Discussions

Hmm. I see two potential problems here. The first is that if you theoretically make ice using the refrigerator in your home then the net effect will be to heat the building. Refrigeration systems work by transferring heat from one place to another. Your refrigerator transfer heat from inside the ice box to the air surrounding the refrigerator. Due to system losses, the net effect of refrigeration systems is to generate heat. So if you make ice in your home, then transfer that ice to somewhere else in your home, use a fan to blow across that ice (the fan generates a little more heat), the net effect will be to make the entire home a bit warmer.  The second problem is that even if you source ice elsewhere, or think of it as sort of local to the room, it’s likely to be a very small amount of cooling. Conventional cooling systems are measured in tons, with each ton being 12,000 btus of cooling capacity per hour. This is the amount of heat that it takes to melt 2000 lbs of ice over the course of 24 hours. This is just a massive amount of cooling capacity. Most residential systems start at 1.5 tons of cooling, and are often in the 3-5 ton range. Each pound of ice in the cooler will probably be worth ~ 200 btus of cooling (assuming you start at 10F, it takes .5 btus to raise the ice temp 1F, 144 btus to melt the ice, and 1 btu per degree temp increase of liquid water). Divide this by an 8 hour night, and you’ve got 25 btus/hour per pound of ice. At 10 lbs of ice per night you’ll end up with 250 btus/hr. The smallest window ac units are in the 5000 btu/hr range.

Assuming moderate occupancy and little to no ventilation, there’s basically no chance that you’ll be in a room that would be considered safe. I suppose the great thing about having a CO2 monitor is that you can demonstrate this to people in charge of the building, but it’s not really an experiment that’s worth running. Can you get room unit specifications? It’s really worthwhile to pursue these.  <boring anecdote alert> I was in a school last week that had room heaters that essentially blow airflow across a heating coil. However, the units had openings to the outside so some fraction of outdoor air was being drawn into the room. I pulled specs on the unit, which had max airflow of 1000 cfm. However, I didn’t have time to measure airflow from the outdoors, and the unit specs don’t appear to have them. My guess is that under current operation the unit might pull 20% outdoor air, which would be 200 cfm – better than nothing but insufficient for the room demand (~400 cfm ventilation). My point is that many people are in the dark about the mechanical devices they’re surrounded by every day. In contemporary schools with ducted ventilation systems it’s likely that most of these systems will be central, so it’s also important to get specifications on these. Pulling specs will allow teachers to quickly determine if the system could conceivably pull enough ventilation air to be safe. However, it doesn’t absolve us of the responsibility to verify proper ventilation rates to every room.  Specific to school communications – I’m seeing a lot of statements designed to reassure teachers that have very little real substance. For instance, districts might say something like, “we’ve increased our ventilation system to 100%.” The claim is probably rooted  in the fact that some ventilation systems will fix airflow as a constant but allow users to program some fraction of air to recycle through filters to avoid overwhelming improperly designed heating and cooling equipment. But what quantity of airflow = 100% in this context? What if the 100% of the system capacity is 1/4 of the flow rate required to keep occupants safe?  TL:DR takeaway: Get equipment specs Verify safety on a room-by-room basis

Really good question! This is very difficult to answer because of unknowns. Basically, the vast majority of schools are either unventilated altogether or under-ventilated. So assuming the number of occupants in the classroom is the same, it’s likely that the larger classroom would have a slight advantage over the smaller one.  However, it’s worth contextualizing this a bit better. The bottom end of ventilation that I mention in the video is 3 ach. For most classrooms this will mean at least 250 cubic feet of air per minute (cfm), and many classrooms will need a lot more. It’s just an insanely high ventilation rate relative to what most classrooms are currently experiencing. The protection afforded by room size is really low, and larger rooms will tend to have more occupants which will offset this protection. The focus here needs to be on getting the ventilation airflow right. I’m working on another video outlining how to cheaply verify ventilation for laypeople. The cliff’s notes are that people should stock up on CO2 monitors. Basically the best way to tell if a space is underventilated is to use CO2 thresholds. Normal outdoor levels of CO2 are in the range of 400 ppm. Indoor environments in the range of 800-1000 ppm are considered underventilated. For classrooms in the time of covid I would use the lower threshold and make 800 ppm the cut-off where either people leave the space or increase the ventilation rate. As with other supplies during covid, I’m really shocked that as of last week one could still readily purchase reliable co2 monitors. 

School Ventilation
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School Ventilation
10
15

Hmm. I see two potential problems here. The first is that if you theoretically make ice using the refrigerator in your home then the net effect will be to heat the building. Refrigeration systems work by transferring heat from one place to another. Your refrigerator transfer heat from inside the ice box to the air surrounding the refrigerator. Due to system losses, the net effect of refrigeration systems is to generate heat. So if you make ice in your home, then transfer that ice to somewhere else in your home, use a fan to blow across that ice (the fan generates a little more heat), the net effect will be to make the entire home a bit warmer.  The second problem is that even if you source ice elsewhere, or think of it as sort of local to the room, it’s likely to be a very small amount of cooling. Conventional cooling systems are measured in tons, with each ton being 12,000 btus of cooling capacity per hour. This is the amount of heat that it takes to melt 2000 lbs of ice over the course of 24 hours. This is just a massive amount of cooling capacity. Most residential systems start at 1.5 tons of cooling, and are often in the 3-5 ton range. Each pound of ice in the cooler will probably be worth ~ 200 btus of cooling (assuming you start at 10F, it takes .5 btus to raise the ice temp 1F, 144 btus to melt the ice, and 1 btu per degree temp increase of liquid water). Divide this by an 8 hour night, and you’ve got 25 btus/hour per pound of ice. At 10 lbs of ice per night you’ll end up with 250 btus/hr. The smallest window ac units are in the 5000 btu/hr range.

Assuming moderate occupancy and little to no ventilation, there’s basically no chance that you’ll be in a room that would be considered safe. I suppose the great thing about having a CO2 monitor is that you can demonstrate this to people in charge of the building, but it’s not really an experiment that’s worth running. Can you get room unit specifications? It’s really worthwhile to pursue these.  <boring anecdote alert> I was in a school last week that had room heaters that essentially blow airflow across a heating coil. However, the units had openings to the outside so some fraction of outdoor air was being drawn into the room. I pulled specs on the unit, which had max airflow of 1000 cfm. However, I didn’t have time to measure airflow from the outdoors, and the unit specs don’t appear to have them. My guess is that under current operation the unit might pull 20% outdoor air, which would be 200 cfm – better than nothing but insufficient for the room demand (~400 cfm ventilation). My point is that many people are in the dark about the mechanical devices they’re surrounded by every day. In contemporary schools with ducted ventilation systems it’s likely that most of these systems will be central, so it’s also important to get specifications on these. Pulling specs will allow teachers to quickly determine if the system could conceivably pull enough ventilation air to be safe. However, it doesn’t absolve us of the responsibility to verify proper ventilation rates to every room.  Specific to school communications – I’m seeing a lot of statements designed to reassure teachers that have very little real substance. For instance, districts might say something like, “we’ve increased our ventilation system to 100%.” The claim is probably rooted  in the fact that some ventilation systems will fix airflow as a constant but allow users to program some fraction of air to recycle through filters to avoid overwhelming improperly designed heating and cooling equipment. But what quantity of airflow = 100% in this context? What if the 100% of the system capacity is 1/4 of the flow rate required to keep occupants safe?  TL:DR takeaway: Get equipment specs Verify safety on a room-by-room basis

Really good question! This is very difficult to answer because of unknowns. Basically, the vast majority of schools are either unventilated altogether or under-ventilated. So assuming the number of occupants in the classroom is the same, it’s likely that the larger classroom would have a slight advantage over the smaller one.  However, it’s worth contextualizing this a bit better. The bottom end of ventilation that I mention in the video is 3 ach. For most classrooms this will mean at least 250 cubic feet of air per minute (cfm), and many classrooms will need a lot more. It’s just an insanely high ventilation rate relative to what most classrooms are currently experiencing. The protection afforded by room size is really low, and larger rooms will tend to have more occupants which will offset this protection. The focus here needs to be on getting the ventilation airflow right. I’m working on another video outlining how to cheaply verify ventilation for laypeople. The cliff’s notes are that people should stock up on CO2 monitors. Basically the best way to tell if a space is underventilated is to use CO2 thresholds. Normal outdoor levels of CO2 are in the range of 400 ppm. Indoor environments in the range of 800-1000 ppm are considered underventilated. For classrooms in the time of covid I would use the lower threshold and make 800 ppm the cut-off where either people leave the space or increase the ventilation rate. As with other supplies during covid, I’m really shocked that as of last week one could still readily purchase reliable co2 monitors. 


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