Is it a good or bad idea to hook 2 new batteries in parallel to use for a forced air furance?

Definately help the battery life.

Two batteries in parallel is a good thing.

The only problem I see about having 2 batteries in parallel would be when they run out of power, then you're done with those 2 batteries until they're recharged. If you run one battery at a time, you have some idea of how much time you have left once you start on the second battery.

For that reason alone, I typically run a single battery at a time.

Key word is new, or at least a fairly new battery. I hooked up two in parallel and one had a bad plate and drew the voltage down. With two good batteries you will have longer life.

I have had two hooked together since using them that way last winter. I most recently charged them during the summer. Now when I went to charge them both have open cells. Coincidence? I do not know. Anyone have any ideas or opinions?

If they were not hooked up to anything and were open circuit, they should have been ok if their state of charge was good.

How old were the batteries?

They were not hooked up to anything except each other. I had charged them this summer with no problems. I recently tried to charge them again and the charger showed F03. Open cell. So I removed the cables between the batteries and tried again and both batteries showed open cells. They are identical batteries. One was purchased in 06 and one Sept 07. Just over the one year replacement warranty.

Doh. I don't see how leaving them in paralell but open circuit should have harmed anything. Open circuit - no current drain.

So with 2 new 27 group batteries in parallel be the way to go?

Yes, I believe 2 in parallel are the way to go! I spent some time [favorite search engine here] battery maintenance and such last spring. I have a 24V trolling motor on the boat and my batts didn't hold charge. I learned a bunch about proper testing, maint, etc. I tested with a cheap hydrometer and found bad cells on each batt, they were 3yrs old, but clearly my being gone most of a year and the batts sitting in the basement not being topped off each month etc help their demise.

I also use them in parallel in the house and we built a new one this fall. They will be in the open to make sure we keep them warm. [last house we had them nicely concealed in a storage bin where they stayed cold] Recharge them asap after use, and if they aren't used for a month, charge them. That will give you the best life possible.

If you can, run a generator a few hours each day and a charger to top them off and you'll be fine. Our hunting shack has tons of windows, bad insulation, the batts are outside, and a forced air furnace. We use 2 batts and run the generator about 1 hour in the morning and maybe 4 at night and never run out of juice.[1 batt won't make it if its at all cold!]