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Comment by ahaucnx | Hacker News Reader

Author of the article, happy to answer any questions.

A bit of background, my house is in Northern Thailand that has similar levels of air pollution every year than what we see currently in North America and Canada and it took me a few years to get my house to zero PM even when having the worst wildfires around. So before installing a positive pressure system and making my house air tight in the process, I experienced with all types of air purifiers but did not find anything that works consistently at very high pollution levels.

The experience with these wildfires let me actually found AirGradient and we are proud of open sourcing many of our air quality monitors.

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Seems like the big drawback of the positive-pressure approach would be flushing the heating/cooling out, too (as you've said in the article). That drawback would seem to be proportional the flowrate of the positive-pressure airflow.

So what if you redo your experiment with lower positive-pressure flows? This is:

1. Keep the 280-m^3/hr purifier on the whole time.

2. Start with the 220-m^3/hr positive-pressure system running.

3. Wait for the particulate-level to equilibrate, then record the particulate-level and the pressure-difference (between outside and inside).

4. Reduce the air-flow of the positive-pressure system by, say, 20-m^3/hr.

5. Loop back to Step-3 while the air-flow of the positive-pressure system isn't negative.

With the objective to be to determine how low the positive-pressure system's flowrate can get while still maintaining its benefits, reducing heating/cooling losses.

The system that I am having in place now is connected to our indoor air quality monitor and constantly regulates the fan speed of the PPS. By measuring the CO2 levels in the room, I can also detect when the room is actually occupied. This works extremely efficient and by playing around with the parameters can be optimized for cleaner air vs energy efficiency (or both).

So in a way it does exactly what you say and finds the ideal equilibrium to keep just the right amount of pressure needed.

Air to air heat exchangers will help a lot with energy loss.

Now that we have a CO2 meter, I am strongly considering wiring it to an air exchanger and having a PID controller just keep the house co2 below 600-800 ppm, and the pm 2.5 below 10 ug/m3.

> Seems like the big drawback of the positive-pressure approach would be flushing the heating/cooling out, too (as you've said in the article).

There are lots of enthalpy recovery systems on the market, some that can recover >75% of the 'waste' energy, though mostly aimed at larger customers. You need to do a cost-benefit analysis to see if it makes sense to spend the upfront cost.

Yeah, looks like you're right: [waste-heat recovery systems](https://en.wikipedia.org/wiki/Waste_heat_recovery_unit ).

Guessing that there are two main scenarios there:

1. A leaky house where positive-pressure causes flows to come out of various leaks all around.

2. A well-sealed house where positive-pressure causes flows to leave through a well-defined channel.

The first-case might be harder to recover heat/coolness from, as the (high/low)-temperature air might leak in a way that'd be harder to make use of it.

The second-case would seem to offer a stream of warmer/cooler air that could be used with a heat-exchanger or heat-pump or something.

To note it, well-sealed houses might have issues with relatively poor ventilation, making positive-pressure more desirable -- much like how the OP describes wanting to keep CO2-levels down with their positive-pressure system.

Do you have any issues with mold after air tightening the house? Here in Finland pretty much every house needs constant airflow/intake and the houses built to breathe to fight against mold. Im curious if youve had any issues, especially due to living closer to the tropical zone.

When I got my basement damp-proofed against a nasty mould problem there were three options for how it could be done. In increasing order if effectiveness, they were: 1) they could re-plaster with a sealant; 2) they could mount plastic stand-off sheets to the wall with plasterboard covering; 3) they could air-tighten and install a positive pressure system.

So PPSes are sold as a damp prevention mechanism; as long as you install one when you air-tighten it's doing the job that natural airflow would otherwise take care of.

Also worth noting: each successive option added a zero to the price.

He won't because it's Thailand. The air inside the house won't be significantly warmer than outside for a prolonged period of time. Not so in Finland.

Your best option is a centralized or decentralized automatic ventilation system with heat exchanging and humidity control. On top of that, the better the insulation the less the air is cooling down near walls/windows and the less likely the humidity is condensing into water causing mold. And then last, the less humid the air inside the house is, the more it can cool down before condensation is happening - so keep the humidity low not only with automatic ventilation but also by making sure to not cause humidity by cooking, showering (and breathing, haha) and if you do, air out for just a few minutes.

What do you mean by humidity control? a central one?

I am not saying you don't need one, but here in Sweden modern houses do not use humidity controls and are still air tight and mold free, maybe at the cost of letting more air and energy be swapped. Older houses have local vents at the baths which are humidity sensitive and will blow out excess humidity quickly, again at the cost of wasted energy.

I suppose that controlling humidity centrally will somewhat save energy, but at what cost and complexity?

Well, unless you stop breating, showering, cooking, keeping plants indoors, etc. you will have to somehow moderate/control the humidity as it will grow over time until the dew point close to walls/windows or other cold surfaces is reached.

The reason why houses in Sweden can be mold free is because people get taught that they need to air out their house on a regular basis. And if you look it up, it tells you to air out not during noon (even though it's warmer so you lose less energy) but in the morning and late evening - specifically for humidity control.

So...

> maybe at the cost of letting more air and energy be swapped

Yes, exactly that. But since we are in the context of air pollution, opening windows to air out isn't a great idea. That's why I mentioned an automatic ventilation system (which pretty much always contains filters).

> Older houses have local vents at the baths which are humidity sensitive and will blow out excess humidity quickly, again at the cost of wasted energy.

Not only that, fresh air will also be sucked in from somewhere else outside - which then again brings the air pollution inside.

All new houses here have mechanical ventilation, usually with heat recovery and heat pumps but without filtering (excluding the heat pump's coarse filter for dust).

I got curious and read a little, it is generally accepted here that higher pressure in the house can actually lead to mold, maybe because there is no moisture control and it is expected that some of the moisture generated in the house will be driven into the walls, probably assuming the walls are not perfectly sealed. On the other hand air pollution in Sweden is a non-issue, except maybe for a couple of streets in Stockholm.

Yeah that makes sense - if you have clean air then why would you need to filter. :)

Ideally you would have a an air tight house with a mix of interior and exterior insulation based on your min/max temperature deltas and their dew points to keep the sheathing from being too cold

That alone is insufficient. Think about the bathroom: the air will be hot and very humid. The walls would have to be way too hot to have a dew point that isn't reached when then hot&humid air cools down at the wall.

So you are left with reducing the humdity by airing out in some way (with windows, with a very non-airtight building, or using automatic ventilation). Or, of course, you can use a dehumidifier.

The problem is solved, barring serious complications with your home's envelope, with mechanical ventilation systems that give you control over how and where air is being exchanged in your house, while filtering it as well. That's assuming your building envelope doesn't have any serious issues; if water is leaking into wall assembly because of a poorly installed roof or gutter system, for example, no amount of ventilation is going to keep things from rotting away in a few years.

Anyhow, although leaky houses can help deal with moisture, that's pretty much an incidental effect. Leaky homes also comes with significant expense in the form of heat loss, worsened indoor air quality, pathways for bugs to enter the home, etc. There's a reason why we've been moving towards ever-tighter houses, even with the upfront costs and effort required for air sealing, insulation, and mechanical ventilation.

In the GP's tropical climate, I'd expect that humid air leaking into the house does very little to actually dry out the wall cavities; if anything, there's a good chance that those leaks would do the opposite and support any mold growth.

HVAC engineer here. I'd be interested to know if you've done any measurements with outside air coming through standard F5 filters. This is the typical setup for most ducted AC I come across. Much lower pressure and cheaper than HEPAs. It would be good to know what air quality results as a baseline and also in conjunction with in room purification if you have any data.

Edit: I'm in Australia. Here's a comparison of filter types for different locations: https://www.filtermakers.com.au/selection-guide/

No, we only tested with true HEPA filters.

One important thing to point out with PPS systems is that you need to get the air from fully polluted to zero polluted in ONE pass as unlike air purifiers, there is no recirculation. So the filter quality is super important.

Agreed. Unfortunately we usually only see HEPAs in hospital theatres, CSSDs and the occasional lab. Air quality standards are still very lacking here. They haven't been updated since COVID or the east coast bushfires in any way.

Not sure how to read that guide, but would an F5 filter out smoke well?

No they wouldn't. Typically outside air systems shut off in fire mode, but they often keep running if there's smoke due to air pollution. If you need to deal with this you need a HEPA.

An alternative for critical facilities like datacentres, is they often just go into recirculation mode. This wouldn't work in locations with persistent air polution.

I have a pet peeve about graphs with bad / non-existent labeling.

Here is how to read the graph: The black line is the outside air PM2.5. The green-yellow-orange-red bars are the indoors air PM2.5.

I have air filters in all my rooms. I guess my questions are:

1) What should I do to get up to PPS? Should I just run a tube to their inlets through my window?

2) How is heating / cooling handled? The weather here swings wildly. I'd be reluctant to pump hot or freezing air in. Might work well in spring / fall which are peak allergy season, though.

3) What's a good way to monitor air quality on the cheap?

I'm looking for opinionated recommendations.

1) The first DIY PPS that I built used a small duct fan drawing air from outside and pushing it through a hepa filter to the inside. It is important to use duct fans as you need to create air pressure more than air flow and box fans are not built to create the amount of pressure required.

2) Ideally you get a PPS system with a heat recovery unit. Also important is to have the PPS demand controlled based on an indoor air quality monitor that measures PM and CO2 and then only pushing in the right amount of air to meet your target values for PM and CO2. This way you can save a lot of energy costs and temperature differentials.

3) Thanks for asking ;) We maintain popular open source air quality monitor kits that are very easy to assemble and use industry grade sensors [1]

[1] https://www.airgradient.com/open-airgradient/kits/

I was looking for something like this.

Friendly note: "Join thousands of makers builing their" that's probably meant to be "building".

Thanks. Fixed.

I've been looking for an inexpensive PPS with heat exchanger that fits in a window (preferably an Andersen window, the kind that opens sideways, but maybe that's too much to ask). Since there don't seem to be any on the market (for any kind of window) I'm working on a design of my own. I think that two CPU coolers connected by their CPU plates with one pushing air out and one drawing air in would make a cheap heat exchanger. Then possibly running a large hose from the outflow of that to the air intake on a small HEPA air purifier would create a simple, cheap PPS. I know the pressure would not be high inside the house, but it would be a cheap way to introduce fresh air without losing heat/cooling and would help create a small positive pressure, without having to cut big holes in the walls.

CPU coolers are tuned for 300 watts cooling at a delta of 60ish degrees centigrade (100ish Fahrenheit).

You'd be operating at a much lower delta, split over two coolers. At best, the transfered power would scale with the delta, due to tuning it might scale worse.

I think you might be better off just skipping the heat-pipes of cpu coolers and using aluminum plates positionned in both airflows. CPUs need heat-pipes because the heat source is sooo small. Here your heat source will be much bigger, especially in terms of surface area.

Interesting, so in Winter, say the outside temp is 20F and inside temp is 69F, that's a 49F difference. In Summer, say the outside temp is 85F and the inside temp is 68F, a 17F difference. Maybe I'd get at best 50% heat recovery in the Winter and 25% in the Summer? That's a lot better than no recovery. I like the aluminum plate idea, maybe two big heat sinks with long fins attached back to back with the air blowing through the fins would work better and be cheaper. I like the fact that the CPU coolers are somewhat plug and play though, the fans are already integrated and power supplies are easily available. Both approaches are fairly easy and cheap to try. Wikipedia offers some insight on heat pipes:

https://en.wikipedia.org/wiki/Heat_pipe

So what is the working temperature range for CPU cooler heat pipes? Though they maybe be optimized for 100F temperature difference, CPU's also don't always run at full speed and maximum power. So perhaps CPU coolers would still be useful at lower temperature differentials (and lower absolute operating temperatures.)

> I like the aluminum plate idea, maybe two big heat sinks with long fins attached back to back with the air blowing through the fins would work better and be cheaper.

That's not quite what I meant. I effectively meant: just make one big, wide heatsink. Then have the intake run through the left side of the heatsink and outtake through the right side. No 'heat transfer between the heatsinks' needed.

Hmmm, there would still need to be heat transfer inside the heat sink from one side to the other via the bottom plate. Maybe that would still be more efficient than the transfer via a larger contact area between two heat sinks back to back? Not sure...either would probably work. What I can build might depend on the geometry and size of the heat sinks that are easily available. One big heat sink would make construction a little easier.

It’s great that PPS can reduce PM2.5 and CO2 to extremely low levels. I live in an area with industrial pollution (coal etc) and am concerned about VOCs, NOXs, SO2s etc as well. Wouldn’t I bring in more of those if I constantly bring in outside air even when the pollution is high outside? Unfortunately I haven’t been able to exactly identify the chemical that’s causing issues for me yet. I’d love to hear your thoughts on this situation.

Have you explored the effects of a localized positive-pressure system?

Like, say the positive-pressure system has a duct that pumps outside-air directly to locations where a resident spends a lot of them time -- like their bed, a work desk, or a couch in a living-room. Could they get reasonable air-quality that way, even if the system isn't strong enough to clean up other areas of their home that they might spend less time in?

Yes, this is what I am doing in my house. Pump the cleaned fresh air into the peripheral rooms (mostly bedrooms) and then this actually pressures the whole house (living room, bathrooms etc).

What was the filtration level of the filter system? What was the orientation/positioning?

I’m wondering if the airflow was in a loop but not mixing the entire room, or the filter wasn’t Hepa (or close), or it wasn’t reaching its claimed throughput/filtration, etc.

I recently installed a Merv13+Merv16 filter on my newly upgraded HVAC system and am able to drive PM2.5 to 0 with a minimal fan setting of roughly 2ACH. (Granted, we haven’t had wildfire smoke that strongly since installing it, but I induce bad air from time to time and it cleans it up quickly.)

What kind of NOx and SOx readings do you get? (I assume the AirGradient sensors can measure that?). Do you think an activated charcoal filter is a worthwhile addition?

What do you need for a 700sqft space built tight?

So its going to make the minisplit work harder but to what extent at 30 and 40 degree air outside?

I have seen PPS systems work from small rooms to whole sports halls. If you have a large temperature/humidity differential between inside and outside, I would recommend looking for units with heat recovery system.

what about Radon?

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