Heat transfer from roof loop to tank below expectations.

[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.

[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.

[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?

[Naptown]

WHAT WISDOM ARE WE REFERRING TO
NOTICE ALL CAPS
































































just kidding lol

[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.....)

[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.

[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.

[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.

[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.

[MikeSolar]

That sounds like you have the right idea.

Only one small problem:
Mixed water does not stratify. That would violate the infamous and repressive laws of thermodynamics. If you add heat to the top of the tank (via heat exchanger, electric element, gas flame or any method that does not require moving the water) then as you heat the top water only it will become stratified. And if the thermostat is at the top, the bottom will get heated only by conduction and it will take a loooong time to "mix" to a uniform temperature.

Hopefully that was just a poor choice of phrase on your part.

Trying to type and watch spain vs portugal.........not very easy.

You are right, poor choice of words.

[inetdog]

It simply takes longer for mixed water to stratify. Sorry for the long sentence.

That sounds like you have the right idea.

Only one small problem:
Mixed water does not stratify. That would violate the infamous and repressive laws of thermodynamics. If you add heat to the top of the tank (via heat exchanger, electric element, gas flame or any method that does not require moving the water) then as you heat the top water only it will become stratified. And if the thermostat is at the top, the bottom will get heated only by conduction and it will take a loooong time to "mix" to a uniform temperature.

Hopefully that was just a poor choice of phrase on your part.

[MikeSolar]

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.

I'm not sure I understand you but it looks like you agree with the way the two companies I referenced do it. the real benefit, aside from always having the coldest water going to the panels (it is not mixed at the bottom at all), is that, if you assume, that there is no back up tank and you want to do hand washing or take a shower, there is a higher degree of probability that (after 30 min in the morning from a totally cold tank) the thermosyphon system will give you enough hot water to do the job. Not returning it to the top means using a backup, whatever that is, to get the temp to 120F. It simply takes longer for mixed water to stratify. Sorry for the long sentence.

[MikeSolar]

Here is where the problem lies with returning to the top. (and the second returns about where I reccomend)
It's morning and the tank is stratified with 120 deg water up top and say 70 degrees at the bottom.
The controller senses a differential of 10d and starts up
So now you have 75-80 degree (probably closer to the 75d once the system stabilizes) water mixing with the 120d water at the top and cooling it down where if it were reintorduced lower you would actually be heating the lower portion without disturbing the top stratified layer. pulling from the bottom and returning to the top is never a good idea.

Agreed, assuming that you have a control that doesn't check the tank top temp. The Resol controls I use will not turn on the second pump until the liquid coming back is above tank top temp (if I set it up that way). I only use the 2 pumps system when I have a lot of collectors and I need a storage volume that doesn't allow for an internal HX. Damn those ASME rules, they are so archaic. Europe has a much better system, build it, test it and rate it for pressure, none of these CRN numbers. But that is a different discussion.

If I am using just a thermosyphon system with only the solar pump, there will be no charging of the tank until the temp at the exit of the HX is higher than tank top. That will happen quick enough if there is nothing taking heat out of the solar line and it needs no controls.

[inetdog]

Pulling from the bottom and returning to the top is never a good idea.

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.

[Naptown]

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?

Here is where the problem lies with returning to the top. (and the second returns about where I reccomend)
It's morning and the tank is stratified with 120 deg water up top and say 70 degrees at the bottom.
The controller senses a differential of 10d and starts up
So now you have 75-80 degree (probably closer to the 75d once the system stabilizes) water mixing with the 120d water at the top and cooling it down where if it were reintorduced lower you would actually be heating the lower portion without disturbing the top stratified layer. pulling from the bottom and returning to the top is never a good idea.