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I have a forced air heating and air conditioning system that performs poorly on the second floor: It is way too warm in the summer and a little too cold in the winter.

I live in a 1920s masonry row house that has one vent going to each of the three upstairs bedrooms and four vents on the first floor. One of the first floor vents is on one of the three ducts leading upstairs.

There is a single 11"x13" return vent at the base of the stairs on the first floor. There is no return upstairs.

Thinking it was a insulation issue, I had a cellulose blown in the attic a few years back, as this post suggests. This definitely helped reduce the severity of the issue, but there is still room for improvement.

What remedy / remedies would provide the most benefit? Feasible remedies I can imagine are:

  • Install dampers / new registers on first floor to restrict airflow there
  • Add a second floor return duct (in front or middle bedroom)
  • Add another upstairs supply duct (in front or middle bedroom)

I have read conflicting reports about whether adding an upstairs supply or return would provide any benefit, and that closing off registers can cause issues.

Below is a quick and dirty diagram of my HVAC system. The blue rectangles are first floor vents branched off of the trunk in the basement, and the pink rectangles are second floor vents also branched off the trunk in the basement. The only exception is the overlapping pink and blue vent, where a single branch has a first floor and second floor vent. All of the vents are on or near the floor. First floor is open, stairs are open both upstairs and down, and another image showing the upstairs bedrooms is included.

Diagram of HVAC system Diagram of HVAC system upstairs

  • Perhaps find an ideal location for the thermostat which will provide the optimal function possible. In both situations, the thermostat being closer to the upstairs space should improve it a bit - cheap-cheap option. Where's the stat now? Do you have efficient windows up stairs? Is your duct work installed atop/above the blown in insulation? – ChiefTwoPencils Feb 23 '15 at 3:35
  • Thermostat is downstairs on the opposite wall of the staircase. I could certainly move it, but wouldn't that just lead to the upstairs being the desired temp and downstairs being overcooled/heated? The duct work doesn't go to the attic at all. – canzar Feb 23 '15 at 4:34
  • Not necessarily. I had a similar issue. We ended up doing a lot more for making the home more efficient overall but one of the cheapest and easiest things was to move the return and stat; they're poorly placed. A space surrounded by conditioned spaces have small loads on their own. The second floor is acting as a huge air gap between your roof and the down stairs. It can never run enough to cool/heat the upstairs space if it's being controlled by the better conditioned downstairs; it doesn't know it's hot/cold up there. Even with your other options you'd need a stat there or force it to run. – ChiefTwoPencils Feb 23 '15 at 7:27
  • Is the stairway open? And where are the walls and openings like doorways, arches, etc.? – wallyk Feb 23 '15 at 16:26
  • Downstairs is completely open, the stairs are open, and I added an overlay indicating where the upstairs bedrooms are. – canzar Feb 23 '15 at 17:40
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I would guess that modifying the duct work in a 1920s masonry construction home will be prohibitively destructive and involving removing or damaging irreplaceable trim that contributes to the building's character.

Mini split heat pumps:

An alternative, which will not be inexpensive but may be significantly less destructive, would be investigating installing a multi head mini split ac unit with one head in each bedroom. Such a unit costs perhaps $2500 to $5000 for the unit, installation extra and depends on the difficulty of install. If you live in a heating climate, especially one with cheap electricity in the winter or might install solar and have sunny winters, you should likely invest in the heat pump versions. Models are available that operate down to -13 F now (Mitsubishi M-Series) in single head. Supposedly multi-heads that operate that low are coming out this spring.

Advs:

Individual temperature control for each bedroom - Really high efficiency available for both heating and cooling - Avoid destruction of interior trim

Disads: - Potentially a lot of $$$ - Still a newer technology in the US (used all over Europe and Japan). If you plan to sell soon, recovering your investment at resale could be a challenge - Some people hate the look.

Other than minisplits:

Air leakage:

I'd suggest you have a blower door test performed. All the insulation in the world doesn't matter if you are changing over the air in the house too quickly. If you didn't air seal the attic before filling it with cellulose, you could have this problem. An infrared gun on a cold night can help show these leaks too. Insulation installers will say things like seals in air but cellulose, fiberglass insulation, and mineral wool insulation are actually very air permeable. If you find that is the problem, you could move insulation out of the way in the attic to use canned foam (get a commercial gun, much nicer job to do than with home depot cans). If you can't move the insulation, you could install a vapor permeable sheet membrane like Tyvek or a smart vapor retarder like IntelloPlus on top of the insulation to block air movement. Do not use a vapor impermeable barrier like black plastic or visqueen (clear plastic); you could trap moisture and cause a mold problem. Attic access hatches are also causes of large volume air leaks. There are special hatch covers you can buy or build to address this problem.

Duct work in unconditioned space:

Do you have any ducts in unconditioned (uninsulated) space like an open crawl space or an unconditioned attic? If so, you could be losing huge amounts of heat to the crawl space or attic. If your ducts are older, they likely aren't air tight either. You could use specialized foil tape or duct mastic to seal the joints and you could insulate this duct work, though it won't be fun work. If your ducts are really leaky, you could get big gains in system performance from this work.

Humidity control:

Have you measured your humidity when your system is running? If your humidity is high in the winter or summer, you will feel colder or hotter. Your humidity could be high in the house for many reasons (improper shower venting) but dirt crawl spaces are a big source in the summer (not in the winter so much). Seal off the dirt crawl space to eliminate humidity, mold spores, radon, etc. from getting in the house. A radon membrane company, in areas with radon, might be a cost effective contractor to use.

  • I hadn't thought of an air leakage test -- I'm not sure whether there was a membrane installed with the cellulose, but I don't think there was. I will check with and infrared thermometer. I don't have humidity issues, my ducts are all in conditioned spaces, and I don't think a mini split is a good fix (natural gas here is much cheaper, money for the install) – canzar Feb 23 '15 at 12:51
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    Infrared thermometer is probably a waste of time but if you have it, I guess it is your time. You need a blower door test and fluke IR gun pictures. Your local utility or city may offer these as part of a subsidized energy audit. – Keith Hoffman Feb 24 '15 at 0:31
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    Thank you for the idea: I did a poor man's blower door test with a couple window fans and an infrared thermometer, and found a couple of issues: a drafty window, my front door, a closet hatch to the attic, and the floor in the rear bedroom. Thanks again. – canzar Feb 24 '15 at 2:05
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    Wow, canzar. That's awesome. But note: if you found those with box fans, they were big leaks! – Keith Hoffman Feb 25 '15 at 4:42
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Without seeing the layout of the house, materials, etc., and its external environment—trees, exterior finishes, etc.—all I can do is speak in generalities.

Cooling

By providing adequate cooling upstairs, downstairs—especially with an open stairway—should need minimal targeted cooling. It might be sufficient to mostly close downstairs registers and fully open upstairs registers. An additional upstairs air return near the ceiling would better remove hot air, especially if the downstairs return is closed. Replacing dampers which don't have variable restricters is always a good idea, but just setting a book on each or taping some cardboard over those not needed would at least prototype whether a permanent register damper is worthwhile.

Heating

Setting the registers wide open downstairs and minimally open upstairs should achieve the opposite. 1920s architecture probably intended for the bedrooms to run a bit cool in the winter since those were primarily for sleeping—and while under heavy blankets—not casually reading or playing video games. To bring the bedrooms closer to modern expectations, it might be enough to replace the windows to upgrade them or just add winterproofing (a layer of plastic plus airspace).

General

If the airflow between floors is limited or there are areas closed off from participating, then additional ducting and vents might be called for. Or maybe keeping more doors open is enough. There ought to be a rational air movement direction in each room so that heated or cooled air is injected furthest from the air return and has one clear path to the return. "Ram-scoop"-style diverters on registers to move air in the most efficient direction to condition the majority of air in each room are inexpensive and highly effective.

From a triage perspective, anything which decreases the hvac system's airflow literally chokes its performance. Be sure that all air filters are clean and properly installed. Be sure that the plenum, all access doors, and all ducts are tight and not leaking air. Also check that there are not any crushed or mangled ducts and that critters have not built obstructions or died in anywhere in them. Inspect the blower itself for any maintenance-related inefficiency like a slipping fanbelt, loose pulley, or inadequate lubrication. If all is well, it might be worth upgrading the blower (maybe along with the furnace) for improved airflow.

  • It's not always a great idea to close off a significant number of registers. If your AC compressor and blower aren't undersized, you might actually make the problem worse. Most central ac compressors and blowers are oversized. – Keith Hoffman Feb 23 '15 at 8:26
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    @KeithHoffman: Agreed. I didn't mean to suggest register blockage as a permanent solution--only a potentially useful experiment to see if the situation improves. Presumably then, one could moderate the airflow with a variable damper. But we're flying blind here without details of the interior space, registers, heat sources, etc. – wallyk Feb 23 '15 at 8:30
  • So it sounds like restricting airflow to the first floor would provide the most cooling benefit and also be the least expensive option. Of the three ducts going upstairs, one has a vent in it downstairs on the wall. Should this one be permanently covered? I will add a diagram later today. – canzar Feb 23 '15 at 13:00
  • @KeithHoffman: Should the closing of vents be coupled with a decrease in the fan speed? – canzar Feb 23 '15 at 22:25
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    @canzar: If anything, the fan volume should be increased. The idea is to create more air movement through the areas needing the extra heat or cooling. – wallyk Feb 23 '15 at 22:43
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Sounds to me like you need the system balanced. A common issue in all homes new or old is that the thermostat satisfies before the upper floor is properly conditioned. Another issue is that normally the main-floor return air is located near or under the thermostat. I would recommend getting damper-able registers for the entire main floor, and especially the basement if you have not already.

Closing/dampering down the registers closest to the thermostat will make the biggest difference for your issue. Also if you find any rooms that are on the warm side you can damper down the register slightly until the desired amount of airflow is achieved. Every room of the home will experience different heat loss/heat gain so every room will require a different amount of air flow. Factors like distance from your furnace/air-handler and amount of windows in a room will also affect this.

Another thing you should make sure is to always close your basement registers for proper cooling in the summer time. The air conditioner is only sized for top 2 floors of your home. Additionally the cooling being supplied to the home will migrate to the basement anyways. If the registers are left open, you will not have proper cooling on your top floor. Some people opt to keep there basement registers closed all year round as the basement normally will not have a great amount of heat loss, and so they don't have to remember to open and close them between heating and cooling season. Also adding a upper floor return air will greatly increase air circulation upstairs. You should think about adding one in the future, however this is not the easiest of tasks.

Additionally, make sure you have a clean furnace filter in. Dirty filters will cause massive restriction and will greatly increase the static pressure of the system. 1" thick filters should be changed every. 1-3 months. Buying the best or tightest filter is not recommended as they create massive restrictions when using a 1" filter. If you have a 5" filter you should change it every 6 months. These thicker filters will cause less restriction however you can only use them if you have the appropriate filter rack.

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I have the same exact problem. My system is in a closet near my stairs, with several returns downstairs and no returns upstairs. I've been told that the biggest problem is that there are no upstairs returns. The system was designed to heat the house before AC systems were popular. After previous owners installed the AC they decided not to spend the extra money to redo the vents, which would require tearing up the walls.

This is an interesting read http://www.askthebuilder.com/air-conditioning-blues-return-air-problems/

My HVAC guy told me the best thing to do is to move my system to my attic, have everything re-wired and plumbed up there. They would then use the existing ducts that currently take the air upstairs and use them to send air downstairs. But they would cut new ceiling ducts upstairs that would run through the attic space directly to the unit (abandoning the old vents, which I guess would require some dry wall & paint work to cover up).

To do this, they would cut a hole in the ceiling at the top of my stairs to get the system into the attic, and then use that hole as a new air return. He quoted me $4500 to do that (this is after I paid them $7000 for a replacement HVAC system). Since that was a bit more than I was willing to dish out at the time, I'm trying to install a new air return myself to see if that alleviates my problem.

To do this I purchased a long flexible 6" duct and and 6" 240 CFM duct fan. The next time it gets hot, I will temporarily rig up the duct to pull air from the ceiling upstairs, run the other end of the duct to the opening of one of the downstairs air returns and connect the duct fan. The fan will pull the hot air downstairs and feed it back into the coils, replacing the hot air upstairs with cooler air from the vent.

Of course this is just a cheap ($40) way to test if a new air return will help. Both of those items are easy to find on amazon. If it does help, you can certainly hire someone to install it so it looks nice. Luckily, I'm able to fish the duct all the way from the air return box, through the floor and into the master bedroom. I should be able to cut a new hole into the air return box and feed the duct in. At that point I might not even need the powered duct fan.

If I still need the powered duct fan, they come with wires that you can hook to your HVAC system so that the fan only comes on when your blower is running, or when your AC is on. This is a hard test to be scientific about, because you need to try to judge for yourself if the room is cooling off any faster than without the return. You can certainly get a cheap thermometer and plot the temperature change over time, though you'll probably want to do that on two separate days to get a good idea how much it's helping.

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