tank in a tank heat exchanger

[Lloyd]

Hi Bruce,

here is a try.

Buy 20 foot lengths of type L hard copper, then solder on a cap at one end. fill the pipe a couple of inches shy full, of fine silica sand, then solder on another cap.

Build yourself a white oak jig to the radius you need for the bends to fit the tank. Then with map gas heat the radius and bend it around the jig.

Cut the caps off each end; blow/wash the sand out. HD sells a copper swagging tool for about 9.99, that you use with a hammer, then map gas the end, and then swagg the end up, then solder up. No fittings needed.

You can do the whole Heat/X in copper, with no fittings.

Lloyd

Hi Guys,
I'm finishing construction of my big (128SF) drainback solar panel for hot water heating (space and domestic).  I have built the 700 gallon insulated storage tank, ready to put in the liner.  My water pressure is just 12 psi, from a hill mounted tank. 

I'm interested in the "batch" tank within my tank heat exchanger for my domestic hot water, but am having a hard timing finding an affordable tank. I can't use a water heat tank since the outside of those tanks would corrode when submerged.  New galvanized steel tanks are out of sight, cost wise, and stainless steel pressure tanks seem insane as well.

My last resort is a copper tube exchanger, but in winter my incoming water is near freezing, so according to the figures I've seen, I would need at least 180 feet of 1/2" copper ( as 6-8 coils in parallel) at 2 GPM.  (38F to 105F).  This is $360 just for the soft copper, perhaps $420 all together for copper cost.  I think that even 180 feet is marginal.

Any ideas would be appreciated.

PS-  I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year. 

[BruceM]

Thanks for all the ideas, guys.  My head is spinning with alternatives, now.

Best Wishes,
Bruce

[LowGear]

PS-  I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year.

I read that charcoal filters don't like hot water but your system might really profit from a temping valve and a whole house sized filter on the hot water side.

Casey

[BruceM]

I'd rather skip the plastic for hot water and avoid the whole filtering issue (yes, hot is a problem for carbon filters, and no, I don't have the extra psi to spare for one anyway).  My water is fabulous- no ag or chemical contaminates, no iron or manganese sulfate like some other areas west of here. 

Watts 3/4 and Honeywell tempering valves are both pretty low head at (about half a psi) my modest flow rates.  More of a problem for the in floor heat as the circ pump is presently only 10 watts and gutless.  I'd have to use a much stronger pump or I may do something custom via a 3 way motorized ball valve and a Picaxe chip.  I like the EL-Sid 10 watt pump I'm using now- it has two h-bridges and with twice the usual number of poles of the "electronic-magnetic" motor, it has less pulsing of the water and doesn't make the 1" Pex pipe hum where I strapped it inside a wall in the kitchen. 

[glort]

Not sure I fully understand the requirements and setup here but instead of a tank in tank set-up, what about a Flat plate heat exchanger coupled to the heat source being the main tank and then the load? Would work a lot better than copper coils etc and be easier to install.... If it's practical in the application.
If a really large one was acquired, it may just be possible to submerge the thing and still get enough heat out of it.

[Dualfuel]

Dear Bruce,
I am having trouble pinpointing exactly what you are building on the builditsolar website...Gary has many articals. Although, like you wrote, its sure is a wonderful educational resource.
Could you link me to a specific desiign you are working with?
Thx.

[BruceM]

My collector is copper pipe with aluminum fins, per Gary's prototype:
http://builditsolar.com/Experimental/CopperAlumCollector/CopperAlumCol.htm

The difference is that my panel is turned sideways, so the risers are horizontal.  This is called a Hiser configuration (for horizontal risers).
Here's an example-
http://www.builditsolar.com/Projects/SpaceHeating/Hiser/Hiser.htm

My panel is 7 runs of 1/2" copper, 32 feet long in parallel, with a return of 3/4" copper across the top.   It is mounted on the wall with 1/4" per foot of run drain back slope, or 8" higher at the far end.  The Hiser project above was not a drain back system.

Here's a good article on some tricks for doing drain back system:
http://www.builditsolar.com/Projects/WaterHeating/DBAirLine/DBAirLine.htm

Because I have little vertical between the panel and tank, I added an optional air bleed tap on the highest point of the panel, in case I need to assist the drainback with an air bleed solenoid.

My tank is unusual in that it is all below my shop floor, except for the last foot of height.  The extra foot lets me use a non-submersible pump, without a hassle, since the pump can be mounted just below the water level, outside the tank. 

Here's an example of an EPDM lined homebuilt tank:
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm

We just put in the liner today, and 31 of 55 inches of water in the 800 gallon tank. 

Best Wishes,
Bruce

[LowGear]

Nice job Bruce.  I can dream that good but somehow when their feet hit the ground they don't often come this good.  Excellent.

Casey

[BruceM]

Here's the state of my solar project as of today.  Panel up but not plumbed all the way yet,  insulated storage tank built and liner in but not trimmed.  

Thanks, Casey.  It's a struggle for me now, MS related memory problems make me feel lost in the details most of the time, and the physical work is getting so painful I'm going to be avoiding big projects for some time, I think.

I decided to stick with copper for the domestic heat exchanger/preheater, no pump.   Spendy but low risk and very long lived.  The house in-floor heat will be direct from the insulated storage tank, no exchanger needed, same for the shop heat.  Shop heat plumbing deferred until I see if I do have excess capacity next winter.  If I have excess capacity, I'll run the shop heat whenever the storage tank hits 140F.

Best Wishes,
Bruce

[BruceM]

Here's the completed an installed copper heat exchanger. 

The system is working now, and though the sun angle is getting increasingly off axis this time of year (intentionally)  I still get 10 degrees of heat gain at 3.1 gallons per minute.  I did some calculations and it would take 50 feet of Slantfin Fineline 30 radiant baseboard to radiate that many BTUs at 140F and that's at 65F room temperature!  Good grief, I need a new plan for shop heat/heat dump. 

I have run into an issue with the twinwall polycarbonate glazing.  After stagnating for about 5 warm sunny days straight while I was fiddling with my new custom pump controller electronics and a new EMI filter install,  the material cracked where bulging from expansion along the top of the panel.  Not sure show I'm going to solve that issue just yet.  Tempered glass would be nice but my quote for 4 pieces of 4'x8' tempered glass was $1800 plus tax.  The twinwall Lexan was only $324.  The twinwall gets pretty wild and wavy with one side heated to 200+F.  The outer layer does not get over 150F.  So it's a structural issue-  it does not expand in a flat and uniform manner, and while held loosely to allow expansion, the the edges still must be constrained in a straight line.  The joys of DIY design...

[BruceM]

I also ran into an issue with running my in floor heating system from the hot water storage tank (no heat exchanger, atmospheric pressure only).  When using the propane hot water heater as a secondary booster, so much air was released on raising the water temperature that the system would air lock due to lack of circ pump head sufficient to push water higher than the air trap above the water heater.  Bleeding the air by temporarily pressurizing and venting it would only solve the problem for 20 minutes, if the propane water heater was on.

  My solution was to modify my Watts brand air vent valve by JB Welding a brass tube port on the former air vent opening, and connecting to a 7 gallon portable compressed air tank converted to be a vacuum tank with gauge.  The Watts air vent now "sees" the system as being pressurized and continues to function normally despite the water being at atmospheric pressure only.  This solves the problem and lets me continue to use my very low head 12V EL-SID pump to heat my house.  Most of the heating season I will be leaving the propane water heater off and bypassed, so air will not be released into the system from heating in the water heater.  The heating of water in the big storage tank does not cause the problem since it is vented to the room.   Hopefully I won't have to recharge the vacuum tank more than a couple times a heating season.

[BruceM]

I got quite a surprise a few days ago.  I checked out the system in preparation for this year's heating season, opened the valves, purged air, turned on the pump for a test run.  Everything A-OK, house manifolds hot, system running absolutely quiet.  Turned off the thermostat.  

I noticed later that day that the floor was pretty warm near the manifold- but thought it was just a delayed heating from the short test run.

The next day, I noticed that the floor was STILL warm, and in fact warmer even further from the manifold.  This despite having windows slightly open day and night.

On the third day my house was getting uncomfortable- up to 78F.  I went and checked my 800 gallon storage tank- sure enough, 25 degree drop in temperature.  The manifolds were hot in the house - hot water was flowing even though the circ pump was off!

The in-floor heat system presently is a non-pressurized almost closed loop which has a buried hot water tank, then the hot water rises to the circ pump, then up above the propane tank which stands on an 18" pedestal.  This 8 foot rise, plus the cold return from the house (a couple feet above the shop) and back into the buried tank is apparently enough to thermosiphon pump my whole house in floor heat.  Slowly, but it does pump a lot of BTUs.

I was able to stop it manually by closing a valve, but now my design dilemma is how to make it stop automatically.  I'm thinking perhaps a vertical loop in the cold return before it's returned to the storage tank.

It also makes me wonder if instead I should raise the hot water line up through the ceiling into the insulated attic and dispense with the circ pump altogether?!!  The thermostat could control a motorized ball valve instead.

Thermosiphon flow sure is contrary- it won't when you want it to and will when you don't!

[BruceM]

I tried putting anti-thermosipon loops on both the cold return and the hot supply to my in floor heat; it still would not stop circulating once started by the pump.  

I finally got it fixed by adding a swing type check valve, mounted so that the swing flap has to be lifted vertically by the pump to make it flow.  That was enough restriction to stop the thermosiphon.

I also added a water catching bottle to the air bleed-vacuum system; same as brake bleeding bottle. Didn't want my steel vacuum tank to rust out.

A lot of plumbing and purging but now I'm ready for winter and will find out how my tank and hot water panel size work out for meeting my space heating needs.  I'm still using the patched twinwall polycarbonate glazing, with some 2.5" galvanized steel strips held down over the former bulge/cracks, which are silicone caulked and foil taped.  Not pretty but it works and will let me continue testing the system through the winter.  Single glazing with 3/32 polycarbonate is probably my best bet.

[Ronmar]

Yep, a simple swing type check valve will stop the thermosiphon in it's tracks:)

[Derb]

For your tank within a tank system, Thought about using an old Electric Hot Water Cylinder? I have used a couple now for indoor pot plant holders (when cut in half). The ones I have used are low pressure cylinders for use with a header tank in roof, about 180 litres, are copper inner tank with insulated material glued onto outside and clad with galv sheet steel. It only takes me about 1/2 hour total to cut off cladding with a skinny disk on grinder and then attack insulation with a sharp spade. This would give you rust resistance along with all the threaded pipe fittings you would need.