I live in Seattle where AC is only needed for a handful of days each year. A friend has a 20 year old gas furnace needing replacement, and he would like to update his system, including adding AC. A local company bid a system that includes a 5 ton Bosch (generation 2) variable speed heat pump, a new 80% single-stage gas furnace and an A coil. Am I right to be concerned that while the heat pump is variable speed, the furnace has a single speed blower? My primary concern is that such an approach may result in humidity problems. That said, given that humidity is really only a concern when running the AC (I think), and given the low number of days that AC is needed here, should we be concerned about this approach? - Thanks in advance! - Jim

I should add that he is being quoted a total cost of $15000. He also got a bid for a matched system, by which I mean a variable speed compressor paired with a ducted variable speed indoor air handler, but the costs for the latter system is over $20000 and a bit out of his budget comfort zone.

  • do not add info in comments ... please edit your question instead ... then delete the comment
    – jsotola
    Commented Apr 27, 2023 at 18:57
  • If it were me, I'd go with a variable speed blower, but that would just be my personal preference to give more even heating/cooling. Is there a reason that your friend isn't planning on using the heat pump to provide heat as well? I'm guessing that in the relatively mild Seattle area that you could probably get by with only the heat pump for both heating and cooling and forego the furnace other than for a blower.
    – Milwrdfan
    Commented Apr 27, 2023 at 19:19
  • Yes, the heat pump only bids have been over $20000, more than his budget allows.. This surprises me given that an indoor air handler seems far less complicated than a gas furnace. After all the indoor air handler is just a fan and a coil.
    – Jim Luby
    Commented Apr 27, 2023 at 19:24
  • There's more to be concerned about with this setup than just humidity. Does the single-speed 80% furnace have the smarts to regulate the output of the heat pump? Or will it, as I suspect, simply call for either max output or nothing from the heat pump? If it can't modulate heating or cooling from the heat pump, a variable speed compressor that will only run at full speed is a waste of money and inefficient.
    – MTA
    Commented Apr 27, 2023 at 20:10
  • 1
    @Tiger Guy : I concur and will discuss this matter with my friend. - Thanks - Jim
    – Jim Luby
    Commented Apr 27, 2023 at 20:44

1 Answer 1


Let's start with what information you need to choose AC system

Zone: what climate zone do you live in ? I show Seattle to be in zone 3.

Lets assume your home is 3,000 sqft

For that a 5 Ton AC is a good choice

Next: what type HVAC? Super or normal. That is determined by the SEER rating and other gadgets like variable speed compressor. The cost difference in SEER rating is substantial. But higher SEER will save you on electricity so they say. Really ? how much. Here you go. The cost difference btw SEER 15 and SEER 20 is about $5,000. Your electricity saving between Seer 15 and SEER 20 is about $200* per Year ??? How many Years it takes to recover the higher cost ?

Just for clarity the SEER 15 will cool or heat your home at same levels as SEER 20, so it is just the electricity consumption.

Humidity: Seattle is in humid to very humid area. The AC will ONLY remove humidity when the cooling is running. So if you want more constant humidity removal instal a dehumidifier that can run all the time independent of the AC.

Variable speed Fan in the heat exchanger cabinet. It is nice to have but not must have.

To summarize: I recommend 5Ton, SEER 15/16, no variable speeds on anything, it is more simple unit and easier to repair.

  • Thank you for your valuable input. Almost all sites I’ve visited show Seattle being in zone 4. Also, I just received the house square footage which is 2500 sq feet. Does this information change your suggesttion for a 5 ton AC? - Thanks again!
    – Jim Luby
    Commented Apr 27, 2023 at 22:21
  • @JimLuby Recommended AC Unit Size: 4 - 4.5 Ton Recommended Furnace Size: 113,000 BTU
    – Traveler
    Commented Apr 27, 2023 at 22:30
  • Thanks very much!
    – Jim Luby
    Commented Apr 27, 2023 at 22:31

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