How to prevent overheating?

[J.P.M.]

Collector is about 2 x 4 feet, very roughly. Never got up on the roof to measure it.

I have the model SID5PV pump. Curve below. Fluid dynamics was never my strong suit. Enlighten me, master (Midnight Diner music plays softly in the background).

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.

[Guido in Boston]

You're welcome.

If the air bleed and the check valve haven't been inspected or serviced in a while, I'd suggest they're at least due and probably causing problems.
If the air bleed is not bleeding air sufficiently, that may be the source of at least some of your overheating due to a vapor lock that forms and then comes/goes from slow/partial valve operation.
What's the collector surface area ?
Which El Sid pump do you have ? Seems like most of them are pretty low head for that much elevation change for one thing. Also, if it a low head pump, besides needing a bigger pump to overcome the elevation head when filling the system, it may produce insufficient head to maintain a decent flowrate.
It's neigh onto impossible to troubleshoot or analyze a plumbing system remotely with incomplete information.

Without editorializing: To your questions, the answer to both is no.

Collector is about 2 x 4 feet, very roughly. Never got up on the roof to measure it.

I have the model SID5PV pump. Curve below. Fluid dynamics was never my strong suit. Enlighten me, master (Midnight Diner music plays softly in the background).

[J.P.M.]

Thank for the very educational post. Would love to install PV and expect to do so in the next decade once grid-connected PV is permitted. Yes, now that you say the check valve is fluidically after the circulator pump, I do have one.

The panel is 2 stories above the HW tank, on the standing seam roof. Not accessible without a 2 story ladder and good sticky shoes.

Mismatched tempco was the bane of my existance for more than a year - major failure analysis program on MX. Did you ever read the old Design News magazine "Sherlock Ohms" column? I wrote a good one about that subject for them, among others.

Since you seem quite knowledgable, got any background in residential/small-community scale pyrolysis of waste plastic (washed up on beach) to make liquid or gaseous fuels? Not enough mass/time for a waste to energy plant...

You're welcome.

If the air bleed and the check valve haven't been inspected or serviced in a while, I'd suggest they're at least due and probably causing problems.
If the air bleed is not bleeding air sufficiently, that may be the source of at least some of your overheating due to a vapor lock that forms and then comes/goes from slow/partial valve operation.
What's the collector surface area ?
Which El Sid pump do you have ? Seems like most of them are pretty low head for that much elevation change for one thing. Also, if it a low head pump, besides needing a bigger pump to overcome the elevation head when filling the system, it may produce insufficient head to maintain a decent flowrate.
It's neigh onto impossible to troubleshoot or analyze a plumbing system remotely with incomplete information.

Without editorializing: To your questions, the answer to both is no.

[Guido in Boston]

Thank for the very educational post. Would love to install PV and expect to do so in the next decade once grid-connected PV is permitted. Yes, now that you say the check valve is fluidically after the circulator pump, I do have one.

The panel is 2 stories above the HW tank, on the standing seam roof. Not accessible without a 2 story ladder and good sticky shoes.

Mismatched tempco was the bane of my existance for more than a year - major failure analysis program on MX. Did you ever read the old Design News magazine "Sherlock Ohms" column? I wrote a good one about that subject for them, among others.

Since you seem quite knowledgable, got any background in residential/small-community scale pyrolysis of waste plastic (washed up on beach) to make liquid or gaseous fuels? Not enough mass/time for a waste to energy plant...

[J.P.M.]

Yes, licensed plumber for sure. I will sweat a fitting or 2 for a repair, or tape some threaded fittings but would not do an installation. Not sure I have a check valve. I have an air-vent at the top of the panel to allow refilling. Where would check valve be located? what is the purpose? Wish I could get rid of the CPVC but Cu doesn't do well when concrete is poured around it.

One of the other engineers I used to work with sent me this today. Promising but electronic. The analog has a max on time of 10 s so I'm asking about the (undocumented) digital one - WTFM completely, I say.
https://www.grainger.com/product/13X...p;gclsrc=aw.ds

No insurance here in the hurricane zone - cost is 50% of property value per year - hahaha. Naked is more financially responsible unless you are a business.

I used to work at a defcon on Peacekeeper/MX even way farther back in the day. Tell me about QC and DCAS. But we won and the Ruskies lost the USSR. And it was decommissioned so even better.

Sounds like your wife might have worked with Abiomed - have good friends there. Cheers!

The primary purpose of the air bleed valve at the top of a system is to allow trapped air that's evolved out as the water heats up to escape and so avoid an air lock at the top of the system as the air accumulates over time and so completely stops or at least impedes the flow. A temperature stagnated collector system is the result.
An air bleed will also, as you write, make the system easier to fill (and drain) but a simple ball valve will do the same thing for many years without failing.

A check valve is a usually necessary and essential component of most flat plate, fluid circulating solar water heating (or other liquid) systems where the storage tank is below the level of the flat plate collector(s).
It's a one way valve that prevents water from flowing in the opposite direction from the way the pump is pushing the water.
Without a check valve, most, or all, or more than all of the thermal energy gained when the system is heating water can and will be lost beginning when the pump stops due to thermosiphoning.

Check valves are usually placed in fluid circuits near pump outlets in such a way that reverse thermosiphoning will be prevented - at least until the check valve develops a bypass leak from fouling and then it needs to be serviced or replaced as I mentioned in a prior post.
Question: What's the elevation of your storage tank relative to the bottom of your collector(s) ?

On copper and concrete, the way I learned it, unless the concrete has a lot of fly ash or somehow has a lot of sulfur in it, copper and copper tubing (both annealed and drawn types) usually get along well with concrete as lots of old, large and still operating radiant slab hydronic heating systems with copper tubing will attest.
Concrete poured around copper piping systems or ornamental work will bubble off some H2 gas but that's usually not much of a problem as long as the H2 evolves out of the pour.
The bigger problem with copper tubing embedded or set in concrete is most often that the differential thermal expansion of the copper tubing is not properly accounted for and stress corrosion cracking of the copper is the usual result. Plumbing codes handle that situation well for residential applications.

You may be thinking of aluminum in contact with concrete which requires some considerations.
Aluminum reacts with the alkalis in portland cement type concrete and big time hydrogen gas evolution is the result.
Over time this causes the aluminum to corrode and the aluminum oxide material, has a larger volume than the aluminum it evolved from. That greater volume must be accommodated for somehow and nature takes it by cracking or spalling the concrete. In addition, any trapped and evolved hydrogen gas that gets stuck in the concrete can do the same.
Now since concrete has poor strength properties in tension, the concrete runs a real risk of structural failure from either spalls or cracks or both.
Another possibility is that aluminum in contact or close proximity to other metals (rebar, clamps, etc.), especially if there are any chlorides in the mix (think chlorinated H2O to mix the concrete), will result in galvanic corrosion of the aluminum with more volume changes and other associated corrosion problems.

Bottom line, IMO only you'll be safer if you get your system examined by a knowledgeable solar pro and talk to the AHJ.
Or, better and safer yet, if you still want to fool around with alternate energy, stop throwing good money after bad, scrap the solar water heater, quit fooling around with stuff above your pay grade and get a PV system/ You'll be $$ ahead for a lot less PITA.

[Guido in Boston]

You're most welcome.

I'd expect 3 valves plus a couple of drain cocks ought to do it but I'd still respectfully suggest getting a licensed plumber to do the work and also run it by the AHJ before you start.
Just don't skimp on valve quality - that's little more than foot shooting false economy.
While you're at it, add a check valve that works or at least change out the one you may have. If it's more than a couple of years old, you'll most likely find that it's failed partially open from fouling due to mineral deposits and become useless.

Yea, a lot of folks are not favorably inclined toward the AHJ's and often look n them as an impediment to progress. Unfortunately or not, if/when a disaster takes a property or any part of it, insurance carriers usually look for unpermitted improvements as a possible reason to deny a claim.
More just sayin' stuff.
Besides, a lot of times AHJ's do actually know some stuff, like most jurisdictions not liking non-code or otherwise inappropriate stuff from the automotive world for residential service that probably isn't ASME rated and probably also not suitable for potable water service.
A lot of times they can be a source of probably correct, pertinent and practical information.
After dealing with all kinds of code inspectors, including my own QC people, I found an inspector is like a Marine in at least one respect: the inspector can be your best friend or your worst enemy and you get to choose which.

Off topic, my wife's a retired Nurse-Practitioner who specialized in heart transplant and early lvad implementation and applications. She got to work with a lot of manufacturers and designers of medical equipment back in the early days of Lvad's.
We've had some interesting discussions over the years about the similarities between fluid mechanical systems and the human cardio-vascular system. We've both learned a lot from the discussions.
WAY back in the day (as an advisor-assigned side project during grad school days) I got to contribute to the design of one of the first blood heat exchangers used in open heart surgery applications. My part was as one member of the team that did some fairly extensive measurement work on the transport properties of human blood which, curiously enough, had not been extensively investigated up to that time, but essential to heat exchanger design. Most of it was pretty boring but one interesting thing we found and quantified - that was pretty much already known but had not yet been well quantified for heat exchanger design - was that human blood is thixotropic. Fun stuff that with 5 bucks will now buy me a cheap Starbucks.
FWIW, I believe the result of all our work might still be on display in the SUNYAB engineering library.

Yes, licensed plumber for sure. I will sweat a fitting or 2 for a repair, or tape some threaded fittings but would not do an installation. Not sure I have a check valve. I have an air-vent at the top of the panel to allow refilling. Where would check valve be located? what is the purpose? Wish I could get rid of the CPVC but Cu doesn't do well when concrete is poured around it.

One of the other engineers I used to work with sent me this today. Promising but electronic. The analog has a max on time of 10 s so I'm asking about the (undocumented) digital one - WTFM completely, I say.

https://www.grainger.com/product/13X528?gucid=N:N:PS:Paid:GGL:CSM-2295:4P7A1P:20501231&gclid=Cj0KCQjw06-oBhC6ARIsAGuzdw2amutcaao0Oh9Fqsjf5Y3a3rgs_dvtX2oQcxYUlBrL9Oa-b1ih4E4aAnpjEALw_wcB&gclsrc=aw.ds

No insurance here in the hurricane zone - cost is 50% of property value per year - hahaha. Naked is more financially responsible unless you are a business.

I used to work at a defcon on Peacekeeper/MX even way farther back in the day. Tell me about QC and DCAS. But we won and the Ruskies lost the USSR. And it was decommissioned so even better.

Sounds like your wife might have worked with Abiomed - have good friends there. Cheers!

[J.P.M.]

Thanks for the response JPM, well thought out and communicated. AHJ is a joke in this locale.

I was not intending to remove the existing T&P if that's what you thought. Retired P.E. here as well, small world (medical device/product design, not plumbing).

Lacking a commercially available low-temp T&P, or an NPT- or sweat-fitting compatible packaging for an automotive thermostat, I probably will add another HW tank and hope for the best. Will be a project to figure out how to isolate the second tank when it's not needed in the winter with the fewest number of ball valves.

You're most welcome.

I'd expect 3 valves plus a couple of drain cocks ought to do it but I'd still respectfully suggest getting a licensed plumber to do the work and also run it by the AHJ before you start.
Just don't skimp on valve quality - that's little more than foot shooting false economy.
While you're at it, add a check valve that works or at least change out the one you may have. If it's more than a couple of years old, you'll most likely find that it's failed partially open from fouling due to mineral deposits and become useless.

Yea, a lot of folks are not favorably inclined toward the AHJ's and often look n them as an impediment to progress. Unfortunately or not, if/when a disaster takes a property or any part of it, insurance carriers usually look for unpermitted improvements as a possible reason to deny a claim.
More just sayin' stuff.
Besides, a lot of times AHJ's do actually know some stuff, like most jurisdictions not liking non-code or otherwise inappropriate stuff from the automotive world for residential service that probably isn't ASME rated and probably also not suitable for potable water service.
A lot of times they can be a source of probably correct, pertinent and practical information.
After dealing with all kinds of code inspectors, including my own QC people, I found an inspector is like a Marine in at least one respect: the inspector can be your best friend or your worst enemy and you get to choose which.

Off topic, my wife's a retired Nurse-Practitioner who specialized in heart transplant and early lvad implementation and applications. She got to work with a lot of manufacturers and designers of medical equipment back in the early days of Lvad's.
We've had some interesting discussions over the years about the similarities between fluid mechanical systems and the human cardio-vascular system. We've both learned a lot from the discussions.
WAY back in the day (as an advisor-assigned side project during grad school days) I got to contribute to the design of one of the first blood heat exchangers used in open heart surgery applications. My part was as one member of the team that did some fairly extensive measurement work on the transport properties of human blood which, curiously enough, had not been extensively investigated up to that time, but essential to heat exchanger design. Most of it was pretty boring but one interesting thing we found and quantified - that was pretty much already known but had not yet been well quantified for heat exchanger design - was that human blood is thixotropic. Fun stuff that with 5 bucks will now buy me a cheap Starbucks.
FWIW, I believe the result of all our work might still be on display in the SUNYAB engineering library.

[Guido in Boston]

Thanks for the response JPM, well thought out and communicated. AHJ is a joke in this locale.

I was not intending to remove the existing T&P if that's what you thought. Retired P.E. here as well, small world (medical device/product design, not plumbing).

Lacking a commercially available low-temp T&P, or an NPT- or sweat-fitting compatible packaging for an automotive thermostat, I probably will add another HW tank and hope for the best. Will be a project to figure out how to isolate the second tank when it's not needed in the winter with the fewest number of ball valves.

[J.P.M.]

Yes, snowbird - Boston summer, South in the winter. Quest que c'est AHJ? The rural "Global South" is not as technologically advanced as the rest of the world and monopoly electric companies in small communities can do what they want. Like prohibiting grid tie PV. Which might actually make business sense if some idiot does a DIY grid tie that f*&#s up the grid...

I really think a low-temp (150F) T&P is the best solution but I can't find one. Best I have found is a pressure-washer overtemp device but it has a plastic barb fitting on the outlet that I am very reluctant to incorporate in my home. Any thoughts on customizing the opening temp on a T&P. The other option I would like to use is an automotive thermostat which has the correct opening temp but I can't figure out how to package it for a residential water system. They have rubber hose and hose clamp packaging.

AHJ == Authority Having Jurisdiction.

Usually "the building inspector" or the county or city building inspection dept. - the folks you go to when you need a building permit and also the ones who tell you that you can't do something stupid but can't always explain why it would be dangerous to do so.

As for pressure and temp. relief valves with a lower temperature rating, I've never seen or used or specified such devices but I've been told they exist, mostly for plumbing systems with plastic components. I believe relief valves with pressure and temp. ratings of 100 PSI(G) and 180 F are about as low as you'll find.

Look Guido, I'm not trying to be more Catholic than the Pope here but having been around lots of industrial and commercial energy systems and designed my share of them and seen what can happen when people do dumb and sometimes dangerous chit out of ignorance, laziness or hubris, I've found it safest to not get too fancy or reinvent the wheel, especially when safety is involved. and if you don't think safety is involved in relief valve matters, then you're out on the ignorance limb with a saw.

What you have is a system that has either too small a tank for the collector surface or too large a collector surface for the tank or a collector array that's in need of a higher tilt to avoid summer overheating or a lot of other things. Without seeing it, it sounds like the system design was not well thought out or somehow changed the design after installation.

In any case the last thing I'd suggest is messing around with the relieving devices on the system beyond ensuring they are working as intended and are fit for the intended service.

IMO and meant respectfully, but as a retired P.E. who designed a lot of refinery and power plant equipment, I'd humbly suggest you get some advice from someone who's competent and experienced in solar thermal design and pressure vessel technology to help you sort out what you have and what you want to do.
Without such advice, I see the probability of something going wrong as being greater than I'd be willing to accept.

Take what you want of the above. Scrap the rest.

[Guido in Boston]

You're welcome. Where do you live that prohibits grid tie PV ?

Most any plumbing scheme is possible and usually not a problem with a bit of thought by experienced pros. Just use licensed professionals and check with the AHJ for potential unknown and quirky problems (such as cleaning seasonally unused tanks).
Having done it for going on 17 years at current residence, if any way possible, I'd still consider covering a portion of the collector surface. Cheap, easy and flexible. Just sayin'.

BTW, just curious, your screen name says you're in Boston, but the system's in FL ?

Yes, snowbird - Boston summer, South in the winter. Quest que c'est AHJ? The rural "Global South" is not as technologically advanced as the rest of the world and monopoly electric companies in small communities can do what they want. Like prohibiting grid tie PV. Which might actually make business sense if some idiot does a DIY grid tie that f*&#s up the grid...

I really think a low-temp (150F) T&P is the best solution but I can't find one. Best I have found is a pressure-washer overtemp device but it has a plastic barb fitting on the outlet that I am very reluctant to incorporate in my home. Any thoughts on customizing the opening temp on a T&P. The other option I would like to use is an automotive thermostat which has the correct opening temp but I can't figure out how to package it for a residential water system. They have rubber hose and hose clamp packaging.

[J.P.M.]

Only one collector.

Thanks for the PV suggestion, which does make sense, except PV connection to the grid is prohibited where I am. So it would have to be entirely stand-alone and probably not worth the capital and labor expenditure.

The "second holding tank" option sounds promising, another 30 gallon one, and I wonder if it is possible to plumb it so it's active (in-line) in the summer and inactive (drained, bypassed) in the winter?

You're welcome. Where do you live that prohibits grid tie PV ?

Most any plumbing scheme is possible and usually not a problem with a bit of thought by experienced pros. Just use licensed professionals and check with the AHJ for potential unknown and quirky problems (such as cleaning seasonally unused tanks).
Having done it for going on 17 years at current residence, if any way possible, I'd still consider covering a portion of the collector surface. Cheap, easy and flexible. Just sayin'.

BTW, just curious, your screen name says you're in Boston, but the system's in FL ?

[Guido in Boston]

At this time, since covering part of the collector system seems to be out, my guess is your best bet would be to either get a holding tank and so increase the system volume or, skip the holding tank and decrease the collector area if you have more than one collector.

But if you get a holding tank, you'll need to estimate the system output so you can size the tank. Too big and you'll wind up with tepid water (which may be OK if it's only to preheat water and then boost it the rest of the way with electricity or some fossil fuel fired device).
Or, too small a tank and you may not eliminate the whole problem.

For as (in)efficient as this system probably is, you may be $$ ahead not to mention grief with a PV system that meets some of your household electrical needs and also heats your potable water using a conventional electricity fired tank type water heater.

Only one collector.

Thanks for the PV suggestion, which does make sense, except PV connection to the grid is prohibited where I am. So it would have to be entirely stand-alone and probably not worth the capital and labor expenditure.

The "second holding tank" option sounds promising, another 30 gallon one, and I wonder if it is possible to plumb it so it's active (in-line) in the summer and inactive (drained, bypassed) in the winter?

[J.P.M.]

I have a small PV panel next to the solar thermal. It runs a low voltage pump (El Sid) that circulates the water through the solar thermal panel when the sun is up, and doesn't when the sun is down - clever. No this is a professional installation. No controller. Not thermosiphon - see PV/pump description above.

At this time, since covering part of the collector system seems to be out, my guess is your best bet would be to either get a holding tank and so increase the system volume or, skip the holding tank and decrease the collector area if you have more than one collector.

But if you get a holding tank, you'll need to estimate the system output so you can size the tank. Too big and you'll wind up with tepid water (which may be OK if it's only to preheat water and then boost it the rest of the way with electricity or some fossil fuel fired device).
Or, too small a tank and you may not eliminate the whole problem.

For as (in)efficient as this system probably is, you may be $$ ahead not to mention grief with a PV system that meets some of your household electrical needs and also heats your potable water using a conventional electricity fired tank type water heater.

[Guido in Boston]

So, do you have a differential controller ? If not, is this a thermosiphon (gravity) type heater ?

is this a DIY system ?

I have a small PV panel next to the solar thermal. It runs a low voltage pump (El Sid) that circulates the water through the solar thermal panel when the sun is up, and doesn't when the sun is down - clever. No this is a professional installation. No controller. Not thermosiphon - see PV/pump description above.

[J.P.M.]

Panel is on inaccessible roof. House is in hurricane zone. Electronics are not good in extremely rural and salt-air zones.

So, do you have a differential controller ? If not, is this a thermosiphon (gravity) type heater ?

is this a DIY system ?