Losing heat at night

[Hotnbothered]

Hi and thanks for reading. I have a pumped solar hot water system which is new two panels on the roof feeding a hw storage tank 500 litres topped up by a boilerand there seem to be problems which the engineer is confused about: at night the solar panel ssensor reads hotter than the tank and we have used a inordinate amount of gas far too much.

When I check The panel even at the end of the night before sunrise the ppummp is not running but the roof pannell sensor Is as I say hotter than the very well insulated tank.. I feel that the pump must be on a timer but the tank hot water is still convecting through the roof.....

Any ideas

gas

[Hotnbothered]

Forgot to mention that the 500 ltr tank is 5m below the sp in the basement.

[Ampster]

Seems like you may need a check valve to prevent the hot water from going to the panel. It has been years since I put a hot water solar n but I recall reading about some kind of anti convection loop that also accomplished a similar result.

[Hotnbothered]

Thanks for that Ampster, I'll check with the installer. ....

Any other ideas out there?

[peakbagger]

A very popular failure method of SHW systems is failed check valves. Even if the pumps are not running, the system will naturally thermoshyphon moving warm water from the storage tank to colder panels at night. That would explain a high gas bill, you are effectively heating the outdoors with warm water you heated with gas at night.

In areas with freezing conditions (like mine) the loop up to the roof is anti freeze but it exchanges heat with potable water. On a cold night the potable water side can freeze and then burst the pipes leading to a flood. I was at long term solar outfitters "yard" sale about 10 years ago. They were cleaning out a storage unit they had used for many years. One of lots for sale was several SHW heat exchangers. They were a common design used in early SHW systems and consisting a bundle of 1/2" pipes with return bends on them, every one of them had at least one and sometimes several return bends split from freeze damage. I also have a friend that installed a SWH system about 35 years ago along with several neighbors. His is still running but at least two of the systems had the same check valve failure. He visually confirms that his check valve is functioning every few years. I also had one fail on my system about 5 years ago. I was lucky and caught it before any damage was done.

If it is bad check valve, the only way to stop this condition until you replace the valve is to shut the flow at night off with an isolation valve if there is one. Some of the modern systems have a packaged pump, valve and controller all in one block. Changing a valve can be difficult as the piping was set up to occupy a small space rather than make it serviceable.

You also could have a faulty controller but that is easier to diagnose as you can hear the pump running when its cold out at night. If its an old style controller it could be a stuck relay or bad thermistor input but I think newer ones are solid state so more reliable.

[J.P.M.]

As others have written, in all probability you are reverse thermosiphoning fluid.

If the system is new, ensure you first do indeed have a check valve installed. Then check to see that it's installed correctly (check valves can be installed backwards blocking flow in the desired direction and only allowing reverse flow. If those are not the situation, see if the check valve is fouled in some way. If the system has new components as I believe you wrote, installation debris like filings/metal bits, solder snot that breaks off, whatever, finds it's way to the check valve which is a great debris catcher. It then fails partially open - a situation that only takes a small amount of stuff.

On the gas usage, how much is an inordinate amount ?And for how long ? Coincident with the new panels ?

While it sure looks like your problem is check valve related, there's a chance the problem may be with the control system thinking, for any number of reasons, the collectors are warmer than the storage. Have the control system checked for improperly located or poorly installed temperature sensors, wiring that's not or improperly connected, etc.

Whatever the cause, know that check valves are indeed high on the list of reasons for failed/improperly operating solar thermal systems. Worse, yet, they can partially fail (and always silently) so without some diligence by owners, which is rare to nonexistent, a system will sit on a roof wasting energy and the owners will never know it. I'd SWAG about half the direct type solar thermal water systems in my HOA are non to very poorly operational because of faulty check valves. The owners are usually clueless about it and some don't care when informed that there's an energy hog on their roof. Check valves need regular service around here as the water is, like many/most places, pretty hard, making problems with scaling/fouling more prominent.

[Hotnbothered]

Thanks all for your help so far..... I have a couple of questions especially to JPM. SWAG? Is this an acronym or do you mean insulate like lag. Also what is HOA please?
Should the check valve which in England UK well call a one-way valve where should this be placed on the cold feed to the panel the return or BOTH?
Thanks once again. I want to understand this system its pros and cons and especially so that if a problem exists I can at least talk with the plumber with some knowledge. My plumber seems very confident and bullish but I need to resolve this issue.
Thanks again all.

Ps you asked how mucg gas I used. A five foot lpg gas bottle in 6 weeks for sixty shower and three baths, I go to the gym a lot. Otherwise no significant heating other than the air above my house!

[peakbagger]

SWAG - Scientific Wild Assed Guess. Used in the engineering/science profession to preface a somewhat hopefully educated assumption of an observation. Engineers especially those with professional licenses generally need to back up their assumptions with calculations and analysis which can get tedious and expensive. Generally when SWAG is used its a qualifier that we are skipping the calculations and analysis and giving an initial assumption based on past experience.

In the case of the malfunctioning SHW system we could come up with a proposed testing program to determine if there is a technical issue and if so what it may be. This can take time and cost money using resources that a typical poster on a forum may not have. The alternative used by myself and others is make an educated guess based on observations of past experiences with similar systems to point you in the direction where the problem may be. Hopefully you will understand the system a bit more and be able to apply that to your system.

If I had access to a thermal camera I would probably just stand in a location with good view of the panel on a cold night and look at the thermal profile of the panels relative to the surroundings. If I see that the panels are warmer than the surroundings on a cold night that points me to a problem that heat is circulating from a warm part of the system to what should be a cold part. I would move down to the where the SHW pump and tank is located and rule out that the controller is not malfunctioning. I also would probably use the thermal camera to see the relative temperatures of the heat transfer fluid heading up to and returning from the roof. If the line going up to the roof at night is warmer than the line coming back that is another indication that thermosyphoning is most likely happening. At that point you could drain the system and do a detailed inspection of the check valve but based on past experience I would have a spare in hand or in the rare occasion if it was rebuildable I would have rebuild kit ready so I did not need to leave the system non operational while I was determining the condition of the original check valve.

Note there can also be another similar but different thermosyphoning issue related to a vertical pipe coming out of a warm tank. If the pipe goes up vertically from a hot tank there can be internal heat circulation in the vertical tube even though there is no flow in the overall loop. On hot water tanks some are equipped with "heat trap" valves that are a special type of check valve with a light spring or a weighed check that stays closed unless there is actual draw on the system. The alternative is a loop of piping where warm water is trapped in top of the loop to keep this from happening. In this case a thermal camera pointed at the roof would observe that the upper return line of the panel and possibly the top header near the pipe would be warmer than the surroundings.

[Hotnbothered]

Just a quick thanks for that peakbagger I will get back in a day or two as crazy busy at the moment.... Heat camera wonder if there's an app or something anyway as Patten said I will return...

[J.P.M.]

SWAG - Scientific Wild Assed Guess. Used in the engineering/science profession to preface a somewhat hopefully educated assumption of an observation. Engineers especially those with professional licenses generally need to back up their assumptions with calculations and analysis which can get tedious and expensive. Generally when SWAG is used its qualifier that we are skipping the calculations and analysis and giving an initial assumption based on past experience.

In the case of the malfunctioning SHW system we could come up with a proposed testing program to determine if there is a technical issue and if so what it may be. This can take time and cost money using resources that a typical poster on a forum may not have. The alternative used by myself and others is make an educated guess based on observations of past experiences with similar systems to point you in the direction where the problem may be. Hopefully you will understand the system a bit more and be able to apply that to your system.

If I had access to a thermal camera I would probably just stand in a location with good view of the panel on a cold night and look at the thermal profile of the panels relative to the surroundings. If I see that the panels are warmer than the surroundings on a cold night that points me to a problem that heat is circulating from a warm part of the system to what should be a cold part. I would move down to the where the SHW pump and tank is located and rule out that the controller is not malfunctioning. I also would probably use the thermal camera to see the relative temperatures of the heat transfer fluid heading up to and returning from the roof. If the line going up to the roof at night is warmer than the line coming back that is another indication that thermosyphoning is most likely happening. At that point you could drain the system and do a detailed inspection of the check valve but based on past experience I would have a spare in hand or in the rare occasion if it was rebuildable I would have rebuild kit ready so I did not need to leave the system non operational while I was determining the condition of the original check valve.

Note there can also be another similar but different thermosyphoning issue related to a vertical pipe coming out of a warm tank. If the pipe goes up vertically from a hot tank there can be internal heat circulation in the vertical tube even though there is no flow in the overall loop. On hot water tanks some are equipped with "heat trap" valves that are a special type of check valve with a light spring or a weighed check that stays closed unless there is actual draw on the system. The alternative is a loop of piping where warm water is trapped in top of the loop to keep this from happening. In this case a thermal camera pointed at the roof would observe that the upper return line of the panel and possibly the top header near the pipe would be warmer than the surroundings.

FWIW, Nicely put.

On heat traps off/around domestic how water tanks, I've found them to be about as effective as a check valve for the hot water supply line to the rest of the dwelling, and work best when the leg height from the high point in the loop to bottom point in the loop is ~~ half the tank height (~ 75-80 m. for my tank). Less will still work, but will not be as effective at reducing heat loss as a greater vertical distance. As might be expected, that loop to the house outlets becomes less effective as the tank fluid temp. increases, but I've not quantified how much by measurement, but calc wise it's related to the temp. diff. of the tank water to the line water and the change of fluid density with respect to temp. and to some extent the rate of change of that rate of change of density as f(temp.), at least for incompressible fluids.

For the collector loop, either for direct or indirect systems, because the vertical distance from storage to collectors is usually 5-10 m or more, short loops (that is < that 5-10 m) don't work too well, and loops that great create their own added problems. So a check valve or other means such as a motorized ball valve is required. Example: I've got a thermosyphon height trap (a loop) on my direct system collector loop that runs from ~ 20 cm above the tank high point to ~ 15 cm from the floor, a distance of ~ 150 -160 cm or so. The collectors are ~ 7 m or so above the bottom of that loop. Without the check valve, the loop is useless to prevent thermosiphoning when the collector temps are < the storage temp. All collector lines are insulated w/ 1" closed cell foam.

I change out the check valve as a matter of routine system maint. about every 2 yrs. or sooner as I've got hard water. It's easy to spot when it fails. Things get warm where they're supposed to be cold at night. The check valve is integral with the pump. I had a separate, additional spring check valve in the system, but found it fouled/failed as quickly and was a PITA to change out, so, I eliminated it. I also tried swing (gravity) type valves, but the mineral buildup on the gate quickly made them ineffective. Point is, locate and plumb the check valve so it's accessible for service/cleaning or changeout.

[J.P.M.]

Thanks all for your help so far..... I have a couple of questions especially to JPM. SWAG? Is this an acronym or do you mean insulate like lag. Also what is HOA please?
Should the check valve which in England UK well call a one-way valve where should this be placed on the cold feed to the panel the return or BOTH?
Thanks once again. I want to understand this system its pros and cons and especially so that if a problem exists I can at least talk with the plumber with some knowledge. My plumber seems very confident and bullish but I need to resolve this issue.
Thanks again all.

Ps you asked how mucg gas I used. A five foot lpg gas bottle in 6 weeks for sixty shower and three baths, I go to the gym a lot. Otherwise no significant heating other than the air above my house!

Hot'n: I apologize for doing what I dislike when it's done to me, that is, using abbreviations and rudely assuming the meaning is known by all.

Peak bagger has done a better job of explaining SWAG than I ever could. The other acronym I used in my post, HOA == Home Owners Association. That's often and most commonly the term applied to the governing body for planned communities in the U.S., and probably other countries as well. One other common term I and some others use is PITA == pain in the ass.

As to check valve location: My check valve is integral to my pump, but depending on piping layout, to prevent thermosiphoning, the actual location of a check valve relative to other components is not that important as long as the direction of flow is correct. An exception to that would be for drainback or drain down systems. There, and obvious reasons, don't put the check valve between the collectors and where they will drain to. but the use of a check valve in such systems to prevent thermosiphoning alone is unnecessary. That's not the type of system what you have anyway, at least that's my understanding from what I've read. Unless the application makes it inappropriate, a common location, at least for most spring type check valves is close(r) to and above the pump outlet. When I was fooling around w/check valves and thermal loop heights etc. the additional check valves were always close to and downstream of the pump outlet.

Gravity swing check valves are obviously dependent on gravity keeping them closed when the pump is not operating so they must be mostly or entirely vertical. Or at some angle to vertical for some weird (or inappropriate) application for the chosen valve, but that's off topic for this discussion.

More than one check valve is not always a good idea for several reasons, one of them being if one valve fails it'll be hard to know it. More than one is probably an unnecessary redundancy. Also, depending on the piping layout, even though it's perhaps unlikely to occur or cause problems, it may be possible to isolate a portion of the line so that when it cools down the invoked stresses from thermally induced dimensional changes might cause problems. To repeat, unlikely, but possible. Also, any check valve will increase pressure drop in the system. That will reduce the fluid flow rate and that will probably decrease or at least not improve the system thermal performance. Depending on how much the flowrate is decreased, that decrease will also most likely have a detrimental effect on the system fouling rate, resulting in further thermal and fluid-mechanical performance decreases.

Be careful about how you handle system thermal expansion. For either open or closed loop systems, thermal fluid volumetric expansion must be considered for all loops. Same for drainback systems but not quite so critical and as more of a check for off standard/upset conditions, again, depending on the design. Expansion tanks are a common way to address the situation.

I'm sure not there, and NOMB ( None Of My Business), but sometimes folks who are unsure of the territory they're in are brusque and blustery. Perhaps that's part of what's up with your plumber. Or, maybe he thinks your a fool and suffers fools badly. Is he good at giving you understandable answers to your questions ?

Lucman probably has more information than I have, and I'm pretty sure more experience at installation and troubleshooting of solar thermal systems than me. He may choose to offer correction/comment/additions to my above spoor.

Good luck.

[LucMan]

Hi and thanks for reading. I have a pumped solar hot water system which is new two panels on the roof feeding a hw storage tank 500 litres topped up by a boilerand there seem to be problems which the engineer is confused about: at night the solar panel ssensor reads hotter than the tank and we have used a inordinate amount of gas far too much.

When I check The panel even at the end of the night before sunrise the ppummp is not running but the roof pannell sensor Is as I say hotter than the very well insulated tank.. I feel that the pump must be on a timer but the tank hot water is still convecting through the roof.....

Any ideas

gas

As others have mentioned the check valve is suspect. What troubles me is the fact that you say that in the morning the collector sensor reads warmer than the tank, not likely unless there is a problem with the sensor. Check your manual for thermistor specs.
Some controllers use a different thermistor for the tank and the collector. NTC 10k thermistors are are rated at 10K ohms at (25C) 77 degrees , I have seen PTC thermistors used in combination with the NTC's. The cables connected to the controller are color coded gray for one type and black for the the other. NTC 10K for tank (gray), PT 1000 black for collector ,check your manual . Worth a check if they have been reversed you will have a problem. It's also possible that they may have been spliced to increase their length and not properly identified at the connection.
Here's another check, the pump packs generally have a ball valve installed internally under the thermometer above the pump, turning the thermometer opens and closes the valve. Turn it on and off to make sure the pump is not running or to prevent thermosyphon.
The pump is controlled by the differential controller ( no timer) when it senses via the the thermistors that the collector is 7-10 degrees F warmer than the the tank it turns the pump on and circulates the heat transfer fluid.
Is the boiler connected to the top coil in your storage tank?
Again study your manual and check your installers work.