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.
Best Answer
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