HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Questions on how we spend our money and our time - consumer goods and services, home and vehicle, leisure and recreational activities
rich126
Posts: 4475
Joined: Thu Mar 01, 2018 3:56 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by rich126 »

My lazy fairy cheap solution was to install a window unit in my bedroom upstairs since I like it cool and didn't want to freeze the downstairs nto get the entire upstairs cool. As a single guy that worked for me but for others that isn't optimal.
----------------------------- | If you think something is important and it doesn't involve the health of someone, think again. Life goes too fast, enjoy it and be nice.
User avatar
18_bank_accounts
Posts: 42
Joined: Fri Jun 02, 2017 11:37 am

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by 18_bank_accounts »

Similar situation. We installed new ductwork, a new furnace, a return upstairs, and a two zone system. It was not cheap at $18k. We also had to sacrifice a closet.

The two smart thermostats can keep both upstairs and downstairs at the exact same temp easily.
mary1492
Posts: 716
Joined: Thu Oct 17, 2019 3:02 am

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by mary1492 »

.....
Last edited by mary1492 on Thu Sep 29, 2022 2:31 pm, edited 1 time in total.
User avatar
bobcat2
Posts: 6076
Joined: Tue Feb 20, 2007 2:27 pm
Location: just barely Outside the Beltway

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by bobcat2 »

mary1492 wrote: Sat Jun 25, 2022 2:49 pm
rich126 wrote: Sat Jun 25, 2022 12:47 pm My lazy fairy cheap solution was to install a window unit in my bedroom upstairs since I like it cool and didn't want to freeze the downstairs nto get the entire upstairs cool. As a single guy that worked for me but for others that isn't optimal.
It's actually a very good solution. We have an empty nest and two zone A/C for a 3600 sf one story ranch home. We are very frugal with the central A/C as it spikes the electric bill.

Last summer I decided to purchase a small inexpensive window unit for our bedroom - it's really the only place we need it at night when it's unbearable and impossible to sleep. This took care of everything for us - barely any impact on the electric bill, and we're very comfortable at night. During the day, we have fans that are fine for keeping the air moving.
:thumbsup :thumbsup

We did this in April. One of the best $400 investments we have ever made. In our case it was a Windmill window AC and we couldn't be more pleased. Also, it doesn't look bad at all.

BobK
In finance risk is defined as uncertainty that is consequential (nontrivial). | The two main methods of dealing with financial risk are the matching of assets to goals & diversifying.
tortoise84
Posts: 464
Joined: Thu Nov 19, 2020 10:03 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by tortoise84 »

Yeah, I just use a portable AC (my HOA does not allow window ACs, but they are better than portables). I got this refurbished Toshiba dual hose inverter AC for $300. I originally got it just for emergencies in case my central AC broke down, but I was so impressed with its efficiency that I'm using it to spot cool upstairs rooms. On low speed mode it only draws 300W and is fairly quiet, yet it still puts out 53F air.
Ophiuchus
Posts: 80
Joined: Tue Jun 25, 2019 10:10 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by Ophiuchus »

The Critic wrote: Thu Jun 23, 2022 8:07 pm Long time reader, infrequent poster - but have seen some good HVAC advice given...

As the title states, I have a 2-story, 2300-sq ft house where the 2nd story is always 6-8 degrees warmer than the 1st story. We are in the market for a new HVAC system since the system is 25 years old (original) and has a refrigerant leak. We have received bids from several contractors and all but one have told me to live with the issue. However, if the situation can be improved (not necessarily perfected) for a modest cost, I'm interested.

Here is some additional background:

- All of the contractors said the ductwork going upstairs is a bit undersized and redoing the ductwork isn't something I'm interested in.
- One contractor suggested adding an air scoop to bring more flow upstairs and supplement it with a damper for the winter months. I proposed this idea to the other contractors and none of them were interested in it, not sure why?
- Airflow to the second story seems to be lacking. In particular, there are two upstairs rooms facing south that get very warm during the afternoons and are coincidentally, the two rooms where airflow seems to be most lacking.
- The entire house only has one return. It is a 20x25 size and located in the stairwell area.

The current system is a 4-ton, single-stage a/c and downflow furnace. My original thought was to get a 2-stage system since the longer runtime in Stage 1 may help with equalizing the temperature differences. However, given my existing airflow problems, could this make the situation worse? Some folks have warned me that the blower runs slower in Stage 1 and this will compound my airflow issues to the second floor.

Thank you.
Had the same thing out our old house. After weighing the options and cost, we ended up putting in the flow diverter. We could adjust like 70% upstairs or downstairs. It didn’t completely solve the problem. Went from 10 degrees difference to 3 degrees. This was enough to make me comfortable. Cost around 550 bucks
mrc
Posts: 1908
Joined: Sun Jan 10, 2016 5:39 am

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by mrc »

rgs92 wrote: Thu Jun 23, 2022 10:14 pm This is very hard to do. We had the same problem, and every HVAC person we asked said to put in an independent split-duct system in a desired area of the house that need separate temperature control.
This is what we did as well. Before we installed the split system in the master bedroom, it was awful. We ran the blower all the time, set the thermostat at 68F, froze the downstairs, and had summer electric bills to prove it. All to achieve 76-78F upstairs. After installation, we could run the blower on auto, set the thermostat to 75F which keeps the downstairs cool and dry. In the afternoon and overnight, we run the split which keeps the bedroom a comfortable (and dry) 70F. We use much less electricity now as well. The only drawback to this solution is that the other bedrooms (without the split) remain warm. But no one stays in them. If we had kids living in those rooms, we would have added a head to those rooms.
By the time you know enough to choose a good financial adviser, you don't need one. | bogleheads.org is my advisor: The ER is 0.0% and the advice always solid.
User avatar
stickman731
Posts: 418
Joined: Sun Jan 01, 2012 9:42 am
Location: New Jersey

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by stickman731 »

Had similar problem in my 2300 sq. ft. front to back split level in NJ. Installed an attic fan to control temperature in attic, installed ceiling fan in bedrooms; then closed all but one of the downstairs vents in the summer as cold air settles in the downstair family area. Problem solved. Temperature set at 72'F.

In winter open the downstair vents.

PS: I have a 15 x25' bonus room that I added a Fijutsu Heat Pump that was never tied into main system. If you do change / upgrade your system remember to check with your local utility to see if they have any rebate deals particularly on energy efficient models.
RandomPointer
Posts: 320
Joined: Sat Nov 23, 2013 12:38 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by RandomPointer »

I had a similar problem. One day, I fixed my attic. I sealed all the gaps in the attic. There are gaps that allows hot air to escape from the attic to the living space during summer, and let warm air to go into the attic during winter.

After I sealed the attic, and add a lot of insulation, the temperature difference between the first and second floor is 2F at most.

Searched for attic air sealing in YouTube, and add enough insulation. That might be a cheaper option in the long run.
Topic Author
KittenDad
Posts: 7
Joined: Mon Aug 10, 2015 8:26 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by KittenDad »

Some updates:

Had an insulation contractor stop by. We found the attic is already insulated to R-30 (8") and most of the gaps were already sealed. They offered to add additional insulation (to R-50 spec) for $2K, but I have a feeling that it is diminishing returns.

After further discussion with my wife, I think installing a ductless mini-split in my upstairs office is probably the easiest option for alleviating the upstairs cooling issue. 2 of the upstairs rooms are fine, it's my office and the room adjacent to it that are a problem...mostly my office. So, any recommendations for mini-split units? The contractor I am working with offers Mitsubishi and Fujitsu.

For the central system itself, I'm probably going with a single-stage 16 seer system with a 95% furnace. The contractor has offered to include an air scoop and manual damper in the quote plus the OEM's 10-yr extended labor warranty.
User avatar
tyrion
Posts: 1423
Joined: Fri Feb 08, 2008 2:33 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by tyrion »

I have a fairly similar setup -
2100 sq ft on 2 floors
4 ton central AC
Return air only at ground level on the first floor and insufficient ducting for cooling to the second floor (was originally a heat-only setup)
Upstairs rooms get pretty toasty by the late afternoon, including my office

I've tried running some fans to circulate air between the floors. It helps and keeps it within a few degrees difference between the floors until 2 or 3pm, then the disparity grows.
When I check the surface temp on the upstairs ceiling it's consistent and basically room temp. A bit warmer around fixtures, but no obvious holes in the insulation.

Will try a portable AC unit tomorrow (from our last house, currently in storage in the garage) to see if that helps enough with the upstairs, but I think a multizone mini split is probably the best answer.

Wondering whether it's worth it to just replace the existing 13 seer 4 ton central AC and reuse the concrete pad and 40amp circuit, or it would be better to add a mini split to supplement the existing central AC.

We also have a great whole house fan and use that almost all the time at night, although it would be nice to just cool the bedrooms with zone cooling as an alternative for hot nights.



OP, I hope you'll keep us updated with your choice and progress. And I would go with 2 or more head units if that second upstairs warm room is ever occupied.
CRC301
Posts: 193
Joined: Sat Feb 14, 2015 12:31 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by CRC301 »

I've been meaning to share my experience with a similar situation. It's documented fairly well over on hvac-talk forum: TL;DR; I went with a single medium-static concealed duct unit installed in the attic (Daikin FDMQ) and couldn't be happier. On paper its sized to handle the load of the second floor on its own at design conditions but we haven't tested that. The existing basement system is still connected to the same 2nd floor areas so the 2nd floor is getting cooling from both systems. The modulating minisplit works wonders here as it is able to modulate and adapt to the varying cooling requirements of the 2nd floor when the basement system kicks on based on the 1st floor thermostat.
The Critic wrote: Mon Jun 27, 2022 6:49 pm Some updates:

Had an insulation contractor stop by. We found the attic is already insulated to R-30 (8") and most of the gaps were already sealed. They offered to add additional insulation (to R-50 spec) for $2K, but I have a feeling that it is diminishing returns.

After further discussion with my wife, I think installing a ductless mini-split in my upstairs office is probably the easiest option for alleviating the upstairs cooling issue. 2 of the upstairs rooms are fine, it's my office and the room adjacent to it that are a problem...mostly my office. So, any recommendations for mini-split units? The contractor I am working with offers Mitsubishi and Fujitsu.

For the central system itself, I'm probably going with a single-stage 16 seer system with a 95% furnace. The contractor has offered to include an air scoop and manual damper in the quote plus the OEM's 10-yr extended labor warranty.
Prior to installing the above linked system for my second floor, I did a ton of research. Mitsubishi and Fujitsu are both great mini-split manufacturers (so is Daikin for that matter). As always, the installing contractor is more important, so if your trusted contractor carries both brands, pick the one with the lower price and/or better manufacturer/labor warranty. Also, it's a (maybe) big bonus if the contractor is "Mitsubishi Diamond" or "Fujitsu Elite"; the best warranties are handed to units installed by those "tier" contractors. Here is a GBA forum that discusses both of those brands' minisplits: https://www.greenbuildingadvisor.com/qu ... ing-output.

Not sure if you had read about them, but if the wall-hanging variety aren't aesthetically pleasing, they offer in-ceiling "cassette" models; they sit (mostly) flush with the ceiling. They blend in easier.

Did your contractor do a room-by-room heat load calculation? Any wall/ceiling-hung unit is likely to be oversized for non-master-bedrooms/offices. There's another option with minisplits: "concealed/slim ducted" indoor units. You can put them in wall cavities, closets, or attics and the single unit can serve more than one room via carefully designed ducts. A "medium-static-pressure" unit could probably serve all the rooms on your second floor (a proper heat-load calculation and duct-design is very important for this). Another thing worth mentioning is the multi-indoor-head to single outdoor unit models are really enticing but they are a tradeoff to one-to-one minisplits; a one-to-one system will always be more efficient and better at modulating to hit set point. Further, a single concealed/slim ducted unit serving multiple rooms with ducts may be the same and or less cost than a multiple wall/ceiling hung setup. Ask your contractor about it.
Topic Author
KittenDad
Posts: 7
Joined: Mon Aug 10, 2015 8:26 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by KittenDad »

Update:

Thank you again for all of the feedback. After reviewing all potential options, it became clear that short of re-doing the ductwork (with adequate returns) for a properly zoned system, this issue will never be 100% resolved.

We decided to replace the existing central HVAC system with a new single-stage Rheem RA16/R95T setup, add an air scoop/manual damper to boost flow to the second floor and install one Mitsubishi mini-split into my upstairs office.

My south-facing upstairs office has always been the warmest room and the mini-split should address this issue. The other rooms upstairs are not nearly as bad.

The work is getting done in the next few weeks and I will report back with the results. I think this should resolve the issue without needing to perform invasive ductwork repairs.
Topic Author
KittenDad
Posts: 7
Joined: Mon Aug 10, 2015 8:26 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by KittenDad »

CRC301 wrote: Mon Jun 27, 2022 11:33 pm Did your contractor do a room-by-room heat load calculation? Any wall/ceiling-hung unit is likely to be oversized for non-master-bedrooms/offices. There's another option with minisplits: "concealed/slim ducted" indoor units. You can put them in wall cavities, closets, or attics and the single unit can serve more than one room via carefully designed ducts. A "medium-static-pressure" unit could probably serve all the rooms on your second floor (a proper heat-load calculation and duct-design is very important for this). Another thing worth mentioning is the multi-indoor-head to single outdoor unit models are really enticing but they are a tradeoff to one-to-one minisplits; a one-to-one system will always be more efficient and better at modulating to hit set point. Further, a single concealed/slim ducted unit serving multiple rooms with ducts may be the same and or less cost than a multiple wall/ceiling hung setup. Ask your contractor about it.
They did not. Unfortunately, availability of the various mini-split options seems to be a challenge right now. I would have preferred the non-hung options, but they would have also been more expensive.

My contractor offered to install the single-room Mitsubishi MUZ/MSZ-JP09WA mini-split for $3k (parts and labor), which is an outstanding price considering that it'd cost me $1800-$2K to perform a DIY install of that unit. It is the smallest size unit they have and with the variable features, I'm hoping that the sizing issue will be less apparent.
Last edited by KittenDad on Wed Jul 06, 2022 12:34 am, edited 1 time in total.
CRC301
Posts: 193
Joined: Sat Feb 14, 2015 12:31 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by CRC301 »

KittenDad wrote: Tue Jul 05, 2022 5:55 pm
CRC301 wrote: Mon Jun 27, 2022 11:33 pm Did your contractor do a room-by-room heat load calculation? Any wall/ceiling-hung unit is likely to be oversized for non-master-bedrooms/offices. There's another option with minisplits: "concealed/slim ducted" indoor units. You can put them in wall cavities, closets, or attics and the single unit can serve more than one room via carefully designed ducts. A "medium-static-pressure" unit could probably serve all the rooms on your second floor (a proper heat-load calculation and duct-design is very important for this). Another thing worth mentioning is the multi-indoor-head to single outdoor unit models are really enticing but they are a tradeoff to one-to-one minisplits; a one-to-one system will always be more efficient and better at modulating to hit set point. Further, a single concealed/slim ducted unit serving multiple rooms with ducts may be the same and or less cost than a multiple wall/ceiling hung setup. Ask your contractor about it.
They did not. Unfortunately, availability of the various mini-split options seems to be a challenge right now. I would have preferred the non-hung options, but they would have also been more expensive.

My contractor offered to install the single-room Mitsubishi JP09 mini-split for $3k (parts and labor), which is an outstanding price considering that it'd cost me $1800-$2K to perform a DIY install of that unit. It is the smallest size unit they have and with the variable features, I'm hoping that the sizing issue will be less apparent.
That seems like a good price for that mini split install! :sharebeer Does it come with a good warranty (parts & labor)?

The main issues with oversizing is short-cycling and bad humidity control but I think your situation has some mitigating circumstances (see below).

You didn't mention, but I'm assuming you are going to leave the central air system connected (via duct) to the office even with the mini-split right? In that case, the humidity issue shouldn't be a large concern as your whole-house system should handle that (if it's sized appropriately).

Short-cycling is probably more of a concern; it's bad for wear & tear on the system components and can also lead to wide temperature swings in the area. Luckily, the mini-split you are installing is fully modulating (I think I read 3,800-10,000 BTUs). So my advice would be to leave it turned off in the shoulder seasons and let the whole-house system heat/cool that room. Once you start noticing the room not being heat/cooled properly, turn on the mini-split. At that point, it's most likely the time of year that the system will modulate and have longer run times (which is what you want).
Topic Author
KittenDad
Posts: 7
Joined: Mon Aug 10, 2015 8:26 pm

Re: HVAC Issue: Upstairs is significantly warmer. Will a 2-stage A/C make this worse?

Post by KittenDad »

CRC301 wrote: Tue Jul 05, 2022 9:07 pm That seems like a good price for that mini split install! :sharebeer Does it come with a good warranty (parts & labor)?

The main issues with oversizing is short-cycling and bad humidity control but I think your situation has some mitigating circumstances (see below).

You didn't mention, but I'm assuming you are going to leave the central air system connected (via duct) to the office even with the mini-split right? In that case, the humidity issue shouldn't be a large concern as your whole-house system should handle that (if it's sized appropriately).

Short-cycling is probably more of a concern; it's bad for wear & tear on the system components and can also lead to wide temperature swings in the area. Luckily, the mini-split you are installing is fully modulating (I think I read 3,800-10,000 BTUs). So my advice would be to leave it turned off in the shoulder seasons and let the whole-house system heat/cool that room. Once you start noticing the room not being heat/cooled properly, turn on the mini-split. At that point, it's most likely the time of year that the system will modulate and have longer run times (which is what you want).
The installing contractor is part of the Mitsubishi Diamond program, so 12 years for parts, but only 1 year for labor.

Yes, the central system will remain connected to the office. The mini-split is intended to "supplement" the central system, so I expect usage to be limited to the warmest days of the year.
Post Reply