Adapt an electric water heater for use as heat storage by adding a coil

Copper coil to heat household water tank

(This document was translated using Google Translator from the original spanish article)

Install a system for using solar energy or biomass to produce hot water in winter is profitable within a few years (particularly in areas where solar power is not enough), but can be very expensive for many, especially in developing countries.
With this tutorial you intend to use an old water heater 200 liters. of 'Cointra' brand and add a coil to heat the water inside. Quite a challenge considering we have to keep the seal and put it in without enlarging the tank ;-)-.
Before deciding on this, I had several options for heating water for hot water in a simple and economical, each with its pros and cons:

• Buy me a plate heat exchanger high performance to bring the heat to the tank. This solution has many drawbacks, it affects the lime, is expensive (between 60 and 300 euros), you need a second engine to circulate the water tank (thereby generating mousse) and if you want to give priority to the ACS have to put a three-way motorized valve to divert excess heat after reaching a certain water temperature.

A copper heat exchanger standard.

• Instead of buying it, make myself exchanger based on many welded stainless steel plates with a small separation between them, with the difficulty of pores and low performance if not enough solder. It looked very laborious, so I decided to do a simpler, with a tube inside another tube from the water that surrounds the central tube would be heated enough to temper the water (or so I supposed). So I did as you see in the image below, the tube surrounded by yellow foam as insulation:

Photos together with Autostich (R)

This solution was a failure, the steel tube just transferring the heat, getting at most a 15% return in the transfer to the tank. Also the thing was complicated with a second circulator motor to carry the hot water tank. And over the water with air bubbles out, like peroxide, produced by the second engine churning the water ...

The part that requires maintenance on a boiler: The magnesium sufferer resistance

• Finally, after much racking my head trying to look for cost effective, reusable and efficient to the problem of heating the water from a pressurized tank without coil in the end I turned on the little light, why not put a coil to the tank? That idea was dismissed at first by the fear of losing the tightness of the shell (protection vitrified steel to avoid rust), and it was likely he would have to pierce the cap, weld, paint it again, etc, and although he could do Thus, in fine stainless steel tubes. soldiers found a better way, use the brass stand-suffering and endurance to pass the tubes:

In this way get a reusable coil (if we change the sufferer, the deposit will last for many years, but if damaged, can be welded in another tank used) to be high performance copper and with sufficient flow tube 12 mm., avoiding the costs and problems mentioned above.

Level of difficulty: 7 for little hands.
Material cost: € 30 Copper (8 mts. Diameter copper pipe. 12) and accessories, 100 € deposit of 200 lts. Second hand.
Timescale: approximately 5 hours. (3 if you are skilled with copper).
Quality results: 9, heat recovery almost as good as reservoirs for commercial Solar Power 200 lts. for 600 €.

However, this solution is not always applicable, need some skill with the drill, namely solder, patience and ability to make a coil that enters the mouth of the shell, a bigger tends to be bigger mouth, so it is better not to go to a deposit of less than 150 liters (rare).
Coil efficiency
I thought the coil straight pipes will make a circuit that top-down (does not require bending the tube) to be efficient it is best to have a lot of contact surface and that makes the hot water goes first to the area more heat (the more difference between water temperature and the coil current, the amount of heat generated will be higher), then let the residual heat in the coldest area (the heat is distributed by stratification, the water gets warmer up near the exit of hot water.)
You can also buy coils coiled ready for solar power, something better performance than the straight that I have prepared for this tutorial:

Or make one yourself by filling it with sand to not close the bending and heating to facilitate bending around a big tube, if I knew this on the moment of building, I would make it this way.


And the other option is to make stainless steel., Soldering coil tube to the top, but the difficulty is much greater than having to weld elbows close together and then use the thin tube, would also have much lower performance than that of copper . Although you can do, the difficulty up to 9 ;-).

Getting your hands dirty
We emptied the tank and release the mouth of the tank maintenance:




Take this opportunity to take a look into the shell, in this case is in good condition except for the side boards on which notes that the painting jumped in some areas, probably due to excessive pressure and not using safety valve in installing the previous owner. In any case the sufferer is protecting magnesium oxide fine and still in good condition after two years of use.


The welded joint of funds side, it seems that magnesium is accumulated above the board, protecting:


We take to clean sludge and lime with a water hose and a rag:


Now we get to the stand. After racking his head for a while I saw that from the hole where the meeting is hosted and screw the center is about 12 mm., Which could use the measure tube.
The first is to remove the rubber seal and cut resistance, as the tubes pass through your site:

First with a little bit, and then with the bit more to allow us to support (in this case could be used in dia. 12), drill and drill the holes through which he passed the resistance. The most important thing is not to damage the ring seat. In the center is a screw, leaving a sufficient space.
With the drill at low speeds and tight little, the hole is easy to practice in a soft material such as brass.

Subject piece with the screw at the base of the sufferer to make the holes, make them as straight as possible. Check that the tube passes and go over the hole if necessary:

And this is the result. You see the pod temperature sensors and the sufferer back.

Starting to weld copper pipes, brass stand. The tubes had used the rusty them stored in the open, with steel wool cleaned the ends and gives the flux to ensure that the tin-silver solder paste well.
(I feel the quality of the photos, the time change ;-)


Heat is applied with a torch until the color changes a little tube and is applied without exceeding the tin, that will not be a drip:




Before going, I'll put the lid (no gasket, very important, so it does not burn). Modified to do the holes in the lid through which passed the resistance cables to allow the passage of the tubes of 12 mm with a file:

And we find that passed correctly. I had to change the nut and round and then turn it into a bolt by applying a cutting disk 1 mm., Since sticking to the tubes (this nut tie support and assure no leakage between seal and reservoir cap) :

Now prepare two tubes with the break as possible to make the journey up and down the tube, sorry no elbows 180. This operation has to be made ​​with each pair of tubes we want to add, welding a first hand flat on the table gives us a lot of work to make the coil.

Heat with the torch (careful not to burn anything that is close);

Tin generously applied (not dripping) and wipe clean with a cold after the weld:

Just put the two tubes welded one side, so that the curve is attached to the tubes, it is important to be compact or not we enter the mouth of the tank.
I have used 8 tubes, but can reduce the number if we see many difficulties not to exceed the maximum diameter that has to have the coil.

This is how it is becoming the top, where we have fewer hurdles to place.

And little by little, I reach the end, in this case I could get 8 tubes of 84 cm. long, each pair positioned so that surrounded the base, and with the help of tools (as we see a screwdriver may be worth it for many things, lol) I separate the tubes to finish welded in place because we must be careful not applying heat to the solder already taken place. Copper is quite flexible, so no problem.


In this picture we can see the details of how it has been the base of support, respecting the diameter that marks the highlight of the cover (which is where it is pinched together).
We can also make some a little shorter for the curve to pass through the interior of another curve, more compact.

The problem I see now is that I left very short tubes on the bottom, and we put together before welding because otherwise we can not put it once welded parts, but there is a certain danger of overheating and burning.
Get it wrong and ruin the rubber seal, would desoldering (heat and pull the pieces with pliers to release them) and get another board of 3 mm. thick or manufacture of a larger board.
The solution I see is putting the board in place but isolated from the tubes and cap. For this I used toilet paper ;-) (also used wool fiber or other insulating material).


We put everything in place, with adapters to take the tube to 1 / 2 inch, which is the tube that is often used in plumbing.

Apply heat from the side, avoiding heating the plate (and thus the board):

We apply the tin, let it cool and check how it has been the board, after removing the paper I find that perfectly fits your site:


For best performance with this 7-meter tube, I bent a little tubes out on top, to cover more water.

If we see that fit together perfectly at the mouth of the shell.


Connecting to the network (8 bars) I see that does not lose any welding; important because if water could be mixed with the Secondary and Primary would have to re-review.


Now it is back in place, weighing about three kilos, making it difficult to maneuver to put the screws again. I've been tying first one side then the other. When hit, recommended gradually tighten all is well positioned to cover.



We fill the water tank by opening the hot water faucet until the water runs through it. Then you close it and if it misses to the top ... Eureka!


And it is the coil in place. I tried it and has great performance, and only need to add all the accessories needed for heating and heating water with low heat coil, giving priority to the ACS (this is complicated, which will be a new challenge that I love ...).
Using the thermostat KELD for solar energy and three probes, we compare the temperature of the coil in the fire with water, return, and a third probe activate a 3-way valve to divert the primary hot water heating once the deposit has reached the desired temperature. But that in next posts ...
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Responsibility of the author:
This tutorial, like everyone else, are made ​​publicly available for one that may come in handy, I am not responsible for what happens on the bad or good use of these instructions, each must use their common sense and knowledge to safeguard their property and physical.