Pellet stove in basement that is connected to the main duct work in the home. Return on the stove as you can see is a flex duct. Any issues with this besides the bad installation (loose, dips)?
Flex duct can be used as return air however I am not a fan as it restricts the air. I would call it out as something to be upgraded myself. and it would fix the dips and loops as well.
I don’t think that you can connect a return and supply duct to a wood/pellet stove. That to me is a safety issue.
I can be done if installed correctly. CSA B365-10 has installation instructions for this style of set up. From the picture it does appear to be done properly.
I don’t have my B365-10 with me, but I am pretty sure if a wood burning appliance (even pellet) when using the cold air return the return must be fully boxed. Not just sheet metal on bottom of floor joists. I am on my phone looking at it so I couldn’t get a good look, but it doesn’t appear to be a fully boxed in return.
It is fully boxed in Timothy.:mrgreen:
Thanks for the help. Buyer said he is going to replace the flex duct. I don’ t understand the concern with hooking up the stove with the supply and return ducts? Does it matter the heat source, gas, oil, propane, wood or pellet?
Consult pellet stove manufacturers installation instructions re duct sizing. For a typical forced air gas furnace, it is preferred to have as low pressure on the return side as possible, which translates to significantly larger and shorter return ducts than supply ducts. I suspect the manufacturer of this pellet stove will agree 
The return looks to be smaller than the supply. It is connected to what is probably a joist space (14X? ), not the main return air duct. Flex ducts are more resistant to air flow even without dips.
One thing I would suggest to him, is that he does not have to pull return air out of the existing ductwork, just out of the area to be heated. Why not run a 24 X 8 drop to a short 24 X 8 return air duct that covers at least two joist spaces, use joist liner to a an upstairs wall and make a return opening through the floor between the studs. I have attached a sketch if my description is not clear.
If the appliance is approved for supply and return ducts then it should be ok if installed properly.
I didn’t think that such appliances (wood/pellet stove) existed aside from wood/pellet furnaces.