"i'm going to hook 2 solar heaters together and then to my house and was wondering how long is too long for the duct work to be  ?
the output duct is going to be about 12 feet long do you all think this will be ok ?
i know the shorter the better.
my intake duct will be about 5 or 8 feet long... duct size is going to be an 8" flex insulated duct... solar heater size is 3' X 7' thats each heater
thanks"

Solar air collectors where the air is fan forced usually want about 2.5 cfm per sqft of collector area, so for yours that would be 3*7*2*2.5 = 105 cfm of flow through the collectors for good performance.

For 20 ft of 8 inch diameter flex duct at 105 cfm, the duct calculator listed below gives:

Pressure loss 0.007 inches of water
duct velocity = 300 ft/min

These are both minimal, and unless your fan is really really sensitive to pressure loss, that should all be fine.  There is probably quite a bit more loss in the collectors themselves than this duct.  There is a Duct calculator is here if you want to try other duct sizes:
http://www.connel.com/freeware/airduct.shtml

You can try this rough check to see if you have enough flow rate.  See how long it takes the outlet duct to fill a 50 gallon trash bag.  50 gallons is 6.7 cubic ft, so it should fill up in 6.7/105 = 0.064 minute (about 4 seconds) -- if it takes much longer than that, you are not getting enough flow through the collector for good performance.  If it fills it up faster, thats OK.

On the heat loss from the duct:
If the duct is insulated to R3(just a guess), and Tduct = 110F, Toutside = 30F, duct surface area of 25 sqft, then, the duct heat loss is:

Qloss = (25 sqft duct surf)(110F-30F)/ R3 = 670 BTU/hr

To put that in perspective, your collector might be gaining about:
Qcollector = (250BTU/sqft-hr)(42sqft)(0.5efic) = 5250 BTU/hr
So, the exit duct is losing about 13% of the heat you are collecting -- not a killer loss, but, I'd say more insulation would be good if you can manage it.  Pile tree leafs on it :)

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The bottom line answer is that it should work OK :)