the only way to catch an intermittent fault.

Also, there is a Resol blog out there where the factory guys help out with troubleshooting and configuring the controls.

Evac tubes should really use one of the ultra temperature solar fluids, tubes go way over 300F when the flow stops. Or keep adding booster to the fluid you have when the ph drops into the 7's. With ph dropping that quickly it usually indicates over-heating.

zxcv_1984

Ok, here's an update! Intermittent issues sure make you use your head

Last weekend, I happened to come by the controller as the collector sender T was hiking up.
Pump was working, confirmed both by the controller status and by removing the pump screw and observing the impeller running. Infeed and outfeed T to the DHW tank were around room temperature.
The circuit pressure was about 10-15 psi over the temperature I previously set it to.
As the collector T was still at a very safe level, I quickly hooked the priming gear and pushed fresh glycol in the circuit. Collector T started to decrease, as DHW tank infeed T started to rise.
I unhooked the priming pump from the circuit, and voilà!

I checked the setup periodically during the week, as far as I could see the system worked as it should've.

Now, 11:45AM this morning, collector T is at 185C, system is on safety shutdown, DHW tank infeed and outfeed T were at room temperature, circuit pressure at 10 psi over the set pressure.

I'm thinking somehow air makes its way into my system, which would explain the increase in temperature. ...unless there's a vapor lock? I'm still looking for a chart telling the boiling point vs pressure for 50/50 propylene glycol.

The circulation pump runs, but absence of circulation (unless I charge back the circuit) would also be coherent with the presence of air.

...Can air enter a pressurized glycol circuit? Guess I'll walk the whole circuit and look for leaks.

All ideas are welcome, I have limited experience, but I'm willing to work to set this system back to top shape.

hot_rod

What pressure are you filling the system, cold?

I'd run 30- 40 psi. But adjust the expansion tank pre-charge to 30psi first. Higher operating pressure makes air removal easier, and raises the boiling point of the glycol, when the collectors stagnate. 35 psi fill pressure raises the boiling point to around 280F

Reoccuring air is generally caused by the system going into stagnation. When the tank temperature is met.
then on next start up, you have an air lock.

Is there an auto air vent on the collector? That eliminates that vapor pocket caused by stagnation. Use a Caleffi solar vent, rated at 360F operating temperature on the collector.

That brass fitting on top of the expansion tank is a dual check valve assembly. It allows you to remove the exp tank without losing any system fluid. It also checks fluid pressure in the tank so you don't get sprayed with glycol when you remove it.

Take it apart on the large nut. That leaves one check on the tank, one on the system. There is a fiber washer in that union nut. Try not to rip it. If so make a new one from auto parts gasket material. Water meter EPDM gaskets fit also.

Set the tank to 30 psi, fill the system to 35. This puts a small amount of glycol in the tank for both cold contraction, and as air comes out. it's called a "safety seal".

Under the foam cover should be an automatic air purger #6. stick a wire into the hole, it may be plugged with glycol sludge.

Do you have the over-heat functions enabled in that Resol control?

zxcv_1984

@hot_rod wow, thanks, I'll get busy working on your recommendations ASAP!

Charging pressure (cold) is currently 25 psi.
I don't think there's an auto vent, and the two collector arrays are installed horizontally, so I have 3 highest points, if that matters.

Overheat function is enabled.
When stagnation is met (NOT overheat, just DHW tank hot enough that nothing else can be pumped from the collector), a solenoid valve divert the flow, and the excess heat is pushed to the 1000 L thermal reservoir used to heat the house.
Did I get you right here? Just not sure I understand "stagnation" as I should in your reply.

9h00AM this morning, collector T was climbing steady, already at 70 degC. As I was hooking up the charging pump to do as I did last time, I tried switching the WILO pump in high speed (it's a 3 speed pump). The collector T almost instantly started dropping, DHW tank infeed T rose quickly to match with collector temp minus a few degrees.

I'll definitely check that pump impeller, part of my troubleshoot.

hot_rod

There is usually a flow meter inside those pump stations also, #5. That is the best way to assure the pump is moving flow. If the pump is stuck or an air pocket, no flow on the meter. It should move as you change pump speeds?

Are you running on the variable speed function? That is the best way to maximize the solar harvest. The pump speeds as the collector temperature increases and slows if a cloud comes by, and as the sun drops in the sky.
It prevents the pump from shutting on and off so much. Extends pump life and saves energy also.A cruise control for your solar Put the pump on speed 3 and use variable speed function.

After the tank is hot and it diverts to dump zone, what controls that function? Does it shut down based on a high temperature?

Stagnation happens any time the sun is on the collector and the pump is not running, for whatever reason.

Typical flat glass covered collectors will stagnate about 325- 350F or higher. And that happens in seconds on a hot sunny day.

Evac tube can reach stagnation temperatures above above 500°F!

The additional pressure prevents flashing to steam. the Euro systems I visited tend to run around 60 psi. which is why you see a 90 psi relief valve on those Euro designed pump stations.

High pressure on the glycol loops makes some folks nervous as a leak could force some glycol int the DHW side of the solar tank. Always use the non toxic solar fluid.

Caleffi sells a Grundfos replacement for the Wilo. A bit higher performance also Its a 15-58U, U for union connection so it fits into the pump station 1" G unions.

Find them online at SupplyHouse and others

zxcv_1984

Good day,

@hot_rod Variable speed function is running, thanks for asking!
The flow meter may be out of order, but I might try just take it out and cleaning it up before anything else. The sight glass is real dirty.

I'll look for the Caleffi replacement.

As for stagnation, I'd say from experience that with everything in right working order, the system never goes into stagnation, as the excess heat is dumped into the heating system thermal mass.

Now, with the intermittent trouble I've had the past few months, it happened quite a few time. Never before, though.

The decision to dump to heating thermal mass and eventually shutdown if collector T goes over a set temperature is managed by the Deltasol controller. Attached pictures: current flowmeter and pump specs.

@hot_rod

hot_rod said:

Take it apart on the large nut. That leaves one check on the tank, one on the system. There is a fiber washer in that union nut. Try not to rip it. If so make a new one from auto parts gasket material. Water meter EPDM gaskets fit also.

I'm not sure I understand: why should I take off the tank? What do I need to inspect there?

zxcv_1984

..Checked the pump impeller tonight. Beside a small piece of dirt stuck inside, looks fine to me. Charged the system back to 35 psi. I'll see how it behaves tomorrow.

hot_rod

You need to isolate, or remove the expansion tank and adjust the air pressure to a few lbs below 35 psi.

If the tank has the original 12 psi pre-charge and you fill to 35 psi, you reduce or lose that expansion tank capacity, squeeze the air pocket.

If the fluid is brown colored, sharp acidic smell, it has been cooked. The ph is another indicator. if ph is in the 7's, time to get new glycol.

A small spinning turbine wheel inside those flowmeters

Some pics of what happens in a diaphragm type expansion tank.

Any idea how many gallons of fluid in the system? Most package systems come with an over-sized solar expansion tank, not knowing how much fluid will be used in the piping. It looks like a #30 tank, that should handle 15- 20 gallons of solar capacity expansion, even at 35 psi.

Raising the fill pressure reduces the expansion tank capacity, so keep a 5 gallon bucket under the relief valve discharge

Does your flush cart have a filter on it? Get a standard cartridge filter from a plumbing or hardware store to clean out as you flush. I got this stainless steel type many years ago. It handle jobsite abuse better than plastic ones. And high temperature fluid. Two garden hose adapters and washer hoses

zxcv_1984

System works just fine since September 25th. I keep on monitoring. @hot_rod I'll have to adjust the expansion tank pressure while disconnected as your described. Right now tank pressure hasn't been touched since I increased the system pressure from 25 to 35-ish psi.

I've observed some of the brownish stuff you mentioned in the charging system tank. Part of my glycol has been burned, that's for sure. If I take a sample from the circuit it still runs yellow/green and smells sweet.

I'm gonna test pH soon and decide whether to replace the glycol or not.

Any suggestion of pH reader for infrequent uses?

One last question: you mentioned the possibility of using inhibitor booster instead of replacing glycol. Can you recommend any inhibitor booster? I'd like to explore the alternative, but can't find the product anywhere.

hot_rod

Do you know the brand of solar fluid you have? You can often get a boost kit from the manufacturer.

I used this Fernox product, see if it is still available in a concentrate, it is compatible with most all the propylene glycols.

Dow has boost available also. If you find a glycol supplier, most large cities have one or more. They do custom blending and could sell you a small quantity of the additive.

Dow suggest replacing the fluid if the ph drops into the 7's.

What you find with fluid that has dropped from 10.5 new - to 7's is it takes a lot of boost to bring it back. So flushing and starting over may be a better option.

If you need new glycol, the stuff we sell at Caleffi is a bio, corn based glycol. It has a little better temperature range for solar, also.

First step, check the ph of what you have.

Meters from 30 bucks to several hundred. I like this for a middle of the road meter. I like the feature that tells you when to recalibrate.

zxcv_1984

Glycol check:
pH reads at 8,6
Density is at 50%.

I'm impressed. Doing my tests with litmus strips I couldn't read right because of the propylene glycol color, but last measurement (6 months ago) I thought that I was anywhere between 8 and 8,5.

Gonna keep this glycol a bit longer and keep checking it periodically.

@Hotrod glycol brand is Recosol. I'm gonna try to find a glycol distributor near me and see if I can get some inhibitor booster, see if it makes sense economically.

hot_rod

Are they Apricus evac tubes? That is an Australian company, so the AU glycol would have been shipped with it.

I don't know if this is an ethylene based glycol? That should not be used with potable water unless you have a double walled heat exchanger.

Might be time to start over with a power flush, with a garden hose, then add a propylene based solar glycol?

https://www.recochem.com/wp-content/uploads/2021/12/311_EN_DEG-Diethylene-Glycol_Product-Data-Sheet_1.pdf

zxcv_1984

The evac tubes come from SunMaxx kits. Original tubes, still going strong.
The system is over 15 years old, so the heat transfer fluid is definitely not the original, OEM stuff.
It's propylene glycol all right, that I sourced from a local heating supply vendor.
Exact name is Recofreeze PG.
I just couldn't get my hands on "solar grade" propylene glycol when I sourced it, a few years ago.

https://recochem.com/products/recofreeze-pg-fluids/

zxcv_1984

Update:

As my intermittent issue popped again, I jumped on the opportunity to try to solve this once and for all.

First, for those of us who, like me are less experienced, be advised that electronic pH-meter are NOT calibrated right out of the box, and they need periodic calibration. It's all on me, I was a bit too enthusiastic when I got it and skipped reading the instruction.

That being said, great news for me, the latest glycol check with a properly calibrated pH-meter gives healthier number:

Glycol check:
pH reads at 9,2
Density still at 50%.

Glycol is good, and drains properly, so clogged conduit looks out of the hypothesis list.
I checked the voltage at the pump, the controller still energizes the pump as it should. Also checked off the list.
I also opened the pump to check the impeller integrity, which is good. The impeller turns easily, no trace of grime, dirt or any other out of place stuff in the pump body.

With a glycol system emptied from its fluid, I measured expansion tank pressure at 20 psi-ish, so I cranked the pressure back up to 32-33 psi (thanks @hot_rod for pointing that out to me earlier in the thread).

I filled the system back and pressurized it to 35 psi.

Fluid would run and heat would transfer when running on the charging system.

Now, running on the circulation pump for a while: glycol out of pumping station is hot (40degC+), glycol in is measured as room temperature.

Flowmeter works.

I opened the vent screw on the pump, to discover that it turns intermittently. Or stop, and runs again with a little push from a screwdriver.

So my guess is that the pump is the culprit. I'm ordering a new one, hope this clears this thing once and for all

hot_rod

I think you are correct about the lethargic circulator.

The least expensive fix is buy a complete Star 16, like this one on Amazon, and just switch out the motor, the two alan bolts, which it sounds like you have already opened. The Star 16's are getting harder to find, I think most of the Wilo PSC circs have been discontinued.

Or a complete new pump with a union body. Caleffi has a Grundfos replacement. They used to be the yellow label Grundfos sol

ar version, with a higher operating temperature. I'm not sure if we still get Grundfos solar circulators, or it is a standard 15-58.

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