# Water-to-water Heat Exchanger. Wow, soft copper pipe is expensive



## turbotech (Mar 25, 2011)

With the current prices on soft copper pipe I am not excited about making a heat exchanger for a storage tank. I could go with a aluminum heat exchanger and sacrificial rod, but that requires more maintenance and more failure possibilities. 
It looks like a 3/4" x 60' coil is about $150 and need 4 coils comes out to $600 to make two 120' coils. That has a surface area of 105 sq inches. That is crazy to me. A copper sheet measuring 12"x24" and is .080" thick costs $110. I could TIG weld a box that is 4" x 4" x 12" using the plate to get 192 sq in total. That is a higher surface area than two 180' coils which is 160 square inches.

The box would be equivalent to two 3/4" coils that are 218' long each!

I am thinking of going the copper box route. Any thoughts on why it would be a bad idea?

EDIT: Corrected box size


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## SmokeyTheBear (Mar 25, 2011)

turbotech said:
			
		

> With the current prices on soft copper pipe I am not excited about making a heat exchanger for a storage tank. I could go with a aluminum heat exchanger and sacrificial rod, but that requires more maintenance and more failure possibilities.
> It looks like a 3/4" x 60' coil is about $150 and need 4 coils comes out to $600 to make two 120' coils. That has a surface area of 105 sq inches. That is crazy to me. A copper sheet measuring 12"x24" and is .080" thick costs $110. I could TIG weld a box that is 4" x 4" x 12" using the plate to get 192 sq in total. That is a higher surface area than two 180' coils which is 160 square inches.
> 
> The box would be equivalent to two 3/4" coils that are 218' long each!
> ...



You might want to redo your 60' copper tube coil as the exterior surface area is (assuming the 3/4 is an OD measurement) 3.14159265 x .75 x 60 x 12 (1696+) square inches and that is much larger than the 192 square inch box.


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## turbotech (Mar 25, 2011)

I just realized that when I thought about the real world size. I forgot to do the  times 12 for inches. It comes out to 1,271 sq inches. Your equation is has .75 where it should have (.75/2)^2. Thank you for checking this for me. I will see what a larger sheet costs to make a 12 x 12 x 12 box.


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## afblue (Mar 25, 2011)

I am not understanding your math. 3/4"x60' to me is 1697 sq inches of surface area.   (pi)dh=3.14*.75*720=1696.46 sq inches 

I do agree with you copper is rediculously expensive. I imagine 50 years from now when I pass away, my grandchildren are going to gut my house and make a fortune


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## SmokeyTheBear (Mar 25, 2011)

turbotech said:
			
		

> I just realized that when I thought about the real world size. I forgot to do the  times 12 for inches. It comes out to 1,271 sq inches. Your equation is has .75 where it should have (.75/2)^2. Thank you for checking this for me. I will see what a larger sheet costs to make a 12 x 12 x 12 box.



Your calculation for the surface area of the cylinder should be pi x diameter x length or circumference x length.


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## turbotech (Mar 25, 2011)

I think I am losing it. You two are right in that it is pi*D. I posted the cubic area. I will try again.


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## tom in maine (Mar 25, 2011)

Copper is expensive. But experimenting on heat exchangers is more expensive.
Copper tubing is the best bang for the buck. I have been doing this for over 30 years.
Smooth copper does it the best for the range of temperatures that you need to work with in thermal storage systems.


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## turbotech (Mar 25, 2011)

Tom in Maine said:
			
		

> Copper is expensive. But experimenting on heat exchangers is more expensive.
> Copper tubing is the best bang for the buck. I have been doing this for over 30 years.
> Smooth copper does it the best for the range of temperatures that you need to work with in thermal storage systems.



I think you are right. The cost for the sheet would be much more expensive than using the copper pipe coils. I guess I had a case of sticker shock and wishful thinking. Now I just need to find a good place to get the pipe.


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## turbotech (Mar 25, 2011)

The other thing I was thinking about was trying to use a water to water heat exhanger like they use to bridge to the house heating water. Then a pump and stuff is needed on the tank side. It didn't seem worth it.


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## turbotech (Mar 25, 2011)

The best deal I can find on 3/4" x 60' of type L is $160. Looks like I am already over budget on the storage tank with the 4 copper coils that I need.
Is Type L the best one to use for this application?


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## bupalos (Mar 25, 2011)

Relatedly, I've got extra powder coated steel panel radiators that I'm trying to figure out how to use. Who here thinks it's just a super great idea for me to use one of these as a heat exchanger submerged in an open tank. Is that just going to rot?


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## turbotech (Mar 25, 2011)

It is going to rot. The question is how fast. Being open I would say it will rot fast. Have you thought about doing a closed system with an anode?


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## PassionForFire&Water (Mar 25, 2011)

Aluminum only has 60% of the conductivity of copper.
The thinner the tubing wall the faster the heat exchange.
Redo the calculations with this in mind


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## turbotech (Mar 25, 2011)

I lookup the conductivities for copper and it shows 232 BTU/(hr*foot*deg_F). From what I read the max burn time for the Tarm Solo 40 is around 4 hrs. I should be able to calculate what length coil I need at a minimum. I see a deltaT of 20 degrees used a lot. With the given burn time and deltaT I should be able to figure out the length in feet I need.

I was thinking of using the Type L coil because it is thinner wall for faster transfer.


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## afblue (Mar 25, 2011)

232 BTU/hr*ft^2*F I think is the Steam to water coefficent. Water to Water for Copper is 60-80BTU/hr*ft^2*F. The way to calculate the loops you need are 2 numbers. First is the BTU/hr that the boiler produces to calculate the gpm to flow. So the Solo 40 is a 140,000BTU/hr. The formula is BTUper hr/500*delta T=GPM So I get 14GPM. Now you need to have the cross section to flow that. You want to have the water flow through at less than 4ft/second, optimal is 2ft/sec. 3/4 type L flows 3.2-6.5 respectively. so if you shoot for the middle of 3ft/sec, then 14gpm/4.85=2.9 rounds up to 3 separate 3/4" loops. 140K/3=46,700BTU per coil. Thats 33.4 sq ft per coil of copper. pi*d*h tells me that each coil needs to be 170ft longer at full boiler output, and no house load. Thats 510 ft of copper which is really $$$. Now that is worse case scenerio. If you have a 40-50K BTU load running while you are charging the tank, then you can take that off the top of the boiler output. Then you are talking about ~350ft of copper. The delta T would also be alot higher when the tank is at 100F at the beginning of your burn, so if you time the loading correctly, as the tank is getting to maximum temp the fire is about to burn out, or the its at least coals before the boiler goes to idle. 

Real world experience from the guys here will tell you what you can get away with, but my guess is going to be around 400ft of 3/4" copper.


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## NHFarmer (Mar 25, 2011)

I will give you my real world system. I have a Tarm Solo 40 and a 950 gal. non pressurized storage tank. I bought 20' lengths of 1/2" copper pipe, I used type L pipe and hand bent them with a conduit bender. I made 4 coils and connected them in a parallel setup using 1" copper pipe. I used a total of 320' of 1/2" pipe plus 10' of 1". I run my tank up to 170* and the boiler will idle some but it is really no big deal. I run the tank down to 120* minimum and It will heat up with no problem. I heat with CI radiators so I can utilize lower temps. This is the third season with this setup and it has work out well for me.


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## turbotech (Mar 29, 2011)

I have been asking around about copper pipe and may have a lead on some 3/4" type K that is new cut-offs. They are left overs from water supplies.
I read afblue's post and it makes sense what I need. Where can I find the correct heat transfer values for the K type that is thicker wall?

The Tarm Solo 40 states 140K BTUs, but is that input BTUs? Assuming 80% efficiency and that being input BTUs, then it seems like 3 coils of 135' each would be plenty even without any other loading being turned on. Each coil would being flowing 3.4 ft/sec using 14 GPM as the overall rate.

14    gpm        
1.871657754    Ft^3/min        
610.3750127729    ft/min        
10.1729168795    ft/sec    Single 3/4â€ copper pipe    
3.3909722932    ft/sec    Three 3/4â€ copper pipes    
2.5432292199    ft/sec    Four 3/4â€ copper pipes    

26.6666666667    80% efficiency    3 coils -> sq ft per coil    
135.881104034        length in feet

Overall, it looks like I will use 3 coils of type K copper pipe that are 135 feet long.

Moving on to sizing the DHW coil and side-arm.............


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## turbotech (Mar 29, 2011)

For the DHW I plan on using the existing tank and using a side-arm. The thought is to use a 2" copper outer pipe and a 1" inner pipe (maybe larger on the inner pipe). The unpressurized tank will need coil(s) for the DHW heater. I want to be able to have the conventional heating element come on if the storage tank is not heating the DHW high enough. Which is the best way to do this? Pre-heat with a coil and a seperate side-arm coil? Mixing valve?


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## Singed Eyebrows (Mar 29, 2011)

Sink a cast iron radiator in the tank. Cast iron has 3 times the surface area of steel. Cast iron will also resist corrosion etc that will chew up steel. Look on Ebay for rads near your house, I've seen them go for $25.00, Randy


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## turbotech (Mar 29, 2011)

Singed Eyebrows said:
			
		

> Sink a cast iron radiator in the tank. Cast iron has 3 times the surface area of steel. Cast iron will also resist corrosion etc that will chew up steel. Look on Ebay for rads near your house, I've seen them go for $25.00, Randy



I looked at the coefficients for cast iron. For water to water it is 40-50 (Btu/ft2 hr oF), and for copper it is 60-80 (Btu/ft2 hr oF). So copper is much better. I would need a few good sized radiators to equal the copper pipe. With the copper I shouldn't have to deal with chemicals. Have you tried the cast iron radiator in a tank?

I would need a cast iron radiator with an equivalent surface area of a 4.6 foot square box.


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## Singed Eyebrows (Mar 29, 2011)

turbotech said:
			
		

> Singed Eyebrows said:
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I have not tried this, I run pressure storage. Even if 2 rads were $100.00 though this would be far cheaper than copper. I'm sure they would last for many years with or without chemicals without any kind of real investment to lose, Randy


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## Duetech (Mar 30, 2011)

Sorry to say I don't share  the "real world" consensus concerning coiled copper. Seems like a terrible waste of cold cash. It would be a lot of work but hard copper and solder will make a submersible heat exchanger of probable equitable (or beter) output for a lot less money. Look at an air/water heat exchanger like used in a fossil fuel furnace plenum. A custom "built to your size tank" exchanger using 1" reduced to 1/4" and back (like in the above mentioned exchanger) will extract or collect large btu's. It is labor intensive but you can place your exchanger in the tank right where you want the conversion to take place and not through a portion of the tank that is prone to rob efficiency. The hard copper exchanger will take up less room and cost less to build. IMHO


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## turbotech (Mar 30, 2011)

Cave2k said:
			
		

> Sorry to say I don't share  the "real world" consensus concerning coiled copper. Seems like a terrible waste of cold cash. It would be a lot of work but hard copper and solder will make a submersible heat exchanger of probable equitable (or beter) output for a lot less money. Look at an air/water heat exchanger like used in a fossil fuel furnace plenum. A custom "built to your size tank" exchanger using 1" reduced to 1/4" and back (like in the above mentioned exchanger) will extract or collect large btu's. It is labor intensive but you can place your exchanger in the tank right where you want the conversion to take place and not through a portion of the tank that is prone to rob efficiency. The hard copper exchanger will take up less room and cost less to build. IMHO



I priced hard copper and soft copper. Soft copper was cheaper. You can get hard copper a lot cheaper where you are? How much less it it than soft copper?


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## afblue (Mar 30, 2011)

turbotech said:
			
		

> Cave2k said:
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I second your findings, the cheapest place that I have found hard and copper coils and pipes. Yes  soft type L coils are $3.18/ft, hard copper L piping is $2.83/ft, but if you need 2 90deg @ $1.13 each fittings every 8ft. then you are talking effectivelly $3.11/ft for hard with fittings in a serpentine configuration, and 30 solder joints in 60ft. If you were to make a 3ft by 7.75' grid with 7 branches, you are talking over 40 joints and it costing $3.27 per ft. 

Not to mention with hard copper in a grid, or serpentine, every joint adds more head pressure.  Soft copper is smooth laminar flow. simple, and alot less hassle to solder and install with alot less points of failure.

I know copper looks to be ridicoulously expensive, but if you ever take apart the system and scrap it, its going to be worth more in scrap 20 years from now then when you buy it now.


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## markmudd (Mar 30, 2011)

Real world example, I have 500 gallons non pressurized storage and 120" of 3/4 copper in parallel hookup.  The coils are supplied by 1" lines from 150,000 BTU boiler (non-gasser). 1 three speed pump does all flow(not a primary seconday plumbing setup).        I can heat tank up from 125 or so to 170 Deg. with hot fire within 3-4 or so hours while supplying water to air heat exchanger assuming my house temp is in normal range (67 to 70) to begin with.     I will get usable heat for exchanger for 9- 11 hours depending on how cold it is outside.    Wish I had radiant in the floor, it would be longer between burns.   Also have 120 ft of 1/2 inch copper in storage tank for DHW and it works fine too.


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## turbotech (Mar 30, 2011)

mocus said:
			
		

> Real world example, I have 500 gallons non pressurized storage and 120" of 3/4 copper in parallel hookup.  The coils are supplied by 1" lines from 150,000 BTU boiler (non-gasser). 1 three speed pump does all flow(not a primary seconday plumbing setup).        I can heat tank up from 125 or so to 170 Deg. with hot fire within 3-4 or so hours while supplying water to air heat exchanger assuming my house temp is in normal range (67 to 70) to begin with.     I will get usable heat for exchanger for 9- 11 hours depending on how cold it is outside.    Wish I had radiant in the floor, it would be longer between burns.   Also have 120 ft of 1/2 inch copper in storage tank for DHW and it works fine too.



Thank you for posting actual results. Is that two coils that are each 120 feet long in parallel?
What is the tank material and what type of insulation?


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## markmudd (Mar 31, 2011)

I have two 60 ft coils, 120 ft total.  They are supplied by 1 inch copper line in parrallel, in that water does not run thru first one and and then the other, but that each coil recieves same temp water by dividing the 1 inch line into two 3/4 lines then reuniting lines after coils.    I know That is less than a lot of folks with technical expertise recommend, but it works good for me.   It is controlled with valve in that I simply adjust flow to tank coils based on heating situation and then whole system slowly cools and supplies heat after fire is gone.   Obviously not a high tech system and I would probably use primary secondary type of plumbing if I had to redo but I did not know of this site when I did it.   I ahve a royal 6150 boiler which is a good unit, but again not a gassifier.       Missouri climate is moderate and I have lots and lots of wood.   

TAnk is 500 gal made out of 2x4 with 3/4 plywood and 2.5  inches of polyurethane insulation and 1 inch of blue board foam on exterior (between 2x's) and 1 inch blueboard on interior behind liner oer 3/4 plywood with osb on exterior.   Have  a single top center support to keep tank from speading on top and 2x4's run horizontal and are bolted at corners.    For top I layered 5 1 inch thick blueboard sheets with EPDM on innermost top panel. 

Do not recommend  use of blue board for high temp.  Use the Polyscianate stuff (spel?)  I also messed up on installing my liner and need to re do it to have a better seal around top edge.  but it holds heat well.  Plan to fix liner this summer.


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## Duetech (Mar 31, 2011)

turbotech said:
			
		

> Cave2k said:
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Foot for foot it is probably not cheaper going to 1" hard copper from soft but the point is to look at a heat exchanger like they use for air/water as in a furnace plenum/boiler conversion. In a simple 1" 20" x 24" air/water exchanger there are usually two 1" tubes and about 16 quarter inch tubes to transfer water from one 1" tube to the other 1" tube. The smaller tubing is what passes through the fin medium that transfers the heat to the air as it passes through the fins. (the heat is transfered through surface area...imagine trying to put 240 feet of copper tubing in a plenum for heat transfer...lol). If a person were to build a similar tubing arrangement for heat transfer inside of the storgae tank it would of course be bigger but would not need 240 feet of 1" tubing and water is probably much better than using fins. Example: in a 4' x5'  by 5' tall tank you would position a home made exchanger near the top and one near the bottom of the storage tank. Each exchanger could have three (or two* if you know you can get the tank water to flow at a rate to keep up with the heat transfer) 1" pieces about 5' long and fourty-eight (or 36*) 1/4" pieces about 4' long (to go across the tank and back* and across again). That is a lot of cutting and 1' to 1/4' tees and a lot of soldering but the 1/4" copper will give you 4x of the surface area of 1" copper and the heat transfer would be superior to 1" copper. If a person were adept at bending they could eliminate a lot of soldering. In my area 1/4" is cheaper than 1" and that is what I was refering to about being cheaper to build. Soft copper could probably be used but then you have a lot of flaring and fittings to account for. Sorry for the confusion.


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## jebatty (Apr 3, 2011)

> The Tarm Solo 40 states 140K BTUs, but is that input BTUs? Assuming 80% efficiency and that being input BTUs ....



This is the output rating, not input. You probably can use 70-75%, not as an efficiency number, but as average output over a burn. At high burn the Solo 40 can output more than 140,000 btuh, but that won't last very long. Humming along at around 100-110,000 btuh for a good part of the burn should be about right.

As to actual output, I determined that last year when I had a flowmeter installed, since removed.


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