How to prevent overheating?

[Guido in Boston]

Interesting you bring that up. I noticed that the solar panel in and out ports on the tank are backwards with respect to the circulator pump. The cold water in and hot out are correct.

https://www.thermomegatech.com/product/pump-thermal-relief-valve/

[Guido in Boston]

65 gallon now! NO WAY unless you don't use any hot water for 3-4 days.
Check the piping it sounds to me like the the solar heated water is being short circuited within the tank sending hot water back to the collector instead of the cooler water from the bottom of the tank being heated. The stratified water at the top of the tank is being "superheated".
Recommended collector for 65 gallons is 32 square ft.

Interesting you bring that up. I noticed that the solar panel in and out ports on the tank are backwards with respect to the circulator pump. The cold water in and hot out are correct.

[LucMan]

65 gallon now! NO WAY unless you don't use any hot water for 3-4 days.
Check the piping it sounds to me like the the solar heated water is being short circuited within the tank sending hot water back to the collector instead of the cooler water from the bottom of the tank being heated. The stratified water at the top of the tank is being "superheated".
Recommended collector for 65 gallons is 32 square ft.

[Guido in Boston]

Yes, indeed !! And that 120 F number you write of is so far from the 180 - 190 F. you seem to be getting only points out that - as I wrote in a prior post - something in your numbers is simply not adding up.

I didn't calculate the daily tank addition much better than back of envelope stuff.
That was an educated dart throw based on modeled irradiance data for Miami from the Solar Radiation Data Manual put out by NREL for a south facing surface at a 20 degree tilt to the horizontal which shows and August daily average insolation input of 5.6 kWh/m^2/day. which ~ = 1,775 or so BTU/(ft.^2*day) and at an unlikely optimum system efficiency of something like 40% or a bit more. That's what people close to the equator probably can expect for water heating collectors most anywhere. The output is more dependent on solar availability, collector design and collector orientation than latitude.

Maybe the thermostat that regulates the tank's electric heating element is not functioning correctly - or set too high - or both. If the thermostat is heating the water to, say, 160 F and the collector boosts it another, say 25 F, that'll get 160 + 24 = 185 F. water.
Smaller thermal losses in the summer or losses a bit higher in the winter might make that a bit higher or lower.
Can you do a little self help and disconnect the tank's heating element for a couple of days, allow the tank to cool some and see what happens in terms of tank temp. on a sunny day ?

Or, are you sure that the collector area is only 8 ft.^2 ?
That almost seems like a system meant for camping or some portable type operation.
I'm reasonably certain that 8 Ft.^2 of collector surface in So. FL won't get 30 gal. of water to 180 F. unless it starts the day at around 155. Check the tank thermostat.
Any data sheets or mfg. cut sheets you can share ?

While there is a thermostat and electric heating element in the tank, I have the breaker off at all times so no BTUs from electricity. I also monitor my power usage daily and it's all refrig and A/C.
The collector may be larger - what's next one up? 3x5? So 15 sqft vs. 8, or roughly double. No idea who made the collector. The tank is a Lochinvar FTA066KS - 65 gallons (my bad, again, but in the wrong direction). Angle from horizontal is 5 rise and 12 run.

[J.P.M.]

So far, nothing seems to explain how the water in the tank is overheating. I agree that adding another tank in the summer would improve the situation. The baseline (cold) temperature of the water is about 85F so 24F rise in the course of a day would be 110F. Even a 50% increase in thermal BTUs is 85+36=120F which is nowhere near the 180-190 I get in summer.

Not sure if you are calculating BTUs for SoFla latitude correctly - I'm at 24.5. Just out of curiosity, what do people closer to the equator get for BTU input to solar thermal systems?

Yes, indeed !! And that 120 F number you write of is so far from the 180 - 190 F. you seem to be getting only points out that - as I wrote in a prior post - something in your numbers is simply not adding up.

I didn't calculate the daily tank addition much better than back of envelope stuff.
That was an educated dart throw based on modeled irradiance data for Miami from the Solar Radiation Data Manual put out by NREL for a south facing surface at a 20 degree tilt to the horizontal which shows and August daily average insolation input of 5.6 kWh/m^2/day. which ~ = 1,775 or so BTU/(ft.^2*day) and at an unlikely optimum system efficiency of something like 40% or a bit more. That's what people close to the equator probably can expect for water heating collectors most anywhere. The output is more dependent on solar availability, collector design and collector orientation than latitude.

Maybe the thermostat that regulates the tank's electric heating element is not functioning correctly - or set too high - or both. If the thermostat is heating the water to, say, 160 F and the collector boosts it another, say 25 F, that'll get 160 + 24 = 185 F. water.
Smaller thermal losses in the summer or losses a bit higher in the winter might make that a bit higher or lower.
Can you do a little self help and disconnect the tank's heating element for a couple of days, allow the tank to cool some and see what happens in terms of tank temp. on a sunny day ?

Or, are you sure that the collector area is only 8 ft.^2 ?
That almost seems like a system meant for camping or some portable type operation.
I'm reasonably certain that 8 Ft.^2 of collector surface in So. FL won't get 30 gal. of water to 180 F. unless it starts the day at around 155. Check the tank thermostat.
Any data sheets or mfg. cut sheets you can share ?

[Guido in Boston]

Yea, I stated something along those lines in my 09/22,8:35 P.M. post when SWAGing about a 24 F. rise from what is perhaps something like a 6,000 BTU daylong input from an 8 ft.^2 collector to a 30 gal. tank but I didn't get a bite on that from the OP.
Also, with that much piping through a 2 story piping arrangement, I'd bet that pump wouldn't produce as much flow as a simple thermosiphon system would produce with the tank at the top of the collector.
Besides, it's an older system that probably needs service and my guess is it wasn't the best designed or high quality system to begin with peddled to someone who didn't have the necessary information to make a good decision.

So far, nothing seems to explain how the water in the tank is overheating. I agree that adding another tank in the summer would improve the situation. The baseline (cold) temperature of the water is about 85F so 24F rise in the course of a day would be 110F. Even a 50% increase in thermal BTUs is 85+36=120F which is nowhere near the 180-190 I get in summer.

Not sure if you are calculating BTUs for SoFla latitude correctly - I'm at 24.5. Just out of curiosity, what do people closer to the equator get for BTU input to solar thermal systems?

[J.P.M.]

First thing that comes to my mind is that a 2 x 4 flat plate should not be able to overheat a 30 gallon storage tank.

Yea, I stated something along those lines in my 09/22,8:35 P.M. post when SWAGing about a 24 F. rise from what is perhaps something like a 6,000 BTU daylong input from an 8 ft.^2 collector to a 30 gal. tank but I didn't get a bite on that from the OP.
Also, with that much piping through a 2 story piping arrangement, I'd bet that pump wouldn't produce as much flow as a simple thermosiphon system would produce with the tank at the top of the collector.
Besides, it's an older system that probably needs service and my guess is it wasn't the best designed or high quality system to begin with peddled to someone who didn't have the necessary information to make a good decision.

[Guido in Boston]

First thing that comes to my mind is that a 2 x 4 flat plate should not be able to overheat a 30 gallon storage tank. My first question is how old is the system? Second, is this a new problem or has overheating been an issue since the initial installation?

Installation was in 2017. Winter home, and this is only the 2nd summer I have been there. Same problem last summer but to a lesser degree.

[LucMan]

First thing that comes to my mind is that a 2 x 4 flat plate should not be able to overheat a 30 gallon storage tank. My first question is how old is the system? Second, is this a new problem or has overheating been an issue since the initial installation?

[J.P.M.]

Thanks for taking the time to work with me on my problem, JPM, and for providing some fine education at the same time. Very much appreciated. Will have a look at the text you recommend.
Kind regards,
Guido in Boston, PE

You're welcome.
I was taught and mentored by many engineers who shared many things with me.
One of them was an obligation to share what they'd learned and shoot straight.

Again,

Good Luck,

J.P.M.

[Guido in Boston]

Thanks for taking the time to work with me on my problem, JPM, and for providing some fine education at the same time. Very much appreciated. Will have a look at the text you recommend.
Kind regards,
Guido in Boston, PE

[J.P.M.]

No disrespect intended, but I think you misunderstood, or I poorly described my problem. The one time the panel-mounted T&P lifted was years ago, and as I said, my bad (pump off, system full). After the T&P was replaced all was fine. The problem I am trying to fix now is only a summer problem - winter performance is excellent.

In summer there is so much solar BTU input that in the course of a 9am-3pm sunny day the water reached 190 out of the faucets - I have not let it get any hotter for fear of something bad happening

OK.
Bottom line and then I'm done.
Aside from replacing the system with PV, the simplest solution as I suggested previously is to cover part of the collector in the warm season.

I'm aware of your weather, but a properly secured, corrugated panel will most likely not take off in a hurricane. Secure it well and tether it
You'll just need to place and remove it in the fall and the spring.
I know, a PITA for access. Life is not perfect.
Access to equipment is one part of good design solar peddlers often don't consider.

Next workable solution is also as I previously suggested: To either get a bigger tank or another tank to act as a buffer.

I can't add anything else to the discussion other tan to get a decent undergraduate text on solar thermal processes if you're curious. Duffie and Beckman is probably the best bible for that purpose. A free online download is available.

Good luck in your search for a solution.

J.P.M.

[Guido in Boston]

Which valve is operating (lifting)?

If it's the one at the top of the collector, my guess is it's probably doing so because the flow is either low or plugged for reasons previously described and so the water is just sitting there in the sun getting hot and going nowhere. The low flow or not flowing water then gets to the temperature set point of the valve and the valve lifts. Cooler water replaces the sewered water and the process continues.

Get the system checked out by someone knowledgeable in how solar flat plate water heaters work and if you choose to keep it, get it fixed. It's not functioning as intended and add-on devices won't change or fix it.

The 6,000 BTU energy addition is a daylong estimate for the collector system, and to stress, is only a dart throw.
But since it looks like the system is not functioning, it doesn't make much difference.

No disrespect intended, but I think you misunderstood, or I poorly described my problem. The one time the panel-mounted T&P lifted was years ago, and as I said, my bad (pump off, system full). After the T&P was replaced all was fine. The problem I am trying to fix now is only a summer problem - winter performance is excellent.

In summer there is so much solar BTU input that in the course of a 9am-3pm sunny day the water reached 190 out of the faucets - I have not let it get any hotter for fear of something bad happening

[J.P.M.]

For your questions, there are 2 (code) T&P valves - one on the tank and on on the top of the panel, next to the air bleed. Some years ago I left the system full and the pump off (my bad) and the T&P on the panel blew for some hours since nobody was home, eroded the elastomer seat and would no longer seal. Replaced.

So, 6000 BTU/hr or per sunny-day-length? The HW tank is a Lochinvar electric heat (1500W) one but the breaker is off.

Which valve is operating (lifting)?

If it's the one at the top of the collector, my guess is it's probably doing so because the flow is either low or plugged for reasons previously described and so the water is just sitting there in the sun getting hot and going nowhere. The low flow or not flowing water then gets to the temperature set point of the valve and the valve lifts. Cooler water replaces the sewered water and the process continues.

Get the system checked out by someone knowledgeable in how solar flat plate water heaters work and if you choose to keep it, get it fixed. It's not functioning as intended and add-on devices won't change or fix it.

The 6,000 BTU energy addition is a daylong estimate for the collector system, and to stress, is only a dart throw.
But since it looks like the system is not functioning, it doesn't make much difference.

[Guido in Boston]

Question or 2 on the relief valve that's lifting (operating):
1.) Where is that valve located ?
2.) Is there more than one relief valve on the system ?

On pump curves: Every manufactured pump has a pump curve plotted by the manufacturer as a 2 dimensional graph. The abscissa is the flow rate the pump will produce - usually in G.P.M. or litres/time period (sec., minute or hour depending on the application). The ordinate is the "head" or pressure drop that the pump will successfully work against to produce the flow rate shown from the graph at the point where the two parameters intersect on the pump curve.

For the SID5PV pump, the curve tells us it will sustain a flowrate of ~ 2.5 G.P.M, when the flow is frictionless (zero pressure drop) - an impossible situation but it's a base # so to speak. At the other end of the curve, the other limit is the maximum "head" or head pressure the pump will produce when operating under "deadhead" or zero flowrate conditions. You might visualize that as how high a standing column of water you'd see if the pump's outlet were to be plumbed to a clear vertical pipe of sufficient height so as not to overflow.
From the SID5PD pump curve, that water column looks to be something like maybe ~ 2.2 ft. in height - from looking at the point where the pump curve meets the ordinate axis at the left side of the graph.

Any other point on the curve represents the flowrate attainable by the pump given the pressure drop induced and that flowrate as the fluid is forced through the fluid conduit and all the associated valves, fittings, branches. tees, diverters, filters and other things that cause pressure drop.

All that comes down to my suspicion that, because you probably have at least a P.S.I . or so of friction loss even at a low flowrate (BTW, 1 P.S.I. of pressure drop ~ = 2.3 ft. of head). your system doesn't have too high a flowrate. However, if you have 30 gal. of hot water at something > 200 F from 8 ft^2 of collector surface, some things are not adding up. On a hot summer day in FL, your system - if it was operating at a good overall daily thermal efficiency of, say, 50% - which is a high dart throw estimate - might add something like 6,000 BTU of heat (a back of the envelope calc, but probably optimistic) to that water. But, low flowrates tend to deduce thermal efficiency and at close to stagnation flowrates, efficiengy approaches zero meaning no heat gain.
If things were running at 50% thermal efficiency, on a hot, sunny summer day in FL, that would raise the H2O in a 30 gal. tank by something like ~ 24 F or so before consideration of standby losses. So, where's the rest of the heat coming from ?
Does the tank have another energy source like electricity or natural gas ?
If so, what's that sources thermostat set at ?
Things are not making sense.

As I wrote, it's difficult to analyze what's going on without being there.
Get a plumber on site who knows something about solar water heating and have the system analyzed or as I suggesed get a PV system .
I'm not getting anywhere with this line of reasoning.

For your questions, there are 2 (code) T&P valves - one on the tank and on on the top of the panel, next to the air bleed. Some years ago I left the system full and the pump off (my bad) and the T&P on the panel blew for some hours since nobody was home, eroded the elastomer seat and would no longer seal. Replaced.

So, 6000 BTU/hr or per sunny-day-length? The HW tank is a Lochinvar electric heat (1500W) one but the breaker is off.