If you are planning on upgrading your 80% efficient furnace (the kind that has the flue that goes up through your roof) and use a natural gas water heater, “orphaned flue” is a term you will need to know.
The flue gasses from a new high efficient furnace are so cool they can be vented out a sidewall through PVC pipe. By removing your old furnace from the flue that was shared with the water heater, is effectively “orphaning” the water heater on the flue.
Flues are sized to accommodate the flue gasses of all the appliances they serve, so by removing one of the appliances, which also happened to produce the most heat, could have an effect on the flue conditions.
So now your existing flue is too large for just a water heater. Does it even matter?
If your flue runs through a brick chimney it’s going to be a problem. A too large flue can mean the water vapor in the flue gasses condense on the inside of the flue. This condensate combines with acidic compounds created in the combustion process and eats away the mortar and masonry holding the chimney together. As the water on the brick freezes and expands the brick starts to pop off.
Back drafting of the flue gasses can also occur because the water heater is not producing warm enough flue gasses to stay buoyant and exiting out the top of the chimney. When the gasses become too cool, they will flow downward and back into the house. This means the air quality in your home is jeopardized and sometimes dangerously so. As you probably know, flue gasses contain carbon monoxide, a dangerous substance that should never be inside your home.
What is the solution?
One of the best ways to avoid these problems is to have flue liner that is sized properly for the orphaned water heater installed. This will help keep the chimney warm and allow normal venting and eliminate moisture concerns. The average price to have a flue liner installed is $600 – $800 when replacing your furnace and air conditioner or $1200 – $1800 if done by itself.
Another solution is to replace the gas water heater with an electric one, but this won’t be any cheaper, and you might not have a big enough electrical service to handle it.
When you are shopping for a new high efficient furnace, make sure the company you are working with addresses the orphaned flue problem. If they tell you they “think” your flue will be okay, send them packing and call the experts at Santa Fe Air!