There is no rule of thumb for sizing based on square feet. A Manual J load calculation should be done for both the house and the addition. There is software that can help you do this yourself. Look up HVAC-Calc.

It's going to be hard to find a furnace that has a small enough capacity for the addition, but a large enough blower capacity to cool everything.

If you could find such a furnace, you would need to then install a zoning system and wire it to the thermostats so the big zone only started the system in cooling mode (and boiler in heat mode), and the small zone could turn it on in both heating and cooling mode.

This whole thing seems like a bad idea. It probably wouldn't work very well even if an expert installed it. DIY will make matters worse. The best way to do this is to put in a small furnace and A/C unit for the addition, and then add an air handler and separate A/C for the rest of the house. And have a pro involved with the design and installation of the system. Do it right or don't do it at all.

This post was edited by hvtech42 on Fri, Nov 7, 14 at 11:38

Where is current furnace located?

Where is current ductwork located?

New addition on crawl space or slab?

Post back.

IMO

^I assume by furnace, he means boiler, since he mentioned radiators. So, no existing ductwork.

Sorry about that, I wrote this quickly at work and mixed up terms.

Yes, existing boiler (not furnace). So no ductwork yet. New addition with crawl.

hvtech42 - I appreciate what you are saying about a smaller unit for the addition and a separate AC unit for the rest of the house. In a way, I am struggling to accept it as truth. But i am hoping for a creative solution, like finding a variable speed/capacity unit that would automatically adjust in a 2 zone situation and not cycle too frequently, or something like that.

I know this might not be possible from the strictest of senses, but I am hoping to make it work in some way if I get creative.

Any other suggestions?

I just reread your original post with square footages and actually, it might be feasible. I can't imagine your addition will need more than a 40K BTU furnace. You can get 40K BTU furnaces with 3 ton drives, and I doubt the whole house will need more cooling than that. You could install a zone system that would close off the ducts to the main house in the winter, but open them up in the summer. You would want a 2 stage or modulating furnace, since even 40K (the smallest size available) would probably be oversized for the addition alone.

The ducts in both the addition and main house would need to be sized correctly for whatever size A/C unit will be installed. You don't want it to sound like a hurricane in the addition when the air to the rest of the house is shut off. This is not a DIY job, and neither is installing zoning.

Still, though, I stand by my original recommendation of separate systems for the addition and main house. Much less to go wrong. My recommendation would be to just put in a small system with the addition when it's built and leave the main house alone. Then, some other time, when you have the money to do it properly, put in a separate central air system with air handler for the main house. No matter what you do I would leave the radiators in place.

hvtech42 - fantastic advice. Thanks for giving it, even if you didn't think it was the best solution.

Last question - My father has past experience in HVAC from the 1980s. He has also even installed units more recently that work just fine. He's from the "old school" mentality where heat just needs to heat. If it isn't perfect, it isn't the end of the world. (I tend to agree) He is a GREAT resource and if I can do this without hiring a "professional" it would be fantastic. I am doing all of the work on the addition myself except for the foundation to save money cause we don't really have any.

Do you have any suggestions for resources to get up to speed so I *might* be able to do your first suggestion, with my dad's help?

Any suggestions for specific units that are reliable and not overpriced that would fit your first idea of a 40k unit with a 3 ton drive?

Any suggestions for what to buy and how to setup the 2 zones?

I know I am asking a lot. I appreciate your help IMMENSELY.

Oh, and I forgot to mention, if I got a 2 stage furnace, do you really think I still would need to adjust the duct size all that much or would the furnace just always live in the lower stage, thus not creating any hurricane type situation?

There is no "adjustment" of the duct size, you always need to start by doing a Manual J load calculation to determine the right size of equipment, and Manual D calculations to figure the duct system. The 40K BTU and 3 ton were just numbers I was throwing around as an example. I was just guessing, which is exactly what you should NOT be doing when planning a new system.

The problem with the setup you are proposing is that you will need expensive equipment to do it properly. 2 stage/modulating furnace and zoning systems do not come cheap. If you went with separate systems, you could just go with basic, no frills equipment for each part of the house that would be cheaper, easier to install, and easier to service.

There is no "HVAC 101" resource that would singlehandedly teach you everything you need to install this system correctly. It takes years of training and on-the-job experience. The best place to get information will be the documentation for the products you are considering. As an example, I am linking below an installation manual for a zoning system. I would read through it and make sure you understand everything and would be capable of following those instructions yourself.

Here is a link that might be useful: Lennox Harmony III Installation Manual