If the furnace draws more power than you anticipated the solution may be more batteries to provide that power.
Or try this, If you currently have two Group 24.. You will find the GC-2 Golf car battery is nearly exctly the same footprint, only a bit taller (Quite a bit in fact) If you have the head room (MEASURE TWICE before you buy).
G-24 = 75 amp hours + a 2nd one in parallel totals 150 amp hours (Slightly rounded)
GC-2 = 230 amp hours.. Plus a 2nd one in series to make 12 volts = 230 amp hours. over 1 1/2 the power.. PLUS it is a true deep cycle battery.. Which has other advantages. Cost is not much different than a pair of G-24's either.
