[danans]

The way I like to describe HRV/ERVs to people who don't know about them is:

"Imagine you could open a window to get fresh air into your house and stale air out, but when you did so, most of the heat/humidity would stay in during the winter, or stay out during the summer, leaving you just the fresh air".

In terms of the effect on environment inside a house, I usually say:

"Imagine it's always a fresh-air spring day inside your house"

This is a great project. One problem with ERV/HRV systems right now is that they are very expensive niche products. While this system doesn't achieve the extremely high heat recovery efficiencies of counter-flow units, the perfect is the enemy of the good, and this seems like it could be orders of magnitude cheaper.

[pzduniak]

>that they are very expensive niche products

My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model that I can directly control over MODBUS / 0-10V signal (even turning it into a "dumb" unit). Most of the expenses were running the ducts. YMMV

It's just awesome. Every single room has fresh-smelling air and after fine tuning all my heating systems with algos implemented in Home Assistant - I'm getting ~60-100ppm over outdoor CO2, perfectly clean air, temperature within 1C of the set value, on-demand humidity extraction after showers etc. All it needs to be properly overengineered now is a bunch of dampers and per-room CO2/humidity feedback :)

[tomashubelbauer]

I wish this was as common as having a fridge in the house. The productivity gains from people not being sleepy and tired from shit air would be insane.

[tashoecraft]

When having my mini splits installed I pushed for an erv system in the bedroom. The installer had only ever done them in commercial units, and he hemmed and hawed about it, but I had two c02 monitors in my room showing it getting to above 2000 whilst sleeping. I've noticed a big improvement in how groggy I feel in the morning.

[BizarroLand]

If there were good enough ERVs that could be installed by the DIY'er for about the cost of a cheap refrigerator (~$600 or so) then they would be more common.

I get that manufacturing the transfer plates isn't simple or cheap, but other than that one thing they're basically fans, they shouldn't cost that much.

[michaelmior]

> after fine tuning all my heating systems with algos implemented in Home Assistant

I'd be very interested in hearing the details of this.

[pzduniak]

I'll definitely prepare a longer write-up when I have everything figure out, but here's a summary:

I have 4 systems:

- Komfovent HRV for ventilation

- NIBE F-series heat pump for floor and water heating

- Vaillant gas boiler that "supports" the heat pump

- Samsung multi-split AC units

HRV - Komfovent uses the same controllers in all of their units, so you get all the communication goodies you'd want - though it took me a long while to figure out that basic features need to be toggled on :) There are existing YAML presets for their C6/C6M controllers on HA forums. The only caveat is that if you want to feed it a virtual thermostat, you need a stuff a device simulating a 10k NTC inside of the ventilator. Otherwise it's just a single Ethernet cable.

Heat pump - I'm not exactly sure if I'm happy with NIBE, but thanks to the community the integration ended up being quite easy. I wasted a bunch of money on their MODBUS40 just to learn that you need to use a certain MODBUS address in the internal bus to make certain registries writeable (eg. thermostat values) - so I took an ESP32 with Ethernet, a galvanically isolated RS485 dongle, a 12V to 5V converter and used https://github.com/elupus/esphome-nibe. The firmware extracts my templated HA sensor's value and feeds it to the heat pump as a virtual thermostat.

Vaillant uses this weird "eBUS" protocol, there's a bunch of cheap PCBs that you can use to connect to it - I'm using https://github.com/danielkucera/esp-arduino-ebus. That's the last system that I haven't touched :)

Samsung ACs use their MIM-B19N modules installed in the outdoor units. There's some magic around enabling remote control, but once you plug their diagnostics device into their indoor units, you can flash all of them at once. I had to mess around with internal NASA addresses to have all the units appear at once.

For indoor sensors I have 3 types:

- AirGradient units measure CO2, tempeature, humidity, PMx etc. - these are mounted at ~150cm and feed the "current house temperature" template.

- I have like 8 Everything Presence One devices, powered by a custom PCB that converts 12V/24V sent over wired alarm cables to the device. They have built-in temperature, humidity and motion sensors. These are mostly installed for motion sensing and their height makes the temperature measurements quite useless.

- Everything else (and most importantly bathrooms) is done using custom ESP32-C3 devices that use SHT31 sensors to measure humidity and LD2412 for movement sensing. Also using the same adapter PCB for powering.

Thermostats are synchronized across all the devices with HA scripts. The HRV specifically uses its own wired temperature sensor to determine if it should enable heat recovery ("free cooling mode"), since its extracted air temp is always a bit lower than room temp (laziness :-)). "Current temperature" template fed to other heaters is derived from multiple room temperatures (currently using an average), with rooms "ignored" if AC is heating there (or was turned on recently). Ventilation has 2 modes set up - 20% and 80% - with the latter toggled by a bathroom humidity threshold.

There are 3 remaining things I want to set up:

- auto switching to gas heating if it's cheaper / the house is running on batteries - so far I've only imported electricity / gas prices into HA and quickly realized that I'm missing a power monitor on the heat pump circuit

- dampening of air ducts to reduce the temp drop when high humidity extraction boost gets triggered

- using more of the HRV range by auto-adjusting fan speed depending on real CO2 values - there's max 2 ppl at the house most of the time, so even at 20% the HRV is quite wasteful

[michaelmior]

Very cool! I think one of my biggest things is that I need my sensors to be able to manage things well.

[semi-extrinsic]

As a comparison for a "dumb" system, our house has a Nibe F110 that handles all air extraction and energy recovery (only for water heating) and a single Mitsubishi mini-split AC. And the bedrooms have some small electric panel heaters. I just cannot be bothered to fiddle with the smart house stuff, this solution works just fine for us.

[karussell]

How is the noise of these systems? And how often/long need they run to give you clean air and humidity extraction. Is there extra noise when it is windy outside? Is it installed inside the windows somehow or do I need to drill through the wall?

[sgerenser]

Typically 2 holes, one for air intake and one for air outtake are drilled through the wall. They’re often installed when a home is being built or heavily remodeled. They can be installed after the fact (especially if you have an accessible basement or attic) but it might be a bit invasive running ducts where you need them.

[ericd]

Alternatively, we got one piped into the HVAC ductwork. It's not as optimal as its own ducting, but it's much easier/cheaper than running a bunch of new ductwork.

[organsnyder]

That's what we did. Our house was built in 1916, but the weather sealing we've done combined with six people breathing in it led to quite high CO2 levels. We have a traditional ducted air conditioning system in the attic (heat is hot water radiant), and added the ERV there. It made a massive difference.

[ericd]

Yeah, same here on most counts, even going down to 3 ACH on the blower door test made CO2 levels pretty high, now they’re barely above outdoor.

[danans]

> My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model

That is just the HRV, not the design of the system, the ductwork, and the installation. All those add up. In new construction those costs can be shared with the regular HVAC system design, but in a retrofit its far more expensive

[tinco]

Anyone know if there's a good way to control 0-10V dampers? I looked for a solution to control 5 dampers but I didn't find anything, so I started to design my own a couple years ago but never finished the project. I'm having a hard time imagining something so common doesn't have a common solution.

[pzduniak]

If you're OK with large controllers, the cheap Chinese RS485 stuff seems to work perfectly fine. For "digital inputs" I started out with Polish $150 devices, eventually ended up using the cheapest AliExpress listings for some expansions and so far they've been working exactly the same, with the only difference being the quality of the docs. I'd expect the 0-10V modules to be exactly the same.

[yrcyrc]

Would you have a link/reference?

[pzduniak]

https://www.komfovent.com/en/products/domekt-r-400-f-c6m-573

I think this is the one I'm using for ~200-ish sqm.

[yrcyrc]

Great thank you

[throw0101b]

> One problem with ERV/HRV systems right now is that they are very expensive niche products.

Most building codes in US/CA mandate them since about 2015, so I'm not sure how niche they are (at least in new construction).

Depending on the (air) volumes involved, ERVs can be had for under CA$ 2000:

* https://gasexperts.ca/product-category/air-exchangers/lifebr...

* https://bphsales.ca/collections/high-quality-erv-air-exchang...

HRVs for less, but it's probably worth the extra few hundred for better humidity management.

[danans]

Installation is usually the most expensive part, and can easily send the total price into 5 digits of $, especially in a retrofit, depending on the market.

[batushka5]

These units are ubiquitus in northernish Europe, as any new/renovated building needs them to reach A/A++ energy effiency. Brands like Komfovent, SystemAir, offering 200 m³/h ducted units for 2000 Euros, with efficiency like:

  Outdoors    °C -23  -15  -10  -5    0   25   30   35
  After unit, °C 12,9 14,5 15,5 16,5 17,5 22,6 23,6 24,6

with indoor conditions + 22 °C, 20 % RH

[turtlebits]

ERVs are not expensive, 1500 USD will get you a decent whole house unit. The installation is the expensive part, which this project doesn't change.

[turtlebits]

You have it backwards. Counter flow units have lower efficiency than these "regenerative" type ERVs.

The downside of this is that the high efficiency is limited to small spaces (based on the mass of your core), where counter flow units are great for entire homes.

One point often overlooked with counter flow units, is that you can place exhaust ducts in spaces that you want to purposefully remove air, like bathrooms and kitchens, while providing fresh air to places with little air movement, like closets, basements.

Regenerative core ERVs do little for fresh air circulation.

[llm_trw]

Do you have a source explaining how these work?

Naively allowing the air columns to thermally mix would result in the average of the inside and outside temp. So how does this do better?

[IndrekR]

The direction of flow is reversed every 30s. The cycle is short enough that due to the thermal mass there is thermal gradient within the heat-exchanger. So it effectively works as counter-flow heat exchanger. Same principle (but continuous flow) is used in rotor ERV: https://en.wikipedia.org/wiki/Heat_recovery_ventilation#Ther...

Heat exchanger there is usually an extruded ceramic grid (ERV) or rolled corrugated aluminum (functions closer to HRV than ERV)

[LukeShu]

Counter-flow heat exchangers. A parallel-flow heat exchanger would result in the average, as you say; but a counter-flow exchanger means that as the formerly-warm air gets progressively cooler, it is exposed to progressively colder air.

[gwd]

I've got a counter-flow heat exchanger, but it looks like they're using a different design:

> Each OpenERV TW4 module has a very quiet pair of fans, pointed in opposite directions, and a heat exchanger in a 6 inch pipe, that goes through a wall. The hot, polluted air from inside goes out for 30 seconds, and the heat from it is stored in the heat exchanger.

> Then, the fan reverses direction, moving clean air from outdoors to the indoors. On it's way in, it picks up that heat from the heat exchanger. This type of heat exchanger is called a regenerative heat exchanger, or less commonly, a regenerator. The kind shown in the video is a recuperative type, not regenerative. Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.

> The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss.

https://www.openerv.ca/learn-more

[llm_trw]

Duh, thank you for reminding me air flows in a duct.

[empiricus]

the intuition: if the 2 colums flow in the same direction, the final temp is the average. but if the 2 columns flow in the oposite directions it is posible to fully exchange the temperature

[kccqzy]

What makes outdoor air "fresh" compared to indoor air? You said that the temperature and humidity of indoor air are preserved. So is it just CO2 concentration? Would installing a chemical CO2 scrubber have an effect similar to an ERV system then?

[NavinF]

CO2 ppm is the main number that research studies look at, but it's also a proxy for the "freshness" of the air. All the air quality metrics are correlated. Eg a lot of cheap CO2 meters measure something else like TVOC and convert it to eCO2 using a lookup table.

CO2 scrubbing would be better than nothing, but it's really expensive and won't improve other metrics like TVOC

[runjake]

CO2 and chemical off gassing from indoor items. Eg: manufacturing chemicals from furniture, natural gas, cooking fumes, etc.

[jonstewart]

Planning a renovation of my 1947 rowhome in DC, and I’m really looking forward to adding an ERV.

[bluGill]

You probably cannot do a renovation the tightens up your house enough to matter. Of course you have not specified what you are doing, it is certainly possible to do that, but it is a major effort that makes the house unlivable for a couple months and costs a lot of money. If you don't do that level of renovation your house will have enough leaks that a ERV will not make any difference in air quality (and even that level doesn't always make the house airtight enough to need an ERV). Making a house airtight is very hard - worth doing because of the energy savings, but not easy.

If you are doing that level of renovation is is probably better to just tear down the house and rebuild. The costs will be similar and there are a lot of other things people demand of a new house layout that cannot be retrofitted in the old shell. Often the law will not allow this and so you are forced to renovate just to keep some now illegal feature that is worth keeping, but otherwise a tear down would be better.

[jonstewart]

You're right in the main, of course, that it takes a major renovation to make an old home tight enough to be worth it, but wrong in your assumptions:

* It's a rowhouse, so there are two party walls on either side.

* Brick.

* The front and back walls are half the length (15') of the party walls.

* The house is small, a footprint of 450 sq ft.

* The renovation will extend the back roof line (pitched roof, front to back).

* The attic (with extended roof) will be renovated with a bedroom and 3/4 bath, plus storage and mechanical.

* Plumbing will be replaced.

* Gas boiler (with radiators) and existing central A/C will be replaced with heat pump.

* We'll be out of the house for months.

* It's in a historic district so it can't be torn down.

* Fortunately most of the windows have been replaced before the historic preservation office started cracking down on replacements. We have light-blocking hex blinds that insulate them nicely at night.

* I will try to sneak in a new front door that's the same design as the old one, and fix up the jambs and sill.

* Historically DC is still more heat dominated than cooling dominated, but climate change is tilting the balance. It may be hard to fight the stack effect in the winter but in the summer I hope to run the house at positive pressure. It's when the A/C's been going for days that the air in the house seems stale.

* Even though there's very little insulation today in the attic (just cellulose strewn between joists), it's not expensive to heat or cool, because it's such a small house.

* I care more about the ERV as a luxury good than as a cost effective appliance. I'm quite sure the cost of it will be negligible when compared to everything else in the reno (we're doing the kitchen, too).

Although the cost will be insane, I expect we'll come close to breaking even on value added to the property, given the neighborhood we're in.

[bluGill]

You have mostly made my point.

You have also touched on why I oppose historical districts. Historical buildings should be something people are required to learn about in their history class.

[jonstewart]

I’m with you there.

[BizarroLand]

Anyone can do a renovation that "tightens up your house enough to matter".

Use a qualified professional. Get multiple inputs.

It will cost money, and more money as you approach perfection, but it is doable.

[bluGill]

Not really as the structrure of most houses leaks. It can be done but you are doing a lot of work that is easy to skip

[BizarroLand]

That is true, and I guess technically you don't want a house with no leaks, only with leaks that you have complete control over.

Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.

It can also be put on as insulation in attics and crawlspaces.

Combine that with a full ducting audit if any of your ducting pierces the envelope, a full intrusions audit for power boxes and the like, and new windows/house sheathing/ proper roofing, you can get very close to as good as a new built.

Most of these I would not suggest doing as a DIY, therefore I still say it is expensive, and even moreso if you have lathe and plaster instead of more modern drywall or encounter any of a myriad of issues likely to be uncovered when doing these things to a very old house.

[bluGill]

> Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.

A hard maybe on here. Done right it can of course, but you are depending on it filling the cavity without so much pressure that is breaks the walls. The industry as figured a lot out, but this is still compromise and some guessing and so it won't always work (it usually will). There are also many ways they used to put voids in old houses that would not be reached by this.

[colordrops]

I can't find anyone to install one in Los Angeles. Is there a particular climate these are suited for?

[jeroenhd]

Air exchange HRVs are quite common here in the Netherlands. Not exactly ubiquitous, but common enough that you can find them in most recently built apartments at the very least. If you're going to have mechanical ventilation installed, you may as well save a buck in the long run on the heat loss.

There's only so much temperature gradient these setups can handle economically, and it's quite possible that the hot LA summers combined with the cool AC air are too much for such an installation not to leak energy at an unacceptable rate.

Then again, just like with ACs that also serve as heat pumps, it could just be a matter of not enough people (or professionals) knowing about these installations to make it viable to build a business around them.

[TheSpiceIsLife]

HVAC folk ought to know about ERVs.

Type in HRV system in to your preferred search provider and hit the shopping tab.

Here in Australia they range from about 1500 Antipodean Dineros for a single room through-wall mounted systems, and around 5500 upward for a centralised unit. Plus installation costs, but HVAC install is one of my paid activities, so I mostly don’t pay that part.

NB heat pump is a more accurate term for refrigeration type air conditioners, as in cooling mode they’re ‘pumping’ the heat out of the inside environment and rejecting it outside.

But have you heard about brown?

[throw0101b]

> I can't find anyone to install one in Los Angeles.

If you call a random HVAC company, they may not want to deal with "fussy clients" that want something "fancy" like an HRV/ERV. Best to look at folks that perhaps try to adhere to building science more. A quick search for the LA area:

* https://www.jmsacandheating.com/indoor-air-quality/heat-ener...

* https://www.aircomfortexperts.com/additional-products/ervs/

* https://www.azaircond.com/indoor-air-quality/energy-recovery...

* https://www.socalclimatecontrol.com/ervs-and-hrvs-energy-eff...

Or do a dealer search from a manufacturer, e.g.,:

* https://broan-nutone.com/en-us/home/dealer-locator

> Is there a particular climate these are suited for?

Any climate. Modern ones can even handle IECC Zones 6 and 7:

* https://basc.pnnl.gov/images/iecc-climate-zone-map

[driverdan]

That's surprising. Doesn't CA require them in new construction? All CA HVAC contractors should be familiar with them now.

[bluGill]

They have been required by code in all new houses in Minnesota for about 25 years. I'm sure CA requires them too. Though in older houses they are a waste of money as your house already leaks much air. These are a good thing if you have a well sealed house, but older houses universally are not sealed that well and so they won't give you anything.

[zerr]

To be fair, the temperature (coldness) of the outdoor air contributes to refreshing the room as well.

[d--b]

I don't think this is meant as a replacement for windows :D

[freetonik]

In Finland, many people avoid opening windows altogether, and only rely on various devices for mechanical movement of air. I find it quite alarming actually, as many homes have stale air and subpar ventilation systems, always justified by "never let heat escape the house". The idea of opening windows to refresh the air in the house is basically alien to many Finns, while it's a normal thing to do in other countries; in German, there's a term Stoßlüften.

[BlobberSnobber]

Stoßlüften isn’t merely a term, it’s a German religion

[littlestymaar]

> most of the heat/humidity would stay in during the winter

Getting rid of humidity in winter is the main reason why you want to bring fresh air in a house though!

[danans]

Not in cold climates where in winter the air outdoors is very dry. Heating systems in such climates often have integrated humidifiers.

[littlestymaar]

Then it just means that you're overventilating! We emit much more steam (through cooking and showering for instance) than we consume oxygen/emit CO2 so controlling the humidity is the main purpose of ventilation, air renewal comes for free as a byproduct of that.

[manmal]

My system doesn’t over ventilate - we sometimes have too high CO2 in the bed rooms, but humidity is way too low in winters, sometimes below 30%. That means getting sick more often and having irritated airways at times.

[cwillu]

For reference, it is currently 84% relative humidity and -20°C outside where I live. This is about the same absolute humidity as 5% relative humidity at 22°C (i.e., inside).

[littlestymaar]

An all you need to bring that up to above 50% is 2cl/cubic meter of water!

A sponge drying up in your kitchen sink is enough to raise your kitchen's air humidity by 10%. Shut down your ventilation and you'll see, you won't suffocate but instead you'll get mold starting to pop-up. Moisture is the reason why houses have ventilation system in the first place.

As I said elsewhere in this thread, though, there's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to suck way too much air out of your house than needed during the cold days.

[crazygringo]

> A sponge drying up in your kitchen sink is enough to raise your kitchen's air humidity by 10%.

That's simply not correct.

I go through liters of water a day with my two humidifiers just to try to raise humidity by around 20 percentage points. In a small urban apartment that isn't much bigger than some people's whole suburban kitchens.

A damp sponge isn't going to do a thing, and I can't imagine where you would ever have gotten the idea that it would.

Moisture is not the primary reason for ventilation, except above showers -- it's to prevent CO2 buildup along with other toxic gases like CO and VOC's.

[wilted-iris]

I have not observed this to be true. Ventilating enough to keep CO2 low means sub 20% winter humidity in multiple places I’ve lived.

[RGamma]

Even with our cross counterflow enthalpy exchanger it can get somewhat dry in the bedroom in winter. The device in the OP would probably require an additional humidifier.

[szundi]

The less co2 you have the less effect of air exchange does have. Also more co2 in air also decreases this over the decades.

[torginus]

Isn't it the other way around? 50% humidity means that air contains 50% as much moisture as it, at a given temperature. Raising the temperature means that the air can now hold much more moisture.

Bringing in cold air at 50% humidity, then warming it up to room temp makes the humidity fall, leading to dryer air indoors than comfortable.

[Notatheist]

Yep. Humidity will tank to 25% here in winter. I have two humidifiers fighting the HRV continuously when it gets cold. As I understand it an ERV controls moisture as well, but such a module for my system costs over $4000.

[littlestymaar]

But household activities like cooking, showering, drying laundry, or even just washing the dishes etc. generate tons of moisture, and this moisture is the reason why your home has ventilation in the first place: to get it out and avoid mold!

There's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to such way too much air out of your house than needed during the cold days (and it also tend to be designed to suck cold air into dry room first, and get out from wet rooms, when you want it the other way round when it's very cold).

[crazygringo]

Not in the winter though!

In the hot and humid summer you're definitely trying to reduce indoor humidity.

But in the winter when it's bone-dry? A hot shower barely makes a difference.

I keep two humidifiers running all winter long just to bring indoor humidity up to 35% or 40% where it's healthy.

Otherwise it often goes down to 15% or even 10% on cold winter days, which is terribly unhealthy.

[littlestymaar]

> Not in the winter though!

> In the hot and humid summer you're definitely trying to reduce indoor humidity.

No, you can't do that with ventilation when it's hotter outside than inside actually, that's not how thermodynamics works! But we don't care about that, because in the summer you don't have cold walls or window where water vapor can condense and let mold grow.

> But in the winter when it's bone-dry? A hot shower barely makes a difference.

The reason why it doesn't make a difference is because all the moisture is vented away by your ventilation system! And that's because that's what it's designed to do! Stop it and see how it goes! For the record a single wet sponge drying up in your kitchen is enough to raise humidity by 10%! You barely need 2cL of water per cubic meter to have 50% humidity at 20°C.

As I said the problem is that in winter, ventilation system often ventilate way too much.

Also, they are often designed so the cold and dry air enters in the bedrooms/living room and the warm/moist air is extracted in the kitchen and the bathroom, and because of that the rest of the house doesn't get any of the excess moisture of these places. This is done because the designers wanted to make sure that the humidity level never raise too much in the room, because again humidity will ruin your house and health pretty quick (having air that's too dry isn't very good for your lungs, but having fungi spores in the air is much worse!)

[quietbritishjim]

I think you are agreeing with the parent comment with a tone of disagreement.

They say they let cold air in during the winter because they want to lower the humidity.

Then you say that, if you let cold air in (and then let it heat back up again) then you end up with lower humidity.

The only thing you might disagree on is exactly what humidity you would like inside your house. But that's subjective. (As it happens, I agree with them: I often find it too humid inside during the winter, because I've restricted airflow to keep the heat in.)

[jmb99]

My house currently is sitting at 35% humidity while being very poorly ventilated (~900ppm CO2). In the summer, it’s around 50% with the same level of ventilation. This generally has been the case everywhere I’ve lived; in the summer, you’re cooling air, which (all else equal) increases the relative humidity of that air. In the winter, you’re heating air, which decreases the relative humidity of that air.

[littlestymaar]

> while being very poorly ventilated (~900ppm CO2)

Aren't you missing a zero of something? Because 900ppm isn't “very poorly ventilated”.

[crazygringo]

It absolutely is poorly ventilated.

I notice I'm not as mentally focused once it gets to 800 or so.

I have a CO2 monitor to keep it below 600 for productivity and concentration.

Remember, fresh air is around 420.

[littlestymaar]

It's “less than ideal” level of ventilation, but it's also very far from “very poorly ventilated” (it's not even above the target level set by workplace regulation in my country, which is 600ppm above the baseline).

[ZeroGravitas]

ERV is apparently the term used for HRVs that also exchange humidity and keep it in/out as required, though I'm not sure that makes much logical sense since E is for Energy and H is for Heat, no mention of humidity.

Possibly just different terms used in different countries where the humidity is a bigger problem (very hot and/or very cold outside air).

This current OpenERV product appears to use dessicants for this purpose but might be an optional add-on?

[mrspuratic]

A perfect ERV will recover the heat (occurs naturally by a temperature difference) and the latent heat (associated with condensation/evaporation). i.e. water vapour in the outbound flow should be transferred the inbound flow.

This is to avoid energy loss that would occur when moisture condenses (i.e. the latent heat) by using an adsorbant material to capture moisture before it escapes, or a membrane that allows moisture in the outbound flow to pass to the inbound flow.

[esskay]

Does very much depend on your location. In the UK a dehumidifier is still often a good idea in the winter because although our humidity drops, its nowhere near enough. Inside can still be high 50's, in older properties it'll never really go below 60%.

Really a lot of our older homes shoud be retrofitted with a MVHR unit to help things as ventelation is awful in most houses. I'm actually quite surprised a lot of landlords in the UK don't do it as theres always a fight between them and their tenants who don't necesserily want to leave a window open all day in the winter to stop mold.

[ZeroGravitas]

Positive Input Ventilation (PIV) units seem to be a new thing for landlords to battle this problem.

I think it basically trickles in cold (and therefore drier) air into a central space to reduce humidity, like an (slower, quieter) extractor fan in reverse.

Seems a bit of a conflict of interest still, especially if the tenant is paying for heating.