how is HVAC formed, or the The Terrible Horrible No-Good Day
November 7, 2022 6:21 PM Subscribe
A member of the household has covid. He is isolated in his room. Can I close the windows in the rest of the house and depend on the HVAC system pulling in fresh air from outside, or will it be recirculating from all the rooms? It's cold.
It's really important, for logistical reasons, that the rest of us do not get covid, at least not this week.
On the theory that ventilation is good, I have the whole house's windows open. It's cold (but not dangerous cold, this is California. Just unpleasant cold.) I'd like to close them and turn the central heating on, if I can do that without increasing the risk that we'd be breathing recirculated covid exhalations from the sick room. Can I? How does ventilation work?
Also, if it matters: two of us already have colds and ordinarily I'd be wanting us to be as warm as possible.
It's really important, for logistical reasons, that the rest of us do not get covid, at least not this week.
On the theory that ventilation is good, I have the whole house's windows open. It's cold (but not dangerous cold, this is California. Just unpleasant cold.) I'd like to close them and turn the central heating on, if I can do that without increasing the risk that we'd be breathing recirculated covid exhalations from the sick room. Can I? How does ventilation work?
Also, if it matters: two of us already have colds and ordinarily I'd be wanting us to be as warm as possible.
I'm also in the cold part of California & I just have a couple space heaters because I can't afford my PG&E bill when I turn the central heating on :) so if you really don't want to risk it (which is highly advisable) that's an alternative that will also save you some money.
posted by bleep at 6:36 PM on November 7, 2022
posted by bleep at 6:36 PM on November 7, 2022
Best answer: Your HVAC system likely has air return registers in the immediate vicinity of the HVAC unit - for example, in my place in LA (and everywhere I lived in Texas) the furnace was in a utility closet at a T-junction in the hallway, and immediately underneath on two sides was a big air grate. That's where the system pulls in air. It does not use outside air, or the heater/AC would never be able to cycle off. The sucking in of this air does - weakly - suck in some air from outside, but we're talking a bit of a draft, not a massive air-exchanger unless you live in a special facility.
Most systems recirculate, for this reason. They recirculate through an air filter generally. Viruses aren't going to pass through those. Please turn your heater on and be comfortable, all of you. The stress of being uncomfortable is probably a net negative compared to the benefit of having everything open.
What has been recommended for most of the pandemic is to isolate a person in a room and keep the door closed. If you are very concerned you can have them crack a window, as the air coming into their room from the room vent will create a weak bit of pressure that will force air out. If you want to increase that pressure a bit, put down rolled-up towels on the outside of their closed door, like a draft-catcher. It's not like a hospital-grade negative pressure room, but it's likely a smidge helpful - and even hospitals aren't keeping COVID patients in negative pressure rooms in part because there aren't enough and they are generally reserved for people with zero immune system, like bone marrow/stem cell transplant patients.
The threat with ventilation/airflow is when there's a vent blowing air past me, with COVID, and you are at the next table getting my immediate exhalations/talk-spit/coughs blowing on you. This is not what's happening in your house. Like, maybe don't lay on the floor by their door and huff at it like a pet when someone is in the bathroom, but assuming they are not immediately on the other side of the door you will be fine. You may want to have them open the window and blow out the room with a fan before someone else opens the door to go in there, for a couple of minutes, and then both of you can put on masks. Or just don't go in there, and they can ventilate their room to the outdoors before opening the door to come out masked if necessary. If you have a portable air filter or know someone who would loan you one, you can either run it IN the patient's room or just outside their door (between the door and the furnace air intake) as an additional line of defense against escaping particles.
All the US airflow studies that showed aerosol transmission were in open rooms*, and back when we still reported any of this data there were plenty of cases of household transmission WHEN the first sick person wasn't behind closed doors. But I personally know lots of people who did precisely that as soon as someone felt off, especially pre-vaccines, and it did prevent household transmission.
*There were some Asian investigations into shared-hallway or shared-ventilation housing, early in the pandemic, but it was generally not filtered air.
posted by Lyn Never at 6:44 PM on November 7, 2022 [6 favorites]
Most systems recirculate, for this reason. They recirculate through an air filter generally. Viruses aren't going to pass through those. Please turn your heater on and be comfortable, all of you. The stress of being uncomfortable is probably a net negative compared to the benefit of having everything open.
What has been recommended for most of the pandemic is to isolate a person in a room and keep the door closed. If you are very concerned you can have them crack a window, as the air coming into their room from the room vent will create a weak bit of pressure that will force air out. If you want to increase that pressure a bit, put down rolled-up towels on the outside of their closed door, like a draft-catcher. It's not like a hospital-grade negative pressure room, but it's likely a smidge helpful - and even hospitals aren't keeping COVID patients in negative pressure rooms in part because there aren't enough and they are generally reserved for people with zero immune system, like bone marrow/stem cell transplant patients.
The threat with ventilation/airflow is when there's a vent blowing air past me, with COVID, and you are at the next table getting my immediate exhalations/talk-spit/coughs blowing on you. This is not what's happening in your house. Like, maybe don't lay on the floor by their door and huff at it like a pet when someone is in the bathroom, but assuming they are not immediately on the other side of the door you will be fine. You may want to have them open the window and blow out the room with a fan before someone else opens the door to go in there, for a couple of minutes, and then both of you can put on masks. Or just don't go in there, and they can ventilate their room to the outdoors before opening the door to come out masked if necessary. If you have a portable air filter or know someone who would loan you one, you can either run it IN the patient's room or just outside their door (between the door and the furnace air intake) as an additional line of defense against escaping particles.
All the US airflow studies that showed aerosol transmission were in open rooms*, and back when we still reported any of this data there were plenty of cases of household transmission WHEN the first sick person wasn't behind closed doors. But I personally know lots of people who did precisely that as soon as someone felt off, especially pre-vaccines, and it did prevent household transmission.
*There were some Asian investigations into shared-hallway or shared-ventilation housing, early in the pandemic, but it was generally not filtered air.
posted by Lyn Never at 6:44 PM on November 7, 2022 [6 favorites]
Residential systems typically recirculate, even in California, but there's not usually a supply and return duct going to every room, and they'restill taking in fresh air from somewhere, just not all fresh air. If your furnace is within your house somewhere, like in a hall closet kind of thing, there might be a big grille at the the bottom of the closet - that is the entirety of your return air system right there. The expectation is that the doors to rooms will not be airtight enough to keep the unit from pulling in air from those rooms, and interior doors are sometimes installed with a 1" undercut to allow circulation.
posted by LionIndex at 6:46 PM on November 7, 2022
posted by LionIndex at 6:46 PM on November 7, 2022
Lyn Never has it with the fresh air source as well.
posted by LionIndex at 6:47 PM on November 7, 2022
posted by LionIndex at 6:47 PM on November 7, 2022
A quick hack that you can do, and which may have helped keep me from getting sick two weeks ago in a similar situation: put a window fan in the isolation room blowing outward. It doesn't affect room temp too much but it does make a noticable current in the direction you want.
posted by dbx at 6:48 PM on November 7, 2022 [2 favorites]
posted by dbx at 6:48 PM on November 7, 2022 [2 favorites]
Best answer: Yeah, it's the air going OUT the window that is important, and that's why I suggest either only cracking it (you don't want to equalize the air pressure inside and out, and you don't want to bring fresh air in, you want the slightly higher air pressure inside from the heater running to make all the bad air go outside) OR, if you have a fan you can more or less put in the window, like a box fan or one of those fit-in-the-window fans, use it to blow OUT. You want it to exit the premises and not recirculate any more through the house than is incidentally going to come around the room door.
From my experience in Los Angeles, where it's generally very low-humidity when it's cold, opening the window to put a big fan in it will suck all the heat out of the room, and if the patient has the really sore throat of the latest variants, it'll freaking HURT to breathe that cold dry air. So I would recommend the cracked-window method in that case, and a humidifier in there for comfort if you've got one.
I hesitate to mention it, but you say the rest of y'all have colds. Right now the "days to positive home test" rate is running 4-6-days post-symptom-onset and sometimes 3-4 days even for a clear PCR result, so just because only one person in the household has tested positive does not mean the rest of you are in the clear, unfortunately. Keep testing.
posted by Lyn Never at 7:00 PM on November 7, 2022 [7 favorites]
From my experience in Los Angeles, where it's generally very low-humidity when it's cold, opening the window to put a big fan in it will suck all the heat out of the room, and if the patient has the really sore throat of the latest variants, it'll freaking HURT to breathe that cold dry air. So I would recommend the cracked-window method in that case, and a humidifier in there for comfort if you've got one.
I hesitate to mention it, but you say the rest of y'all have colds. Right now the "days to positive home test" rate is running 4-6-days post-symptom-onset and sometimes 3-4 days even for a clear PCR result, so just because only one person in the household has tested positive does not mean the rest of you are in the clear, unfortunately. Keep testing.
posted by Lyn Never at 7:00 PM on November 7, 2022 [7 favorites]
Best answer: No matter what you do, windows closed is going to be higher risk than windows open. But if you're careful, the absolute risk is not that high. Personally, I'd keep the windows at least a little open in California weather, but I have a very strong desire not to get covid (again). But if you're willing to take a little more risk, you can fairly safely do so.
You'll need to investigate the room in which the sick person is located. Does it have an air return for the furnace that sucks air out of the room? Does it have a hot air supply from the furnace that blows warm air in? Neither is good news (the air return because it will suck air with covid through the furnace and your furnace air filter is very unlikely to be good enough to remove all particles that might harbour covid, the hot air supply because it will pressurize the room and cause viral-particle-laden air to leak out under the door). You'll also want to check to make sure there are no vents through the wall into the rest of the house (they are often hidden in a closet).
The best option is to seal off any air vents in the room (a towel over them with something heavy on top will help with an imperfect seal if there are dampers), stick a towel against the bottom of the door, and give the isolated person a space heater to use (it would be a good idea for them to open their window at least some of the time to get some fresh air, since the air will quickly become stale and high in carbon dioxide in their room). Then you can relatively safely turn the furnace on for the rest of the house.
Another option, if you have space heaters, is to keep the windows open for the halls and common areas and keep your bedroom closed with a space heater running.
posted by ssg at 7:03 PM on November 7, 2022 [1 favorite]
You'll need to investigate the room in which the sick person is located. Does it have an air return for the furnace that sucks air out of the room? Does it have a hot air supply from the furnace that blows warm air in? Neither is good news (the air return because it will suck air with covid through the furnace and your furnace air filter is very unlikely to be good enough to remove all particles that might harbour covid, the hot air supply because it will pressurize the room and cause viral-particle-laden air to leak out under the door). You'll also want to check to make sure there are no vents through the wall into the rest of the house (they are often hidden in a closet).
The best option is to seal off any air vents in the room (a towel over them with something heavy on top will help with an imperfect seal if there are dampers), stick a towel against the bottom of the door, and give the isolated person a space heater to use (it would be a good idea for them to open their window at least some of the time to get some fresh air, since the air will quickly become stale and high in carbon dioxide in their room). Then you can relatively safely turn the furnace on for the rest of the house.
Another option, if you have space heaters, is to keep the windows open for the halls and common areas and keep your bedroom closed with a space heater running.
posted by ssg at 7:03 PM on November 7, 2022 [1 favorite]
And I also agree with Lyn Never here that constantly drawing air out the window is an option (and that you need to keep testing, I recently had covid and tested negative on days 3-5 from the first symptom).
posted by ssg at 7:05 PM on November 7, 2022
posted by ssg at 7:05 PM on November 7, 2022
First, being in separate rooms at all is a big help. You're pretty good just from that.
Then yeah if you can close any return in the covid room and block most of the space under its door, crack a window in there and the HVAC will push that room's air outside for you.
If you like, your HVAC system's filtration might also take a quick upgrade, but don't sweat it, getting the basic airflow is the bigger deal.
It likely has a replaceable filter. The one that's in there now is probably 'coarse', for dust and pollen. You might see it rated MERV 2, or MERV 5. I can't tell you what your system is compatible with, but my unremarkable residential furnace is fine with a MERV 11 running a lot during wildfire smoke, and it cleans out 1-micron particles according to my air sensor.
posted by away for regrooving at 9:08 PM on November 7, 2022 [1 favorite]
Then yeah if you can close any return in the covid room and block most of the space under its door, crack a window in there and the HVAC will push that room's air outside for you.
If you like, your HVAC system's filtration might also take a quick upgrade, but don't sweat it, getting the basic airflow is the bigger deal.
It likely has a replaceable filter. The one that's in there now is probably 'coarse', for dust and pollen. You might see it rated MERV 2, or MERV 5. I can't tell you what your system is compatible with, but my unremarkable residential furnace is fine with a MERV 11 running a lot during wildfire smoke, and it cleans out 1-micron particles according to my air sensor.
posted by away for regrooving at 9:08 PM on November 7, 2022 [1 favorite]
(Note: air filters come in different dimensions, ask me if I've ever gotten the wrong size and still have them sitting in the basement.)
posted by away for regrooving at 9:11 PM on November 7, 2022
posted by away for regrooving at 9:11 PM on November 7, 2022
An alternative or addition to opening windows is a Corsi-Rosenthal box. You want to aim for 12+ air changes per hour.
posted by Sophont at 10:10 AM on November 8, 2022
posted by Sophont at 10:10 AM on November 8, 2022
A MERV 13 air filter will capture COVID particles before they can be recirculated via HVAC. If your furnace manual allows it, upgrade to that (or the highest allowed). This would be in addition to having a corsi-rosenthal box or air purifier (as I assume your furnace is not on all the time). I would try and put an air purifier in the room of the person with covid to capture particles in that enclosed space. Then block off the door with a towel and crack a window in that room.
posted by oneirodynia at 3:45 PM on November 9, 2022
posted by oneirodynia at 3:45 PM on November 9, 2022
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posted by Winnie the Proust at 6:25 PM on November 7, 2022