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Community and Q&A

HRV/ERV Alternatives for Small Home in Mild Climate with Fire Season in Mind

cjwagner | Posted in Mechanicals on

Hello- Im building an 1000sqft ADU in coastal California, climate zone 3c and need some advice on mechanical systems. 

All electric, 2 bed, 2 bath ADU.  Slab on grade, open floor plan with loft above, master bedroom off to the side (Floor plan attached). It will be heated and cooled with a minisplit heat pump, dryer ideally will be unvented and I want a decent sized range hood vent. We are experiencing increasingly long periods (3+months) of terrible air quality due to wildfire smoke and my goal is to build this house very air tight and to prioritize indoor air quality. 

With that in mind I am trying to find an economical system for ventilation and perhaps makeup air (more on that in a minute). My energy consulatant didn’t understand why I would want a HRV/ERV installed- maybe because of the small size of unit. She said if it feels stuffy, crack a window. Well that’s not a great option in fire season anymore. 

1st question: What stand alone unit would work best for ventilation that incorporates good filtration. I don’t necessarily think this needs to be a HRV ect because our climate is so mild. For example this unit by field controls looks interesting :

2nd Question: Whats the best way to either incorporate this system for makeup air for bathroom vent van and range hood or to have another system that also brings in CLEAN air. I understand smaller cfm range hoods are preferred for energy efficiency but we cook a lot and I worry just as much about the indoor air quality from poor extraction as energy cost (remember it doesn’t get very cold here) 
Thanks for any ideas – much appreciated!

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  1. user-2310254 | | #1


    You might want to look at the Panasonic IntelliBalance ERV. It's affordable and the latest version has a boost mode. I recall reading a few recent posts with similar questions. (Be sure to search GBA using Google or similar search engine. The site search is not very good.)

    My recollection is that the experts have been suggesting putting a filtration box inline with the fresh air intake.

  2. exeric | | #2

    I concur with Steve's recommendation. Here are two recent discussions I was involved with that may be helpful. I live in a climate in California that sounds similar to yours.

    In my situation I installed an 8" diameter butterfly duct valve in an outside wall close to my range hood. That eliminates back drafting or stalling. You will suck in outside smoke during that interval unless you also put in a filter across it, which I've neglected to do. I think you might be able to use the suggestion of the originator of the second thread and cut a round piece of activated carbon and stuff it into the hole for the butterfly valve. It would have to go on the side of the butterfly mechanism that wouldn't be obstructed by that filter.

    1. exeric | | #3

      I should add that originally I tried to use a Panasonic bathroom fan, which I still have, in conjunction with that opening to get flow through ventilation. It just didn't cut it and that's why I ended up installing the Panasonic ERV at a later date. YMMV.

  3. cjwagner | | #4

    Thanks for the thoughtful replies. I can see how a HRV ect can be integrated with a air filter. I guess this question really boils down to:
    1)are ERV's/HRV's necessary in mild climates for small spaces?
    2)If not how can one filter the outside return air with a ductless hvac system and continuous exhaust ventilation system?
    Eric- I'm curious whats the size/ layout you building is that the bathroom fan didn't cut it. I like your solution for the range hood.
    I would love to hear some success stories of continuous exhaust ventilation systems in small buildings with mild climates if there are any.

    1. exeric | | #6

      There is an issue that many people don't understand about ventilation. You don't need a large movement of air to get adequate ventilation. If you over ventilate you waste energy not only in running the fan but also in cooling or heating the home when the temperature outside is far different from what humans consider comfortable. Winter temperatures have the greatest delta from what people consider comfortable. That's why one doesn't want to over ventilate.

      The second problem is that unless you have a long narrow shotgun house, as they have in New Orleans, there will be rooms that have stagnant air even with a steady supply of air. With only one inlet and one outlet fresh air does not get evenly distributed, not even close. People overlook that probably an equal benefit to the energy exchangers in ERVs/HRVs is that there is ducting that evenly distributes the air.

      The default reason for using Panasonic bath fans with a ventilation function is that you assume there are holes in the house that allow good ventilation "without" an inlet vent. I don't think you can assume that unless your house is poorly constructed and is very leaky. It didn't work for my house which is the opposite of a shotgun house; it is essentially square. A long narrow home would allow good flow through ventilation but most homes do not. If that's your house then go for it and have one powered exhaust on one end and a make up vent on the other. But the makeup vent must be matched to the air flow of the powered exhaust.

      1. exeric | | #7

        A blower door test on a home works on the principle of the controlled and measured flow of air under pressure by a big fan. A house that is very tight will have to have that blower door fan cranked up very high just to get any flow of air at all into that house. A leaky home on the other hand will have the fan moving at a small percentage of the tight home's blower door fan speed to get a similar movement of air.

        What happens if you have a bath fan on high mode for showers that is working efficiently in a leaky house? It works to evacuate moisture very efficiently because there is lots of movement of air. If you tighten that house significantly and use that same bathroom fan (on high) then there will be much less air movement and the home will likely have high humidity problems. Now consider that tight house using that same fan in low speed ventilation mode. It's not going to work at all and will just stall. It doesn't mean the fan will stop but that it will most likely cavitate in the air. Just like you need a makeup vent for a range hood, you also need a makeup vent for bath fan. But you need two different make up vents with different flows just for the one bath fan running at two widely different speeds. The high speed make up vent can do double duty for the range. It will stay closed when the bath fan is selected to the low speed when only the low air movement make up vent comes into play. There's actually a science to all this and it can't be ignored if you want good indoor air quality.

        Those are some of the things one needs to do in trade off for not using an HRV or ERV in a tight home. Of course, if one's house is really leaky then don't worry about any of it and just pay the high energy bills and discomfort that a leaky home results in.

      2. cjwagner | | #10

        I guess my situation is somewhat unusual as it is an open floor plan with loft (so a bit of a shotgun house) with one bedroom off to the side. So single point exhaust-only might work especially if the minisplit register is distributing the air. (there's only going to be one register anyway) I suppose a lunos fan or erv could make up for the bedroom. I totally see your point that a ERV like your Panasonic with well designed ducting would be superior. But a large part of designing the building this way was to have the HVAC be simple. I guess anything that's unusual never ends up being simple.

        1. charlie_sullivan | | #11

          In your case, you don't really need the ERV feature, but a centralized system will give you better filtration capability, and once you have the centralized system, the cost adder for ERV is very small.

    2. creativedestruction | | #8

      "how can one filter the outside return air with a ductless hvac system and continuous exhaust ventilation system?"

      Through-wall passive fresh air inlet(s) with appropriate high-MERV filter.

      1. cjwagner | | #9

        So I have seen passive air inlets by lumos but they dont have great filter (MERV 4 or something) Can you suggest a setup that accommodates more filters. To be truly adequate it would need to be 3 stage including carbon filter. Seems like the really good filters are really only able to be integrated into erv/hrv systems like Eric mentions using. Kinda a shame to have to spend so much when the energy savings from a ERV might not be very substantial in some climates

        1. charlie_sullivan | | #12

          I don't think that will work well. If you put a really good filter in a passive inlet, the air is likely to come in through leaks in the envelope instead. You could do a supply-only filtered air system, and rely on leaks and passive vents for exhaust, but I wouldn't do exhaust only if you want good filtration.

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