Geothermal heat comfort in a 50 year old home with limited insulation and ductwork? - Page 2
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  1. #14
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    Quote Originally Posted by heatpumpguru View Post
    I tend to size around a 1/2 to smaller on the Capes to get run times due to large roof area and thermostat being on the first floor and it is easy to cool. Returns to the upstairs are key,if you do not have enough you may have to stick window units up there.That is really not the goal of a PRO.
    Since this home was never built for central air, it's not likely I'll easily pump enough cold air upstairs. A few years ago I did my own detailed room-specific heat gain calculation taking into account ceiling surface area, window surface area, direction facing, insulation, climate and geographic factors. The estimate -- 7000 to 8000 BTUH for each upstairs bedroom, so I bought two window A/C each 10,000 BTUH and they have been happy, but it currently costs me about $50/ month for 3.5 months to cool upstairs. Add that to about $50/month for 2.5 months for cooling the main floor with one huge 1st floor A/C 24,000 BTU window unit. So the net current annual cooling costs $300/year via window A/Cs.

    I know if my geo is insufficient for summer cooling on the second floor, I might still use the 2nd floor window A/Cs for annual costs of $175, but that only very slightly extends somewhat the time to break even..

    But what I'm REALLY CONCERNED ABOUT is if I will be WARM ENOUGH in the winter, since oil is the commodity that is going to kill me in the next 10 years, not electric.

    As you stated, for a 4 ton geothermal heat unit technically I should have 16 individual 6" dia. ducts in the house. Looking back to my previous comment, I currently have only 11 ducts plus one extra in basement, not 16.

    With only 11 ducts, will a 2 stage 4 ton geothermal fail to push enough hot air and therefore require greatly increased supplemental resistance heat that will up my electric bill over every winter? Is it worth the money to run a bit more ductwork for heating or heating and cooling, given the age of the house.

    My gut feeling is that I am only about 4 ducts short of what I need. What if I install 1 extra duct in each of the two upstairs bedrooms, plus install a passive vent to help return air in each upstairs bedroom on the wall adjacent to the hallway stairway landing, which is a direct route back down for return air. That way each upstairs bedroom has 2 ducts, not one.

    But that is not planning for increasing the main trunk size from which these direct vertical 3"x9" ducts branch off and come directly upwards through a middle wall and exit the FLOOR in the middle of each bedroom. Weird place for heat vents but that is how the existing 2 ducts are laid out, why not 2 more just like them?

    And 2 extra ducts upstairs achieves only half way to the 4 ducts needed to reach the goal of 16.

    So for the first floor, I have to decide where I need the BTUH most. My bedroom already has 2 ducts (13'x13'), but the other 1st floor bedroom (10'x10') only has one. I could add one more duct to that room, since that room has sometimes been lukewarm in the winter anyway (one duct but 2 windows, northwest facing).

    For the final extra ductwork, I might consider 1 or 2 more ducts to the addition area living room (15'x26'). I already have 2 ducts midway on the far outside wall. But the room has a 7 foot slider, an 8 foot picture window, and two small windows. It is warm with oil forced air, but might need more air with lukewarm geo heat, I don't know.

    Anyway, in order to avoid running cross-ways against the rafters and avoid digging up the finished basement ceiling, I could add 2 more ducts right along the wall that borders the addition versus older part of house. One near the slider, where there is heat loss, and one behind the woodstove, which is the greatest distance from all of the existing heat ducts in living and dining area.

    So this would give me a total of 11 existing + 2 upstairs + 3 mainfloor = 16 ducts plus 1 in basement.

    I wonder whether this would "BREAK THE BANK", cost efficiency speaking, to add to the geo install total price by adding 4 or 5 ducts in this manner (two from the main trunk going straight up through a middle wall of the 1st floor into the middle of rooms on 2nd floor, also one duct from main trunk straight up also inside the middle wall to the 1st floor bedroom, and finally 2 ducts only about 3 to 5 feet direct off the furnace room itelf through the former outside wall of house, to the living room inside wall.

    I'm considering asking for a caveat price not-to-exceed on retrofit of 4-5 ducts, price per duct in certain rooms. I want to know all of this before pulling the trigger on something that is NOT BASED ON NEED FOR COMFORT, PURELY BASED ON LONG TERM NEED TO SAVE ENERGY (MAINLY OIL) COSTS. The loan costs will wash out the savings in energy if the install becomes ridiculously expensive.

    For return air, I'm not worried as long as air can get back to the basement. Is this incorrect? I have a door I leave open between main floor and basement.

  2. #15
    Quote Originally Posted by heatpumpguru View Post
    We sell Carrier and they do not really stock the GEO parts
    I think Carrier/Bryant uses Climate Master of course they put ther name on it.Most parts are pretty easy to get around here. Waterfurnace is really easy to get parts from if you are a dealer. I like both units but waterfurnace simplifies things when it comes to service and troubleshooting.

  3. #16
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    Quote Originally Posted by heatpumpguru View Post
    I tend to size around a 1/2 to smaller on the Capes to get run times due to large roof area and thermostat being on the first floor and it is easy to cool. Returns to the upstairs are key,if you do not have enough you may have to stick window units up there.That is really not the goal of a PRO.
    My question to you is, what exactly makes the first floor "easy to cool" for a cape compared to another type of house? Purely the square footage and the fact there is a finished second floor above you?

    Second question -- you said I would need returns for second floor to cool it. Would PASSIVE returns (not connected to return ducts, just room-to-room vent next to stairway) work as good as returns DIRECTLY connected to the cold air return ductwork. In other words, is there real suction going on in the return air ductwork from a second floor, compared to passive return, and does that effectively make the cooling work better?

    And again, the big concern of mine is whether HEATING is adequate for the whole house. The range of bids was 2.5 ton to 4 ton sizing. As a homeowner it is hard for me to know whether the 4 ton bid which includes justification stating results of Manual J with elite software and claims a load of 77,000 BTUH heating is needed, were the INPUTs to the calc. done right -- As a scientific modeller myself, I know model outputs are only as good or crappy as the inputs used -- garbage in, garbage out, they say.

    I have read on SEVERAL geothermal websites that undersized ductwork will lead to noisier airflow, higher duct pressures, and more work required of the blower. And I have also read that with undersized ductwork, any cooler spots in my house may be more of a problem because the duct temperature is cooler with geo than with oil-forced air. (Although 5-10 degrees hotter than a conventional air source heat pump.)

    Would it be worth it to modify ductwork? Should I upsize from 11 to 16 ducts, add a second dedicated horizontal trunk to feed 4 ducts upstairs, instead of 2 currently. And while I'm at it add one back bedroom downstairs and 2 for living room?

    Attached is a drawing of possible additional ductwork. Would that sort of kill the entire cost-savings of this project by jacking up the install cost by 30 percent or more? I don't have a clue because I've never asked for duct renovations. As a homeowner I think I have figured out where to put ducts without disturbing the main floor drywall, but would have to have someone verify that.
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  4. #17
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    Quote Originally Posted by rru2s View Post
    And again, the big concern of mine is whether HEATING is adequate for the whole house. The range of bids was 2.5 ton to 4 ton sizing. As a homeowner it is hard for me to know whether the 4 ton bid which includes justification stating results of Manual J with elite software and claims a load of 77,000 BTUH heating is needed, were the INPUTs to the calc. done right -- As a scientific modeller myself, I know model outputs are only as good or crappy as the inputs used -- garbage in, garbage out, they say.
    If you have past fuel use data (assuming you haven't made major insulation changes) you can also back out the heat loss from the total degree days for each season. It would give you another data point to compare to the manual J. For instance, 666gal*138000btupergal*.82/6600dd=11418btu per degree day. If your design temp is 0, you would just do 11418/24*70=33305btu/hr at 0degF outside temp. Crude, but this is a case where it is almost always better to have the data.

  5. #18
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    Quote Originally Posted by bc3141 View Post
    If you have past fuel use data (assuming you haven't made major insulation changes) you can also back out the heat loss from the total degree days for each season. It would give you another data point to compare to the manual J. For instance, 666gal*138000btupergal*.82/6600dd=11418btu per degree day. If your design temp is 0, you would just do 11418/24*70=33305btu/hr at 0degF outside temp. Crude, but this is a case where it is almost always better to have the data.
    Based on where I live, last year was 5023 dd, so: 666*138000*0.82/5023 = 15000 BTUH per degree day, and design temp. was 0 degrees outside, 72 inside, so 15000/24*70 = 43750 BTU/hr at 0degF outside temp. So that comes in at about 3.5 tons for heating, and the question boils down to whether that heat is distributed uniformly throughout the house.

    Thanks for the information.

    Anyone else have input as to whether cooling performance on a second floor is effective using a room-to-hallway vent instead of direct air return to basement return trunk?

  6. #19
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    I tell people 'put a boiling pot on the stove,the vent is connected to the outside,but the steam does not reall head that way.When you turn the exhaust fan on it overcomes the static pressure and heads tha way. I think you would have high with 4 tons with your duct work,it freezes coils and kills compressors.It is harder to cool a home that is 2 stories then heat it.
    It's NOT the BRAND,it's the company that installs it!!!!!

  7. #20
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    Quote Originally Posted by bc3141 View Post
    If you have past fuel use data (assuming you haven't made major insulation changes) you can also back out the heat loss from the total degree days for each season. It would give you another data point to compare to the manual J. For instance, 666gal*138000btupergal*.82/6600dd=11418btu per degree day. If your design temp is 0, you would just do 11418/24*70=33305btu/hr at 0degF outside temp. Crude, but this is a case where it is almost always better to have the data.

    Since it is not efficient to design a geothermal or a heat pump to try to meet 100% of cold days without ANY auxilliary heat, then can I use the SAME calculation to try to estimate what my BTUH heating load needs are down to a temperature that is greater than 0 degrees F?

    I used hourly temperature graphs from 2006 through 2008 to estimate the number of whole 24 hour periods where the temperature is below 17 degrees F. Some days it was at or below 17 degrees one quarter day, or onehalf day, so adding all this up I get 12.5 days per year. That is about 3.5% of the days in a whole year. Would that be a reasonable goal for the geothermal -- not to need any auxilliary heat for all but 12.5 twentyfour hour periods? Or is that overkill?

    If that is the assumption, then using my ACTUAL OIL USE DATA over 2 heating seasons, and ACTUAL HEATING DEGREE DAYS, I get the following:

    (790 gal./yr.)x(138000 BTU/gal)x(0.82 AFUE)/(5200 deg.days)/(24 hr/day)x(70 - 17) delta degrees = 38,000 BTUH heating load for my house at 17 deg outside temp.

    That would suggest that I need a 4 ton geothermal, because if you look up the specs on either a waterfurnace or climatemaster 4 ton 2 stage unit, the heating capacity is around 37,000 BTUH and change for ground loop heating.

    Is it overkill to suggest that only 12.5 days per year I need auxillary heat? Or is it a good goal? IF I really do need 38,000 BTUH capacity, then I think I need more ductwork.

  8. #21
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    We look for around 200 hours or so on back up.
    It's NOT the BRAND,it's the company that installs it!!!!!

  9. #22
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    Quote Originally Posted by heatpumpguru View Post
    We look for around 200 hours or so on back up.
    Thanks, that makes sense. 12.5 x 24 = 300 hours, so it looks like I do need the compressor to deliver 38,000 BTUH or I could go above 300 hours on backup per year.

    So a 3 ton or 28,000 to 30,000 BTUH heating capacity geothermal furnace might not cut it as far as achieving the goal backup times.

    Now the calculation I did is based on real oil usage, but it does not include something people have referred to as "oversizing penalty" for oil-forced air.

    Should I multiply the "oversizing penalty" fraction x 38,000 BTUH to figure how much geothermal BTUH I would require for periods when temps are equal to 17 degrees?

    The other tweak that might come into play was that in estimating the annual hours below 17 degrees from the climate records, I included a lot of times when the low temp equaled 17 degrees but did not go more than one degree or so below 17 degrees. That might be a marginal backup usage situation at the borderline temp, so ignoring that contribution might reduce the estimated hours per year on backup by a few percent.

  10. #23
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    Quote Originally Posted by rru2s View Post
    If that is the assumption, then using my ACTUAL OIL USE DATA over 2 heating seasons, and ACTUAL HEATING DEGREE DAYS, I get the following:

    (790 gal./yr.)x(138000 BTU/gal)x(0.82 AFUE)/(5200 deg.days)/(24 hr/day)x(70 - 17) delta degrees = 38,000 BTUH heating load for my house at 17 deg outside temp.

    That would suggest that I need a 4 ton geothermal, because if you look up the specs on either a waterfurnace or climatemaster 4 ton 2 stage unit, the heating capacity is around 37,000 BTUH and change for ground loop heating.
    Yes, your calculation is correct. I'll just say that in many cases it does make sense to size the geo system to handle the full design load. It really varies a lot based on climate, load, relative fuel costs, drilling/trenching cost, etc. A good contractor should be able to provide you with estimated fuel costs for various scenarios.

    One other thing that I think gets overlooked sometimes - as your example shows, if you size the system to handle 70-80% of the design load, it will end up doing 95% or more of the heating work over the course of a season. This sounds great, but keep in mind that the loop field must be sized for both peak and seasonal loads. I suspect this makes more of a difference in heating dominated climates where the loop field doesn't get much heat put into it in the summer. In my case, if I sized my system for a 20F design temp, the loop length could hardly be reduced at all, so the only money I would have saved was on the heat pump itself. Good loop design software will account for seasonal and peak loads.

    When sizing the system make sure you're looking at the output at the lowest expected loop temp - capacity drops quite a bit as the loop temp drops.

  11. #24
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    Quote Originally Posted by bc3141 View Post
    keep in mind that the loop field must be sized for both peak and seasonal loads. I suspect this makes more of a difference in heating dominated climates where the loop field doesn't get much heat put into it in the summer. In my case, if I sized my system for a 20F design temp, the loop length could hardly be reduced at all, so the only money I would have saved was on the heat pump itself. Good loop design software will account for seasonal and peak loads.

    When sizing the system make sure you're looking at the output at the lowest expected loop temp - capacity drops quite a bit as the loop temp drops.
    I am not sure how to interpret the detailed loop temp specs in the installation brochures, but the data is there. So I'll pass that along to the contractor doing the proposal.

    Meanwhile, I realized that the loop field might become a limiting factor just as the geothermal compressor if my house is borderline for 3 versus 4 tons heating. Therefore, the driller for the 4 ton proposal contractor submitted 2 separate bids to me -- one for 600 feet in 2 boreholes (which works for 4 tons), and the other for a single 450 foot deep borehole (for 3 tons).

    There is considerable difference in cost. However, if the loop temps get too depressed in the winter, then the performance could suffer. You said that the loop temp might not recover enough in the summer if I don't use the AC capabilities enough. However, my driller commented that if my borehole hits water at only 25 feet deep (which is like my drinking well), that will help thermal transfer. I am located on the downslope of a 350 foot 8 percent grade hill, that makes the hydraulic gradient pretty strong, which in all likelihood will mean a fast horizontal groundwater flow velocity, which because of the heat capacity of water should allow the subsurface temps in the rock around the borehole to recover pretty quickly. Groundwater in my area is 55 degrees year round, which is on the warmer side compared to the midwest and northeast, which are about 5 degrees cooler.

    However, it's just amazing how upsizing from 3 to 4 tons ends up requiring more expensive equipment, a more expensive loop field, and realistically needs more ductwork modifications in my home. I can't wait to see the ductwork estimate -- if the entire project goes upwards in cost by more than a certain amount, there is a chance I could have negative return on investment even if I stay in the house 10 years.

  12. #25
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    if you look at 4 to stuff with Copeland scrolls they fire 70% out of the gate so with 4 tons it maybe a wash V/S a 3 with back up since you will always be firing the 3 ton with the 4 ton.
    It's NOT the BRAND,it's the company that installs it!!!!!

  13. #26
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    Quote Originally Posted by heatpumpguru View Post
    if you look at 4 to stuff with Copeland scrolls they fire 70% out of the gate so with 4 tons it maybe a wash V/S a 3 with back up since you will always be firing the 3 ton with the 4 ton.
    I understand scrolls are used in all 3 systems that were proposed by 3 contractors, and they are very efficient at compression and will have less wear-and-tear than other types of compressors. But almost everyone uses them nowadays.

    I also understand that with a 2 stage 4 ton, the system will run in the first stage when the weather is not that cold, such as spring, fall, and early part of winter.

    But once the weather gets colder, the assumption is the second stage works the major portion of the running hours, does it not? So if a 3 ton compressor can't provide the BTUH then backup would be excessive for a house heat load that comes in at the numbers I have.

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