Unit Heater Venting

[Bojans]

I have a house in the UP of Michigan that only has electric heat. This gets very pricey during the winter. When we are gone we blow out the pipes and only heat the basement (walkout) to ~50 degrees which generally keeps the house above freezing when we arrive. I am considering adding a 500 gal propane tank and installing a hanging furnace (likely 45k btu) to heat the basement.

There is currently an old wood burning furnace in the basement that we do not use (the venting in the house was not done correctly and the heat is very uneven). This furnace is vented through 8" double wall stainless pipe going through the foundation and up the side of the house. I would like to use the existing venting for the new hanging furnace.

So... 2 questions
1. Can I adapt the 4" power vented exhaust from the unit heater to the existing 8" vent? The run from the furnace to the pipe would be ~2' with a 12-14' 8" pipe outside.
2. If I cannot vent directly into the 8" pipe would it be possible to run a 4" vent inside the 8" vent? If so would this need to be double walled pipe or could I use single wall?

 

[6768rogues]

What you use for a vent is up to the listing on the heater you chose. I would look online at different manufacturers' products and read their instructions for venting.
I have two Reznor hanging NG heaters in detached buildings, but propane are available. They power vent up or out the sidewall but have vent length limitations. You might be able to vent one out a sidewall and up to above snow height, but you would have to figure that out from the instructions. One size does not fit all.
Most hanging power vent heaters are not condensing units, so it is likely that you will not be able to use a long vent run without having excessive condensation issues.
If it were me, I would buy a low cost condensing high efficiency residential furnace and use PVC pipe for combustion air and exhaust discharge. You will not spend a lot more money, venting will be easy, it will be suitable for residential use and can be 95% efficient.
Reznor makes heaters that are listed for residential use. Not all are, so watch the listings if you go that route.