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Message
single stage vs 2-stage AC experiment
Posted on 6/29/21 at 10:39 am
Posted on 6/29/21 at 10:39 am
So i have been getting bored and I always need to be tinkering with something. So i have decided to do some experimentation with my HVAC system (also because I absolutely hate my zoning controls, shoulda got Honeywell). My existing system is 6 years old and I'm starting to plan for my next system so I know what I want when it is time and I don't get rushed into something I don't want because it is an emergency.
I know I am going to replace all the zoning controls at some point and if I also go with a single stage AC next time around, I can save some money on controls AND on the AC. So I wanted to see how a single stage performs in my house so I can decide for sure whether a single stage is good enough for me or I just "need" a 2-stage system for humidity control and zoning comfort.
I have a 2-ton, 2-stage HP/AC and 3 zones in my house (2-story with finished basement but basement rarely ever needs cooling).
2-stage setup:
- 1st floor zone - 1st stage cooling (1.33 tons) at 525 CFM
- 2nd Floor Zone - 1st stage cooling @ 525CFM
- Both zones calling - 2nd stage cooling at 800CFM
This works well 95% of the time. Humidity was kept around 45% inside and varied between 42-46% with outdoor temps 75F+ and 60%+ outdoor humidity. The biggest problem was on the 100F days. I needed more than 1st stage to maintain temps in the 2nd zone but could only get 1st stage output.
Single Stage setup:
- 1st zone - 2tons cooling @ 530 CFM
- 2nd zone - 2tons cooling @ 530 CFM
- Both Zones - 2tons cooling @ 800 CFM
I wasn't sure what to expect with this system as obviously the 1st zone run times will get shorter with more tonnage going to it. I wasn't sure if this would impact humidity levels or not. Same for 2nd zone (upstairs). I knew during the day this will hold temperatures fine but I wasn't sure if shorter cycles at night would reduce humidity control. So far, using this single stage setup, my humidity is holding steady at 42-44% (slight improvement) and I have noticed no decreased comfort anywhere in the house other than the basement is slightly colder. We have been getting rain every day for a week or so with cooler temps in the 70s/80s. So this humidity control seems very satisfactory.
All in all, I think for my setup I could be perfectly happy and save a noticeable chunk of change on my next system if I go the single stage route instead of 2-stage.
Any comments/questions? I'm up for trying different things if anyone wants me to test something.
I know I am going to replace all the zoning controls at some point and if I also go with a single stage AC next time around, I can save some money on controls AND on the AC. So I wanted to see how a single stage performs in my house so I can decide for sure whether a single stage is good enough for me or I just "need" a 2-stage system for humidity control and zoning comfort.
I have a 2-ton, 2-stage HP/AC and 3 zones in my house (2-story with finished basement but basement rarely ever needs cooling).
2-stage setup:
- 1st floor zone - 1st stage cooling (1.33 tons) at 525 CFM
- 2nd Floor Zone - 1st stage cooling @ 525CFM
- Both zones calling - 2nd stage cooling at 800CFM
This works well 95% of the time. Humidity was kept around 45% inside and varied between 42-46% with outdoor temps 75F+ and 60%+ outdoor humidity. The biggest problem was on the 100F days. I needed more than 1st stage to maintain temps in the 2nd zone but could only get 1st stage output.
Single Stage setup:
- 1st zone - 2tons cooling @ 530 CFM
- 2nd zone - 2tons cooling @ 530 CFM
- Both Zones - 2tons cooling @ 800 CFM
I wasn't sure what to expect with this system as obviously the 1st zone run times will get shorter with more tonnage going to it. I wasn't sure if this would impact humidity levels or not. Same for 2nd zone (upstairs). I knew during the day this will hold temperatures fine but I wasn't sure if shorter cycles at night would reduce humidity control. So far, using this single stage setup, my humidity is holding steady at 42-44% (slight improvement) and I have noticed no decreased comfort anywhere in the house other than the basement is slightly colder. We have been getting rain every day for a week or so with cooler temps in the 70s/80s. So this humidity control seems very satisfactory.
All in all, I think for my setup I could be perfectly happy and save a noticeable chunk of change on my next system if I go the single stage route instead of 2-stage.
Any comments/questions? I'm up for trying different things if anyone wants me to test something.
3
Posted on 6/29/21 at 10:59 am to notsince98
I would post that to the residential section on HVAC-talk
Posted on 6/29/21 at 11:20 am to CORIMA
I might wander over there later.
Posted on 6/29/21 at 11:32 am to notsince98
your current ac system is 6 years old and you are already planning on a new system?
Posted on 6/29/21 at 12:18 pm to Hermit Crab
quote:
your current ac system is 6 years old and you are already planning on a new system?
Yup. It is such an expensive purchase that every time it rolls around I freak out trying to determine what is the best option for me and having no significant time to research and evaluate. My aspergers really cranks up my anxiety when I can't be sure I'm making a good choice on expensive transactions.
Posted on 6/29/21 at 12:58 pm to notsince98
quote:
My aspergers really cranks up my anxiety when I can't be sure I'm making a good choice on expensive transactions.
Must be my wife's problem too.
Posted on 6/29/21 at 2:55 pm to notsince98
Interesting results. I know you frequent HVAC-Talk, like me, for info to educate yourself, and a common theme for the techs that answer homeowner’s questions relative to multi-story homes and zoning, seem to center around this: 2 stories, 2 HVAC systems - 1 for each floor; and 2 stage or multi-stage systems are preferable when zoning is employed. I think the philosophy of the first, 2 stories, 2 systems is b/c it appears many HVAC techs don’t seem to do a good job with properly installing zoned systems.
Seems like you proved to yourself, at least in your home, your zoning is set up properly, and running in single stage mode is providing the comfort and humidity control you seek without the need for a two stage HVAC. The one quote I received 3 years ago on Trane systems, there was major jump in price going from single stage to two stage system, and not a significant jump in price between the two stage and VFD multistage systems. That surprised me as I thought two stage units would closer in cost to a single stage system. I wonder if that’s the case across manufacturers?
On a non-related topic I had a blower door and duct leakage test performed on my house today - 1993 built, rigid ducts. Duct leakage was very low which is great news, less 5%. Air infiltration into the house as per the blower test also yielded very favorable results, he said much better most new builds, but he was able to demonstrate to me with a smoke pen, while the blower was running and the house was under negative pressure obvious areas that could be sealed to reduce air filtration. Had this done in anticipation of addressing/reducing air filtration, plus adding additional attic insulation, prior to a HVAC changeout this fall/winter.
Seems like you proved to yourself, at least in your home, your zoning is set up properly, and running in single stage mode is providing the comfort and humidity control you seek without the need for a two stage HVAC. The one quote I received 3 years ago on Trane systems, there was major jump in price going from single stage to two stage system, and not a significant jump in price between the two stage and VFD multistage systems. That surprised me as I thought two stage units would closer in cost to a single stage system. I wonder if that’s the case across manufacturers?
On a non-related topic I had a blower door and duct leakage test performed on my house today - 1993 built, rigid ducts. Duct leakage was very low which is great news, less 5%. Air infiltration into the house as per the blower test also yielded very favorable results, he said much better most new builds, but he was able to demonstrate to me with a smoke pen, while the blower was running and the house was under negative pressure obvious areas that could be sealed to reduce air filtration. Had this done in anticipation of addressing/reducing air filtration, plus adding additional attic insulation, prior to a HVAC changeout this fall/winter.
This post was edited on 6/29/21 at 4:15 pm
Posted on 6/29/21 at 4:10 pm to CrawDude
quote:
On a non-related topic I had a blower door and duct leakage test performed on my house today - 1993 built, rigid ducts. Duct leakage was very low which great news, less 5%. Air infiltration into the house as per the blower test also yielded very favorable results, he said much better most new builds, but he was able to demonstrate to me with a smoke pen, while the blower was running and the house was under negative pressure obvious areas that could be sealed to reduce air filtration. Had this done in anticipation of addressing/reducing air filtration, plus adding additional attic insulation, prior to a HVAC changeout this fall/winter.
It is such a comforting feeling to know what you are dealing with in your house. Congrats on getting that done. You'll be saving significant $ on both your equipment costs and energy usage the more you learn.
Posted on 6/29/21 at 4:19 pm to CrawDude
quote:
Interesting results. I know you frequent HVAC-Talk, like me, for info to educate yourself, and a common theme for the techs that answer homeowner’s questions relative to multi-story homes and zoning, seem to center around this: 2 stories, 2 HVAC systems - 1 for each floor; and 2 stage or multi-stage systems are preferable when zoning is employed. I think the philosophy of the first, 2 stories, 2 systems is b/c it appears many HVAC techs don’t seem to do a good job with properly installing zoned systems.
I forgot to address this.
I'm a bit lucky in that my HVAC contractor for my new build oversizes their ductwork as standard design. My ductwork was sized for a 4-ton system but I upgraded my insulation so I only needed 2 tons. I'm in the unique situation where either the first or 2nd floor can handle a full 2-tons on its own.
Zoning can definitely be a pain. I hate the Lennox zoning system as it just doesn't work for my setup and there is nothing I can do about it other than replace it. I have spent a lot of timing learning about zoning and zoning controls. I like it it a lot. I'm not sure how 2 systems would even work for me. My cooling load for my first floor is like .5 ton. Installing a 1.5 ton unit would have made little sense. Then I would have ended up with a 2 ton system for upstairs. 3.5 total tons installed for a 1.7ton cooling load just doesn't add up.
there are definitely exceptions to everything. I know I sure would have preferred the simplicity of controls for 2 separate systems. T-stats are much simpler than t-stats combined with zoning controllers.
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