Heating semi-enclosed outdoor spaces - Covid edition

December 14, 2021 12:52 PM

I'm looking at how to comfortably entertain guests outside during New England winter. I want to understand heating options and air flow in terms of min/maxing the right combination of - Retaining heat - Circulation to reduce Covid exposure - Venting CO2 and other gas byproducts Can it be done?

There are some good answers on here for some parts of the question, but I'm looking for specifics. Here's where I'm at:

There are three spaces I'm considering for small outdoor gatherings.
- Porch, 70 SF, roof cover, two full wall sides, two half (open) wall sides
- Garage (1car), 200 SF, roof cover, front door open, so 3 sides closed but with window on each + side door
- Tent, 100 SF, roof cover, can flap open/close as much as wanted

The principal question is how to balance heat efficiency with ventilation safety (eg if using propane heaters) and making sure there's enough circulation so that people can be there for at least two hours and not be recirculating Covid air?

Sub questions:
- This weekend there's going to be rain, so consider that solutions may be constrained by not only the tripart above, but also by needing to use the roof cover provided. Also, is it safe to run a propane heater in the rain (assume covered, raised off ground)?
- Besides electric space heaters, propane fan heaters and hooded 'stand' heaters, what are some good options?
- Can someone explain how air circulation really works re Covid? Like I get how HVAC in an office/restaurant recirculates the bad stuff so that people get sick. I understand that being outdoors helps with risk being 5/10/20x? safer than indoors. But what about half covered spaces? What factors drive the right kind of safety benefits from air circulation?
- Is there anyway to keep heat while effectively venting CO2 and Covid and do CO2 and Covid have different venting needs?


Thanks in advance

posted by Reasonably Everything Happens to Home & Garden (14 answers total)

Best answer: There is a lot here:

1. There is no binary safe/unsafe relative to air circulation and the potential for COVID transmission. The principal determinant is whether anyone in attendance is COVID positive and at a stage where they are producing a significant amount of airborne virus. This is further ameliorated if they consistently wear a mask. So, the question is based on a presumption that there is potential for an amount of COVID exposure which would be of unacceptable risk in an indoor environment for this duration.
2. As an architect keeping an eye on these things, I have seen no direct research quantifying relative levels of additional risk mitigation for increased airflow / outdoor spaces. It is safe to say that any of the scenarios described would be "greatly reduced" risk. An additional factor is how close people are to each other and for what duration in these situations.
3. Good ventilation is absolutely required for any sort of combustion heater. Additionally, ensure that all heaters, especially combustion based, are well away from nearby surfaces with any potential to burn or melt. Depending on geometry, the notion of a propane heater in a garage or on a covered porch makes me nervous. Under a pop-up tent makes me especially nervous. A propane heater under a large (e.g. wedding reception) tent where the tent material is six plus feet above the heat source is far more comfortable.
4. Comfort / temperature is based on a several factors including air temperature, air movement, radiation exposure to particularly cold or warm surfaces and contact temperature. In any outdoor space you are unlikely to significantly change the surrounding air temperature, and the contact temperature is more or less a given by the clothes people wear and items they hold (hot chocolate vs. iced coffee). So, that leaves radiation and air movement. The more air movement the colder folks will be. Radiation (infrared) heaters are the traditional way to stay warm and the principle that makes camp fires so appealing. There are both electric and combustion infrared heaters available. Electrical infrared heaters which work on a larger scale may require special circuits.
5. The more air circulation the safer from COVID and the chillier the experience, ranging from one side of a garage open to nearly completely outside with fans running. If you are particularly concerned I would suggest aiming for a sensible (as in you can feel it with your sense of touch) level of air movement without it being a strong wind or draft. Which space will work depends on lots of factors including the outside wind that day. Again, I don't think you'll find a quantifiable measure of comparative risk for this.
6. CO2 from a heat source will generally vent up, as well warm human breath (initially), so high spaces with good air ventilation/removal above are theoretically promising, but again, I don't know of supporting data.

Conclusion: being in a space where you can feel the air moving is probably maximizing your reasonable protection and means that you are reliant on a radiant heating source for comfort. Look for infrared space heaters and don't place them where combustion products are coming into close contact with nearby surfaces.
posted by meinvt at 1:21 PM on December 14, 2021 [1 favorite]

Can someone explain how air circulation really works re Covid? Like I get how HVAC in an office/restaurant recirculates the bad stuff so that people get sick.

No you don't and neither does anyone else. It's still an open question. A modern A/C system even in a single family home has ducting that moves air dozens to low hundreds of feet before being recirculated, so if 6 feet means anything then A/C systems airflow plus air exchange means that A/Cs are not a valid source of disease transmission. And from the CDC's own site :"there is no definitive evidence to date that viable virus has been transmitted through an HVAC system to result in disease transmission to people in other spaces served by the same system."

Most people probably watched Outbreak - it's a line in the movie. In any case, as long as the system is running, then it's moving air enough to protect from disease. A standard A/C system will do like 2 air exchanges per hour, so that's your baseline. It's when the system is off and windows are closed that can be an issue, as that would be 0 air exchanges per hour.

So with that being said, the real hazards are electrocution due to frayed cords, carbon dioxide, and fire danger associated with outdoor heating. It's black Friday so the 7ft propane heaters can be as low as $100 so if you've got space to store it, they work great. They can also handle rain. The danger is putting full sides down with tarps or plastic and then running in basically an enclosed space. As long as you have some open space for air to blow in, you are getting lots of air exchange and there shouldn't be any buildup of COVID or any other disease.
posted by The_Vegetables at 1:25 PM on December 14, 2021 [1 favorite]

I've been using electric blankets for socializing outside and they work great. I just wrap myself up like a human taco. My nose (and toes after a while) still get cold but everything else stays nice and toasty.
posted by RobinofFrocksley at 2:29 PM on December 14, 2021 [7 favorites]

Electric blankets in a roofed but otherwise open porch have been the best thing we've tested so far. No risk of carbon monoxide poisoning, no dealing with gas canisters or smoke, and it's heating you directly rather than blowing off a ton of waste heat.
posted by aspersioncast at 2:54 PM on December 14, 2021 [5 favorites]

Heating an outdoor space in New England winter is futile and expensive. And the closer you get to inside the higher the risk is. Heat the people, not the space: electric blankets, hand warmers, and hot tea. It's the best you can do and reasonably cozy (I did it all winter last year and we had a fun time. Was it indoors? No. But it was safe and better on the wallet than anything else.)
posted by epanalepsis at 2:57 PM on December 14, 2021 [5 favorites]

This may sound a little alarming, but masks and open air MAY NOT BE ENOUGH.

Scientists were quite unsure about how COVID spreads during most of 2020. WHO came up with the 6 ft social distancing thing after initially stating that COVID was NOT airborne (and later changed the advice). Turns out that WHO, and indeed, a lot of the medical establishment used the WRONG SCIENCE to determine how droplets spread after a sneeze, and whether COVID virus travels via aerosol or droplets. Later it quietly updated its guidance on COVID being airborne via aerosols.

And to keep in mind that in August 2021, multiple people caught COVID at a Las Vegas MLM convention and at least 5 died. Masks were mandatory, but not vaccination.

With that said, patio heaters and/or IR porch heaters, mask and vaccination highly encouraged or mandatory. Lots of coffee and hot cocoa, and disposable hand warmers. It will be expensive.
posted by kschang at 3:07 PM on December 14, 2021 [3 favorites]

I'm just as much an epidemiologist as anybody, these days, so, not at all. The porch and Garage sound reasonably safe for dispersion of viral particles. Outdoor air is quite dynamic. If you use a heater that burns fuel, use a Carbon Monoxide monitor, easy & cheap. I meet weekly with a dance group; we open the windows all along the long wall. We are all vaccinated and wear masks. No one the the group has been diagnosed with Covid; we have been dancing again since Memorial Day, @ 15 - 20 people.

A neighbor has a weekly bonfire, in Maine, including winter. They have put down bricks and pieces of concrete as a fire base, and build a large bonfire, burning scrap wood, pallets, whatever.
posted by theora55 at 4:02 PM on December 14, 2021

Realistically, by the time you get to trying to balance airflow versus retaining heat, are you really outside anymore? You're just creating a different kind of indoors. You'd be just as well to meet indoors and increase ventilation by opening windows and doors, adding fans, etc as you would be to create tented spaces with similar levels of ventilation to what you could achieve indoors.

Using heated blankets and similar in an open outdoor space makes much more sense.
posted by ssg at 4:23 PM on December 14, 2021

Response by poster: And to keep in mind that in August 2021, multiple people caught COVID at a Las Vegas MLM convention and at least 5 died. Masks were mandatory, but not vaccination.

This was a day long event inside with lots of people. Am I missing the relevance; seems like a different thing.
posted by Reasonably Everything Happens at 7:55 PM on December 14, 2021 [4 favorites]

If you are willing to throw money at this then a cadillac solution is a Herman Nelson (IE: a ducted indirect fired diesel heater). It's basically a portable forced air furnace. At full chit a small one would keep space B or C at t-shirt temperature (though they can also be throttled back or controlled via thermostat) and also force a complete air exchange every couple of minutes making the difference in Covid risk between enclosed space and outside negligible. It'll burn 1.5-2 gallon of fuel per hour and 100% utilization. The hot air produced is dry and free of hydrocarbons and combustion by products though the space around the unit can smell a bit (usually they are placed 10-30 feet from the conditioned space for safety, smell and noise reasons). They can be rented anywhere construction happens near or below freezing. Because the actual heater is remote from the occupied space they are very safe; no worries about burns or fires. The fans within are electric so you'd need either grid power or a generator.

Note that these are different from "hotdog" or "torpedo" heaters which are direct fired and dump the exhaust into the conditioned space (also those heaters are pretty loud).

PS: Other people make them as well, the name has become genericized.
posted by Mitheral at 12:25 AM on December 15, 2021

@Reasonably Everything Happens -- you're right, that convention was attended by thousands in a big hall with big production, a big stage, and a lot of screaming and shouting and cheering. But I'm sure they have different events and they never bothered finding out which event was the real superspreader event. The relevance is merely that mask is not enough. please both vax and mask, but I'm sure you already know that.
posted by kschang at 2:56 AM on December 15, 2021

Best answer: While there is a lot we still don't know, it is still possible to evaluate the relative risk of aerosol transmission in various settings. Since summer 2020, when the aerosol modeling error kschang alludes to above became clear, a bunch of researchers have been working to model virus particle spread. For example, a team at (mostly) CU Boulder boiled down their model to this worksheet.

Because we don't know how many particles you need to inhale to get infected (or how that number changes with prior infection, vaccination, age, etc), and we also don't know how many viable virus particles a sick person emits, it's impossible to assign an absolute risk but I don't think it's unreasonable to use these kinds of tools to at least make better-informed decisions (eg, maybe you could cut the risk in half at the cost of everyone being very cold).

At an intuitive level, the best proxy is probably secondhand smoke. Is it better to hang out with a smoker in a tent, just because the tent is outside? Probably not meaningfully. Opening a tent flap would help. Opening a flap at each end would help more. Forcing air circulation with a fan would help even more, but now you're definitely cold - unfortunately, you're probably cold in direct proportion to how well you're avoiding secondhand smoke. You could drop your exposure to zero, for all practical purposes, by standing at opposite ends of an outdoor basketball court. It's very likely that there's a continuum of risk at every in-between setup.

As others have pointed out, the most important factor is whether or not there's a contagious person present, which is something you can contain (to an extent) with testing -- the orchestras I've played with this month are using tons of BinaxNOW kits to check the wind and brass players as we come into rehearsal, for example - that's on top of mandating vaccination, and having everyone wear a mask when not actively playing a wind instrument. We're still just multiplying lots of probabilities together hoping we get closer to zero, but if we're going to play that game I think these models are helpful.
posted by range at 6:55 AM on December 15, 2021

Hot water bottles are another way to heat the person rather than the space. Amazon has basic 2L bottles without covers as cheap as the $6-7 range, with covers around $9-10.

We're considering getting the all-in-one version of the Chinese diesel heater that is very popular for vanlife/RV and tiny homes right now, but that will require some work to appropriately duct and vent, and I don't think is going to play any significant role in aerosol mitigation.
posted by Lyn Never at 10:30 AM on December 15, 2021

Best answer: I was at an event last week that had a tent over a concrete tennis court and the temp inside the tent was colder inside than outside. My understanding from talking to an engineer is that the concrete made the area colder than wood or dirt floor would. So that may influence your decision.

I wonder if a kotastu plus individual electricity blankets would work? We have been having a lot of outside events here (in Canada) like bonfires or sitting in a screened porch without really trying to hear the space. People just dress for the weather.
posted by saucysault at 5:09 PM on December 15, 2021