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Old 09-25-2008, 02:57 PM   #1
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Anyone know proper size for room a/c-heat?


I posted this question below (see "Choosing correct size of thru the wall a/c-heat?") inquiring how to determine the proper size for a thru the wall heat pump with backup electric heat but got no response. The space is over my attached garage and has 530 sqft with 7' ceilings and both sloped and vertical side walls and one north facing window. There is R-30 insulation in the flat ceiling and R-19 in the sloped with air baffles. The knee and vertical walls have R-15. I live in central IL and the roof has full sun one side in the morning and one side in the afternoon. The online charts seem to suggest around 12,000 to 13,000 BTU for the a/c. Is 12,000 correct or should I go up one size to 14,000? I am considering the Amana model PTH153B50AM (rated at 14,000) or the Amana PTH123B50AM (12,000) both with 5 kw backup heat. Will 2000 BTU make any difference? Thanks for any help you can give me.

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Old 09-28-2008, 07:55 AM   #2
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Anyone know proper size for room a/c-heat?


I discovered the room floor/garage ceiling has R-13 insulation between them if that helps anyone make a recommendation. Surely someone on this forum has some guidance on this.

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Old 09-28-2008, 08:31 AM   #3
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Anyone know proper size for room a/c-heat?


Duane, wile I don't feel qualified to answer your question Let me say this. If the footprint it the same get the smaller one, then if its not big enough pull it out and get the new larger one. Then sell off the small one.
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Old 09-29-2008, 07:48 AM   #4
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Anyone know proper size for room a/c-heat?


Yes, both units are the same physical size. I just don't want to have to go thru the process of trial and error and have to try to sell one if it's the wrong size. I just figured some of the pros on this board could give me an informed opinion of one size over the other. Maybe the question is not as simple as I first assumed it to be. I thought if they could give all these responses on sizing whole house units, etc., sizing a room unit would be a piece of cake. I guess my lack of knowledge on the matter is showing.
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Old 10-02-2008, 03:08 PM   #5
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Anyone know proper size for room a/c-heat?


Just curious - does anyone on this forum know the answer to my question, or do they just not wish to respond? Or has it been asked too many times before? I didn't see any similar posting. Surely it can't be that difficult (except for me). Sorry for the sarcasim.
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Old 10-02-2008, 06:41 PM   #6
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Anyone know proper size for room a/c-heat?


i would go with the 14,000 btu unit your talking 1 ton vs 1.2 tons.maybe upgrade/add to the attic ceiling which is going to pull and the unit espacially in the heating season with an unheated garage.definitly carpeting in the space for the winter.where you install the unit is going to effect the heating .if you can consider the sun hitting it during the day that would give you a better running heat within.did you consider a ductless HP unit with one condenser with 2 evaporators they would be up on the wall and you won't have that heat sink the in the wall unit will produce during the off cycle.another consideration is 2-7000 BTU units one on each wall..this would be an easier placement if you went with the 2 evaps with the ductless the air handlers can go anyplace on the wall in the room...even in the ceilings they mount between the joists on some options.
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Old 10-03-2008, 07:21 AM   #7
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Anyone know proper size for room a/c-heat?


Thank you for responding Biggles. Yes I considered the split unit but didn't really want to mess with the line set. I can't really put in two smaller units because I basically have only one exterior wall, the north wall. All the other walls are inside the attic enclosure area. It sounds like the 14,000 would be my best bet, especially the room will only see intermittent use. Yes carpet is the plan for the floor except for the bath which is located over the heated part of the house. Thanks again for your ideas and guidance. Much appreciated.
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Old 10-04-2008, 10:17 AM   #8
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Anyone know proper size for room a/c-heat?


Well I will take a stab at it.

Heating and cooling have nothing to do with sq ft. It has everything to do with cu ft. I did a 1200 sq ft home and had to have a 5 ton furnace. This would be overkill for the average 1200 sq ft home. This one had 25 foot vaulted ceilings. If there were floors built in then the home would have been well over 3000 sq ft. I calculated it based on cu ft which is the way it should be done.

Deciding what size furnace or air conditioner is not exact science. There are lot of factors that need to be taken into account. Windows, insulation, weather, trees, type of building, traffic in and out, wind, basement/slab/frame foundation, roof type, and much more. I could go on an on about the variables.

A rule of thumb is alway go for larger. With air conditioners they need to be matched with the cfm of the furnace or you could end up with icing.

As for BTU's I figure 5 btu's per 1 cu ft. This means a 2000 sq ft home with 8 foot ceilings would require a 80,000 btu furnace. If it was in a mild climate and didn't require lots of air conditioning, was well insulated, minimal windows, full basement, then a 60,000 btu unit might be fine.

If it was in Fairbanks Alaska and had no basement, out in the wind, lots of windows then a 100,000 btu unit would be appropriate.

I know this is not an answer but it is giving you an idea of what you might need to heat that space.

You space is well insulated but you are also above a garage in a cold climate.

Hope this helps.


Last edited by Marvin Gardens; 10-04-2008 at 10:21 AM.
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