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I am currently upgrading my furnace from an oil furnace/AC combination to all heat pump. I am considering two stage vs single stage heat pumps.

I understand that two-stage will supposedly give me more 'comfort' (for running on low stage instead of one shorter single stage). However, one of the HVAC company representatives that came out to give me an estimate for a heat pump explained that he doesn't think a two stage is worth it for my house. Here is why;

I have a colonial style single family home (2 levels plus a finished basement, built in the 70s). The way the duct system is setup, the main level heats and cools fine, but the upstairs gets significantly less airflow (with bedroom doors open). Even with an oversized unit and blower, the air coming out of the vents is weak compared to the first floor. With the way my ducts are setup currently, he said that the second floor will not cool well in the summer if a two stage is running on the low stage. In essence, in order to get efficient cooling upstairs I'd need to be running at higher speeds. He was the only one that presented this opinion of several other people that came out.

Does this guy have a point, or will a two stage still give me better 'comfort' even with really low air flow on the second level?

The home is approximately 1600 square feet above ground, and then another 800 square feet for the finished basement. There is currently a 3.5 ton unit a several companies recommended a 3 ton unit.

Note: I am not asking for the ideal solution to my heating/cooling problems. I am asking of single stage and dual stage, which would do better cooling and heating the entire house more evenly in my situation.

  • A zoned damper system might be the right answer for you. Depending on how your ducts are already configured, this could be super-simple. – longneck Apr 11 '16 at 15:16
  • Unfortunately this is not an option because the duct work is split in many different places as opposed to a few central locations. – n00b Apr 11 '16 at 15:18
  • Depending on the house, it might be worth it to do multiple zones with multiple smaller units. You haven't told us much about the house, so it's difficult to contradict somebody who's actually surveyed the home. – Tester101 Apr 11 '16 at 16:12
  • If your ductwork is defective, your ductwork will be defective with any system attached to it. If correcting the ductwork is not in the plans, you should probably consider a different system entirely, such as mini-splits, that does not depend on the defective ductwork. – Ecnerwal Apr 11 '16 at 18:15
  • @Tester101 I added the square footage. Let me know if youd like me to add more. – n00b Apr 11 '16 at 18:25
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The answer will depend on how much temperature drop/rise there is inside the ducts. A two stage cooling system means that the heatpump will either drop the air return temp a little bit (stage 1 is running, half cooling) or it will drop the air temp a lot (stage two is running, full cooling). Either way the fan moves the same amount of air. In stage 1 mode the net effect will be for it to spend more time with the fan on, since to hold say 70F it will have to spend a while pushing 65F air through, vs a single stage system that puts out 60F air which will get back to setpoint faster. More air circulation (assuming its all a nice 65F) should actually be better for comfort in your restricted upstairs area vs spending a short time blowing 60F air until the downstairs is comfortable, leaving the upstairs without much time to get cooled air.

If the ducts are soaking a lot of the cooling up (i.e. the downstairs is getting 65F air but the upstairs is getting 68F air) then no, the first stage of a two stage unit will not give a lot of comfort and a system pushing colder overall air (i.e. single stage) all the time would be preferable.

In any case (if you get this system or not) most ducts, even in older homes, have some sort of internal damper to control flow to each run/room. These might not be accessible if you finished your basement and covered them all up, but that's for a different question about exposing louvers and patching sheetrock ;-). Further, your vents in each room should have a shutoff (not preferable vs in-duct damper, because it's noisy) which would allow you reduce the airflow to the downstairs rooms. This would be the solution if you find that running the system just doesn't get the upstairs rooms comfortable. Start by reducing flow to the one or two ducts closes to the thermostat, and continue until your system runs long enough to cool the upstairs without leaving any of the first floor rooms too frigid.

edit: grammar and clarification

  • thank you for your response. Did you mean to say "no the two stage will not give..." in your second paragraph? For the record our system does not have dampers as far as the HVAC rep and I saw, but thats a helpful reminder none the less – n00b Apr 14 '16 at 19:58
  • I did mean first stage and not using a single stage system, thanks. fixed. – Jeff Meden Apr 14 '16 at 20:00
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I agree with everything noted by Jeff.
Further to this, you should consider cutting in dampers to aid in your airflow distribution - this should be a relatively easy task. Start with the rooms where you are overcooling the most and work back from there.

I would note that dampers may not help with everything -you will struggle to balance the system if your ducts going upstairs are small or follow torturous routes, or if you have take-offs to the ground floor super close to furnace. Air takes the path of least resistance unfortunately!

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There is no doubt that a 2-stage unit will more evenly condition your space, for heating or cooling, that is what they are designed for. The full/partial compressor load capability, combined with variable fan speed commonly offered on 2-stage units, allows the unit to run for longer periods. That reduces temperature swings and would provide more even cooling of your space.

As a bonus, they are cheaper to run, eventually the additional cost will be offset by energy savings. read this

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