Replace duct work or whole system?

Discussion in 'HVAC Heating & Cooling' started by ITSec, Feb 14, 2012.

  1. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    I need some advice from the experts, should I replace the duct work or the whole system?

    I currently have a 3 ton Trane compressor and First Co hydro heat system installed in 2002, the house is ~1,500 square feet ranch. The indoor unit is placed in the attic with 14" return duct and 6" supply ducts. I'm in zone 4 mixed-humid. Heat load 34,000 BTUs and cooling 25,000 BTUs.

    I was told the inefficiency was due to the system being too big, I need 2.5 ton and not a 3 ton. The supply duct is too small, I need 16" supply and not 14"; as well as the supply ducts being small.

    To replace the system with a Trane dual fuel system it would cost $12,099. It includes gas/electrical lines, XV80 furnance, XL15i heat pump, hyperion air handler, XL900 t-stat and all new ducts (2.5 ton, 15 SEER, 80% AFUE). Without new ducts and use current ducts $8,000.

    The builders placed a piece of sheetmetal on 2x4 for return duct
    [​IMG]

    Return duct is under other supply ducts
    [​IMG]

    Return duct
    [​IMG]

    Return duct connection to air handler
    [​IMG]
  2. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    Service side of air handler
    [​IMG]
  3. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    Price out what a 2-ton 2 or three head ductless multi-split would cost before sinking any money into the duct-haired Medusa.

    A 3-head Daikin 3MXS24JVJU or Mitsubishi MXZ3B24NA would blow the socks off any ducted system on whole-system efficiency in the cooling season, and can be cost-competitive with 80% gas burners for heating the place too if you're electricity prices are even slightly lower than the national average. (It really depends on your utility pricing.) I'd expect it to come in under $10K, maybe even under $8K.

    The fact that the air handler & ducts are in the attic, above the insulation adds at least a half ton to the load numbers, and a 2-ton continuously variable speed compressor on a ductless system should be able to handle the whole thing, and if it doesn't quite make it during the worst cold snap of the season you have the hydro-air system as auxilliary/backup. If it's an open floor plan you might even do it with a single-head 2-ton mini-split in a house that size in a VA climate, but the ability run up to 3-heads off the same compressor is sometimes useful if you have a full basement or multi-story or lots of doored-off areas. (My uncle in WA, zone 4C heats his open floor plan 2-story with just a 1-head 2-ton mini-split and has never been more comfortable, but his sleeping loft/bedroom areas have better than average convection with the lower floor. He hasn't fired up the ducted propane furnace since.)

    The better split systems have efficiencies approaching (and sometimes exceeding) geothermal heat pumps for whole-system efficiency, with none of the design risks.
  4. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    What is the driver for wanting to replace the unit/system? Is it comfort (winter/summer/both)? Is it worn out? Is it particularly inefficient? Or is it unable to handle the load at times? Does the home currently have a gas furnace/water heater or not? If you are adding gas and didn't have it before don't forget to factor in the monthly base charge when figuring actual operating cost--probably in excess of $25/month.

    Is the current unit running non-stop in the hottest summer weather AND still unable to hold setpoint late in the afternoon? If it is still cycling on and off and you have a feel for how much of the time it is on, and how much it is off at your peak load, then you can work out how much oversized it is.

    I can't speak to the heat pump side of this and it probably figures into the sizing. The calculated cooling load of 25,000 btu/hr is 2.08 tons. The estimation techniques generally oversize things so a 2 ton unit will probably do the job. However, I can understand why one is hesitant to risk so much money on something that has any risk of being undersized. If a 2 ton unit will work instead, then the duct mismatch goes away. (The size ratio from 2.5 to 2 is about the same as the area ratio of 16" vs. your existing 14" duct.) Undersized duct is typical of home construction unfortunately.

    I suspect that 3 tons is considerably oversized for 1500 sq. ft. ranch in that climate unless the house is poorly insulated/sealed, although as Dana notes the attic install can cost you a portion of the available load. The question is would a 2 ton handle it. I've got a 4 ton unit in a not particularly efficient 1994 construction 2800 sq. ft with a similar climate where we get high humidity and up to 105-110 F at times, and it is about a ton oversized. My heating load is right at 50,000 Btu/hr at 0 F...determined by plotting actual daily gas use vs. ave. daily temp for a winter.

    The reason I stuck with a 4 ton unit when we changed out was that the old unit originally couldn't keep up on 105+ days when we moved in, and I didn't want to risk undersizing to get down to the next frame size: 3 ton. But that was before I did some maintenance on the system, some duct insulating/sealing, sealed window leaks, improved insulation in several spaces, and balanced the registers. After that it was keeping up on the hottest days. We weren't the original owners so I don't know what the original performance was, but I have indications that the original AC compressor and coil were replaced the year after construction. My guess is that they started with a smaller unit, but due to poor HVAC layout/design, poor sealing, inadequate insulation, and tall open floor plan it couldn't keep up and they finagled a replacement unit out of someone.

    The attached furnace in your climate (similar to mine although Virginia is significantly milder in both Winter and Summer) will most likely be sized by the air handler required for the AC. That is what I learned when I sized my system, I couldn't go as low as I wanted on the furnace size. (The next blower frame size and therefore furnace size started at 3 tons AC rather than 3.5 tons.) However, I went with two stage air and two stage gas 95% AFUE, so the gas almost always runs in single stage other than a few hours each year, and the second stage AC only kicks in on the 95+ or so summer afternoons. I rejected the heat pump option because electric is high here, gas is cheap, we already had gas, and the majority of folks I know who tried retrofitting from gas to heat pumps were disatisfied with the comfort level of the new install.

    Ours is a Ruud unit (identical to Rheem right down to the model numbers) and it set us back ~$7,500 last winter before a $1,000 rebate on the dual stage HVAC, and the Federal tax credit.

    I don't have any experience with ductless systems.
    Last edited: Feb 15, 2012
  5. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    Dana - the ductless system seems like a great idea. I've been reading about HVAC systems and how they should be installed in the conditioned area and ductless seems to solve that problem since my crawlspace is open as well as the attic.

    Bison - My current system does not keep the house warm during the winter months and during the summer it's never cool. The system is constantly running during both seasons. Unit will not heat house above 73F (winter) and will not cool below 82F (summer).

    I spent money on getting the windows replaced with Sunrise Restorations, the salesman said it would cure my ploblems with the HVAC system. Since I'm still paying for the windows, I don't want to blow my monthly budget.

    The cheapest option is to replace the ducts myself and have someone build a new plenum, but will that fix my problem?
  6. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    Replacing the ducts may help, but if they're not meticulously sealed and insulated it could even go the wrong way rather than fix it. Designing the ducts to comply with ACCA Manual-D would be the starting point.
  7. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    I was thinking about paying for one of those online duct designs, but I'm a little skeptical about paying for that type of service. Cost is $300 and comes with materials list.
  8. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Is it presently all heat pump (with supplemental strip heaters) or does it include gas?

    73 F seems a bit high as a winter set point, but if the house is drafty and has cold sections then you probably need the higher setting to get those areas into the comfort zone. I take it there is no humidifier for winter? That would move the comfort zone up a few degrees as well. And from what I've heard and read about heat pumps the air temp from the ducts is much cooler than with furnaces. I run about 69 F with a furnace.

    Not being able to get below 82 F in summer is an even more telling concern. I sure wouldn't reduce the tonnage of the AC unit with that issue.

    I don't think the actual problem has been identified with certainty yet. Is the house the same vintage as the HVAC or is it older? Has the HVAC always performed this poorly?

    Perhaps as the estimate suggested it just needs new duct and distribution work because of original poor design. But if so the whole distribution system should be looked at to make sure it is properly balanced and isn't short circuiting much of the flow back to the unit, etc.

    You've already done windows so there may be bigger problems with uninsulated areas/walls or high air leakage into the home.
  9. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
    To me, that system must have a poor duct design, was never setup correctly, or something else is wrong with the system.

    I'm in the same climate, 2300 sqft + 1150 sqft finished basement, 44 year old house, single pane windows, etc. and my system is a 2-stage 2-ton/3-ton unit. The previous unit was 3 ton A/C only. New one is a heat pump. All last summer, it ran in low stage (2-ton) probably 90% of the time. About the only time that I heard it go to high was when it would first fire up and try to overcool the coil as part of the dehumidification method. It also maybe went to high a few times on the hottest days. Had no problem maintaining 77-78F.

    Most heat pumps due deliver lukewarm air, but mine has an option to avoid it. Mine is a York Affinity and has what they call a "hot heat pump" mode. It essentially monitors the refrigerant conditions as well as blower speed and attempts to maintain 100+F air outlet temperatures. It isn't possible under all conditions, but I believe it to be true during this entire season. I'll have to measure the temp sometime, but I know it is north of 100F as it doesn't feel like a heat pump and the temperatures feel nearly as warm as the old electric furnace (that one had about 45-50F rise, outlet temperature ~115F+).
  10. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    The builder's of this development used hydro heat for heat in these houses, so I do not have a heat pump or supplemental strip heaters. The 2 story houses in the development have their hot water tanks in the attic. I have a gas water heater in a closet in the center of the house that is connected to coils that run through the air handler in the attic. Hot water ~140F goes up through the air handler and back into the water heater at a cooler temperature. The air handler and water heater constantly stay running trying to keep the house warm.

    My wife jokes, saying it's warmer outside than it is in the house.

    I don't think the actual problem has been identified with certainty yet. Is the house the same vintage as the HVAC or is it older? Has the HVAC always performed this poorly?
    - The house was built in 1988 and hvac unit was replaced in 2002 with the same spec as the one that was removed. It has always been poor and I just dealt with it, since I'm getting older I would like to be at least somewhat comfortable in the house.

    You've already done windows so there may be bigger problems with uninsulated areas/walls or high air leakage into the home.
    - In the attic I have about 8-9 inches of insulation and the exterior of the house no wrap or insulation, just vinyl siding with 1/4 overlap on plywood.

    More pictures to give you guys an idea how this house was built.

    Bypassed junction and straight into florescent lighting
    [​IMG]

    Crawlspace: they nailed a 2x4 to a 4x6 so floor joist could have support
    [​IMG]
    Last edited: Feb 15, 2012
  11. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Ok, I understand now, that's what those insulated water lines were for. I've not seen one of the hydronic heaters before. It the water heater burner is running constantly then it should be getting within a reasonable proximity of the system's design heating output.

    Nothing wrong with wanting it to be comfortable. If it fails that then little else matters.

    Ouch! Am I reading this right: no insulation in the exterior stud walls? If that is the case then I would put my money toward insulating them before I would spend anything on the HVAC. Hopefully Dana will weigh in on this and how badly it impacts the sizing, but that would make the effective wall R value with studs no better than R3 (probably 2-2.5 if my calcs are right.) The attic insulation isn't great, but half my home has a cathedral ceiling that is limited to similar effective depth.

    I've got some areas in the walkout basement that were finished in the original build, but that were not insulated as best I can tell. This seems to be true where the plumbing and sewer vents run (not even insulated on the back side against the exterior), and on some stud walls against the vertical concrete. I've insulated a few. One of these days I'm going to snap and make inspection holes everywhere down there to determine just how much of it needs to be insulated.

    Hey, I think the same clown installed my bathroom lighting. I had to add junction boxes and mounting studs on several of mine when I replaced the dated original fixtures (el cheapo hollywood lighting bars.) The wires were just shoved through a hole that appears to have been made hastily with a screw driver. The fixtures themselves weren't even properly secured to the wall.
  12. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    You may have/be using more sophisticated dehumidification controls on your 2 stage unit, but on mine the easiest way to improve the dehumidification was to reduce the blower flow with jumper switches. Mine runs almost completely in low stage and on mild days I was disastisfied with the dehumidification. The problem was two fold: first the low stage air flow is proportionately greater than the high stage, and second the installer had also left the jumpers set to the default max (for a 5 ton rather than 4 ton.)

    I noticed the first issue because the unit was slow to produce condensate on the low stage. So I used some tuning jumpers to reduce the flow. Later I deduced (because it wasn't written anywhere) that the main jumper switch setting was at the default for the larger unit. So I put the other jumpers back to nominal, then reset this latter set. Both accomplished the same thing, improving dehumidification in low stage. This likely reduces the efficiency somewhat, but the lower blower speed seems to offset the compression penalty. I reached the latter conclusion when comparing the effective kw/ton in high and low stages. (This wasn't figuring cycle length or any of that, just comparing a few minutes of lined out operation in high and low.)

    That makes sense. You will lose some efficiency holding the higher air temp and it will limit the capacity/crossover point when it gets particularly cold, but comfort is the key.
  13. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,486
    Location:
    Cave Creek, Arizona
    I may be a bit confused. You say the system is running all the time without satisfying the thermostat, but you also said it was oversized. That does not make sense, because an oversized design will usually turn on and off too frequently in order to balance itself with your needs. A smaller unit should require smaller ducting than an oversized one, so I cannot tell if your resized design is proper or not.
  14. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    HJ - that is true since you have brought that up. Maybe it a combination of not enough insulation and improper ducts. The 3 ton system needs 1,200 CFM and with the 14" duct it's only 800 CFM, then all the bends and loops of the return duct it probably getting less than that. The air handler only has 1 speed. I'm no HVAC expert but from what I have learned so far I know it needs 1,200 CFM.
    Last edited: Feb 16, 2012
  15. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    More important than the insulation, try AIR SEALING. Looking at how the place was built, fixing all the air-leaks you know about first, then hiring an air-sealing & insulation contractor to do the rest with blower-door testing & verification and IR imaging would CERTAINLY be cost-effective. It would lower the heating & cooling loads and increase comfort to boot!

    The dust-darkening of the insulation around that hole in your ceiling at the light fixture is evidence of the massive air leakage at that point, and that's probably one of hundreds of mid-sized holes in the pressure boundary of the house.
  16. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Having gone the air sealing route I doubt it will have nearly as much impact as insulating if the walls are indeed uninsulated. Isn't the majority of measured air leakage in homes due to things like gaps at the bottoms of studwalls and rim joists anyway? That's what I recall from studies.

    Perhaps the best advice at this point is to have a professional energy audit done. That should include leakage rate testing as well as evaluation of the insulation and ductwork. I've never had one of these done, but assuming it is competently performed it should be able to put some numbers in place for the impact of the problems.

    Not to put words in other contributers' mouths, but I have the impression that we could all agree that the HVAC unit itself does not appear to be the problem based on the information provided thus far.
  17. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    The big holes at the bottom of the house, and those at the top are what drive air infiltration from stack effect. You have to fix them both, and we have clear (photographic) evidence of signficatn holes at the top, including the dust-darkening of the fiber indicating a history of air flow. If that's just one-off it's not huge, but if (as indicated) that it's typical of how the house is built, the blower door should tell-all.

    Yes, the band joist is often a huge (usually untreated, even in brand-new homes) air leak that is often several times that of all windows & doors combined.

    Insulating uninsulated walls is a no-brainer investment, and also slows down air infiltration, since otherwise each stud bay is essentially a flue with it's own stack effect drive. They will all leak, some more than others, but with high-density blown insulation those convective losses are cut by more than 95%. (But with low density goods it's cut maybe ~50%.)
  18. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Leave the unit alone, insulate and seal as mentioned and turn up the water temperature a bit.
  19. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    Spend you money on insulating the walls and the attic. Unless you do that the house is never going to be comfortable.
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