Does anyone have a good algorithm, or can you recommend a site for sizing a
I am working on a DIY design, and want to maximize (or as close as possible
to maximize) the heat transfer, which means I need to determine dwell time
right? I figure I have two choices here...
1) Circulate the glycol from the solar array constantly. When the glycol is
x degrees warmer then the DHW, turn a pump on and circulate the DHW through
the exchanger. Even if my exchanger is inefficient or undersized, I should
still get a good transfer as the glycol will more then likely stay hot for
multiple passes. The downside to this is that with an undersized exchanger,
I may not get to all the heat before it's lost.
2) Build an oversized exchanger, and potentially lose some heat at the end
by allowing the DHW to heat the glycol. How do you determine oversized?
As for circulation, lots of people are into temperature controllers to
control pumps. I built my system using two El Sid pumps and two small
PV panels. When the sun shines, the pumps run. No sun, no pumps.
Partial clouds, or sun low in the sky, and pumps run slow. Idiot
proof, and you will not cook your glycol when the power fails some hot
Now, to your main question about heat exchangers... I build my own too.
I didn't do any sizing calcs, but my main idea was to get a much
larger surface area than some of the small commercial units such as:
The integrated pump / heat exchanger (U tube) that sits on top of
your water heater
Hot fingers that go inside your water heater from the top
I built a system with two heat exchangers, and I'm delighted with how
it works. The primary exchanger gets the hotest glycol, and heats my
120 gal preheat tank. This heat exchanger consists of 6 tubes of
3/8"OD x 20' copper for the glycol, and 6 more tubes of the same for
the water side (which heats the 120 gallon tank). All 12 coils are
manifolded and tightly wound together and the whole thing is submerged
in a stock pot full of water. Works great. Very low pressure drop on
either the glycol or water side. Lots of surface area too. And it is a
true double wall system, with good thermal contact between the glycol
and the water tubes. The main problem is that it was a #$%&*!@% to put
together. I brazed all the manifolds for strength. It was really hard
to get all that copper up to temperature to braze without blowing holes
in the tube from the torch. I needed a large welding tip to heat all
the copper... but get that flame on a tube for more than about 1 second
and poof, it is vapor! And the other problem... how long will it
My secondary heat exchanger takes the warm glycol coming from the
primary heat exchanger and divides it into coils of copper at the
bottom of four 55 gallon barrels. All coils in parallel. The glycol
going to the bottom of each barrel is split further into two -20' x
1/2" OD copper coils. So the glycol goes through 8 coils of 1/2" OD
copper, 20' long, all in parallel. Lots of residence time and surface
area to give up heat. This has worked very well. I have exactly the
same configuration of tubes at the top of each barrel to pre-heat my
cold well water before it goes into the bottom of the 120 gallon
preheating tank... which then goes into my hot water heater.
The barrel system works so well that if I had to do it over again, I
would'nt buy the 120 gallon tank, circulation pump, solar panel (PV),
and the primary heat exchanger. I would have saved me some major
I get 220 gallons of water storage and I insullated the heck out of it,
so it doesn't loose much heat. Again, it is a true double wall design.
I think the only thing I would change in the barrel system would be to
put two small crimps in each of the 8 - 1/2" water tubes, at the 1/3
and 2/3 points down each coil, to mix the central flow of (cooler)
water with the hotter water around the edges. It would introduce a
small pressure drop, but with 8 - 1/2" tubes in parallel, it would be
insignificant. My major question is...How long will the barrels last.
I put in poly barrel liners in each to help, and sprayed each barrel
with zinc rich paint.
Another option for a heat exchanger is the brazed Stainless steel plate
heat exchangers. Not home-made, and they are single wall, but are
designed robustly, so I have been told. You can get lots and lots of
surface area for effective heat transfer. Not a double wall design,
but some say this type of heat exchanger doesn't need to be double wall
because of the heavy gauge of stainless. I don't know what to think
yet about this issue.
There is a long unresolved debate about single wall vs double wall out
there. You will have to decide for yourself what you want.
The advantage of such large surface areas is that your glycol stays
cooler, while still able to transfer large amounts of energy. I have a
friend who has one of the combo pump / heat exchanger units with the
small U tube that sits on top of his water tank. His glycol gets much
hotter than mine! My glycol runs about 15 F hotter than my hot water
tank. This is very good... gives long life for the glycol, preventing
thermal breakdown, and makes for better collector efficiency.
As for the water locked up in the tubes of a larger heat exchanger...
don't worry about it. The larger surface area, cooler glycol and
better efficiency of your collectors more than makes up for the small
losses here. Insulate the daylights out of it... that helps too.
I hope this is helpful.
P.S. I don't think I saved myself a bloomin nickel building the
primary heat exchanger myself, compared to a good brazed SS plate heat
exchanger... but it was (mostly) a lot of fun, and a tremendous