Heat transfer from roof loop to tank below expectations.

[Naptown]

Mike
Why are you adding another layer of complexity, cost and potential failure points, for what would be a very small gain at best. And if it is a single tank system that pumping would be the last thing you would want to do.
BTW I don't use in most cases external heat exchangers and the additional pump required for them.

[Naptown]

I would qualify that with "for solar water heating"

If the goal is to produce hot water to satisfy the consumer of hot water, absolutely right IMHO.

If the goal is to get the largest number of calories of heat into the tank and you don't care about stratification, then pulling the coldest water into the panel and returning the output to the hottest part of the tank will harvest the most total heat per hour from the panels. That sort of need might happen in a room heating application with a baseboard loop rather than a water heating application, and would give you the largest amount of stored heat by the time the sun goes down. (And so it might be specified in an a handbook oriented to HVAC rather than domestic water?)

On the other hand, it is probably just as likely that the writer did not think things through properly.

Were we talking about space heating or water heating.
I don't recall space heating ever being mentioned in this thread.

[LucMan]

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.

Exactly why I install the HX out to the cold water side this keeps mixing to a minimum, the coldest water is at the bottom of the tank, assuming a delta t through the HX of 20 degrees and an entering cold temp of 55 degrees dumping 75 degree water into the top of the tank mixing with 120 + water is not the best solution. I would rather heat the bottom of the tank water with little turbulence and mixing. Efficiency also increases with colder water entering the HX. Most of the new commercial 200 gal +water storage tanks have the HX connections at the bottom of the tank not at the top for that reason. It's really simple hot water rises cold water sinks why fight physics.

[MikeSolar]

Mike
Why are you adding another layer of complexity, cost and potential failure points, for what would be a very small gain at best. And if it is a single tank system that pumping would be the last thing you would want to do.
BTW I don't use in most cases external heat exchangers and the additional pump required for them.

In the case of 1 tank system, I have two models I follow. The first is the Viessmann type of system which is standard European practice, tried and true. Great tanks with big efficient HXs. Those two cases I mentioned are the result of a lot of government funded research into how to get the cost down and increase the usefulness of resource. They have very simple controls and good performance.

I don't think my systems are complex at all for multi tank or larger systems. I only have one extra sensor and the control is set up for it anyway.

[inetdog]

Were we talking about space heating or water heating.
I don't recall space heating ever being mentioned in this thread.

It was not mentioned. I was just trying to figure out where the mistaken "wisdom" might have originated and later been taken out of context.

"This is an area about which even reasonable men might differ, so I am sure we will disagree."

(Note the font before exploding.....)

[Naptown]

WHAT WISDOM ARE WE REFERRING TO
NOTICE ALL CAPS
































































just kidding lol

[inetdog]

what wisdom?

When two groups of presumably authoritative sources give opposite advice about the right way to do something, the generous assumption is that one set is taking correct advice originally offered in one context and uncritically applying it to a different situation.

More of the time there is the more cynical but probably accurate explanation that some published "experts" are just wrong. Particularly those who do not explain how they reached their conclusion.

Is there indeed Wisdom in the world? Does it lead to Happiness?

[LAWindsurfer]

Reason for pre-heat tanks being in parallel.

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.

Let me explain the rational behind two unequal tanks in parallel for the potable water line. The second tank is only 80 gallons because its slightly narrower width, compare to the 100 gallon size, adds the very few inches needed to preclude obstructing the entrance to the laundry room to the point where it would have become impossible to remove the washers or dryers for servicing. The two tanks are plumbed in parallel to not add an additional obstruction in the water supply; the cross-sectional area of the two 3/4" diameter lines, when taken together, substantially equals the cross-sectional area of the 1 1/4" diameter line feeding the gas hot water heater. I believe the collector loop fluid was channeled through both heat exchangers in parallel in an effort to promote equal heating, but I suspect greater heat transfer would be obtained if the heat exchangers were plumbed in series. Whether the collector loop feeds are plumbed in series or parallel will not matter if an external heat exchanger is installed. It seems to me, the owner but not the designer of the system, that there is a general consensus that I need to install an external heat exchanger.

[LAWindsurfer]

Need to refocus on economical ways to improve existing how water system.

It was not mentioned. I was just trying to figure out where the mistaken "wisdom" might have originated and later been taken out of context.

"This is an area about which even reasonable men might differ, so I am sure we will disagree."

(Note the font before exploding.....)

As indicated at the beginning of this thread, the described apartment building solar powered hot-water preheating system does not appear to do a good job of transferring heat from the collectors to the storage tanks. I say "does not appear to do a good job" because the gas savings are no where near a big (% wise) as those that were observed when a solar hot water preheat system was installed in another nearby building. Inasmuch as the amount of storage is undoubtedly less than desired, and all of the preheated water is used up each night, maximum gas savings will be achieved by maximizing the average temperature of the water in the pre-heat storage tanks. To do that, it would seem to me that we want to maximize the amount of heat transferred from the collectors to the water in the storage tanks. Currently, the temperature of the water in the pre-heat storage tanks very rarely gets really hot, 135 oF. One commentator indicated that the temperature drop of the fluid in the collector loop when passing through the heat exchanger should be about 20 oF; now, the dT is about 5 oF most of the time when there is a 50 oF dT between collector fluid and tank water.

Is it correct that there is a general consensus that I need to add an external heat exchanger and small pump? There seems to be disagreement over whether the output of the external heat exchanger should be fed to the top of the tank or mid-way down the tank (by cutting off a portion of the cold feed tube.) My inclination is to feed it into the top and see what impact that makes on performance.

[inetdog]

Currently, the temperature of the water in the pre-heat storage tanks very rarely gets really hot, 135 oF. One commentator indicated that the temperature drop of the fluid in the collector loop when passing through the heat exchanger should be about 20 oF; now, the dT is about 5 oF most of the time when there is a 50 oF dT between collector fluid and tank water.

Is it correct that there is a general consensus that I need to add an external heat exchanger and small pump? There seems to be disagreement over whether the output of the external heat exchanger should be fed to the top of the tank or mid-way down the tank (by cutting off a portion of the cold feed tube.) My inclination is to feed it into the top and see what impact that makes on performance.

Any difference in where you return the heated water to the tank will be small compared to the apparent inadequacy of the heat exchanger.

When the difference between the temperature of the two fluids is around 50 F, and temperature drop of the panel fluid through the heat exchanger is only 5 F, the heat exchanger is not doing its job, for whatever reason. If the temperature rise of the storage side fluid is also small, then the heat exchanger is undersized. If the temperature rise of the storage tank fluid is large, it is a sign that the pump on that side is not moving it fast enough to extract heat from the HX.

[LAWindsurfer]

focusing on solutions

Any difference in where you return the heated water to the tank will be small compared to the apparent inadequacy of the heat exchanger.

When the difference between the temperature of the two fluids is around 50 F, and temperature drop of the panel fluid through the heat exchanger is only 5 F, the heat exchanger is not doing its job, for whatever reason. If the temperature rise of the storage side fluid is also small, then the heat exchanger is undersized. If the temperature rise of the storage tank fluid is large, it is a sign that the pump on that side is not moving it fast enough to extract heat from the HX.

Statement makes sense to me. I'll post to forum again after I install additional heat exchanger. Purchase and install will likely take a few weeks.

[inetdog]

Statement makes sense to me. I'll post to forum again after I install additional heat exchanger. Purchase and install will likely take a few weeks.

If the HX is a parallel flow type, make sure the two fluids are running in opposite directions through the HX.

[LucMan]

Let's get back to the original post this thread has gotten side tracked. There is a problem with the existing system, and it has nothing to do with with HX's. There is no reason why with 240 sq ft of panels & 180 gallons of storage that the water temp is not at least 150 degrees on any given day.
Get back to the basics. What is the flow rate through the collectors? What is the delta T through the collectors & Hx's? Is there air in the loop? Is the controller properly setup and is it functioning properly?
Yes it should have been series piped but there is something else going on here.

[Naptown]

Let's get back to the original post this thread has gotten side tracked. There is a problem with the existing system, and it has nothing to do with with HX's. There is no reason why with 240 sq ft of panels & 180 gallons of storage that the water temp is not at least 150 degrees on any given day.
Get back to the basics. What is the flow rate through the collectors? What is the delta T through the collectors & Hx's? Is there air in the loop? Is the controller properly setup and is it functioning properly?
Yes it should have been series piped but there is something else going on here.

After re reading the original post I agree with the above.
something else is going on here.
first thought would be air or low flow through the collectors and HX.
Take some temp readings on the in from collectors, out from HX and tank temp at bottom and top of tanks.
Please don't rely on sensor readings at this point.
Don't know what pump model you have but make sure it is on High speed
Model #s of tanks, pumps and controls would help also.

[LAWindsurfer]

Time out needed to recompile data

After re reading the original post I agree with the above.
something else is going on here.
first thought would be air or low flow through the collectors and HX.
Take some temp readings on the in from collectors, out from HX and tank temp at bottom and top of tanks.
Please don't rely on sensor readings at this point.
Don't know what pump model you have but make sure it is on High speed
Model #s of tanks, pumps and controls would help also.

I'm preparing a response to these questions and others by assembling a more detailed description of the hardware and system operation. It will be posted in a day or two. Please hold off additional speculation until I've posted that data.