Need some HVAC science help!

Hi guys! So my issue is my second floor upstairs is always hotter in summer and colder in winter than the main floor and basement. I know that's typical but there's always room for improvement right? The main issue is a poorly designed system because my furnace is in basement and any warm air has to get pushed (see photos), all the up from basement and main floor to attic and in attic there's a box that has all the upstairs flex ducts branching off of it. Registers are the round type in the ceilings and returns are at top of the walls, 1 in each room except for bathroom.
Back to the main problem, the air flow is so weak on that floor and none of the returns are drawing in. Can't even hold a single square of toilet paper to the vent! I did an experiment and went to the main floor that has strong flow, I partly covered half of a couple return vents to see if the upstairs returns would draw in any better and they DID! So obviously I have an imbalance in my system. Would leaving this operating this way increase the supply amount upstairs? The supply air is so weak! So if
Them upstairs returns actually functioned better would it increase air flow through the supplies?

Any adjustable butterfly dampers installed on any duct take offs?

Partially close all the supply registers on first floor.
Adjust as needed to boost supply air output upstairs.

Right now, the first floor is getting all the conditioned air, so the thermostat is shutting off too soon.

You can also remove the first floor return vents, and add a piece of cardboard behind them to partially choke them back, since they usually do not have a damper built in.

You can also get magnetic sheets that can be cut to size with scissors, to stick onto return vents to choke them back as needed.
Return vents are less critical on first floor during winter, when the cold air is just flowing down the stairs anyway.

Almost totally close any supply registers in the basement.
Adjust as needed, but usually they are barely needed.
You might need more heat near any exposed pipes near a cold window, etc. or in a basement bathroom.
Leave the basement returns fully open to maintain air turnover for humidity control year round.

You need to maintain a minimum supply and return air flow or you can damage your system (and/or greatly increase your energy consumption).

Preform a temperature rise check once your changes are completed to make sure you have enough air flow. Check your static pressure to make sure it is within spec once you are done also.

The changes I suggested will increase the energy consumption, because right now the thermostat is responding to the toasty main floor, and shutting off too soon to properly heat the upstairs.

Jeff Handy said:

Partially close all the supply registers on first floor.
Adjust as needed to boost supply air output upstairs.

Right now, the first floor is getting all the conditioned air, so the thermostat is shutting off too soon.

You can also remove the first floor return vents, and add a piece of cardboard behind them to partially choke them back, since they usually do not have a damper built in.

You can also get magnetic sheets that can be cut to size with scissors, to stick onto return vents to choke them back as needed.
Return vents are less critical on first floor during winter, when the cold air is just flowing down the stairs anyway.

Almost totally close any supply registers in the basement.
Adjust as needed, but usually they are barely needed.
You might need more heat near any exposed pipes near a cold window, etc. or in a basement bathroom.
Leave the basement returns fully open to maintain air turnover for humidity control year round.

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Yeah the basement is always most efficient temp because the handler is down there and the four ceiling registers are full open but here's the catch, everything with this house is a "catch 22" because if I adjust something it usually means it will affect something else negatively. For example, i realized after we moved in that in my fully finished basement, they didn't properly prepare the copper water lines nor did they do a very good job of insulating the rim joist area. With drywall ceilings there's nothing i can do about it now. My theory is that the area between the basement ceiling and subfloor has a lot of uninsulated supply ductwork in there and that's the only reason no pipes froze in this house after 19yrs. The basement needs to stay warm for this reason which is why I leave all registers completely open down there. Same thing with the main floor there's 3 sinks and 1 toilet that are located on exterior walls. Long story short, I'm really cautious when it comes to messing around with any warm air supplies during the winter months.

If you partially close the registers on the main floor, that will not endanger any pipes on that floor.

Because the furnace will now have to run a little bit longer, to maintain the same temp with less intense air flow.

If anything, you will get more even temps, less colds spells.

And the upstairs will get warmer.

Good advice, but choose to ignore, even though it will work.

Jeff Handy said:

If you partially close the registers on the main floor, that will not endanger any pipes on that floor.

Because the furnace will now have to run a little bit longer, to maintain the same temp with less intense air flow.

If anything, you will get more even temps, less colds spells.

And the upstairs will get warmer.

Good advice, but choose to ignore, even though it will work.

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I'll definitely try it I gotta get that upstairs warmer. With my luck in this house, I bet if I close off a bit them main floor registers the increase in warm air will be forced back to the already hot basement instead of going up the 30ft shaft to attic. haha hope not

Do you have a high efficiency furnace or an ECM blower motor?

pjones said:

Do you have a high efficiency furnace or an ECM blower motor?

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It's a HE furnace

If the basement does get hotter, then certainly you know you can then close the basement heat vents at least partially, until it returns to the temperature you remember from before.

You can definitely close or partially close the basement registers during a/c season.
You don’t want the basement super cold.
The ducts and pipes will sweat and cause mildew.

Run a dehumidifier in summer, let it drain into the sump pump or a floor drain.

But keep the basement returns open year round.
It gets stuffy down there, and humidity high or low can be dealt with by pulling plenty of air through the return vents.

Jeff Handy said:

If the basement does get hotter, then certainly you know you can then close the basement heat vents at least partially, until it returns to the temperature you remember from before.

You can definitely close or partially close the basement registers during a/c season.
You don’t want the basement super cold.
The ducts and pipes will sweat and cause mildew.

Run a dehumidifier in summer, let it drain into the sump pump or a floor drain.

But keep the basement returns open year round.
It gets stuffy down there, and humidity high or low can be dealt with by pulling plenty of air through the return vents.

Click to expand...

Yeah I see what u mean. In the summer I leave the basement registers full open because I use the trunk dampers near the handler to restrict flow to basement and main level so more cool air goes up to second floor. I did have a sweating issue on the ducts in the handler room but after putting a dehumidifier in that room that problem is gone. Also found out after installing the dehumidifier that the A-coil was leaking and the tech said it was likely sending cool air with heavy vapor content in the ducts so that's likely the reason I had duct condensation.
Anyway, as far as return vents in basement, unfortunately there are no returns in the basement. There's a total of 6 supplies but not 1 return for some reason

If you have separate dampers for trunks to basement and main level, you should have motorized zone controls installed.

You can have a thermostat on each level.

Or at least one zone for main level and one for upstairs.

There is usually enough slop opening around the furnace filter slot to count as a return vent.

And you can have your talented buddy cut another small one in the return duct.

But you probably have them somewhere.

Maybe some of your supplies are actually returns.

Jeff Handy said:

If you have separate dampers for trunks to basement and main level, you should have motorized zone controls installed.

You can have a thermostat on each level.

Or at least one zone for main level and one for upstairs.

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If your system is not designed for a zoned setup then don’t add the motorized dampers. Zoned systems need to have bypass dampers and often have a barometric damper setup to maintain a specific static pressure.

It sounds like either your ducts are undersized, you are restricting too much airflow, or your system is oversized.

What is the return air and supply air temperatures?

What is the average run time on a heating cycle, and a cooling cycle?

pjones said:

If your system is not designed for a zoned setup then don’t add the motorized dampers. Zoned systems need to have bypass dampers and often have a barometric damper setup to maintain a specific static pressure.

It sounds like either your ducts are undersized, you are restricting too much airflow, or your system is oversized.

What is the return air and supply air temperatures?

What is the average run time on a heating cycle, and a cooling cycle?

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Damn that's way over my head lol. How do I measure supply and return temps? I have a temp gun so dose this count if in the middle of a cycle shoot the supply and return trunks right at the unit? Run times
vary but it's usually 20min or so it'll kick in for about 10 min when it's 20-30 degrees out

Billbill84 said:

Damn that's way over my head lol. How do I measure supply and return temps? I have a temp gun so dose this count if in the middle of a cycle shoot the supply and return trunks right at the unit? Run times
vary but it's usually 20min or so it'll kick in for about 10 min when it's 20-30 degrees out

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Use a meat thermometer or something like that. The infrared thermometers are good to get a general idea what the temperature of something is but not accurate enough for this application. They don’t do so well when th shiny surfaces. ( if you painted a patch flat black then you may have good luck). Yes, in heating, measure the supply and subtract the return. That is your temp rise. Do not exceed the number noted on the manufactures data plate. If you exceed that number then you need to move more air (open more diffusers).

If the ductwork to the top floor is undersized then there is little you can do to improve that without replacing or adding ducts. Attempting to reduce the return air on the lower floor is not a bad idea but you need to keep an eye on both tour total external static pressure and your total CFM. Air out requires air temp in so don’t choke it off too much.

Some types of motors do better than others. If you have an EXM blower motor then they do not like to push high static pressures. It ramps up their power consumption and can cause them to burn out prematurely. You will pay highly for this. They are not cheep and can negate your energy savings for having a HE furnace to begin with. Using a high MERV rating filter can cause this also if the installer did not account for this when the system was installed.

ECM motors are constant torque motors (some others are constant CFM). To put it simple, If you throttle back the air then it will ramp up to move the same mass. This could lead to higher static pressures which leads to higher operating cost and higher wear and tear. If the motor can’t keep up then you can overheat the heat exchanger and cause damage to those components. This is why I’m saying proceed with caution. You probably won’t kill it over night, it may take a year or two, but you won’t be happy with the results if you make a mistake. Next time you have the tech come out let them know what you do and even show them to what extent you adjust things so they can have a closer look at the system next time they service it. Ask for a qualified senior tech to look at your equipment and let the dispatcher know what you want them to check so they can send the right person.

20 minute run time doesn’t sound so bad. If your AC didn’t run long enough then it won’t remove the humidity from your house properly and then when it does run you can get duct sweat forming. Lowering the airflow too much can cause similar issues in cooling if you don’t freeze the coil up first.


I’m not sure if what temperature scale you are using but 10 minute run cycle is getting to be on the short end of things. Not the end of the world for the furnace but it hinders the ability of the AC to do it’s job properly if it only gets to run for 10 minutes.

If your return air is over sized, and hopefully that is all that it is, then blocking off some return air sounds like a good solution. Again, just be careful with how far you go and be aware of the effect you are doing to the system when you make those changes.

pjones said:

Use a meat thermometer or something like that. The infrared thermometers are good to get a general idea what the temperature of something is but not accurate enough for this application. They don’t do so well when th shiny surfaces. ( if you painted a patch flat black then you may have good luck). Yes, in heating, measure the supply and subtract the return. That is your temp rise. Do not exceed the number noted on the manufactures data plate. If you exceed that number then you need to move more air (open more diffusers).

If the ductwork to the top floor is undersized then there is little you can do to improve that without replacing or adding ducts. Attempting to reduce the return air on the lower floor is not a bad idea but you need to keep an eye on both tour total external static pressure and your total CFM. Air out requires air temp in so don’t choke it off too much.

Some types of motors do better than others. If you have an EXM blower motor then they do not like to push high static pressures. It ramps up their power consumption and can cause them to burn out prematurely. You will pay highly for this. They are not cheep and can negate your energy savings for having a HE furnace to begin with. Using a high MERV rating filter can cause this also if the installer did not account for this when the system was installed.

ECM motors are constant torque motors (some others are constant CFM). To put it simple, If you throttle back the air then it will ramp up to move the same mass. This could lead to higher static pressures which leads to higher operating cost and higher wear and tear. If the motor can’t keep up then you can overheat the heat exchanger and cause damage to those components. This is why I’m saying proceed with caution. You probably won’t kill it over night, it may take a year or two, but you won’t be happy with the results if you make a mistake. Next time you have the tech come out let them know what you do and even show them to what extent you adjust things so they can have a closer look at the system next time they service it. Ask for a qualified senior tech to look at your equipment and let the dispatcher know what you want them to check so they can send the right person.

20 minute run time doesn’t sound so bad. If your AC didn’t run long enough then it won’t remove the humidity from your house properly and then when it does run you can get duct sweat forming. Lowering the airflow too much can cause similar issues in cooling if you don’t freeze the coil up first.


I’m not sure if what temperature scale you are using but 10 minute run cycle is getting to be on the short end of things. Not the end of the world for the furnace but it hinders the ability of the AC to do it’s job properly if it only gets to run for 10 minutes.

If your return air is over sized, and hopefully that is all that it is, then blocking off some return air sounds like a good solution. Again, just be careful with how far you go and be aware of the effect you are doing to the system when you make those changes.

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I like you're "proceed with caution" approach and thank you for the valuable info. This science is starting to sink in. I proceed with caution not only to avoid damaging my unit but also because this deceitfully, big, beautiful house is a like monster that hates me and I think most of the deficiencies I find are actually helping to avoid other issues so if i over-solve house issues it opens new doors for a new issue. This HVAC for example, i discovered they didn't insulate the rim joist before drywalling and fully finishing the basement and there's 5 plumbing fixtures on the exterior walls around the main floor. Copper lines running along that rim all over down there so how have these pipes never froze after 19 years?!? I think it's due to the fact that the basement is hottest spot in winter and the ducts ran in basement ceiling/subfloor are heating the cavity just enough. Now I could rob some of this hot air to send upstairs but remember when I said solving one issue opens another? I think i may have issues if I start to rob that warm dry basement air.
I know that when I close off the basement and main floor trunk dampers completely I get stronger flow upstairs. Do you think it will help if I tie in to that supply trunk on second floor and install some floor registers up there like Jeff suggested? Seems like a good plan. This will eliminate about 15-17ft of ductwork as it no longer would have to run up to the attic/ceilings. I would leave the returns alone up there. Any basic drawbacks to a something like this?

I did not suggest adding floor registers upstairs.

I said to try to route a heat duct to the cold bathroom pipes enclosure.

And that you might be able to get there from the trunk going upstairs, with a route under the floor.
Instead of trying to pull it down from the attic.

Jeff Handy said:

I did not suggest adding floor registers upstairs.

I said to try to route a heat duct to the cold bathroom pipes enclosure.

And that you might be able to get there from the trunk going upstairs, with a route under the floor.
Instead of trying to pull it down from the attic.

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Right on buddy! This concept could be the key! Such a bad design right now to bring air up so high from the basement. It's sad how many contractors are out there that will do anything to call a job "finished" and get paid