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.
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
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)
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
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.