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  1. #1
    Join Date
    Nov 2011
    Location
    Arlington VA
    Posts
    4

    Confused HVAC repair in preparation for upgrade

    I am looking for advice on options for replacing a failed AC condenser as a prelude to an overall HVAC upgrade. I need to address the AC in the very near term since warm weather (and wife) are on me, and want to ensure the replacement will work properly as part of the future system. I am looking to minimize near term cost since I have not lined up financing for the remodel yet, but am open to investing up front where that makes sense. I also have not completely locked down the remodel design. To add to the fun I have an older home with an existing (poorly designed) addition, oversized HVAC equipment, and undersized / poorly balanced ducting.

    Current configuration:
    Furnace: RUUD UGDA 150E-JR manufactured in 1988
    AC condenser: Carrier 567C048-RCU manufactured in 1988
    AC Coil: matching Carrier 567C048-RCU

    The condenser is effectively failed. It will run for a while, typically 30-60 minutes before popping the breaker on overcurrent. Due to the age, overall poor condition of the condenser I have not investigated repair options.

    I have done a Manual J heat load calculation using HVAC Calc (attached) and get the following:
    Location / design conditions: Arlington VA, Indoor: 75F summer / 68F winter / 50% RH, Outdoor 95F summer / 15F winter / 100 grains RH
    2907 SqFt
    Sensible gain: 27900 BTUH
    Latent gain: 4009 BTUH
    Total gain: 31909 BTUH
    Total loss: 48507 BTUH

    This gives me an SHR of 0.87 and nominal AC size of 2.7 ton. The current equipment appears well oversized at 4 tons of AC and 113K BTUH of heat output. Both the AC and furnace tend to short cycle and the house always stays humid in the summer.

    The duct work has two main branches with one branch (11”x14” supply, 11”x8” return) serving the original house, and one (16” round supply and return) serving the addition. This is out of whack since the load from the original house (17K BTUH total gain, 27K BTUH loss) is greater than the addition (15K BTUH total gain, 21.5K BTUH loss). There are no dampers to allow flow balancing. One room (a foyer with large windows and south facing skylights) has no supply or return at all.

    I have also done a Manual J load calculation for the house after the planned remodel and get the following (same design conditions as above)

    Zone 1
    2365 Sq Ft
    Sensible gain: 19917 BTUH
    Latent gain: 1843 BTUH
    Total gain: 21761 BTUH
    Total loss: 36187 BTUH
    Resulting in SHR 0.92, and nominal 2 ton AC size

    Zone 2
    1131 SqFt (but large windows and two fireplaces)
    Sensible gain: 18555 BTUH
    Latent gain: 3025 BTUH
    Total gain: 21580 BTUH
    Total loss: 31429 BTUH
    Resulting in SHR 0.86, and nominal 2 ton AC size

    Based on my current and projected future needs I am inclined to replace the failed AC and the coil with a 2.5 ton unit and keep my furnace for now, then upgrade to a high efficiency furnace (~50K BTUH) and add a 2 ton AC and 40-45K BTUH furnace for the second zone as part of the remodel. Due to the house layout and construction (masonry) it is not practical to do much with the duct work in the original house (zone 1), but (I may be able to add some return ducts on the main floor and get a small supply and return to the foyer. The duct work for the existing and new addition (zone 2) will be completely redesigned.

    I have had one HVAC contractor look at the project so far. I asked him to give me estimates for 1) replacing the AC and coil only, and 2) AC, coil and furnace. For both cases I requested estimates for both mid-range and high efficiency units. The tech was pretty skeptical of my plan and gave me a hard sell to replace my whole system now. He took two weeks to get back to me with a bid and only provided a single option:

    AC: Carrier Performance 16 24ACC630A003 2 1/2 ton
    Coil: CNPVP3617 3 ton
    Infinity VS modulating gas furnace 59MN7A080V21-20
    Infinity programmable thermostat

    The bid was for quite a bit more than I was looking to spend immediately, did not break out the cost of the components, and included the caveat “Existing duct system is not designed for this size equipment [contractor] is not responsible for this system making the house comfortable and or working properly.”

    I am planning to ask for a few more estimates and am well aware that I only sort of know what I am doing here. Before I press on I was hoping the experts could provide me with a reality check on a few key points:

    1) When I looked at the Carrier Performance 16 specs on the Carrier website the only controls that were listed were the Edge and Comfort thermostats. Will this unit work with the Infinity thermostat? Alternately, if I chose not to upgrade the furnace yet will the Performance 16 work with my existing “dumb” thermostat? Can the Infinity thermostat control the RUUD furnace (single stage heat with two speed blower for heat and cooling)?

    2) The Performance 16 is listed as SEER 16.5. Looking at my utility bills over the last several years my (very rough) estimate is that I am using ~3900 KWH per year for AC. What is the likelihood that going with a higher SEER AC such as the Carrier Infinity system would pay for itself within 5-10 years assuming $0.09/KWH? My instinct is that upgrading would not be justified by cost savings alone.

    3) Does the Infinity series have features that would mitigate having an AC unit that is somewhat oversized for the final projected load that might justify upgrading?

    4) The 59MN7A080V21 furnace seems moderately oversized for the current house and way oversized for the zone it will serve eventually. If I understand the specs right this is a 80K BTUH unit that can modulate down to 40% of capacity or ~32K BTUH. Since the estimated heat loss for the zone it will serve is only 31.5K BTUH the furnace will likely operate at the lower limit pretty much all the time and wind up just cycling on and off defeating the whole purpose of going with a high efficiency unit. Even if I do replace the furnace now would it make sense to go with a smaller unit? Any recommendations on size or model?

    5) Is using a lower capacity AC and coil with a high capacity furnace a problem? The specs for the UGDA 150E-JR show 2030 CFM for cooling (it is probably more like 1800 CFM since it was installed with an intake on only one side). The pressure drop tables for the CNPVP3617 coil top out at 1400 CFM. I expect that at 1800+ the pressure drop will likely exceed the 0.50” rated drop for the blower. Any problem going with CNPVP3621 coil to reduce the pressure drop (it is also a better mechanical fit for the duct plenum and the price difference seems negligible)? Alternately, can I have the installer rewire the thermostat to use the lower blower setting for both heat and cooling?

    6) Is it feasible to purge and reuse the existing coil (R22) with the new condenser (R410A)? I realize this is a rotten idea from an efficiency standpoint. However, it would allow me to defer the coil and furnace sizing issue for now.

    6) Is it customary to include a caveat to the effect that the contractor is “not responsible for the system … working properly”? I have asked the contractor for a clearer explanation regarding what problems I can expect to encounter but haven’t heard back yet.

    Sorry for the long post. I am trying to provide all the info up front. Thanks in advance.
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  2. #2
    Join Date
    Jul 2006
    Posts
    1,091
    Is it customary to include a caveat to the effect that the contractor is “not responsible for the system … working properly”?
    If I had a home owner do his own load and told me everything you wrote in your post..... not customary, but necessary! You warranty your design, I warranty my design. You need to let the man do what he knows.

  3. #3
    Join Date
    Nov 2011
    Location
    Arlington VA
    Posts
    4
    Thanks. I understand your point. The thing is, the contractor didn't suggest redoing the load calc himself, and has not come back with a "your calculation is suspect because ...". I can't really tell if he even reviewed / considered mine - the equipment sizing makes me think maybe not.

    I certainly don't expect a simple AC and coil replacement to fix the duct layout problems. I do expect the replacement system to work at least as well as the current system used to before it failed. Can you suggest some specific questions I should be asking?

    Thanks.

  4. #4
    Join Date
    Jul 2006
    Posts
    1,091
    Have you had someone out to see why the breaker trips? That would be my first step as a home owner. It might be something simple, weak breaker, loose connection. A simple fix may ease the sense of urgency.

  5. #5
    Join Date
    Nov 2011
    Location
    Arlington VA
    Posts
    4
    No I haven't this time. I would rather invest in a replacement system than put $ into a repair if I can find a combination that will work. The breaker did fail on the unit in ~2007 and I replaced it to fix the problem - naturally the breaker was the last thing I checked then. The condensor fan motor bearings failed in mid-2010 and I replaced the fan motor and capacitor and bought myself another year. The heat exhanger fins are pretty dinged up (unit has wire guards vs louvered cabinet and is in a high traffic area). I could not find a SEER rating for the current system but I am guessing ~10 when new, probably running ~5 now. When the contractor looked at the system his first reaction was "yep it is time for a replacement" based on general condition.

  6. #6
    Join Date
    Nov 2011
    Location
    Arlington VA
    Posts
    4

    Wink

    I met with the rep from a second HVAC contractor this afternoon and experience and outcome was 180 out from first. Second guy came across as much more expert and professional. He took about twice as long on the house walk through and did more listening than talking. He reviewed my load calc and confirmed that the assumptions were reasonable and results were consistent with other houses in my neighborhood (co had done previous work in my subdivision). Offered to do his own Man J, but did not see a need to.

    After discussing my objectives, and problems with the current HVAC system, he gave me a range of options and pros and cons of each. We would up going with a Carrier 2 ton 24ACC6 (sized for post addition load) and 3 ton, 21 in frame CNPVP03621 coil (to accommodate current furnace and air flow). I'll keep the existing furnace for now but add a damper on one of the supply branches to improve air distribution. Contractor is providing a satisfaction guarantee with no no caveats.

    Models selected are similar to recommendations from first contractor. As far as I can tell the first company would have been a few hundred $ cheaper for comparable system but second wins hands down on confidence and professionalism. Immediate result is a smaller job now but definitely the inside track as HVAC contractor for my addition. Install is sked for Monday. Contractor is xxxx and Sons. Rep turned out to be one of the sons, but I didn't realize until I saw card stapled to the contract. Encouraging.

    Question for the community: First company came highly recommended (good rank on Angie's, very positive referral from co-worker) but I was surprised and disappointed by their service and responsiveness. Probably would have had my business by presenting more options and answering a few questions. I don't know whether my experience was representative, or due to specific tech / circumstances. Should I provide feedback to the co, or just let it go?

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