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[bcanode]

Playing with a temp gun and wanted to see some others opinions.
Old house built around 1868
Outside temp 7degrees.
Temps leaving the stove at time of reading was 158. It's set @ 180-160

Forced downdraft furnace.
Supply 154
Return 138
temp drop 16

Heat Register temps
104       (22' from furnace)
113       (11' from furnace)
2 @114 (13' and 22' from furnace)
120       (15' from furnace)
122       (5' from furnace)

Avg temp loss is 23.5

With the duct system being as old as it is would it be worthwhile to replace with insulated flexible duct? Any recommendations?

[juddspaintballs]

Is it keeping your house warm currently?  If so, there's likely little reason to change out your entire ductwork.  You can wrap the bubble foil insulation around metal ductwork and duct staple and tape it in place. 

My other house that I heated with my boiler was built in 1875 and still maintains the original windows.  I did replace all of my ductwork, but that was to get rid of the ancient oil furnace and install a heat pump system.  I built my plenum to fit a 22x22 boiler coil at the same time and my FIL designed the ductwork to keep air velocity constant through all vents for a very even heat/cool throughout the entire house.  He used flexible insulated duct throughout the entire attic for the upstairs and metal duct in the basement with no insulation.  The supply and return trunk lines through the pipe chase into the attic are also insulated flexible duct of some enormous size like 12" or 14" ID but that was because it was easier to drop it down through the two floors from the attic since there's no access into the pipe chase from the living floors.  Even just paying the materials cost at cost through his (very large) company and him installing it all for free, it cost me nearly $1000 in duct work.  If what you have in place is currently heating your home sufficiently and you're not going through a significant amount of wood more than you really should be, I'd keep the money in your pocket.  You can wrap that foil insulation over your ductwork if you want to see if that helps. 

[bcanode]

With the current temps it's hard to get the temp above 71.
Windows have been replaced before we bought.
Crawl space is not insulated.

[hondaracer2oo4]

Solid metal ducts are much more efficient then flex duct as far as air flow. All joints need to be covered with mastic. After that I would slide insulated flex duct over the existing piping for insulation. You are losing your heat out of the house. Biggest bang for your buck is attic insulation. Second is sill plate sealing and insulating. Also all air leaks from inside and outside need to be sealed. Your numbers are decent. I get anywhere from 160-95 out of my vents running 185 temp and depending on how far the register is from the hx.