My home is 3600 sq/ft, 2-story brick house in Houston, TX.
Currently I have 2 single stage 10-seer Trane units with a 100K BTU furnace with 4-ton blower in each: one for upstairs and one downstairs.The units have lasted a very long time. Both were installed when the house was built in 98 and are the old R-22 coolant types.

Over the last several years, I have had to have the units serviced each year to top off the coolant, or they may begin to freeze up in the summer (about 3 lbs. coolant each). The cost of the R-22 has really started to rise and recently the downstairs froze up within 2 months of topping off, so I have now decided to replace it.

In the process of getting quotes and researching my options, I have decided I should replace the entire system and not just the condenser and coil. In addition, after pricing them, I'm seriously considering the option to replace both systems at the same time. Replacing both reduced the cost per unit if doing them at the same time and also alleviates some concern over having to replace the second system in the near future, given how old they are.

Anyway, I ran across another option that was proposed to me that is very intriguing, but I have some reservations about it. The option proposed is: Instead of replacing the 2 existing systems with single-stage 14-seer, perform a conversion to one 2-stage, 16-seer system, that is zoned separately for the upstairs and downstairs with separate thermostats, etc.

Now as I mentioned before, I currently have 2-3.5 ton systems. From what I can determine (online), based on the 3600 sq/ft, I really only need a total of 6 tons. Furthermore, the current returns feeding the systems are not likely giving me the full capacity anyway. Currently the upstairs has a 12x12 and a 12x24. The downstairs has a single 20x30 return. Both seem to suck air and are (in my opinion) too noisy.

My concern is that the company believes I can go from the 2-3.5 ton systems into a single 5-ton dual stage, using zones to balance the upstairs and downstairs.

Based on the information I have given, does it sound feasible to reduce from 7 tons to 5 tons if the system is zoned and dual staged? Will the new system have enough tonnage to prevent it from running in 'high mode' a large percentage of time?

  • Yes it is feasible to go to a smaller system. You stated that you have excess capacity now. Going to a smaller unit 16 seer from 10 will also reduce your power bill. As you know R22 is being phased out and some shops have been charging +100$ per pound , a single compressor provides less starts and stops also. I would take a close look at what your demand #'s are now to be sure but I have seen higher efficiencies in the split systems than I expected. – Ed Beal Apr 7 '17 at 19:04
  • Thanks for the comment Ed. Yes, from my calculations It seems I would normally need 6 tons for the entire house. The company that proposed this solution is suggesting a 5 ton for the duel speed system. So, that's a difference of only 1 ton. I'm trying to determine if the efficiency of the dual speed with 16 seer efficiency rating, along with splitting into 2 zones will make up the difference of 1 ton because of its efficiency and therefore be sufficient for my needs. What I don't want to do is go that route and find that the 2 speed system is running in high mode 80%+ of the time. – pczeus Apr 7 '17 at 19:19
  • @pczeus -- are you thinking of a variable air volume zoning system, or a single compressor/condenser feeding two separate evaporators? I'd go for the latter over the former in a heartbeat if at all possible -- variable air volume zoning is always a compromise... – ThreePhaseEel Apr 8 '17 at 1:30
  • For what it's worth a 1 ton 10 seer unit has the same cooling capacity as a 1 ton 16 seer unit, the difference is the amount of power consumed. split systems are more efficient (multiple evaporators) partially because of the reduced ductwork with multi story homes the advantage is bigger in my opinion because of the ease in balancing the temperature with separate thermostats and air handlers. 5 ton in Texas may be a bit small on the days (over 100 F) for 3300sf. With a single evaporator but may be fine with split evaporators and air handlers the cooling is at different levels for each floor. – Ed Beal Apr 8 '17 at 2:00

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