6 ms·

>why is everybody using recuperators when a regenerator would be mechanically much simpler? They exist, search for "ductless HRV." They're simply a small fan

>why is everybody using recuperators when a regenerator would be mechanically much simpler? They exist, search for "ductless HRV." They're simply a small fan that reverses every few minutes and a heat storage material. They also have wireless ones that coordinate the fan direction between them.

kragen 3y ago
thanks! don't you need at least two fans and two chambers of heat storage material?
- schiffern 3y ago
  That's how the wireless kind works. One unit blows in while a second identical unit (theoretically located across the house) blows out, and they switch roughly every minute. This balances the airflow, so you don't get non-regenerated (or more accurately, less regenerated) airflow "somewhere else" like you do if you use a single unit. "in thermal regenerators, the hot and cold fluids pass through the same channels in the packing, alternately, both fluids washing the same surface area. In recuperators, the hot and cold fluids pass simultaneously through different but adjacent channels."[0] So these regenerative-type HRVs alternate the flow direction of the air, using the packing material as a thermal "battery" to warm the incoming fresh air from the outgoing exhaust air. While they are indeed simpler, my understanding is that they don't really perform well. There's no humidity recovery, the CFM is too low, the cheap units don't do any active pressure balancing, you can't continuously exhaust from bathrooms/kitchens, and the lack of net flow to distribute air throughout the house means you're mostly exhausting the fresh air you just brought in. Not great. If you're going cheap, skip the regenerator-type and the HRVs altogether and go straight to a jury-rig ERV.[1] Though again, ideally you can run flex duct (or temporarily on the interior, even just dryer hose) to pull exhaust air from the opposite side of the house, and specifically from bathroom/kitchen areas. However, first you should test your CO2 levels and/or do a blower-door test. Most buildings aren't tight enough to need an ERV anyway! [0] https://www.thermopedia.com/content/1087/ https://www.thermopedia.com/content/1087/ [1] https://www.youtube.com/watch?v=CnLwJxCbxfM https://www.youtube.com/watch?v=CnLwJxCbxfM
  - kragen 3y ago
    this is great information, thank you! i'm surprised there's no humidity recovery; i'd think that, with a regenerator, humidity recovery is unavoidable even if you don't want it. maybe you're talking about cases where humidity or ΔT is so low that you don't get any condensation? https://news.ycombinator.com/item?id=38746150 https://news.ycombinator.com/item?id=38746150 points out that a little desiccant can solve that problem
mrspuratic 3y ago
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).

kragen 3y ago

thanks! don't you need at least two fans and two chambers of heat storage material?

schiffern 3y ago
That's how the wireless kind works. One unit blows in while a second identical unit (theoretically located across the house) blows out, and they switch roughly every minute. This balances the airflow, so you don't get non-regenerated (or more accurately, less regenerated) airflow "somewhere else" like you do if you use a single unit. "in thermal regenerators, the hot and cold fluids pass through the same channels in the packing, alternately, both fluids washing the same surface area. In recuperators, the hot and cold fluids pass simultaneously through different but adjacent channels."[0] So these regenerative-type HRVs alternate the flow direction of the air, using the packing material as a thermal "battery" to warm the incoming fresh air from the outgoing exhaust air. While they are indeed simpler, my understanding is that they don't really perform well. There's no humidity recovery, the CFM is too low, the cheap units don't do any active pressure balancing, you can't continuously exhaust from bathrooms/kitchens, and the lack of net flow to distribute air throughout the house means you're mostly exhausting the fresh air you just brought in. Not great. If you're going cheap, skip the regenerator-type and the HRVs altogether and go straight to a jury-rig ERV.[1] Though again, ideally you can run flex duct (or temporarily on the interior, even just dryer hose) to pull exhaust air from the opposite side of the house, and specifically from bathroom/kitchen areas. However, first you should test your CO2 levels and/or do a blower-door test. Most buildings aren't tight enough to need an ERV anyway! [0] https://www.thermopedia.com/content/1087/ https://www.thermopedia.com/content/1087/ [1] https://www.youtube.com/watch?v=CnLwJxCbxfM https://www.youtube.com/watch?v=CnLwJxCbxfM
- kragen 3y ago
  this is great information, thank you! i'm surprised there's no humidity recovery; i'd think that, with a regenerator, humidity recovery is unavoidable even if you don't want it. maybe you're talking about cases where humidity or ΔT is so low that you don't get any condensation? https://news.ycombinator.com/item?id=38746150 https://news.ycombinator.com/item?id=38746150 points out that a little desiccant can solve that problem

mrspuratic 3y ago

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).