# Hot water to forced air

wrote:

900 btu/cu ft which is 31,783 BTU/cu meter - more or less.
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Well, you're wrong again, this time on two points. First, in the example I gave you, the metal cubes were replaced 5 times. That is a long way from infinite.
Second, in the heat exchanger in the furnace duct, we sure do have infinite cubes. It's called airflow and as long as the system is running, new colder air is constantly moving across the heat exchanger as long as the system is running. Just like one cold metal cube replaces the one that has warmed 1 minute in the simple example. Imagine little blocks of air being changed every minute. Then every 10 secs, then every sec, etc. There you have the furnace heat exchanger.
Now the problem here is that again, you never answer my questions. Do you or do you not agree that by replacing those metal cubes that have warmed slightly with new cold 0F cubes the 500F cube is cooled faster than just leaving one cube there for the full 5 mins?
Here's the experiment again:
Let's say you have 10 identical masses in the form of metal cubes. One of those cubes is heated to 500F. The others are all at 0F. I do the following two cases:
A - I take one of the 0F cubes and place it on top of the 500F cube and leave it there for 5 mins.
B - I take one of the 0F cubes and place it on top of the 500F cube for 1 min. Then I replace it with another 0F cube for 1 min. I do that for the same 5 min period.
Which results in the 500F cube being cooled off the most? Which results in the most heat transferred?
Do you agree or disagree that the answer is B? A simple yes or no at this point please. This is a question an elementary school kid could answer based on experience. And one that a high school science student could answer based on science.
And if you agree the answer is B, which we all know it is, then what is happening is exactly the same as in the heat exchanger in the furnace duct. The only difference is that we're using air instead of metal cubes to cool the heat exchanger. We don't have infinite air, nor is that required. The MORE air we put throught the heat exchanger, the MORE heat that is transferred. If we put enough air through it we've extracted all the heat.

OMG, you are so totally confused here it's unbelievable. A car radiator has air flowing through it just as the heat exchanger in the furnace ducts does. Both are infinite in the sense that the airflow is continous as long as the systems are operating. Exactly like it would be in the metal cube example if I kept replacing those metal cubes every 1 min for the duration of the experiment. I actually can't believe you just made this even more bizarre claim. This from the guy that is always slamming the US over being behind in technology? You really are lost in the wilderness here.

I can't understand any of this? First, a radiator from a car would work under exactly the same principles whether in a car or a furnace duct. The physics don't change. Second, there is no need to use an unspecified car radiator. CL provided a data sheet to a heat exchanger manufactured exactly for the purpose of being a water to air exchanger for this kind of application. We've given you the link 3 or 4 times now. The datasheet clearly shows that as you increase airflow the BTUs of heat transferred increase. You can see that the increase slows as a decaying exponential, which is EXACTLY what I told you 10 posts ago. But you won't even address that datasheet because there is no way around it. You just trim it from every post.
Here it is one more time:
The spec sheet is right here: http://www.heatexchangersonline.com/airtowater.htm
Look at the first heat exchanger. Operating with 140F incoming water at 6 GPM, the performance is listed as:
Airflow CFM BTUS
500 28 600 29 700 34 800 36 900 37 1018 38
Your problems with science here are many. One big one is that you don't understand the difference between heat and temperature. Another is that you think the physics of heat transfer change whether the heat tranfer is used to warm a house or to cool a car because in the car it's "waste" heat. And you can't grasp the concept of infinity very well either.
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<%-name%>

Yes, it's for 6 GPM of water flowing through the heat exchanger. And it shows water at temps of 140F, 160F, and 180F. And HOW MUCH HEAT IS TRANSFERRED VERSUS AIRFLOWS OF 500, 600, 700, 800, 1000 CFM
As the airflow goes up, so does the BTUs of heat transferred. Do you agree that is what it shows or not?
And yes, if you made another chart that showed say 3GPM per minute of water flow instead of 6GPM, the water would be coming out cooler. You would then have LESS heat transferred. And you'd have a similar table showing that as airflow increases from 500 to 1000CFM, the amount of heat transferred increases with it.
That table for 6GPM isn't some unique point where special rules of physics apply. It shows how the heat exchanger works and is an operating point within the scope of this discusssion, where if the OP built his system he could get 30K BTUS of heat out of it.
Look at the other heat exchangers that are larger and show greater flow rates. Find me one that does not show that as you increase airflow, the BTUs of heat transferred go up.

Now I agree that is one missing piece of information that should be spec'd on the data sheet. Without it we don't know the specific air entry temp that resulted in those BTU numbers. However, it doesn't matter. Whatever the air entry numbers, clearly:
The spec sheet does state:
" These air to water units are ideal for heating water in residential and public buildings. Compact and lightweight, these units are designed to maximize heat transfer by utilizing a series of 3/8" copper tubes with a high density of aluminum fins. These fins are spaced in such a way that the fin density is an impressive 12 fins per inch. This unique design allows for heating loads of 50,000 to 60,000 Btu per square foot (please see performance chart below)."
And with water entering at 140F to 180F it's obviously being used in that chart to heat air. At that point, it does not matter what the incoming air temp is. Let's assume for the moment that the particular chart used 65F air. The chart shows the BTUs of heat tranferred versus increasing airflow. As you increase airflow, the BTUs go up. Now, if the incoming air were cooler, say 40F, you'd transfer more BTUs of heat because the temp differential between the hot and cold sides of the heat exchanger is greater.
If the incoming air is 100F, you'd transfer less BTUs because the temp differential is smaller. You would then have a very similar chart that starts out with a given BTU of heat transfer at 500CFM and INCREASES as you increase the airflow to 600, 700, 1000 CFM. Capiche?

Sure does. Do you agree that this chart also clearly shows the BTUs of heat transferred INCREASED steadily with the airflow? That MORE heat is transferred in that table as the airflow increases? That it is that way for the table on every size of heat exchanger? Yes or no?

No shit Sherlock. But remarkably, the application the heat exchanger is targeted for is exactly what the OP was looking for. And the water flow rate and the airflow rate are within the range that a system to heat a home via solar would be. In other words, that chart shows exactly what the OP could use. It's not some curve from some bizarre or unique heat exchanger.

I can't understand why you can't understand the simple concept that a heat exchanger, be it this one, or a car radiator has a CONTINUOUS MASS moving through it. We don't have a single hand, we have say 100 people with hands at 98F, that each place their hand on the warm stove for 30 secs. That transfers MORE heat than one person leaving their hand there because new hands at 98F are constantly replacing hands that have warmed slightly. The greater the temp delta, the greater the heat transfer. You stated that yourself. You wind up with 100 people with slightly warmer hands instead of one person with a hot hand. Does that mean less heat is transferred? Of course not because the mass of those 100 hands more than makes up for the fact that they are only slightly warm. Capiche?
Same thing applies to a home radiator on a hot water or steam system. The more air you move past it, the lower the air temp will be, but the more heat you will extract from the radiator and the cooler the exit water temp will be.
Same thing applies to a car radiator. And it matter not a wit that it's waste heat being transferred to the atmosphere, so please don't start that again.

There you go again. It's been stated to you many times now by Mark, CL, and I that the object of the excercise was to heat the house. Which means getting the most heat out of the heat exchanger, not the hottest air. You are confusing TEMPERATURE with HEAT. Try looking up the definition.
As one more example, I could go down and turn the blower down on my furnace to 100CFM instead of 1300CFM. Assuming it would not kick off due to the high limit safety switch, I would then be getting very hot air out of it. Does that mean I'm getting MORE heat and it's going to heat my house faster, etc? No. I'd be getting LESS heat even though the small amount of air coming out is a lot hotter. Capiche?
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wrote:

There is the whole aspect of diminishing returns that both Trader and Harry are forgetting about - PERHAPS Harry more than Trader.
The difference in efficiency is real, but how great? = particularly since we are working with a closed loop system, and the heat not recovered stays in the system, raising the input temp the next time 'round, which will increase the efficiency. It's not like we are wasting any heat (or at least not huge amounts) by not recovering it on the first pass.
And passing more air through the exchanger does increase the BTUs extraced - but by how much, particularly when lowering the "dwell time" does reduce the efficiency somewhat.
At what point does the loss of efficiency from the one side over-balance the loss of theoretical heat transfer from the other.
My gut feeling is the gains/losses from either side of the arguement are close enough that there will be a fairly broad band of overlap where the difference in heat transferred will be RELATIVELY insignificant in the grand scheme of things. You get to the point where "close enough IS close enough.".
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On Jan 11, 10:11 pm, snipped-for-privacy@snyder.on.ca wrote:

Give me a break here. I haven't forgot about diminishing returns:
A - I can refer you back many posts ago where I said that the more airflow you pass through the heat exchanger the more heat you get out and that the additional heat gained decays exponentially with the increased airflow. And yes, if you put enough air through it, all the heat is extracted and the outgoing water temp and the air temp are equal and no more heat can be extracted. I clearly stated that many times here too.
B - I posted the data from the heat exchanger data sheet you provided which fits the application nicely. It shows that additional heat is still being extracted at over 1000CFM. Yes, the additional gains are diminishing.
C - This started over the issue of placing dampers in the airflow stream. I said you might as well just use max airflow, because there is no downside to doing that from the heat exchange standpoint and you will have heat exchange at a maximum. I think you agree with that, no?

It doesn't increase the efficiency. That water leaving the heat exchanger from the furnace is now HOTTER and will consequently pick up LESS heat going through the solar array, which is just another heat exchanger. Put 65F water into the solar array and you get max heat out of it. Put in 150F water, or whatever the max temp it is capable of achieving, and you get zero heat out of it.

You sure are wasting it, per the above.

Totally wrong, per the above.

It doesn't

Your gut is wrong. Sending that water back to the solar array with all the heat extracted from it in the furnace is how you get the maximum heat out of it, which is what the OP wants. If that isn't possible, the next best thing is to get as much heat as possible extracted from it. And you do that by using maximum airflow.
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On Fri, 13 Jan 2012 07:36:14 -0800 (PST), " snipped-for-privacy@optonline.net"

Not totally wrong, if when you decrease the air flow you also increase the surface area - so you get the same CFM flow but at a lower velocity.. All goes to the TOTAL design of the system. Can't just change one parameter and expect to totally optimize the system.

I more or less agree with you - but with a solar "loop: if the heat is not all extracted the first time round it just means the fluid will be hotter coming by the next time - which may increase the efficiency so more heat is extracted next time through.. Depends a lot on how much solar input he is getting.
And as far as the dampers - I NEVER advocated putting dampers in the MAIN airflow. My damper suggestion was to get away from the "restricted airflow" straw man somebody put up, putting the heat exchanger "off-line" to avoid the restricted air flow. And even then - yes, they are likely pretty close to superfluous.
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On Jan 13, 5:16 pm, snipped-for-privacy@snyder.on.ca wrote:

But we are not changing the surface area. The surface area is fixed for whatever heat exchanger he uses. We're just changing the volume of air passing through. Back to the original point that harry still disagrees with. And that was if you increase the airflow through a water to air heat exchanger, you get more heat out of it. The increase in heat that you get trails off exponentially until you finally have the air exiting and the water exiting at the same temperature. At that point you've extracted all the heat and the heat transfer has reached the maximum. Increasing it beyond that doesn't extract any more heat. I think we agree on that, don't we?
You agree that the same applies to a home radiator? The more air you put through it, the more heat you will transfer?
You agree the same applies to the auto radiator? The more air you put through it, the more heat you will transfer? (assuming of course the heat is there, ie the thermostat is wide open, car block is hot, etc)
You agree that is what the data sheet for the heat exchanger you provided shows?

No, that is not correct. You get the most heat transfer when the temperature delta is the greatest. If you send the water back to the solar array at say 110F instead of 90F, it will pick up less heat when it passes through the solar array. The amount of heat picked up is not fixed, it's directly affected by the temp of the incoming water. The cooler the water I put in, the more heat I can extract from the solar array.

Yes, I acknowledged that in previous posts. I originally thought you were advocating putting them in the main airflow, which is why I said just put all the air through the heat exchanger. It's simple and you get the max heat extracted. You can see from the spec sheet on the heat exchanger the consequences of reducing the airflow. A typical furnace could move ballpark 1000CFM in heating mode. From the spec sheet, that gives 38K BTUs. If the airflow is cut to 500CFM, which is still 1/2 the available airflow, you get 28K BTUs out of the heat exchanger. i don't know what you think about giving up 1/3 of the available heat by restricting the airflow, but it doesnt' sound like a good idea to me.
I still have doubts about the viability of this thing based on how much heat you could get out of it. The OP never gave us data on the solar array. I'm betting that it's sized to a water heater and not capable of anywhere near the flow rate for the curve in the heat exchanger spec sheet. To get the 38K BTUs that heat exchanger has 6GPM of 140F water flowing. As I said before relative to a water heater, that's an entire tank of hot water in 8 minutes. Meaning I doubt the solar array has capacity anywhere near that. It's likely sized to a water heater application and could have 1/10th the heat output. In which case, the heat for the furnace likely isn't worth the trouble.
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wrote:

Both theories are valid - but when the rubber hits the road - or in this case Fluid 1 hits fluid 2 - how much difference will each make???
The only way to know is by setting up a system and testing. Not DIFFICULT to do as a test before final install with a variable speed blower and heat exchanger, and a flow control valve to contol both water and air flow. Don't need to measure air temperature change/heat absorption because if the heat leaves the water, it went to the air. You need to be able to measure inlet and outlet water temps and water flow, as well as air flow. That's all.
I'd definitely be doing this before making any mods to the heating/air handling system in the house.
And PERHAPS a thermosyphon circulation system would be self optimizing???? Just a thought.
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On Fri, 13 Jan 2012 07:58:10 -0800 (PST), " snipped-for-privacy@optonline.net"

No I'm not. I'm just saying there are two things that could affect the total efficiency and heat output of the system, and they could be , to a point, at cross purposes.

No, they do a design analasys - and then they test it to prove it - and they DO instrument their design - at least in the primary stages.
After they have established what works and how - and what assumptions are valid - they go on experience. Often the documented experience of others.
At this point we do not have documented experience of anyone having done this - so doing a bit of experimenting is valid.

Scientific method - defined: a method of research in which a problem is identified, relevant data are gathered, a hypothesis is formulated from these data, and the hypothesis is empirically tested.
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On Jan 13, 5:29 pm, snipped-for-privacy@snyder.on.ca wrote:

I don't need to do that to know that the more air that flows through a water to air heat exchanger, the more heat that is transferred. And that the cooler the water temp is that is entering a solar array, the more heat you get out. Both up to the total max heat tranfer that is possible of course, which would be when the water exiting is equal to the air or solar array temp.
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Try sticking to the example, instead of changing it. We're not talking about changing the burner rate. We're talking about changing the AIRFLOW rate of the furnace with a given burner rate. Whatever the burner rate, you get more heat out of that example of a heat exchanger the MORE air that flows through it. Yes, that air will be at a lower temperature, but given the increase in air mass, it still picks up MORE heat going through the furnace. Capiche?
And where do you address the whole rest of my post? As usual, you don't. You just try to steer the discussion somewhere else.
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wrote:

exhaust air thats too cool will make occupants feel cooler espically at higer speed airflows.
a modern high efficency forced air furnace is a excellent example, during cool down the blower remais on and can feel cold if your standing in front of the air vent
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On Fri, 13 Jan 2012 07:54:42 -0800 (PST), " snipped-for-privacy@optonline.net"

"total system design" needs to be considered - not just one parameter.
IF, for example, a dual path heat exchanger is used, so the water flows at half speed, yet the same amount of water flows, you MIGHT get better efficiency AND more heat output.

You get more OF the heat into the house (higher efficiency), but less TOTAL heat into the house.

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On Jan 13, 5:20 pm, snipped-for-privacy@snyder.on.ca wrote:

But nobody is talking about using a dual path heat exchanger, nor is there any reason to use one in this application. And you can't find a water to air heat exchanger where you don't get more heat out of it the more air you put through it. The spec sheet you provided clearly shows that. Nor can you find a solar panel where you don't get more heat out of it the cooler the incoming water is. Those are the parameters that matter.
I can't get harry to agree that with a simple car radiator or home radiator, that the more air you move through it, the more heat you get out, until you hit the max. The max is where the outgoing water temp and air temp are equal. I thought you agreed on that point, no?

I'm not sure efficiency is the right term, but I agree that with the valve nearly closed, you extract all the available heat from a tiny bit of water and you get little heat into the house. And how do you get more of the heat from that water to air heat exchanger into the house? By increasing the airflow, per the datasheet. Agree?
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Harry,
Yes you want the two media in intimate contact but you then wrongly conclude that means the velocity should be low.... So you think lower CFM through a radiator pulls more heat out of it, obviously wrong. When does your car overheat?
The flaw in your thinking is that you don't consider that there is always more air to replace the air that is moving away and the colder the air in the radiator, the more heat it pulls out.
You are the one confusing heat and temperature.
The usual case for a heat exchanger is is higher CFM = more BTU exchanged and lower temperature.
Take care
Mark
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And there you go again. Just like Mark said, you are confusing temperature and heat. Let's recap. The OP is proposing to take a heat exchanger similar to a car radiator and place it in the return duct of his furnace. He will pass hot water from solar panels through the radiator.
Is the goal here to get the air as hot as possible? Clearly not. The objective is to get the most heat out of the water and into the air. By moving more cold air through that radiator the temperature differential across the heat exchanger is higher and MORE heat is transferred. Yes, the air coming out or the radiator will not be warmed as much as if there were less air flowing, but there is enough additional air so that it more than make up for the lower temperature and MORE heat is transferred. That you can't grasp this most basic concept just totally discredits you.

Again, you don't understand the difference between heat and temperature. Which takes more energy in the form of heat:
A - Raising the temp of 5000 cubic feet of air 1F
B- Raising the temp of 10 cubic feet of air 10F?
Which air is hotter?

Which of course matters not a wit.

I think it's obvious to all who's confused here. Here's another simple experiment you can try. You have a piece of iron that been heated with a torch. You want to cool it off. Do you get it cooled off faster by applying a very small stream of water to it, in which case the water running off is very hot, or by applying a large stream of water and flooding it as much as possible, in which case the water is just luke warm?

I see. So for example we can take all the coils in the AC systems out there and make them 75% smaller and at the same time reduce the blower speed. That should work great. And you throw rocks at Americans for being stupid?

The last sentence is pretty much the only thing you got right here. And it's also why you want the maximum airflow through that radiator. If you slow the airflow to the limit, ie no air is flowing, the air surrounding the radiator equals the water temp in the radiator. You now have max temp air, but 0 air moving and 0 heat being transferred. Now slowly increase the airflow and while the temp of the air moving through starts to decrease, heat transfer starts because you have cold air coming in contact with the radiator. As you continue to increase the airflow you get more heat out, more air out, but the air goes down in temp. The process continues with a decaying exponential in terms of the additional heat that;s available.
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Harry, 1) i'm not going to debate with you, especially since you are rude...
3)think about what i and trad are saying... re BTU and temperature,
which holds more heat in BTU, a small red hot needle at 4000 F or 50 gallons of water at 100F?
which air flow is pulling more BTU out of an exchanger, 100 CFM at 100F or 1,000CFM at 75F?
have a nice day
Mark
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<%-name%>

The only reason you claim it's neither here nor there is because you can't explain it with your rules of physics which don't pertain to this universe. A car radiator is a simple and perfectly valid example of a heat exchanger. The heat from the coolant is transfered to the air moving through the radiator. With the same amount of coolant flow, the more air you move through, the more heat that is removed. That's one reason many cars today are equipped with an auxialliary fan for the radiator. If it's very hot out, you're stuck in barely moving traffic, etc, and the coolant temp rises above the normal range, a switch turns on the fan to move MORE air through the radiator removing MORE heat with it. Doesn't get any simpler than that.
If it worked the way you claim, we'd instead be restricting the air so that the air coming out was as hot as possible. You're confusing temperature with heat. A car radiator with a high volume of air moving through it will transfer MORE heat to the air. The temp of the air coming out will be lower, but there is MORE air and the increase in mass of air more than offsets the fact that it's a lower temp.
Another example would be a simple hot water radiator in a house. With no fan you could hold your hand by it and you'd feel some hot air rising. Now place a fan there blowing lots of air through it. The air will not be as warm. Does that mean that LESS heat is being transferred? Of course not. You're getting MORE heat, but the air is at a lower temp because more mass is being heated. Capiche?

Says who? It depends on what you're using the air for. If I'm using the heat for a chemical process and want it to be 120F, then I want it to be 120F, not 200F.
If the application requires it to be at a certain minimum temperature, eg heating water in a water heater, then yes you would need it to be at a certain minimum temp.
In the stated application, the goal is to transfer the heat to air moving through a furnace to heat a home. As long as the water moving through the heat exchanger is above the temp of the water, you get maximum heat transfer with the fastest airflow, all else being held equal. I've explained this several time now. Let' say the water from the solar collector is at 130F.Start at the limit of zero airflow. The air around the heat exchanger is also 130F because no air is moving. The water temp equals the air temp, no heat is being transferred. Start increasing the airflow and now you will have air coming out that is slightly below 130F and water going back to the collector that is slightly below 130F. Some heat is being transferred. Ramp up the airflow and you get more heat transferred. Will the air temp out of the heat exchanger be less? Sure. Does that mean less heat is being transferred? No, because a lot more air mass is being heated. Capiche?
I've answered any questions you've put forth. Yet I get no reply to mine. I'll state my simple question a third time:
I give you a quiz. Our friend here builds his proposed system. He puts an auto radiator in the return side of his furnace duct work and connects it to a solar collector. The solar collector heats water to 130F and it gets pumped to the radiator. The incoming air in the return duct is 65F. He has a variable speed blower so that he can vary the airflow through the radiator.
Which airflow results in the water leaving the radiator at the lowest temp and consequently the most heat transferred?
A - 1 CFM airflow B - 2 CFM C - 10 CFM D 100 CFM E - 1000 CFM F - 2000 CFM, the max the blower capacity?
Why won't you answer this very simple question?
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wrote:

No, actually we are trying to extracrt as much heat from the water as possible - which can be more air with low net temperature gain - or less air with high net temperature gain. The more air/lower temp actually gets more HEAT than the low air high temperature. If you are sitting in the airstream it FEALS like you are getting more heat with the low flow/high temp option, but in reality, you will heat the entire room or home FASTER withthe more air/lower temp option. In this case, the difference would likely be marginal - but it would be there- and would be quantifiable.

And in SOME engines, even today, if you remove the restriction of the (open) thermostat the water flow increases velocity to the point it does not remove enough heat from the block/heads and you get localized overheating of the engine.

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On Sat, 7 Jan 2012 07:13:25 -0800 (PST), " snipped-for-privacy@optonline.net"

NOT necessarily - with a higher TEMPERATURE primary fluid, the Delta T at the secondary heat echanger will be higher, so the efficiency of the exchanger will be higher, and more heat will be transferred to the secondary fluid.

No, but if the efficiency of the heat transfer can be increased by 50% by having a higher delta T at the secondary heat exchanger, it is POSSIBLE that slowing the flow rate through the primary to increase the primary fluid temperature MAY provide more actual HEAT to the house. It would depend on the efficiencies of both heat exchangers.
It also depends if we are talking a closed series loop, where the waterflow in both exchangers would be equal, or if we are talking separate loops, where the GPM flow through each exchanger was optimized. I'm ASSUMING we are talking a hybrid system, with a frost-resistant primary fluid, and THREE heat exchangers in the equation - solar to primary fluid - primary fluid to heat storage, (water) and heat storage to secondary fluid (air) where the echange fluid flows can all be separately controlled.
NOT a simple question with a simple answer that is always right.

Simple question, with simple answer that is only a SMALL part of a more complex question.
Answer F will extract the most heat from the water IF there is more heat available than can be extracted at 100cfm.. If 100cfm will extract enough heat to get the water exit temperature to within a few degrees of the duct inlet temperature, 500,000 cfm will not extract any more heat - and the return temperature of the water will not be below the 65 degree F air inlet temperature.
If the heat echanger was 100% efficient you would get 65 BTU per gallon (imperial) out of the heat echanger in one minute at 1GPM or 60 GPH primary flow rate. So to answer your "simple" question, we need to know the flow rate of the "primary fluid", as well as it's temperature, to know how much "heat" is available. There is only 1 btu of heat per degee F of temperature change in 10 lbs of water. 10 times the water flow gives 10 times the POTENTIAL heat.
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