4 ms·

I've just installed a pair of these standalone units on opposite sides of a mostly open-plan 40m² ground floor. They can run in intake/exhaust/cycle modes, and

I've just installed a pair of these standalone units on opposite sides of a mostly open-plan 40m² ground floor. They can run in intake/exhaust/cycle modes, and be paired as primary/secondary for contraflow. Too early to see energy benefits yet, but one thing that /really/ helps already is kicking both units into intake when cooking on the hob with the hood fan extracting. I should probably finish that hood sensor (looks at sensor sitting on desk). In trickle/night mode the rated performance is "up to 90%" (at 1.2W and 15m³/h in unspecified test conditions). These ones require a 165mm diameter hole, so the heat exchanger cross section is ~2.7 times the 100mm version, and presumably more efficient. I also installed (refurb, old house) a recuperative MHVR upstairs. This type has two fans for separated permanent intake and exhaust channels. The alternative in my country is knocking 7000mm² holes in the walls (aka passive ventilation). I have to meet various building requirements as a refurb to qualify for energy upgrade grants.

schiffern 3y ago
> one thing that /really/ helps already is kicking both units into intake when cooking on the hob with the hood fan extracting Not sure how much good regeneration is doing[0], but it sounds like it put a band-aid on your problem of inadequate make-up air for the vent! :D Do these standalone units recover heat only (HRV) or heat and humidity (ERV)? [0] https://news.ycombinator.com/item?id=38743885 https://news.ycombinator.com/item?id=38743885
- mrspuratic 3y ago
  I have no external heat recovery system, so yes it's exactly zero while I'm cooking - but a reasonable way to maintain air quality in an open plan, one of my goals. The house also has new external wall insulation and other upgrades, so it's rather more airtight than before (though not formally tested just yet). Otherwise they run in synchronised cycle mode for heat recovery and air movement. I'm not well versed in all the subtleties of theory and current technology, but I don't think regenerative and true ERV go together. If condensation forms in the unit on the exhaust cycle I'd guess does contribute a little to the recovered heat? These units are BSK Zephyrs, with one heat mass component (ceramic honeycomb) and one fan inline in a telescopic duct. To prevent condensation issues they are fitted with a 2° slope outwards, and a humidity sensor with settable thresholds to purge high humidity internal air at full rate (equivalent to about 1.2 air changes/h for me).
- Open_erv 3y ago
  They all are capable of doing so under some circumstances with limited efficiency, because as the air exits the house and cools down at some point water condenses out of the air onto the regenerator media. Then it evaps again when flow reverses. However if you want maximal efficiency you want to add some zeolite 3A adsorbent, which you can sort of do yourself I am pretty sure. You just put some through a coffee grinder, put the regenerator in a garbage bag with it and shake it to get the powder inside the regenerator. It's not idea but it'll certainly help. I tried this with a thermal wheel of comparable size with silica gel and measured the result with AHT10 sensors and it worked pretty good actually. Zeolite lasts longer but you need more. Obviously you gotta try not to get the powder everywhere by getting excess off etc.
  - mrspuratic 3y ago
    Interesting. With EWI and internal insulation the telescopic duct is nearly maxed out as the total wall thickness is close to 500mm, but 50% of that is empty space. I though I might be able to get a secondary heat mass into that in future.
    - Open_erv 3y ago
      I don't like the telescopic ducts because they can leak into the wall cavity. There is nearly no point in adding more mass. It does increase efficiency but the typical regenerators are alumina which already have a lot of thermal mass. The problem is their surface area is too low. I made a simulator in python to evaluate various designs, and the efficiency of them is pretty poor. The main reason the fan reversal time is so long is the fans suck, they take a long time to accelerate, which affects average flow rate, the shorter the period the more time it takes to accelerate. Once the thermal mass of the "batch" of air is relatively small compared to the thermal mass of the regenerator, adding more thermal mass doesn't help. The reason you can't go to town with high surface area is viscous drag. The fans make a lot of noise when they have to work at high speed, which they have to do to produce higher pressures. Fan blade noise increases in proportion to the eigth power of blade velocity. So really really fast. So if you make the holes in the regenerator smaller and the walls thinner to increase surface area, you don't get enough flow. The usual units really hobble at the best of times. If you calculate how much energy (which increases linearly with flow*efficiency) they save a year, they usually actually cost more in amortized total cost of ownership than they save. The best thing is to redesign both the fan and regenerator, that's what I did. There is a nice fan that's produced by a german company that I'm trying to get samples of but they are super slow.

schiffern 3y ago

> one thing that /really/ helps already is kicking both units into intake when cooking on the hob with the hood fan extracting Not sure how much good regeneration is doing[0], but it sounds like it put a band-aid on your problem of inadequate make-up air for the vent! :D Do these standalone units recover heat only (HRV) or heat and humidity (ERV)? [0] https://news.ycombinator.com/item?id=38743885 https://news.ycombinator.com/item?id=38743885

mrspuratic 3y ago
I have no external heat recovery system, so yes it's exactly zero while I'm cooking - but a reasonable way to maintain air quality in an open plan, one of my goals. The house also has new external wall insulation and other upgrades, so it's rather more airtight than before (though not formally tested just yet). Otherwise they run in synchronised cycle mode for heat recovery and air movement. I'm not well versed in all the subtleties of theory and current technology, but I don't think regenerative and true ERV go together. If condensation forms in the unit on the exhaust cycle I'd guess does contribute a little to the recovered heat? These units are BSK Zephyrs, with one heat mass component (ceramic honeycomb) and one fan inline in a telescopic duct. To prevent condensation issues they are fitted with a 2° slope outwards, and a humidity sensor with settable thresholds to purge high humidity internal air at full rate (equivalent to about 1.2 air changes/h for me).
Open_erv 3y ago
They all are capable of doing so under some circumstances with limited efficiency, because as the air exits the house and cools down at some point water condenses out of the air onto the regenerator media. Then it evaps again when flow reverses. However if you want maximal efficiency you want to add some zeolite 3A adsorbent, which you can sort of do yourself I am pretty sure. You just put some through a coffee grinder, put the regenerator in a garbage bag with it and shake it to get the powder inside the regenerator. It's not idea but it'll certainly help. I tried this with a thermal wheel of comparable size with silica gel and measured the result with AHT10 sensors and it worked pretty good actually. Zeolite lasts longer but you need more. Obviously you gotta try not to get the powder everywhere by getting excess off etc.
- mrspuratic 3y ago
  Interesting. With EWI and internal insulation the telescopic duct is nearly maxed out as the total wall thickness is close to 500mm, but 50% of that is empty space. I though I might be able to get a secondary heat mass into that in future.
  - Open_erv 3y ago
    I don't like the telescopic ducts because they can leak into the wall cavity. There is nearly no point in adding more mass. It does increase efficiency but the typical regenerators are alumina which already have a lot of thermal mass. The problem is their surface area is too low. I made a simulator in python to evaluate various designs, and the efficiency of them is pretty poor. The main reason the fan reversal time is so long is the fans suck, they take a long time to accelerate, which affects average flow rate, the shorter the period the more time it takes to accelerate. Once the thermal mass of the "batch" of air is relatively small compared to the thermal mass of the regenerator, adding more thermal mass doesn't help. The reason you can't go to town with high surface area is viscous drag. The fans make a lot of noise when they have to work at high speed, which they have to do to produce higher pressures. Fan blade noise increases in proportion to the eigth power of blade velocity. So really really fast. So if you make the holes in the regenerator smaller and the walls thinner to increase surface area, you don't get enough flow. The usual units really hobble at the best of times. If you calculate how much energy (which increases linearly with flow*efficiency) they save a year, they usually actually cost more in amortized total cost of ownership than they save. The best thing is to redesign both the fan and regenerator, that's what I did. There is a nice fan that's produced by a german company that I'm trying to get samples of but they are super slow.