Replacing a natural gas gravity furnace in an upgraded small older home
I am a regular reader here and have learned a lot on this site. Thanks to all who contribute.
I have been upgrading the thermal envelope in a 960 square foot house in Detroit, Michigan for several years. I have injected polyurethane foam in the walls, R60 cellulose in the attic after air sealing, 2" XPS on the basement walls, and 2" XPS in the joist cavities in the basement. The windows are single pane but with caulked in place storm windows.
I have done a heat loss calculation using the page at buiditsolar.com using R values from coloradoenergy.org. Using a 65% efficiency rating for the existing natural gas gravity furnace, I can get the annual heating cost to match up with reality. The calculated heat loss is 14,300 btu/hr at a design temperature of 6 F. The 6 F was chosen based on the 97.5% temp for Detroit.
My annual heating costs were about $400 this last winter which was a much worse than average winter.
The house has a 768 square foot conditioned basement that has a home theater area and a workshop so total conditioned space is about 1728 square feet. I have a wood burning stove in the main living room but I don't use it and may remove it entirely. The main reason for getting rid of the gravity furnace is to get rid of the large ducts. I really don't want any ductwork in the basement if at all possible.
I was originally thinking about radiant pex in the basement joist spaces, but that is costly and complicated. One of Martin's posts here started me looking at wall mounted natural gas furnaces. The Rinnai direct vent furnaces look interesting. http://www.rinnai.us/direct-vent-wall-furnace/products. They are priced between about $700 for the 8000btu model to about $1250 for the 17000 btu model.
I am thinking I could put in two of the smaller Rinnai units, 8K or 11K BTU, one in the basement and one on the main floor. The basement could be kept cooler when I am not using it and turned on as needed and in very cold weather to add to the total heating of the house. An 11K unit on the main floor and an 8K unit for the basement would be about $1650. Seems tough to beat for a new heating system. Any comments?
Right now I use window air conditioners but would like to put in one or two mini-splits at some point mostly due to noise issues. The high expense of the low temperature units makes me think I might be better off with one or two wall mount gas furnaces (one for basement and one for main floor), and one or two mini splits for cooling only.
I have not done a separate cooling calculation. Is there a good similar spreadsheet for that? Something like a Pioneer 12k mini split is only $525 at http://www.highseer.com/?gclid=CjkKEQjwnqucBRDZvf_rk-fEj7wBEiQA8HDLEu-bX....
How would I calculate the payback on a higher SEER model?
Any feedback would be appreciated. Thanks.
Posted Jun 1, 2014 3:13 PM ET
Edited Jun 2, 2014 5:48 AM ET
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