Heat transfer from roof loop to tank below expectations.

[MikeSolar]

OK than explain why every single tank manufacturer that uses an external heat exchanger directs the flow in this manner.
Doing this was a known quantity thirty years ago when I first got involved with this stuff and hasn't changed since.
Secondly I do not reccomend a second (or in this case third) heat exchanger at all. Waste of money IMHO.
I don't know why on earth they used 2 different size tanks unless there was a space issue. but flow should be able to be balanced through them. Otherwise put the two solar tanks in series with each other and the back up tank and run the loop from the collectors in series with the two solar storage tanks feeding the 100 gal first and proceeding to the 80 then the return to the collectors. Or better yet add a pump and split the tank heat exchangers so each operates on its own pump to the array and will be heated and controlled separately.

http://www.thermo-dynamics.com/solar_boiler.html

http://enerworks.com/products/#resid...-product-lines

Both these companies use the method I outlined and the first one is a 30+ year old company with one of the highest output systems available.

Why do some companies have HX in/out at the top of the tank and some at the side? Company preference and we all know that most small companies just follow what their bigger competitor is doing, saves on research.

TBH, I don't know why they still do it this way but there is a lot of inertia in business and system changes cost money, especially things like UL, CSA or SRCC certs. The question is simple though, is it better to have the hottest water at the top of the tank fastest and for the longest time? If so, what is the best method?

[Naptown]

I agree on having a higher flow rate in internal HX. I do stick by the idea of putting the hot directly at the top for the reasons explained above. Plus, we measure the differential temp at the bottom of the tank so lets assume for the moment that you have cold water at that level, and the top of the tank is 80F. The pump will pump perhaps 120f water down to the tank and if it enters at the 1/2 way mark it will mix with the colder water and the 80F you had at the top could now be less (or it could be more but not 120F). If you draw water off for hand washing, you will get colder water than if you allowed the 120F water to enter at the top of the tank. The mixing would be far less. It works really well and I have done this on a lot of systems.

OK than explain why every single tank manufacturer that uses an external heat exchanger directs the flow in this manner.
Doing this was a known quantity thirty years ago when I first got involved with this stuff and hasn't changed since.
Secondly I do not reccomend a second (or in this case third) heat exchanger at all. Waste of money IMHO.
I don't know why on earth they used 2 different size tanks unless there was a space issue. but flow should be able to be balanced through them. Otherwise put the two solar tanks in series with each other and the back up tank and run the loop from the collectors in series with the two solar storage tanks feeding the 100 gal first and proceeding to the 80 then the return to the collectors. Or better yet add a pump and split the tank heat exchangers so each operates on its own pump to the array and will be heated and controlled separately.

[MikeSolar]

I thought we were talking about the internal static heat exchangers not the second external.
In that case yes slow flow is better and a plate HX will have turbulent flow irregardless.
however I would not bring the return into the top. About 1/2 -2/3 down would be about right.

I agree on having a higher flow rate in internal HX. I do stick by the idea of putting the hot directly at the top for the reasons explained above. Plus, we measure the differential temp at the bottom of the tank so lets assume for the moment that you have cold water at that level, and the top of the tank is 80F. The pump will pump perhaps 120f water down to the tank and if it enters at the 1/2 way mark it will mix with the colder water and the 80F you had at the top could now be less (or it could be more but not 120F). If you draw water off for hand washing, you will get colder water than if you allowed the 120F water to enter at the top of the tank. The mixing would be far less. It works really well and I have done this on a lot of systems.

[Naptown]

If all you cared about was effective transfer of heat through the HX, I would agree with you but not if you want to promote stratification in a tank. You want to keep the mixing to a minimum, take cold from the bottom and put hot in the top without stirring it up too much. If you allow it to mix, the probability will be that the water pulled out of the tank, much of the time, will be below the setpoint to turn on the back up heater. If you let it move slowly through the domestic side of the HX, you may lose a bit of efficiency of the HX but you will increase the effectiveness of the system by keeping the backup off. In the end, that is what is most important.

LA, although it is better to have the HX at the bottom of the tank, it will work quite well at the top. The only difference will be that the pumping power will be slightly higher to overcome the thermosyphoning. The other issue is that having it up there will promote thermosyphoning from a hot tank to the panels at night so I would put a 2ft U-bend on the line going out of the HX to the panels to stop it.

I thought we were talking about the internal static heat exchangers not the second external.
In that case yes slow flow is better and a plate HX will have turbulent flow irregardless.
however I would not bring the return into the top. About 1/2 -2/3 down would be about right.

[MikeSolar]

Most internal heat exchangers are simply a coated pipe within the tank.
Slowing down the flow too much will result in laminar flow in both the collectors and heat exchanger. You want turbulent flow through both for highest output.

If all you cared about was effective transfer of heat through the HX, I would agree with you but not if you want to promote stratification in a tank. You want to keep the mixing to a minimum, take cold from the bottom and put hot in the top without stirring it up too much. If you allow it to mix, the probability will be that the water pulled out of the tank, much of the time, will be below the setpoint to turn on the back up heater. If you let it move slowly through the domestic side of the HX, you may lose a bit of efficiency of the HX but you will increase the effectiveness of the system by keeping the backup off. In the end, that is what is most important. Slowing down the DHW flow does not influence the speed of the solar line. It is preset.

LA, although it is better to have the HX at the bottom of the tank, it will work quite well at the top. The only difference will be that the pumping power will be slightly higher to overcome the thermosyphoning. The other issue is that having it up there will promote thermosyphoning from a hot tank to the panels at night so I would put a 2ft U-bend on the line going out of the HX to the panels to stop it.

[Naptown]

Not the best way. If you have cold water coming in at the bottom and some of it is getting heated by the HX, you would not want to have that newly heated hot water mix with the cold near the bottom of the tank. Having it deposited at the top where it can properly stratify will allow the hottest water to be available to the gas tank. This means the gas tank will not see mixed water coming in and be fooled into turning on for short periods of time (which wastes gas).

We have designed many systems where there is no pump on the potable side and the heat will thermosyphon from the HX to the top of the tank but it takes careful HX design and pipework. This is why I said to keep the pump speed as slow as possible and also use the smallest pump possible to get a high dT across the HX.

Most internal heat exchangers are simply a coated pipe within the tank.
Slowing down the flow too much will result in laminar flow in both the collectors and heat exchanger. You want turbulent flow through both for highest output.

[LAWindsurfer]

Optimizing flow rate on potable side of heat exchanger

Not the best way. If you have cold water coming in at the bottom and some of it is getting heated by the HX, you would not want to have that newly heated hot water mix with the cold near the bottom of the tank. Having it deposited at the top where it can properly stratify will allow the hottest water to be available to the gas tank. This means the gas tank will not see mixed water coming in and be fooled into turning on for short periods of time (which wastes gas).

We have designed many systems where there is no pump on the potable side and the heat will thermosyphon from the HX to the top of the tank but it takes careful HX design and pipework. This is why I said to keep the pump speed as slow as possible and also use the smallest pump possible to get a high dT across the HX.

If I understand the situation correctly, MikeSolar's suggestion to slow the potable side pump down to seek "a high dT across the Hx" may or may not improve things depending on the location of the heat exchanger. If the heat exchanger is located at or above the top of the storage tanks, one wants to have a high flow through it, so to bring through the cooler water. If the heat exchanger is located at or below the level of the storage tanks, where it is immersed in the cooler bottom water, you want to keep the circulation rate low to allow the water to stratify and preclude the mixing that will bring the warmer water through the heat exchanger. I'm not sure I can locate the heat exchanger at or below the bottom of the storage tanks in my situation because of physical space constraints. The most convenient place to locate the heat exchanger in my situation is against the ceiling above the storage tanks; if so done, you want to have good circulation. Locating the heat exchanger down low has the advantage that thermally induced circulation will minimize the electrical power and operating costs for the potable water circulation pump. I need to look at the economic tradeoffs; initial investment in building structure to raise the tanks up off the floor versus continuing operating costs for circulating the water.

[MikeSolar]

I would connect the HX out to the cold water inlet side of the tanks instead of the HW out. Pull the dip tubes and cut them so that they are about 12" from the bottom of the tanks and reinsert. Less turbulence and mixing of hot & cold water in the storage tank this way.

Not the best way. If you have cold water coming in at the bottom and some of it is getting heated by the HX, you would not want to have that newly heated hot water mix with the cold near the bottom of the tank. Having it deposited at the top where it can properly stratify will allow the hottest water to be available to the gas tank. This means the gas tank will not see mixed water coming in and be fooled into turning on for short periods of time (which wastes gas).

We have designed many systems where there is no pump on the potable side and the heat will thermosyphon from the HX to the top of the tank but it takes careful HX design and pipework. This is why I said to keep the pump speed as slow as possible and also use the smallest pump possible to get a high dT across the HX.

[LucMan]

I would connect the HX out to the cold water inlet side of the tanks instead of the HW out. Pull the dip tubes and cut them so that they are about 12" from the bottom of the tanks and reinsert. Less turbulence and mixing of hot & cold water in the storage tank this way.

[MikeSolar]

To MikeSolar: This message was prepared prior to Art VanDelay's 9:50 AM Post

I agree that placing anti-scald valve on output of the gas fired heater would increase storage capacity by allowing solar boost to feed water whose temperature exceeds 120 oF to the gas fed water heater. I need to check local code. As repositioning the anti-scald valve involves lots of plumbing, I'm probably going to wait on it until I see impact of circulation pump with supplemental heat exchanger.

I understand your suggestion on relocating T so extra length on input on 100 gal tank is offset by extra length on output of 80 gallon tank. I need to study the actual configuration of the plumbing to see how best to improve the balance. My original schematic diagram was a representation of the installation that was not intended to indicate actual pipe run lengths. In fact the connection diagram is far more complicated because of the physical constraints of the laundry room.

Thanks for alerting me to the potential of sludge / corrosion should air get into the circulation line. I need to check to see if there is an air vent.

You recommended pumping water from the drain through a heat exchanger and the back into the tanks via the hot water outlet. Does the attached sketch illustrate what you’re suggesting?

thanks again for the help.

Yes, it looks good but you can completely disconnect the original HX in the tanks as the Brazetek HX will do the whole job. Actually, having the original HX in the loop may transfer some heat back to the panels at times so i would disconnect them. barring your ability to use a larger tank, this setup will give you the best efficiency. One other note: try to keep the potable water pump as slow enough to get about 30F dT under steady running conditions. You won't always be able to do it, of course, but it is something to shoot for.

[Art VanDelay]

LAW,

The drawing dated 6-26 looks pretty good.

As I said before, 180 gallons total solar storage is less than half of what you should have. Some excess solar can be transferred to the gas-fired tank.
But as you say, verify operation with the new brazed plate exchanger first.

You can install a 3/4" line from the aux tank to the cold line going to the solar tanks. In that line is a check valve and pump connected to a differential control. The pump kicks on when aux. tank temp drops below solar tank temp. (say delta T=5F ON, 3F OFF)"

It's like you are pretending that the solar tanks are solar collectors, and the aux tank is solar storage.

More discussion about a similar piping diagram is here

[LAWindsurfer]

Addition of supplemental heat exchanger and circulation pump.

OK, It is not imperative but the anti-scald valve (mixing valve) that you have on the output of the storage tanks should actually be on the output of the gas tank, this way you can get a bit better storage capacity. It is actually in our code in Canada to have it set for 50C (120F), not sure about Cali.

Putting the new HX at the bottom of the tanks and piping in a reverse return will make the tanks more even. Reverse return mean that the piping from the solar panels enters on the left and exits on the right so that the pump sees both tank as being the same distance away and will then have more even flow. To do this on your drawing just take the tee that is on your return line from the 80 gal tank and put it on the 100 gal tank so the line comes from the left.

Art seems to like using Taco pumps so the 006 and 009 pumps are Tacos. As you have Grundfos, there is no need to change brands. If the current UPS-15-58 works well on the loops, keep it but remember that it will only work well as long no air gets into the system. This means no air vent on the roof. I have seen it many times but this is a cast iron body pump and sludge will form if air gets in.

The pump on the tank side needs to be stainless steel or bronze such as a UPS15-35 will do the trick. Take the drains out and put brass tees in place for the supply to the pump, then take the pump VERTICALLY after joining the tanks. It should pump into the HX from tanks and a strainer would be helpful. Then, up to the HOT out at the top of the tanks. The existing cold can stay for just cold water only.

The issue of balancing the tanks is actually important because it becomes the difference between using 40% of the resource and 50%. The same is true of the mixing valve. The extra volume in the gas tank can help.

To MikeSolar: This message was prepared prior to Art VanDelay's 9:50 AM Post

I agree that placing anti-scald valve on output of the gas fired heater would increase storage capacity by allowing solar boost to feed water whose temperature exceeds 120 oF to the gas fed water heater. I need to check local code. As repositioning the anti-scald valve involves lots of plumbing, I'm probably going to wait on it until I see impact of circulation pump with supplemental heat exchanger.

I understand your suggestion on relocating T so extra length on input on 100 gal tank is offset by extra length on output of 80 gallon tank. I need to study the actual configuration of the plumbing to see how best to improve the balance. My original schematic diagram was a representation of the installation that was not intended to indicate actual pipe run lengths. In fact the connection diagram is far more complicated because of the physical constraints of the laundry room.

Thanks for alerting me to the potential of sludge / corrosion should air get into the circulation line. I need to check to see if there is an air vent.

You recommended pumping water from the drain through a heat exchanger and the back into the tanks via the hot water outlet. Does the attached sketch illustrate what you’re suggesting?

thanks again for the help.

[Art VanDelay]

LAW,

The 006 is the pump you could add to solve your heat exchanger problem. From my post above:

"But your solution isn't that difficult or expensive. Just add a pump: (above the link is still active for a Taco 006 pump) , and a brazed plate stainless steel heat exchanger: http://www.amazon.com/175-000-BTU-30..._sbs_indust_10 "

[MikeSolar]

Sorry for the delay in updating my post but vacant apartment has taken priority.

The attached sketches provide additional detail as well as no longer implying inverted connections to the heat exchanger. One entitled "as built" shows current configuration. One entitled "add-on heat exchanger" shows proposed revised configuration. The photograph shows one portion of the temperature measurement instrumentation. The temperature of the collector loop is measured using "oven meat thermometers." I suspect they're not particularly accurate, but surprisingly they agree with one another within 1 oF, and it is temperature differential that provides an indication of the amount of heat being transferred from the collector loop to the storage tank. There thermostats were also inexpensive. The top left thermostat is showing time rather than temperature... it indicated about 110 oF.

A recirculation line is necessary for an acceptable hot water delay time as the distance between the water heater and the furthest apartment is more than 100 feet. The line is well insulated, but I suspect there is lots of heat leakage to the individual (inside the wall uninsulated ) risers. It can't be defeated and have happy tenants in front units.

There are 14 bathrooms in the building. There are 17 adults, one child and one infant. There are two, 30 gal/load, top loading washing machines, and about 3.5 loads (~100gals) are run each day on average. On Saturday, they're in nearly constant use between 10 AM and 3 PM. Assuming some pause for reloading between the 30 minute cycle times, thats 2x 8-loads or 16 loads and 480 gallons. Hot water usage is likely to be lower as not everyone does every wash with 100% Hot water. During the week, each machine is run for at least one cycle, frequently with the HOT setting for water wash temperature.

The gas fired water heater thermostat is set to ~135 oF which leads to a bathroom faucet temperature (as I recall) of about 125 oF at full flow. The 135 oF setting was established before the solar boost was installed. At the time, if it was kept much lower, I'd occasionally get complaints of inadequate hot water. I could be set the thermostat lower if the solar storage tanks still contained warm water in the morning, but that seems not to be the case. A thermostat that records max and min indicates that the output of the solar storage tank(s) drops to water-main temperature, ~ 65 oF, during a typical 24 hour period. I expect the storage tanks are at their lowest temperature early in the morning. I am well aware that it would be advantageous to have additional storage, but there was/is no more room in the laundry room or in the garage for additional tanks. The smaller 80 gallon tank was dictated by the smaller diameter required for there to be enough room to move washers and dryers out of the laundry room with having to bring in a plumber.

I am also aware that the unsymmetrical plumbing of the two units undoubtedly results in unbalanced heating of the stored water. However, I don't see that as an major issue as it appears that all of the preheated water is used up by morning. Furthermore, as the heat exchanger temperature differential will be less in the hotter tank than in the cooler tank, less heat will be delivered to the hotter tank and more to the cooler tank resulting in a somewhat self balancing situation. Again, as all the preheated water is being used up each day, I want to maximize the amount of captured heat, and it is not important to keep the tanks of equal temperature or to keep the hottest water at the top of the tank.

Could you please explain what is meant by, "The 006 is fine for the HX to tanks." What is the 006? Regarding the ability of the collector pump to overcome the required "head," ... I don't see the collector circulation pump being a problem as the water coming down from the roof is typically far hotter than the water in the storage tanks.

Art VanDelay said, "The problem with those (Rheem) HXs is a design problem with the amount of heat transfer area." Inadequate heat exchanger performance is what I suspected. Would the situation be improved by plumbing the two heat exchangers in series rather than in parallel? If I added a heat exchanger as suggested, would it be best to put it between the warm water take offs or the cool water feeds or some other configuration. (see second sketch) It would seem to me that the pump could be powered from the same controller as that used for the roof panel pumps, perhaps with an additional relay if the controller relay contacts are not sufficiently rated to run two pumps. How would I compute appropriate pump size and/or GPM flow for this second heat exchanger? I could install a three speed pump like the UPS-15 and just experiment to see which setting gives the best results.

Thanks again for your suggestions and help.




[ATTACH=CONFIG]2114[/ATTACH][ATTACH=CONFIG]2115[/ATTACH][ATTACH=CONFIG]2116[/ATTACH]

OK, It is not imperative but the anti-scald valve (mixing valve) that you have on the output of the storage tanks should actually be on the output of the gas tank, this way you can get a bit better storage capacity. It is actually in our code in Canada to have it set for 50C (120F), not sure about Cali.

Putting the new HX at the bottom of the tanks and piping in a reverse return will make the tanks more even. Reverse return mean that the piping from the solar panels enters on the left and exits on the right so that the pump sees both tank as being the same distance away and will then have more even flow. To do this on your drawing just take the tee that is on your return line from the 80 gal tank and put it on the 100 gal tank so the line comes from the left.

Art seems to like using Taco pumps so the 006 and 009 pumps are Tacos. As you have Grundfos, there is no need to change brands. If the current UPS-15-58 works well on the loops, keep it but remember that it will only work well as long no air gets into the system. This means no air vent on the roof. I have seen it many times but this is a cast iron body pump and sludge will form if air gets in.

The pump on the tank side needs to be stainless steel or bronze such as a UPS15-35 will do the trick. Take the drains out and put brass tees in place for the supply to the pump, then take the pump VERTICALLY after joining the tanks. It should pump into the HX from tanks and a strainer would be helpful. Then, up to the HOT out at the top of the tanks. The existing cold can stay for just cold water only.

The issue of balancing the tanks is actually important because it becomes the difference between using 40% of the resource and 50%. The same is true of the mixing valve. The extra volume in the gas tank can help.

[LAWindsurfer]

Heat Transfer from roof loop below expectations - additional detail 3

Sorry for the delay in updating my post but vacant apartment has taken priority.

The attached sketches provide additional detail as well as no longer implying inverted connections to the heat exchanger. One entitled "as built" shows current configuration. One entitled "add-on heat exchanger" shows proposed revised configuration. The photograph shows one portion of the temperature measurement instrumentation. The temperature of the collector loop is measured using "oven meat thermometers." I suspect they're not particularly accurate, but surprisingly they agree with one another within 1 oF, and it is temperature differential that provides an indication of the amount of heat being transferred from the collector loop to the storage tank. There thermostats were also inexpensive. The top left thermostat is showing time rather than temperature... it indicated about 110 oF.

A recirculation line is necessary for an acceptable hot water delay time as the distance between the water heater and the furthest apartment is more than 100 feet. The line is well insulated, but I suspect there is lots of heat leakage to the individual (inside the wall uninsulated ) risers. It can't be defeated and have happy tenants in front units.

There are 14 bathrooms in the building. There are 17 adults, one child and one infant. There are two, 30 gal/load, top loading washing machines, and about 3.5 loads (~100gals) are run each day on average. On Saturday, they're in nearly constant use between 10 AM and 3 PM. Assuming some pause for reloading between the 30 minute cycle times, thats 2x 8-loads or 16 loads and 480 gallons. Hot water usage is likely to be lower as not everyone does every wash with 100% Hot water. During the week, each machine is run for at least one cycle, frequently with the HOT setting for water wash temperature.

The gas fired water heater thermostat is set to ~135 oF which leads to a bathroom faucet temperature (as I recall) of about 125 oF at full flow. The 135 oF setting was established before the solar boost was installed. At the time, if it was kept much lower, I'd occasionally get complaints of inadequate hot water. I could be set the thermostat lower if the solar storage tanks still contained warm water in the morning, but that seems not to be the case. A thermostat that records max and min indicates that the output of the solar storage tank(s) drops to water-main temperature, ~ 65 oF, during a typical 24 hour period. I expect the storage tanks are at their lowest temperature early in the morning. I am well aware that it would be advantageous to have additional storage, but there was/is no more room in the laundry room or in the garage for additional tanks. The smaller 80 gallon tank was dictated by the smaller diameter required for there to be enough room to move washers and dryers out of the laundry room with having to bring in a plumber.

I am also aware that the unsymmetrical plumbing of the two units undoubtedly results in unbalanced heating of the stored water. However, I don't see that as an major issue as it appears that all of the preheated water is used up by morning. Furthermore, as the heat exchanger temperature differential will be less in the hotter tank than in the cooler tank, less heat will be delivered to the hotter tank and more to the cooler tank resulting in a somewhat self balancing situation. Again, as all the preheated water is being used up each day, I want to maximize the amount of captured heat, and it is not important to keep the tanks of equal temperature or to keep the hottest water at the top of the tank.

Could you please explain what is meant by, "The 006 is fine for the HX to tanks." What is the 006? Regarding the ability of the collector pump to overcome the required "head," ... I don't see the collector circulation pump being a problem as the water coming down from the roof is typically far hotter than the water in the storage tanks.

Art VanDelay said, "The problem with those (Rheem) HXs is a design problem with the amount of heat transfer area." Inadequate heat exchanger performance is what I suspected. Would the situation be improved by plumbing the two heat exchangers in series rather than in parallel? If I added a heat exchanger as suggested, would it be best to put it between the warm water take offs or the cool water feeds or some other configuration. (see second sketch) It would seem to me that the pump could be powered from the same controller as that used for the roof panel pumps, perhaps with an additional relay if the controller relay contacts are not sufficiently rated to run two pumps. How would I compute appropriate pump size and/or GPM flow for this second heat exchanger? I could install a three speed pump like the UPS-15 and just experiment to see which setting gives the best results.

Thanks again for your suggestions and help.




V0-SolarHotWater As Built.jpgV0-SolarHotWater Exchanger.jpgV0-SolarTempIndicate (12) (Large).JPG