I looked at a house today that had a wood burning fireplace insert that had it's own air handler unit to help supplement the gas furnace. Even though it had it's own air handler, the supply duct coming off of the air handler was connected to the gas furnace supply ducts. Unfortunately, the area in the basement where the installation was happened to be too tight to even get into for adequate observation or picture taking. The date code on the air handler was equal to the age of the house so it appears to have been installed at the time the house was built.

Now the furnace and fireplace each had their own flues. But I don't know what type of issues could arise from having a supply duct system for a fireplace directly tied in to the supply ducts for the gas furnace.

Thoughts?

Any chance you were looking at HRV connected to a fireplace, as on page 88 here?

http://tinyurl.com/29g3j3
http://www.efficiencyvermont.com/pag...Appliances.pdf

Looks similar but this unit was directly connected to the gas furnace supply ducts.

This is a growning trend for mfrs. to add hot air ducts to supplement the heat in the house. Some are intended to dump air into the room(s) while a few actually connect to the return trunks. There are several problems with this setup.

The air coming in this bootleg branch is not steady or predictable. Sometimes you have very hot air while other times nothing at all while still you may have cool air being drawn in. This totally screws up the delta T or temp. differential across the heat exchanger. The furnace is set up and tweaked base on the temp. and static pressure of return air. If you pump alot more heat into the return, then it is harder for the furnace to strip heat from the furnace exhaust. Temps rise and components, such as the heat exchanger, may fail or at least have their useful lives shortened. Just to refresh, a cracked HX can draw CO into the home. At the very least, the furnace will not always deliver its optimum performance due to the delta T and delta P swings. Delta P is the pressure gradient. Properly set up, a tech does a "duct traverse" taking pressure measurements in a grid pattern to insure proper air flow. They do all their manual J calculations on duct sizing, materials, take-offs, etc. then somebody throws all that out the window by tacking on a warm air intake into the return. If they attach it to a warm supply, it can create backpressure at the coil & HX. When the heating season is over, someone must remember to close off that circuit so it doesn't participate with cooling cycles. If the return air is too far out of whack, it can lead to failure of the air handler fan. If the Fp draws air in directly from outdoors, dirt and bugs can enter the system not to mention moisture and odors.

These are not tested or listed as a system with any furance and should not be allowed. I'll bet there was no AHJ inspection but if so, he should have flunked it. Also, if the Fp guy is installing it, he could be in legal trouble as most parts of the country require HVAC licensure to do such work.
You get the idea.
HTH

__________________
Keep the fire inside the fireplace!
IAFCI Regional Director & Certified Fireplace Inspector
Certified Master Hearth Professional & HPBA BoD

Interesting. Thanks.

Thanks Bob. The entire setup just didn't seem right.