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  1. #1
    Join Date
    Oct 2004
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    37

    Changing blower speed on Geothermal - will it affect room temps?

    In the final stages about to sign a geothermal contract.

    The one last issue is how much additional ductwork and if it will make the home more comfortable.

    With a 2 stage geothermal, there will be some times where it will run on second stage. Now there are adjustments the installer can make over a range of CFM blower speeds. I thought that for the heat load needs of my house for all but 12 days a year I would not need auxillary electric heat with a 4 ton unit.

    But with a 4 ton unit shouldn't I need to have about the equivalent of 16 ducts?

    I am upgrading a minimum of 2 extra ducts to 13, but thought that I would benefit by one or 2 more.

    I was thinking the furnace could deliver more BTUH if I had more ducts, right?

    But the installer said if he had to increase the blower speed, the air coming out would be cooler.

    Where is the tradeoff here? In practical terms would there be a greater ability to maintain the goal temperature of 70 degrees in cold weather with a higher blower speed and more CFM delivered to the entire house, but otherwise keeping the size of the furnace, compressor, and loop size held constant?

  2. #2
    Join Date
    Dec 2006
    Location
    Middle Tennessee
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    11,347

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    Quote Originally Posted by rru2s View Post
    But with a 4 ton unit shouldn't I need to have about the equivalent of 16 ducts?

    your gonna need to give a lot more info to receive a proper answer to that

    I was thinking the furnace could deliver more BTUH if I had more ducts, right?

    if the ducts are sized properly to start with, you should already be supplying the max. btu's

    But the installer said if he had to increase the blower speed, the air coming out would be cooler.

    are you talking about heating or cooling mode

    In practical terms would there be a greater ability to maintain the goal temperature of 70 degrees in cold weather with a higher blower speed and more CFM delivered to the entire house, but otherwise keeping the size of the furnace, compressor, and loop size held constant?

    the "Proper" blower speed for your particular system whether it's higher or lower than 400/cfm/ton should be determined by a qualified HVAC contractor
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  3. #3
    Join Date
    Mar 2003
    Location
    PA/DE area
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    1,535
    I think I was at your home,and if is the one I am thinking of 4 ton in your home with the ductwork that you have.I wish you luck!!
    It's NOT the BRAND,it's the company that installs it!!!!!

  4. #4
    Join Date
    Oct 2004
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    37
    Quote Originally Posted by heatpumpguru View Post
    I think I was at your home,and if is the one I am thinking of 4 ton in your home with the ductwork that you have.I wish you luck!!
    Tore down the basement ceiling last week so all ductwork could be more thoroughly assessed. I insisted on going with more ductwork. There will be a second basement supply trunk and at a minimum 2 separate supply branch ducts going to the second floor. There will also be a large return coming down from both bedrooms upstairs.

    I was trying to get a new duct (third duct) in the 24' x 15' living room, but there is a cramped workspace to saw out above the cinderblock under the ceiling in the corner of the furnace room to route directly to living room wall above. So for now there will be an endcap installed on the new second horizontal trunk. I also wanted a second duct in the 10' x 10' bedroom since the supply would be routed through the closet in that room anyway, but that can be added later if needed, since 10' x 10' does not usually need more than 1 duct.

    So the final score, old ducts = 11. Definitely planned new ducts = 2. Optional new ducts = 2 more. Total will be from 13 to 15 ducts. That takes it ALMOST up to the goal of 16 ducts to be able to deliver 1600 CFM, 38,000 BTUH heating without auxillary, and just under 50,000 BTUH cooling. The ratio of flow needed for 4 ton versus 3 ton is about the same as 16 ducts versus 11 ducts.

    It could work well as long as the retro is done with the proper supply and return trunk additions and if balance dampers are installed to be able to change the relative airflow of second floor versus first floor between heating and cooling seasons. For this kind of a job, I made sure I talked to a half a dozen customer references who've said that duct retrofits were done very well. Retrofitting cape cod style homes is something that many contractors find to be a major PIA, as I found out from talking on another website to an AC contractor who has done about 30 of them and never wants to do another. Fortunately my brother-in-law does great drywall, so we can put things back together after it's all done.

  5. #5
    Join Date
    Mar 2003
    Location
    PA/DE area
    Posts
    1,535
    Four tons in that home is still too big and the well driller asked me what size I was putting in and I said 3plus and he said you were looking into a 4plus ton and he said to me "is he adding on to the home?"
    It's NOT the BRAND,it's the company that installs it!!!!!

  6. #6
    Join Date
    Oct 2004
    Posts
    37
    Quote Originally Posted by heatpumpguru View Post
    Four tons in that home is still too big and the well driller asked me what size I was putting in and I said 3plus and he said you were looking into a 4plus ton and he said to me "is he adding on to the home?"
    Yea, but he doesn't know my annual heat loss. The oil bills prove it.

  7. #7
    Join Date
    Mar 2003
    Location
    PA/DE area
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    Yes,but he drills around 10 GEO's a week and knows rough sizes of the homes he is working at.It is your home,you can put in what you want!Get a U-tube manometer to check static pressure.The contractor should use a FLOW HOOD to balance the job and give you a report.
    It's NOT the BRAND,it's the company that installs it!!!!!

  8. #8
    Join Date
    Oct 2004
    Posts
    37
    Quote Originally Posted by heatpumpguru View Post
    Yes,but he drills around 10 GEO's a week and knows rough sizes of the homes he is working at.It is your home,you can put in what you want!Get a U-tube manometer to check static pressure.The contractor should use a FLOW HOOD to balance the job and give you a report.
    I recall you quoted 0.1" static pressure and 400 to 950 CFM per ton. That means minimum of 1600 CFM. What happens if you don't get that -- what if it is around 1300 CFM? The reason I am saying this is because the actual install manuals for the two stage units list 2 stage performance at different dip switch settings, which can yield a range of CFM for low and high stage.

    If it wasn't good for the compressor to run the second stage with the blower set so low, then why the heck do the install manuals show that setting option?

    If it wasn't good for the compressor to run the second stage with the blower set so low, then why the heck do the install manuals show that setting option?

    "Heating Settings: The heating setting determines the heating CFM for Tranquility 27™ (TT) and Tranquility 20™ (TS) units. This setting is not used for Genesis (GS) units. Tap 1 is the lowest CFM setting, while tap 4 is the highest CFM setting. Consult submittal data or specifications catalog for the specific unit series and model to correlate speed tap setting to airflow in CFM."

    "Cooling Settings: The cooling setting determines the cooling (normal) CFM for all units with ECM motor. Cooling (normal) setting is used when the unit is not in dehumidification mode. This setting also determines the heating CFM for Genesis (GS) units. Tap 1 is the lowest CFM setting, while tap 4 is the highest CFM setting. To avoid air coil freeze-up, tap 1 may not be used if the dehumidification mode is selected."

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,776
    Annual heatloss doesn't tell you what size you need.

    If you think it does. Tell me what size furnace I have, or need. I use 1,000 gallons a year.

    I can flow more air through 10 properly sized supply ducts, then I can through 16 undersized supply ducts.

    You say your running another duct here and there, but no reference to size, or length.

    Cooling tap 1, is for use in high humidity areas. In those areas, the latent load is high enough that the compressor will not have liquid flood back.

    The amount of air volume needed for 2 identical houses in 2 different locations varies with their location, not their square footage.

    Too many home owners fall for the bigger is better attitude. They usually find out that the promised performance of the bigger unit is not possible, after its too late.




    Heatpumpguru. It sounds like you were out sold by a salesman that tells customers what they want to hear. Not how it will really work.
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  10. #10
    Join Date
    Oct 2004
    Posts
    37
    Quote Originally Posted by heatpumpguru View Post
    Yes,but he drills around 10 GEO's a week and knows rough sizes of the homes he is working at.It is your home,you can put in what you want!Get a U-tube manometer to check static pressure.The contractor should use a FLOW HOOD to balance the job and give you a report.
    Somehow I don't believe that is done by very many of the HVAC contractors doing geo retrofits. In fact, I didn't see that in ANY of the 3 contractor bids, even though geothermal has different requirements for air delivery and BTUH delivery than oil-forced air. But I don't think that one should assume "one size fits all" with regards to home size and geothermal furnace size, since heat loss can be very different from one home to the next of equivalent sq. ft. size.

    I know it's less effort for everyone involved in a retro to just go with existing trunk and the number of room supply branches, but that has just as many potential comfort or cost-related problems as undersizing altogether.

    The trouble with comparing 3 quotes (2.5 ton, 3 ton, and 4 ton) is that I never saw any Manual J numbers from 2 out of the 3 contractor proposals. So I'm tempted to trust the proposal where I am sure they ran the manual J to come up with heat loads and cooling loads to support their recommendations.

    I want to make sure that any businessman doesn't just try to sell me the package that they make the highest profit margin on for the least number of hours and days of installation labor, instead of a goal of balancing installation costs with satisfaction in terms of comfort in WINTER and SUMMER, as well as reasonable ELECTRIC BILLS without too much backup auxillary heat.

    ELECTRIC BILLS --- From my perspective, I got support documentation from Contractor #3 on a proposal that said 4 tons. That was based on Manual J, using Elite Software Development, Inc. That software estimated BOTH heating load and cooling load, and also predicted the oil consumption for my existing furnace and the electric consumption if I install a geo. So that should include the differences in BTUHs required to heat makeup air that goes up the chimney flue for an oil-forced air furnace versus geothermal that does not need to heat an influx of makeup air from the outside due to chimney flue airflow. But since the manual J gave an estimated oil use that was a bit higher than my ACTUAL oil bills for 2 years and ACTUAL heating degree days, I wondered if it was overestimating home heat loss.

    HEATING COMFORT --- Another contractor proposed 3 tons but he said I might not be happy with heating comfort with a 3 ton in my house, because of the fact that I said I found no insulation in the outer walls on the first floor, and said I might do better with auxillary OIL HEAT rather than auxillary electric. However, I'm not sure I should trust this because he has an exclusive maintenance contract relationship with the heating oil company.

    COOLING COMFORT --- All 3 contractors that submitted proposals said I would not have cooling comfort on the second floor with existing ductwork. Also, an un-interested party, an out-of-state contractor who does AC renovations for a living, said he had done 30+ cape cod AC retrofits and they were very difficult to get adequate second floor cooling unless you were gutting and starting over.

    I got a range of bids - 2.5 ton, 3 ton, and 4 ton, but only the 4 ton bid was backed up by Manual J calculations (Elite Software Development, Inc.).

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