Installing a Heat Exchanger for an Outdoor Wood Burning Furnace

Many garages or shops use an air handler like the one below, with a built-n heat exchanger.

 

Typical Heat Exchanger that is installed in the supply plenum
or ductwork (the one that goes to the vents in the different rooms).

This is always the best way because putting the heat exchanger
in the return duct (where the filter is) will cause hot air to blow over
the fan -heating it up instead of cooling it down. 

Installing the Heat Exchanger in the Plenum or Ductwork

If you have Air Conditioning, the heat exchanger must be installed between the furnace and the evaporator coil. Failure to do so may result in freezing of the heat exchanger. If you can’t place the heat exchanger between the furnace and the evaporator coil, you must drain the heat exchanger annually before using the A/C.

The heat exchanger must be installed so that it is airtight. No air must be able to flow around it or out of the ductwork. Use adhesive backed foam tape (used for insulating doors and windows) around the water coil. Use foil tape to seal off the heat exchanger and the hole you make, or consult a local HVAC installer or technician.

Make sure the fittings are easy to get to once it’s installed. Ideally, there should be no splices in any water lines! If you have to splice two lines together, use SharkBite® fittings. The IN fitting should be at the bottom of the lowest point (bottom). If installed horizontally put the inlet at the lowest point – usually 2-3” lower than the other nipple.

You should be able to find a heat exchanger to fit most popular sizes of plenums. If you can’t find one, we can have a heat exchanger specially made at a nominal cost so that you don’t need to have your ductwork modified to accept the heat exchanger by a professional. In all cases, wear protective gear, gloves and glasses and a mask. Edges can be SHARP!

Measure the width of the heat exchanger (Dimension A in next diagram). Measure the thickness of the heat exchanger (Dimension C).

Start by cutting a hole in the side of your ductwork the thickness of the heat exchanger (Dimension C) and the full length of the ductwork (usually Dimension A).

Slide the heat exchanger into the hole for a test fit. Ideally the tubes (D and E) should stick out of the plenum [see diagram on previous page]. The header and tubes (F) can stick out of the plenum as well, if necessary.

As long as the whole coil surface (B) is in the plenum, you should be good even if a little more of the header (F and even G) sticks out.

While test fitting it, try to determine how much tape is needed around it. A different amount (thickness) may be needed on different sides. You can purchase different thicknesses of tape so that it will fit and seal properly.

Place foam tape around the outside of the heat exchanger to seal it off so that it fits tight and air can’t blow by it.

The heat exchanger needs to be installed so it won’t move around. The easiest way is to fabricate some pieces of metal into an ‘L’ shape, approx. the length of the heat exchanger like shown below.


This is what the braces will look like. Very simple. Rivet or screw into place.

Install the braces as shown above. Simply put them in place, the proper width apart (Dimension C). Drill through the duct work and your newly made ‘L’ pieces and pop rivet or screw them into place. You will need four pieces, two each on the top and bottom.

Slide the heat exchanger carefully into the plenum to check the fit again. If all looks good, you can seal off the ends of the heat exchanger and the hole you made in the plenum with foil tape made for ductwork. Input lines should be at the bottom, unless otherwise marked.

Installing a Heat Exchanger that is Longer than the Plenum

You can install a heat exchanger in a plenum that is smaller than the length of the heat exchanger. This is a good ideal because you will get the whole core inside the ductwork. Only the frame will stick out 1" (on each side). Proceed as follows. 

Cut the lines at the top and bottom of the plenum, usually 4 inches long (the width of the heat exchanger).

Cut another line vertically, between the other two cuts, right in the middle as shown.

Slide the heat exchanger through the front hole to the back hole. So that it protrudes front and back.

Ideally the core of the heat exchanger will be entirely within the plenum. (Item B)  

If you want a heat exchanger for a plenum that is 18" x 18", then that is the size of heat exchanger to buy and the outside dimensions will be 18" x 20".

Crimp the sheet metal to the edge of the heat exchanger. This is item G in the diagram above (Page 23). Using channel-lock pliers to crimp it, it makes a very nice professional looking installation. Input lines should be at the bottom inlet unless otherwise marked.

This not only forms a virtually airtight seal, but it also supports the heat exchanger as well.

A transition was built here so that a larger heat exchanger for the outdoor wood burning furnace, could be used with smaller ductwork.

A customer’s photo of his heat exchanger installation for his outdoor wood burning furnace..

Bleeding a Heat Exchanger

You need to have two cut-off valves in the  return line - one with a drain. With the pump running (turn the thermostat up to 90 F so that the pump comes on), simply close the brass valve on the return line quickly, holding it for 3-4 seconds. Then open the valve. Repeat the procedure at least 4 times.

If you’re unsure if all the air is getting out, you can separate the 2 valves at the Pex Pipe between them, after turning off both valves so that water won’t drain out.