caught between contractors
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  1. #1

    caught between contractors

    it appears we need to replace one of our air conditioners. this means we also need to replace our furnace. The units to be replaced are 18yo. 120,000btu gas furnace and a 3 ton AC. We have zoned heating/cooling. we replaced the upstairs units with a 90,000 btu single stage furnace and a 13 seer single stage AC unit. we now need to replace the downstairs units. about 3300 sq feet house. what btu furnace should we get? and should we get a 2-stage or modulating furnace? What seer AC should we get and should it be a single-stage or 2-stage AC unit and how many tons? two different contractors are recommending different AC tonnage and seer, furnace btu's and one recommends a modulating furnace and the other recommends either a single stage or 2-stage furnace. we want to be cost effective and energy efficient at the same time. Additionally, we want to finish our basement. It has one above grade wall and 3 below grade walls. how do we size this equipment to also heat/cool the basement without negatively effecting the 1st floor temperature?

  2. #2
    Join Date
    Oct 2004
    Location
    So Cal
    Posts
    3,433
    And you live where?
    Makes a big difference.
    You will need someone out to perform load calcs to determine what size you need. Guesses don't count.

  3. #3
    I live in the St Louis area. we tend to have two seasons: hot and cold. usually the summers are about 85 and pretty humid. this summer is extremely unusual in that the temps are running about 105 with very low humidity. summer is usually late May thru mid-september and winter is usually mid-October thru late March.

  4. #4
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Your design temperature is around 6 F in the winter and 94 F db/ 77 F wb. I would do a Manual J load calculation or equal. I wouldn't guess because it you oversize your unit.. you will be pretty uncomfortable due to the high humidity. If you undersize... you might be freezing in the winter.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  5. #5
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    2 stage or variable speed is best but more expensive. Depends on your budget and how efficient you want it all. If you don't get a load calc... two stage and variable is definitely more forgiving.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  6. #6
    Sam, thanks for your help. I'm just a homeowner learning the ropes so most of what you replied went over my head. can you please translate?

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,831
    Call Terry http://alrostl.com/

  8. #8
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Quote Originally Posted by lbf4 View Post
    Sam, thanks for your help. I'm just a homeowner learning the ropes so most of what you replied went over my head. can you please translate?
    Sorry, basically I had two points I wanted to get across.

    1. The basic system you can get right now is a single stage 13 SEER AC. The basic furnace is a single stage 80% efficient (AFUE rating) with a constant speed/volume fan. Anything else such as "variable speed", "two stages", "ECM", etc... are optional and will add cost to the system. Just like a car the more you spend the more you get. Maybe ask a dealer for a basic, a medium range, and a premium system, find the one in your budget then use that system as a benchmark for other bids.

    2. A load calculation is very important for where you live because your winters are below freezing and your summers are really humid. If the furnace is too small, you will be cold in the winter. If the AC is too big, then your place will be too humid in the summer. So you need to be just right.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by lbf4 View Post
    I live in the St Louis area. we tend to have two seasons: hot and cold. usually the summers are about 85 and pretty humid. this summer is extremely unusual in that the temps are running about 105 with very low humidity. summer is usually late May thru mid-september and winter is usually mid-October thru late March.

    OMG, st louis area and you put a 90k BTU upstairs in a 3300sqft house. Wow, I thought my old system was oversized. That thing must be like a jet engine firing up or a wind tunnel when running in winter.

    I bet you didn't need more than a 45k BTU furne up there.. and maybe a 60k just for the larger blower.

    Too late now.

    St louis has desing tmerpatures... just as every area does. What the peak record temps are isn't what you design for. You deisgn temps are 94F Dry bulb and 75F wet bulb with 8F winter.

    Depending on shading, window and insualtion a home that size in that cliamte will need a 1.5-3 ton upstairs and 1.5-2.5 ton downstairs. Very unliekly you'll need anything larger. A furnace would be 45-60k BTU down and 45-60k BTU up and upstairs furnace would depend on the equipmet you select and AC size to get a large enough blower.

    120k BTU should easily heat the whole house by itself. 100k or 80k might even heat the entire house.

    Look at it thsi way, if you really needed 120k BTU's for the entire house, you should have a $300-350 peak gas bills for a normal Jan. or Feb.

  10. #10
    thanks for the referral!

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