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View ScottKaye's profile

HVAC questions to the Pro's

by ScottKaye
posted 11-14-2018 05:57 PM

4 replies so far

View OSU55's profile


2498 posts in 2592 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.

View GR8HUNTER's profile


6800 posts in 1315 days

#2 posted 11-14-2018 10:45 PM

not a pro just like to say 1 thing compare apple to apple and not apple to lemon :<))

-- Tony---- Reinholds,Pa.------ REMEMBER TO ALWAYS HAVE FUN

View DrDirt's profile


4600 posts in 4345 days

#3 posted 11-14-2018 10:57 PM

You are definitely on track with the variable speed unit… For us it wasn’t about humidity, but circulation, where our unit will run the blower, w/o heating or cooling, to help keep the temperatures level in the house… especially downstairs would get cold w/o kicking on the heat, as the thermostat is upstairs in the bedroom hall.

Unit size – - look at how cold or Hot do you like it. those are as a difference between outside. If you and SWMBO, like to have the heat at 75 in the winter… stay with the larger unit. If you are frugal and keep to 68 or so, the smaller unit will work.

The Variable speed will prevent the need for cycling on and off with an oversize unit. But the old single speed, that was a big issue because the heat would kick on… Spike the temp, shut off, then equalize and get cold. Your High Efficiency new unit wont have that issue. We dealt with this on the AC side… and would err on having more capacity, then a small unit needing to run and run and run to keep up.

-- “The two most important days in your life are the day you are born and the day you find out why.” Mark Twain

View OSU55's profile


2498 posts in 2592 days

#4 posted 11-15-2018 01:13 PM

There ar t-stats available that turn on just the fan for an amount of time, say 10 min, every hour to prevent hot and cold spots. Also important how the heat stages are wired for a heat pump. Only 5kw is needed for makeup, then an additional 10-15kw available for 2nd stage and emergency. Mine is wired this way so that minimal resistance heat is used and the heat pump does the manority of the work plus prevents the big hot blast, and saves $. The 5kw is all that ever comes on even in single digit weather. Improper t-stat locations create lots of issues. An oversized variable speed unit = burning $ bills.

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