Reply by OSU55

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Posted on HVAC questions to the Pro's

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2494 posts in 2590 days

#1 posted 11-14-2018 09:24 PM

I’ve worked for the largest global scroll compressor mfr (commercial, industrial, residential AC & heat pump systems) for about 30 yrs, in product design, reliability, mfg, and quality. Copeland is the brand name.

Bit hard to tell from your description, but it sounds like you have an upstairs heat pump with some type of variable speed for AC dehumidification, and a downstairs AC unit with a propane gas furnace. This is a bit of a strange combination. It is unclear if you are only replacing the gas furnace or the entire indoor unit or what, so I will speak with some generalities.

It would help to know your location closer than “mid-atlantic”, but most likely you are in ASHRAE climate zone 4A, which is great for heat pumps (could depend whether you are several 1000 ft in elevation). From an operating $ perspective, a heat pump typically is far better than propane, but you would need to do the calculation based on your electric vs propane $. Natural gas is usually a different story – depends on $. Then it depends on upfront $, and I can’t decipher what the choices are.

There isn’t much if any benefit to a two speed/capacity gas furnace.

Many of our system mfr’s use our 2-step or 2-stage scroll in their systems, and there are var speed systems out there (many different flavors). My company would scream blasphemy, but I don’t recommend var speed to anyone – the payback period is measured in decades. The only time I recommend the 2-stage system is in a humid climate, and for comfort only, not really operating $ savings. For the added upfront $, the 2-step systems aren’t cost effective, but they do help comfort a bit more vs lowering the setpoint a degree or 2. FYI, I don’t recommend programmable thermostats either. Jacking your temp setting up and down daily doesn’t save any $ – I’ve done the calculations.

Without knowing the house layout and ducting, and how well the two systems are separated, I have no idea how much the upstairs system would help whole house dehumidification. The more the 2 systems mix air the more it will help. Typically a 2-stage system would be used on the lower floor and not needed for the upper floor unless the 2 systems were very isolated, and possibly not then. Heat rises, so the upstairs system will usually run more and dehumidify better. It is also possible your systems are oversized – a common problem. System installers usually don’t get called for an oversized system, but they do for an undersized one, so they typically go a little extra, which the customer pays for. For a design day, i.e. outside design temperature, the unit should not cycle – it should run all day. This maximizes dehumidification and minimizes starts (electrically expensive) and a smaller unit uses less electricity when running. Of course that does not solve humidity issues on cooler cloudy or rainy days.

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