Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Sizing Room AC

My late Dad's old finished porch (office) room AC --probably from early 1970s--no longer works. It's a Carrier 51-SC1001, rated at 8700 btus. Turn it on, motor goes but no air flow. Anyway the extension has two rooms separated by a partition and door, one room twice the size of the other. the main room is the main concern for cooling. both rooms have dropped ceilings 84" H over which is a sloping, fiberglass insulated extension roof that sees alot of sun.

Main room is 200sq ft, 1400 cu.ft, 130sqft net outside wall, 50sq ft windows, 20 sqft door; Room 2: 100 sq ft, 700 cu ft, 130 sq ft net outer wall; 20 sq ft windows. All on slab. <span style="color:#000000">Longest wall is northern exposure.

Does the 8700btu rating sound good? My Dad was an HVAC engineer but I'm wondering are today's ratings comparable with 40 years ago, or does improved efficiency require a re-calculation? I know from other posts that you want the unit to be small enough to remove enough humidity.

Any recommendations?




  • TechmanTechman Member Posts: 2,144

    A heat load calculation is the only sure fired way of knowing the proper BTU's needed. Todays BTU's are the same as yesteryear's BTU's. Todays more efficient units just means lower electric bills for the same BTU's. So a quick Q here, do you know if the old unit satisfied the area in question? If ,yes, then there you go!Reg.ards
  • D107D107 Member Posts: 1,633

    Thanks for your response. I'd guess the size was adequate;. I would think any slight oversizing of a new unit--depending on what's available--could be handled by putting it on lo-speed.
  • Wayne_16Wayne_16 Member Posts: 130
    Buy to the load

    Oversizing will cause short run times, uncomfortable conditions and possible issues with mold.  Size to the load & only buy a unit as close as you can to that load.  As Techman said, do a new heat calc, as most likely the windows have been replaced, more insulation in the attic etc., requiring a new heat loss calculation.

    The new technology includes the ductless mini-split systems which come in many sizes and configurations.  Some units have the heat pump option which allows the heating load of the area to be covered without running the main furnace.  Just a thought.

     Most of the minisplits have higher seer rating than a wall shaker, operate extremely quiet and qualify for tax credit money. 
  • D107D107 Member Posts: 1,633
    Heat Load

    Thanks Wayne. I wasn't aware that you do a 'heat' calc to figure AC load. The heat loss I did a few years ago totaled 5100btu main room and 2500btu small room.I'll have to check how this is converted into cooling requirements, unless I am misunderstanding your comments. This room is heated by baseboard on its own zone, so that's taken care of.

    The only other option that occurred to me was to add a duct to the existing central AC which covers the rest of the house. It's a Trane TWH739E150CO installed 1990 (with an XV1 500 Variable speed heat pump that my mother uses for the month of Sept if it gets cool.) Mabe it's worth investigating if this unit could handle the extra porch area and if running a new duct is feasible, or maybe just keeping the door to the porch open and letting cool air in from the rest of the house... Must be at least a 3 ton unit. Whole house heat loss is 47K. (on Long Island) (see attached photo of porch. Ideally the dropped ceiling would be removed and those sloping roof rafters would be insulated with foam, but probably not going to happen.)

    Minisplit is certainly an option, but perhaps cost overkill for just those two rooms. WE're guessing that in five years or so the house will be sold. Thanks again.
  • Wayne_16Wayne_16 Member Posts: 130
    Heat Gain/Loss

    There is two different calculations, resulting in two different numbers.  One for the heating load, and then the cooling load.  They both take into account the building construction, insulation and window on all exposed walls, including the ceiling and floor or concrete on grade.  In your case it would be the cooling load. 

    As far as using the existing furnace and AC,  complete a heat gain/loss for the complete building including the porch.  Check the heat gain numbers (AC) against the capacity of the existing air conditioning system.  If the system is large enough to handle the added load, then install supply and return ducts.  May need to add an additional return duct openings in the room attached to the porch as it may be very hard to have a return duct opening placed in the porch.  I do not know what the basement or crawl space is.  Make sure the supply and return plenums are sized to the full capacity of the furnace.  For what ever reason in Minnesota undersized duct is very common.  Adding a supply or 2 off of the existing undersized ducts never works to expectations. But it is cheap.

    Using a minisplit may be in the end a viable alternative.

    Minnesota Wayne
  • D107D107 Member Posts: 1,633
    Heat Gain

    Thanks for the explanation. I see online there is info on heat gain calcs. I did the detailed heat loss 4 years ago as per the IBR Hydronics Institute method and matched it with that done by a top pro's calc used for installing the then-new boiler. (47K loss). Any buildling changes, windows etc were done years before that, so we're up to date.

    The central AC in this split-level is not zoned but I guess it could be by the next owner; there is only one, fairly large return register on the main level, so a 2nd return register might be called for as you suggest. We will have to consider which solution best enhances the sellability of the house as well as does the cooling job. A separate AC unit in the porch would provide 'zoning'; a minisplit in this location--which requires an outside unit close by the porch, may prove inconvenient.

  • Wayne_16Wayne_16 Member Posts: 130
    Morning David

    All understandable, you now have ideas to choose from that will best suit your house and lifestyle.

    Minnesota Wayne
  • D107D107 Member Posts: 1,633
    Final note

    Thanks for all the replies. If this was my house I would do things for maximum efficiency. Given it's my mother's place and she's getting quite on in years, family's trying not to do too many projects. Especially when the tendency today--or at least it was until the economic crisis--was for the new owner to totally redo the house anyway to their design and taste.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!