How to weld a gas boiler heat exchanger: instructions for self-repair

Are you familiar with the situation where you have to repair an expensive part, but the budget for this repair is growing? Surely you know what costs can and should be considered unnecessary. For example, you have a gas boiler and you don’t want to pay up to 30 percent of the cost of the entire device to repair one single part. If you don’t know how to weld the heat exchanger of a gas boiler, just read our article and you will learn.

Thanks to this article, you will learn the procedure for repairing the heat exchanger - a part due to which boilers often break down. We talked about soldering heat exchangers in different ways. Using our recommendations, you can independently repair the failed part.

In domestic conditions, heat exchangers are repaired by soldering. Welding is literally used in manufacturing, sometimes with the help of robotic devices. Leave a heat exchanger with extensive structural damage to the craftsmen, but in the case of small holes in the part, soldering can help. Find out more about this.

Removing the heat exchanger from the boiler

Gas boilers are equipped with heat exchangers, sometimes called boiler radiators, in which some sources transfer thermal energy to others. Most boilers have primary and secondary heat exchangers, while others have combined or bithermic ones.

All types of exchangers are affected by unfavorable factors.The quality of the metal deteriorates, the permeability of the channels decreases, and the water pressure hits the walls. The primary and bithermic ones are affected by the flame of the main burner.

With bithermal heat exchangers, the water supply channel is located inside the heating line - if it were the other way around, the heat exchangers would fail more often

Worn exchangers may develop holes. Considering the cost of this part, it makes sense to repair them rather than replace them outright. Moreover, it is not always possible to find an analogue.

If the heat exchanger in a gas boiler leaks, you will notice leaks near household appliances or an error on the display. If necessary, remove the problematic part.

First do this:

1. Disconnect the boiler from the power supply and gas.
2. Stop the flow of coolant into and out of the heating system. Turn on the tap water supply to the boiler.
3. Drain any remaining water in the device.
4. Remove the boiler lid.
5. Bleed the air through the vent.
6. Check the inside of the device. See if there is moisture anywhere and wipe it away.

To remove the primary or bithermal heat exchanger, free the combustion chamber from the elements that fit it. Remove the pipe that supplies gas to the burner. Disconnect the electrical wires from the combustion chamber. If you forget this nuance, then due to carelessness you can damage them.

Unhook the sensors, heating circuit pipes, remove the pipes of both circuits from the bithermic exchanger. Remove any components that prevent the device from being removed from the boiler.

Disconnect and pull out the combustion chamber. Remove the top and front covers. Remove the hardware under the heat exchanger and pull it out.

Due to water leakage from the heat exchanger, plaque forms and the condition of the boiler elements deteriorates; sensors may fail, after which the furnace will not start

To remove the secondary heat exchanger, remove the board and other electronic elements, unscrew and remove anything that would prevent the unit from being pulled out of the boiler. Isolate the contents of the device from the ingress of water from the exchanger and the channels suitable to it.

But a malfunction of the heat exchanger is not the only reason why a boiler may leak. We described in detail about other possible causes of leaks and ways to eliminate them in this article.

Soldering a gas boiler heat exchanger

All water, air and solids are removed from the exchanger. Blow through a hose by machine, and blow out the residue with your mouth.

Four components are used for soldering:

• soldering tool (gas torch, blowtorch or soldering iron);
• solder;
• flux;
• tools for stripping before and after.

Solder is a material for joining workpieces that has a lower melting point than the base being processed. Solders are usually made from metals. They are produced in the form of wire, embedded parts, pastes, foil, powders, rods and granules. Wires are most suitable for soldering the heat exchanger. Take this one.

To repair the heat exchanger, choose high-temperature solder with a lower melting point, but not lower than 700 °C, from the same material and with similar physical and mechanical properties. The anti-corrosion properties and conductivity of the wire are also taken into account.

Copper, stainless steel and cast iron are common raw materials for exchangers, and zinc is often added to the base. For soldering copper heat exchangers they are often used copper-zinc solders with inclusions that neutralize the dangerous effects of fumes.

Solder in rods is not as convenient as wire solder, but it can also be bent by hand, even with a large diameter, and the resulting solder will be quite rigid

High-temperature hard solders are marked PSR, PMC, PMT, etc.

Copper-zinc is designated PMC and is numbered based on the melting point:

• PMC-36 - from 825 °C;
• PMC-42 - from 833 °C;
• PMC-54 - from 860 °C.

A standard gas boiler heat exchanger can be soldered by anyone, but whether this can be done depends on the materials of the part.

Good solders produce sealed seams. The components of such alloys penetrate into the soldered surface by diffusion, and the base is dissolved in a small amount in the auxiliary material. After hardening, a homogeneous layer appears.

Flux is a substance for removing oxides from the soldered base, increasing the fluidity of the solder, reducing surface tension, and better wetting the workpiece. Universal and special products are combined with heat exchangers. Solder pastes usually contain copper, which is good for the heat exchanger. Mixtures with silver are also suitable.

Preparing materials before soldering

The soldering iron tip is tinned. The tool is heated until the tip is slightly reddened and covered with a thin layer of solder, while the oxidation film is removed. The tip and wire are dipped into flux. At the tinning stage, you can use rosin or resin for this.

After the smoke is released, the sting is held for a few more seconds. Then the soldering iron with solder is dipped into the flux three or four more times.

Devices become tinned after purchase, and then from time to time - with frequent use. The tip of another soldering iron is sometimes used as a basis for tinning.

The photo shows tinning - after this it won’t hurt to press the tip to a wooden board with resin, which will also even out the solder layer

If the condition of the solder is not the best, then it should be cleaned of dirt and oxides. Heat the end of the solder wire to operating temperature and immerse it in the flux, pressing it firmly against the hard surface on which it is located.

If there is no new wire, then the old one can be treated with base cleaning agents, such as:

• abrasive;
• solder paste;
• tinning acid.

Before soldering, remove dust from the surface of the heat exchanger so that sparks do not appear during operation. The area with the fistula is treated with a cleaning sponge or fine-grained sandpaper and wiped with a solvent to remove all chemical compounds.

Then the problem area is heated with a hairdryer for better performance and to evaporate any remaining moisture. Otherwise, it will come out sharply during operation and dislodge the solder joint. After warming up, the area is cleaned again.

How to solder a boiler heat exchanger?

A small fistula on the heat exchanger is found by green spots, but if such a shade is present on most of the device, then the condition of the coating and evenness of color are taken into account.

The soldering agent is chosen according to the situation. A soldering iron is suitable for fine work. A large flame from a gas burner will warm up the problem area well, but the power of the device may not be enough, contrary to expectations. You also need to choose a powerful soldering iron.

You should select the correct nozzle for the burner - the flame should be approximately 2 cm wide, and rotate it for convenience, and while working, hold it so that the fire is distributed evenly over the surface

Most of the work looks simple.Flux is placed on the heated area, after which they begin to solder. The solder is placed on the heat exchanger with its tip, the soldering iron is placed against it and gradually heated. The wire will begin to interact with the flux and base. The remaining uneven mass after soldering is distributed.

In case of gas burner, take a regular standalone or something more productive. For example, powered by a large cylinder. Never turn on the maximum flame mode. Apply solder after the flux color becomes brighter, for example silver instead of gray.

Keep the burner at such a distance that the wire melts more from the heated heat exchanger, rather than from the fire. To treat a small fistula, half a minute of soldering may be enough. Detailed soldering instructions can be found in this material.

Do not overheat the base and solder during soldering - the latter will not be fixed well. Make sure that the resulting structure is not loose and the color is not matte. Soldering ideally restores the entire problem area, and if this is not the case, then another approach will be needed.

Raise the temperature of the soldering iron to 20 degrees above the melting point of the solder. To clean the finished joint, use braids and spring removers - the tools will also remove flux residues. Wipe the exchanger with a damp cloth and remove all microparticles.

For normal operation of the boiler combustion chamber, the soldering area must be cleaned, and some disruption of the shape for the primary heat exchangers does not matter

Water cannot flow through the repaired heat exchanger for another 5-10 minutes. In any case, let the part cool completely. Turn on only clean water on the first day after repair. Make sure there is no rust.

Remove air from the system again and test run the boiler. Check operation at full load. Pass cold and hot water through the exchanger. After several temperature cycles, it may turn out that the exchanger is leaking again.

The repaired part must withstand thermal deformation. To be on the safe side, coat the joint on the exchanger with heat-resistant paint to increase strength. In the following days, retest the heat exchanger.

We also recommend reading about other malfunctions of gas boiler heat exchangers. More details - go to link.

Cold welding as a repair option

So-called cold welding is made on the basis of adhesives. A popular raw material for this is epoxy resin. Do not confuse material and cold welding in understanding the technological process with plastic deformation of metals without heating.

Choose the most moisture-resistant raw materials that the market offers. When starting work, put on gloves and soften the welding with your fingers. Do this until the mass becomes plastic. Place the material on the fistula and spread it over as large an area as possible. Make the layer thick, but not necessarily the thicker the better. Apply with a wooden stick.

Cold welding is used on copper, brass, bronze, cast iron, iron, alloys, and also on ceramics, wood, stone, but the final quality of the joint largely depends on the work itself

Wait for the finished layer to harden and lightly clean the area with fine-grained sandpaper and a damp cloth.

First, wait half an hour for better hardening. The recommended 3-5 minutes are sometimes not enough. Check the quality of the joint using temperature contrast and water pressure.

Conclusions and useful video on the topic

An alternative way to extend the operation of the boiler’s primary heat exchanger:

Soldering a heat exchanger roll with two gas burners:

There are three simple ways to seal a hole in a heat exchanger: with a flame, with a sting, and with adhesive. Most often we are talking about a narrow hole on the primary or bithermic exchanger, so the listed methods are quite sufficient.

Leave complex breakdowns to the experts. But, if something happens, you will find something to solder the heat exchanger with, take it out of the gas boiler and choose the solder yourself. If possible, act independently and remember the importance of preparatory work.

Write comments and ask questions about the topic of the article. If you have a boiler, tell us if anything has happened to its heat exchanger. Write about your experience in soldering and repairing exchangers or other boiler parts. The feedback form is located below the article.