Outdoor Boiler Bypass Kit for Downdraft Furnace

For a downdraft forced air furnace, the water to air heat exchanger must be located below the furnace. But in the off cycle, excess heat will radiate from the heat exchanger and the furnace has a safety feature that will sense extra heat and turn on the fan only mode. This cycle continues and overheats the building. This kit bypasses flow of hot boiler water around the heat exchanger during the off cycle to prevent this overheating.

A downdraft furnace is commonly used in certain manufactured homes that have ducts in the floor.  A downdraft forced air furnace pushes air over a (gas or electric) heat exchanger downward into air ducts located under the floor of the home. These are highly efficient furnaces and their designs have passed the test of time.

However, when adding a water-to-air heat exchanger to such furnace, the heat exchanger often needs to be installed at the bottom of the furnace, instead of the standard plenum location for an updraft forced air furnace. This often requires raising the furnace a few inches to install the heat exchanger, and a box must be fabricated in which to install the heat exchanger. 

The reason the heat exchanger is installed underneath a downdraft furnace is because normally the heat exchanger is installed downwind of the blower fan. In some cases, the heat exchanger can be installed in a return air duct that is above the downdraft furnace and before the blower fan, but this is not often possible, or reliable, because this orientation relies upon the blower fan sucking air through the heat exchanger, and that often will not work. A qualified HVAC contractor may need to be consulted.

If the heat exchanger is installed underneath the downdraft furnace, this will create a problem - but we have the solution! The problem is that when the thermostat no longer calls for heat and the blower fan shuts off, hot water will continue to flow through the heat exchanger under the downdraft furnace. This is a normal and required part of the system since the outdoor boiler pump must run continuously. The problem that this creates is that since heat rises, there will be considerable heat rising from the heat exchanger into the downdraft furnace. These downdraft furnaces have a temperature sensor in them as a safety feature. When that sensor senses higher than normal temperatures, it thinks that there is still heat that must be removed from the furnace so it turns the downdraft furnace blower fan back on to cool it off. Since the heat exchanger continues to radiate heat, this blower fan will run continuously and the home will become overheated indoors. 

The solution is to BYPASS the heat exchanger when the house thermostat turns off the downdraft furnace blower fan. This is accomplished using this product: Kit - Bypass for Downdraft Furnace. 

This Bypass Kit will, once the indoor furnace blower fan shuts off, stop water from flowing from the outdoor boiler through the water-to-air heat exchanger, and re-route the flow of this hot water back to the outdoor boiler.