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Comment by Waterluvian | Hacker News Reader

In addition to a heat pump, I wish my HVAC had a filtered vent from the outside and would be intelligent enough to sometimes just pump in cooler or warmer air from the outside when viable.

I don’t like having to shut off my furnace and manually manage windows. But I also don’t want automatic windows. I just want it to be like, “you’re asking for 21C and we can get there by pumping in outside air rather than using an AC to transfer heat from the inside air to the outside air.”

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We installed something like this, but for the purpose of getting fresh air into the house (instead of circling stale air over and over and increasing the CO2 levels). Our setup is:

Fresh air / circulated air -> Perfect 16[1] -> HVAC

I highly recommend it. You can also get an ERV or HRV to utilize heat/humidity when bringing in outside air.

1. https://www.iqair.com/us/whole-house-air-purifiers

I think systems like this make sense.

I'm not yet convinced that they are worth 3k more than a pico and a fan, at least as a retrofit.

The beauty of a real ventilation system is the heat exchanger. I’m still waiting for someone to make one that I can run by feeding a tube out my window, but until then, you need to spend a bit of cash on a permanent installation.

I was looking for such a minified heat pump as well, no luck.

Or just, a windowshaker A/C unit that has a heat mode.

(That’s uglier, but surprisingly difficult to find).

Exactly, but no luck. Maybe there is a technical reason.

Dunno either. Should be no issues in flipping the whole unit around. Controls/weatherproofing would be on the wrong side. Put some nice indoor plants underneath it because they'll now be self-watering.

You might try buying two CPU coolers with fans and connect them together via their heat sinks with the fans pointing in opposite directions. Then build a housing for it to match your window. That should transfer some of the heat/cool from the outgoing air to the incoming air. Put a filter on it to keep bugs and pollen out of the house (including the outgoing fan because you might turn it off sometimes. It needs at least a no-see-um size mesh screen.)

I read that page, and that sounds like a MERV 16 filter than attaches to a third party fan coil. I can’t tell how it’s any different from, say, an Aprilaire 216 plus its (very cheap) housing, along with smart home integration and a pretty logo, minus the transparent pricing.

One of the differences is how hard you have to push against a regular MERV 16 filter vs. this one. You can generally install a Perfect 16 into an HVAC without having to adjust its settings and thus how much energy is required to run it. I haven’t looked at the April Aire stuff in a bit, so don’t know if they happen to do something similar, but I was looking at a number of different options for a much larger building install, and you can generally price the P16 without having to factor in energy price increases.

My main gripe with the Perfect 16 is that it doesn’t do any of the multi gas stuff ( https://www.iqair.com/room-air-purifiers/gc-multigas ), and is more like running a HealthPro ( https://www.iqair.com/air-purifiers/healthpro-series ).

This is where less gold-plated products may be better! Look at page 6 here:

https://leesheatingandcoolingservice.com/wp-content/uploads/...

The 216 filter has 0.17" wc of initial pressure drop at 1000 cfm as compared to 0.12" wc for its MERV 13 equivalent, the 213. You can just swap it in if you started out with MERV 13 [0] have 0.05" wc of extra pressure budget (and an existing compatible housing), and you can probably save 0.05" wc by just changing your filter more often. Or you can design something like this into a new system, just like you would have to design a Perfect 16 in.

[0] You must have some kind of return air filter to avoid destroying your fan coil. MERV 13 is somewhat of a baseline for a decent system.

Huh, IQ Air does have specs and prices:

https://www.iqair.com/us/whole-house-air-purifiers/buy

That’s… a lot more expensive than Aprilaire for only slightly better performance. Two or three Aprilaire (or Lennox, etc) filters in parallel will dramatically outperform the Perfect 16 and will cost much less. Of course, that would require some custom sheet metal, but the Perfect 16 may also need some custom sheet metal.

chances are these are hydroelectrostatic treated filters, very good performance

Matt Risinger explains how and why he put an ERV in his house in this episode of The Build Show https://youtu.be/OrG7oG8Tvp8. It seems to be the thing to do of late in to help with air exchanges and still meet the passive house standards.

How do these systems cope with various temperatures and humidities? You’ve basically got 4 different modes if your climate zone is diverse enough.

That’s called an air side economizer, they’re very common in commercial buildings but residential units are also available.

Isn’t that effectively a heat recovery ventilation system?

https://en.wikipedia.org/wiki/Heat_recovery_ventilation?wpro...

Yes - these are necessary in houses designed to minimise air leakage, because without them they can end up so well sealed that they do not allow enough air changes per hour.

That's kind of the flipside of what OP is describing. OP is saying if it's colder outside than inside, and you want it to be colder inside, the unit would intelligently just pull in unprocessed (except filtering) outside air.

An HRV or ERV is designed to pull in fresh air without losing heat from inside (or while keeping the heat outside in the summer), by transferring heat between the air being drawn in and that being exhausted.

My heat recovery ventilation system does exactly that. It has a bypass valve to just suck in filtered air when needed, bypassing the temperature exchange mechanism.

My HRV does this and it’ll pull in any temperature air if you tell it to

The cheap DIY is to add WiFi switches to your bathroom fans, turn them on when humidity is lower outside or when the outside is cooler than the inside.

WiFi? Eh. As an ever-so-modest upgrade to the cheap DIY, a Zwave or Zigbee system might be a better choice to integrate with a variety of other things (like thermostats and temperature/humidity sensors.)

Wi-Fi is easier to interface with using any computer, where as those other protocols require a dongle on the computer side.

Sure, but once you have 20 or 30 of those things in your walls, congesting up your network, you’ll wish you went with something that runs at 900 mhz.

>20 or 30.

In my house. 9 X main lights. Kitchen work lights. Extractor fan. Oven Hob (gas) Dishwasher Washing machine Dryer. Fridge. Freezer. Food processor Kettle Toaster. TV. Digi box PlayStation (2 but anyway) Extractor fan Boiler Laptop Printer 5 X lamps.

So I managed to get to 30 things. I'm not entirely sure I'd want to hook up all those things to a network though...

Zigbee also runs on 2.4GHz, but I wouldn't even count WiFi out just yet since those devices are on 2.4GHz where latency and throughput are almost irrelevant (to me) for that application. Plus it's usually cheaper.

The problem is that lots of little wifi devices on your network will slow down everything where latency and throughput is relevant. I guess WiFi 6 will "fix" this, so maybe I'll be wrong when light switches are all WiFi 6, but I'm right for now.

Also, latency is very relevant for IoT stuff. Lights turning off 2 seconds after you push a button is a crappy experience.

Most WiFi switches are based on ESP chips. Meaning they are trivial to install custom firmware on to extend the functionality. I haven’t found any affordable Zigbee or Zwave switches of this kind. Meaning that all the smarts need to be programmed centrally instead of putting them locally on the switch itself.

Or use a cheap zigbee or zwave power plug to switch the fan, then run the logic somewhere else. (I do this)

Presumably you're doing this to pull in air from the outside using negative pressure? I heard this approach isn't optimal because negative pressure means that you'll be pulling in air through the various gaps/crevices in the walls, which is dirty and might have contaminants like mold.

This should not be done because it will draw outside air through the walls of the home leading to moisture problems.

Doesn't this warning then suggest that bathroom ventilation fans should never be used? And should be removed from houses (and building codes) at all expediency?

Yup but no. Bathroom fans are never on continuously.

A better solution is to open one window on each floor in addition to turning on the exhaust.

The suggestion isn't to turn them on continuously (as this would also wear the motor) but in situations that are similar to that of when showering (high humidity or temperature)

Yeah it makes me really upset when the outside dew point is lower than inside (often in the fall) and I've got to dig out my lasko box fan and run it in the windows.

There are several kinds of HVAC systems for fresh air ventilation and free cooling (HRV/ERV) and it was a huge bummer to hear an ac expert say "your tiny house's tiny closet has no space for any new toys at all"

Not to be really dumb about this (I don't have a heat pump, don't know exactly how they work, but have some interest in getting one) -- is it not possible to shut them off entirely for the fall? Are they hard to restart or something? Is it like the obnoxious car systems that always want to be giving you either heat or cool and just refuse to do nothing when it's nice out?

They can easily turn themself off and restart easily.

GP was wanting to take advantage of the more favorable outside air (lower sensible and latent heat) and just do filtered air exchange directly with the outside rather than running the compressor.

I think the main problem would be humidity issues. The outside air swings so wildly in humidity. I was growing weed outside in my state and surprised to watch the humidity on the meter regularly go from 21% to nearly 100% on day night cycles.

Look at dew point or some absolute humidity measure instead. As air gets colder, the amount of water vapor it can hold (the denominator in relative humidity) decreases dramatically.

Introducing cold, saturated air to your warm house will not, in general, make it feel nasty and humid.

Some googling of experts shows it can be quite a concern.

https://www.eng-tips.com/viewthread.cfm?qid=188008

I can only barely understand what’s going on in that link, but I think it really depends on target indoor environment.

While I don’t want the walls sweating, I could imagine there are industrial or commercial environments where moisture level matters a lot. Electronics plant: dry is always bad! Want some moisture to dissipate static. A library wants low, but not zero moisture. A manufacturing plant wants it to be very consistent. A fruit market wants its humidity high.

In a residential environment, it’s a toss up: some stuff wants to be dryer while other stuff doesn’t.

I don’t think you’re likely to make the walls sweat by running an economizer or, for that matter, opening a window while it’s cooler outside than inside. You may end up with more humidity inside than you like, though, depending on the weather.

That link seems to be discussing a situation where there is too much humidity coming in relative to the amount of heat the A/C needs to remove, and that hits my pet peeve about air conditioning design. Air conditioners generally run at a roughly constant coil temperature, which means that there is no independent control over dehumidification versus cooling. If you happen to be at the design conditions, great. Otherwise you don’t end up with the humidity you want.

I know of one system (“dynamic humidity control” by Chiltrix) that adjusts the coil temperature to control humidity. As far as I know, any hydronic system can do this with minimal modification, but I don’t think it’s popular.

I have a similar issue with thermostats in general: set points without a humidity coefficient is a constant battle. 26C and dry is comfortable: Stay off. 26C and humid is sweltering: Power up.

My "smart" thermostat has a settings for this, but I seem to be at its algorithm's mercy instead of specifying: "50% RH or less, no change, 50-70%, -2C to the set points, 70%+, -3C to the set points".

Let alone being able to do dehumidification only.

This isn’t entirely a problem with your thermostat. If it’s 26C and humid, all your thermostat can do is to call for cooling. (Maybe it can call for stage 1 or stage 2. You are unlikely to have anything that can do stand-alone dehumification.) So it can’t actually achieve a comfortable and efficient target.

Would still be more comfortable and efficient if could aim for what I'd be equally happy with: 23C + humid or 26C + dry

That’s just because the temperature swings wildly. Water isn’t going in and out of the air.

Cold air holds less water, so that makes sense.

It is like pouring 200ml of water from a 1000ml jug into a 200ml jug. Same water, higher % fillage (to use a technical term).

If you're in a climate where that is a problem, the solution is to use an energy recovery ventilator (ERV) along with your air conditioning to use the conditioned inside air being exhausted to pre-treat the incoming air using an enthalpy heat exchanger.

Heat recovery ventilation (HRV), which uses a sensible heat exchanger, is sufficient in temperate climates but otherwise you need to be able to transfer latent heat also to reduce humidity, so need the ERV.

It’s called an economizer and is controlled by enthalpy sensors. Look into honeywell jade system, if you are handy then you can DIY it. Controllers are super cheap on ebay. Just be mindful of air balance.

There are machines that do just that. They’re descended from commercial units, and you have to match humidity, but they can be done.

It’s just not been worth the cost, usually.

You must have a well insulated house.. as soon as I turn off my AC or heater my house very quickly becomes the same temperature as outside..

Really? I live in a mostly not very well-insulated 200 year old wood construction farm house and I find that there can easily be a 10 degree F delta between indoors and outdoors. I have heat for the winter given northern climate but otherwise mostly just do regulation by opening or closing windows.

Your house was built before the days of A/C so the builders probably deserve more credit!

My house is also in a forest so the sun doesn’t bake it. That might be part of it.

One thing to remember is that when the air outside is already warmer than the air you want to heat or cooler than the air you want to cool, this will significantly increase the efficiency of the heat pump in achieving that temperature. I would guess to the point where it's not worth complicating the system in this way. (Especially since it will be a relatively rare compared to the reverse situation.)

You can get this in a lot of places, with units that use the inside air to pre-condition the outside air being brought in so you're not destroying the efficiency of your HVAC system. It's called Heat Recovery Ventilation (HRV) or Energy Recovery Ventilation (ERV). Units exist for small residential buildings all the way up to massive commercial units. HRV is used in temperate climates where the humidity outside is generally not a problem, and ERV is used when the outside air needs to be dehumidified also.

It's not usually a feature of residential units, but commercial ones also have a bypass for when the building management system detects (using sensors like wet-bulb thermometers) that the outside conditions are fine to bring inside, which is called the "economy cycle". It switches off the baffles to the HRV or ERV and just brings the outside air in directly.

If somebody was really interested, that could be implemented at home with some extra ducting, some controllable dampers, sensors, a Raspberry Pi, etc.

This “Smart Window Fan” is a window-mounted economizer. If it’s cooler and dryer outside, like at night, it opens its louvers and pulls in cool air and exhausts warm air. It also has a HEPA filter. I assume it has humidity sensors too to avoid running when it’s humid outside.

I am wary of a product sold by only one vendor. Economizers tend to be added to large central ducted commercial HVAC systems and this is the only window unit I’m aware of.

EcoBreeze 2.2 - Nature's Cooling Solutions® https://naturescoolingsolutions.com/ecobreeeze-2/

You should look into a whole house fan.

Can confirm. They are awesome

I have heard to avoid them in areas with high humidity for mold reasons. But otherwise I don’t see why it couldn’t save you a ton on cooling.

> In addition to a heat pump, I wish my HVAC had a filtered vent from the outside and would be intelligent enough to sometimes just pump in cooler or warmer air from the outside when viable.

Hell a modern AC can probably do with only filtered venting, with a heat exchanger which gets disabled when using external air to get to the target temperature.

Thats an interesting idea. Where I live I think you would have to have significantly colder air outside to compensate for the humidity of just bringing in outside air, unless if could be dehumidified. I think there would be a limited window of usefulness.

Hi! We're idiots, and just built a house without paying extra for the automatic window openers.

In fairness, said openers wouldn't be able to talk to our proprietary thermostat.

The heat pump we bought is great, though.

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