HVAC solutions for 1300 sq ft addition

a.fryarJuly 17, 2012

We just bought a house with an unfinished addition and are trying to figure out what the best HVAC solution will be.

The original house is approximately 1500 sq. ft. built in 1940. It has insulation in the ceiling (rockwool maybe?), no insulation in the floor, minimal insulation in the walls, and single pane windows (may be replaced in the future). Currently has an older split system which also may need replaced at some point in the future. It also has a chimney with an Ashley wood burning insert. We do have access to plentiful firewood.

The addition is two story, base floor approx. 1000 sq. ft. and upstairs is an"attic room", 4 ft. side walls approx. 300 sq. ft. Attic room currently has spray foam insulation on the ceiling, base floor has fiberglass bat in the walls, and insulation suspended within the floor joists. It will be connected to the original house through one hallway and the upstairs through a staircase w/closing door from base level. Base floor will contain large master bedroom, his/hers master closet, large master bath, laundry room, pantry, and half bath. Attic room will be used year round as upstairs "den".

Several concerns have been brought up; insulations differences btw new/old, costs of running multiple HVAC machines, and maintaining upstairs comfort year round.

We do have access to a cheap mini-split, would that type of system be appropriate for the upstairs? Will the base floor need separate systems for the new and old sections? Would having 3 "zones" save electricity since we often would only need to cool/heat one area (ie at night the master bedroom).

Ultimately we would like to heat and cool the entire space in the most efficient way, that will also provide reasonable comfort in all sections.

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My suggestion is to install a independent furnace and AC for the condenser. The 300 sq. ft. attic room is small enough that it could be tied into the system of the addition.

In the future you could replace the system in the orginal part of the house. You should upgrade the windows and insulation before replacing the HVAC system.

    Bookmark   July 17, 2012 at 5:22PM
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What is location?

Is nat gas service available to home?

Addition is on crawl space or slab?

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    Bookmark   July 17, 2012 at 10:59PM
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We are in Arkansas. Natural gas is not currently plumbed to the house. I'm not sure if it is available in the area or not since we are just a little outside the city limits.

Both portions are on crawlspace.

I have heard that you often get a 5-10 degree difference in the second story. Will that be an issue if it is on the same system as base floor?

    Bookmark   July 18, 2012 at 12:00PM
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Large 2-story houses where the air handler is in the basement will have problems keeping the temperatures of the first and second floor even. In your case the the upper floor is 25% of the total area of 1300 square feet. If the system is installed on the first floor and you do a good duct design, you should be OK. You still may get a 1-2 degree difference between floors on hot days.

How much insulation do you plan to put above the attic room? Will there be a means of ventalation above the attic room?

    Bookmark   July 18, 2012 at 12:24PM
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You need two separate systems, one for the new addition and one for the original living area.

I would put a high eff forced air HP system in the addition. Will require ductwork. I don't see the heating and cooling for the small attic area as being an issue as long as this area has good insulation properties and good ductwork design.

How is old part of home being heated and cooled?

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    Bookmark   July 18, 2012 at 12:33PM
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Seriously People! You should be considering a Ductless Mini-Split System.

Here is a link that might be useful: Ductless Info and Pictures

    Bookmark   July 18, 2012 at 1:34PM
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The original house is one story, with HVAC ceiling vents. Has an all electric split system, the inside portion is in the attic.

The rooflines of the new and old sections match. The attics of both are separated by a wall. The attic of the original home will not be any sort of conditioned space.

The entire addition at this point is unfinished and we definitely want to get a good idea of what needs to be done in general before we hang the drywall. The attic room is also unfinished. The side walls will be approx. 4 ft. with the ceiling matching the roof slope. Shape is long and thin, roughly 12x30 the stairway included in that. The attic room currently has a green spray foam insulation between the rafters. All finishing work from this point on is up for discussion� So what would constitute "good insulation"? good duct design?

There seems to be a consensus that at least the new and old sections should be separate. I do think that I agree. But for the record what would be the reasoning behind that recommendation?

    Bookmark   July 18, 2012 at 11:41PM
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You should spray foam wherever you can. Upfront costs are higher but have a high return on investment.

A ductless mini split sounds like a good candidate for your home.

Here is a link that might be useful: Ductless Mini Split Guide

    Bookmark   July 19, 2012 at 6:39AM
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The reason for having two systems is get a even temperature distribution in the two structures. You have one structure which is well insulated with an attic room. The other is not well insulated. The heating and cooling loads of each structure is very different. It costs more money, but it is the right way if you want to be comfortable.

The wall insulation should be at least R-13. The attic rafter should be at least R-38 or higher depending where you live. Spray foam insulation is the best.

    Bookmark   July 19, 2012 at 12:29PM
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