Cheap Indoor Air Quality in a Mild Climate
Hello, I am doing a whole home remodel and hoping to achieve a pretty good house. The project is located in Seattle, which has a mild climate and relatively low humidity (albeit higher in the winter, when you want humidity). I have been thinking about HRV’s and dehumidifier’s, both of which are expensive and seem to address problems Seattle homes may not typically have (high heating costs and high indoor humidity). What Seattle does have is occasional (but increasing in frequency) wildfire smoke in the summer. Given all of that, I’d like to ask if I can cheaply and effectively address indoor air quality with traditional kitchen fans and bathroom fans, and a makeup air system?
Specifically, the idea would be to put a passive or active makeup air system that dumps into the return air of my HVAC system, and integrates with the air handler to condition and move the fresh air around the house. The makeup air would only be triggered if I turn on the kitchen or bathroom fans. This way I would be evacuating only when I need/want to, and when I’m drawing fresh air I would have it go through two filters (the makeup air filter, and the HVAC filter). I could maybe turn off the makeup air system during wildfire, but I’m thinking I would just leave it on but use my kitchen/bathroom fans less than usual. (During wildfire, I may still want to evacuate poor quality indoor air and moisture, if I think the filters can do the job of removing the smoke particulates.) This system would require less expensive equipment and less ducting, and I think might actually work better because I’m evacuating intermittently and at higher speeds when I have identified a source of bad air quality. The HRV/dehumidifier does not provide makeup air, and so my house would need to ‘naturally’ rebalance whenever I use kitchen/bath fans, which I think means all that smoky air is coming in somewhere and unfiltered. Any thoughts or musings would be MUCH appreciated. Thank you!
If it matters, the house is 2,100 square feet with a largely open floorplan, and a crawlspace to put all HVAC equipment. The ducting goes throughout the home.
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Replies
tdh2106,
If you are planning to air-seal and insulate to PGH levels, you need a strategy that addresses indoor air quality 24/7, not just to supply makeup air when exhaust fans are operating. Once the house is tightened you will also need ventilation to control humidity. Seattle is known for houses experiencing high indoor RH year round. My sub-PGH house in a similar PNW climate will spend a lot of time during shoulder seasons in the mid-60s RH if I don't ventilate.
Okay, if there are issues with high RH and fresh air, then is the better answer a dehumidifier with a fresh air source? Ultra-Aire makes some that have fresh air intake. When I incorporate your feedback, the ideal set up in my mind would be a dehumidifier that provides fresh air via a fan that runs constantly and boosts higher if CO2 levels rise, and that the dehumidifier function operates separately from the fan and only kicks on if humidity levels rise. Not sure if the Ultra-Aire or any alternative can actually do all of that.
The thing that gives me pause about an HRV is that I do not think it actually addresses the RH issue (exterior air in Seattle is not cold or dry, and so I would presume HRV does not effectively dehumidify) and it feels like overkill to recover heat on a home in such a mild climate. It is, however, cheaper to run than a dehumidifier because it is just a glorified fan.
One thing to note -- I was wrong about ducting the dehumidifier into the return air side of the air handler. That's contrary to code in a lot of places, and also misses the opportunity to have the air handler provide its own dehumidifying when operating in A/C mode. It apparently should be ducted to the supply side.
tdh2106,
The only time you might perhaps need dedicated de-humidification is a short period during the shoulder seasons when there isn't much temperature difference between inside and outdoors. During the winter, ventilation with outside air alone will reduce humidity, even in wet Seattle. Remember that we are talking RH, and that outside air is quite dry once heated.
My suggestion is balanced mechanical ventilation - with or without heat recovery. If you have a high CFM range-hood you may also want dedicated, filtered make-up air for it.
From best to worst options:
1) Balanced ERV with its own ductwork and a dedicated in-line dehumidifier with its own ductwork (no need for ventilating since you have ERV).
2) Ventilating dehumidifier with its own ductwork (this will over pressure the inside of the house but that is not the end of the world and better than underpressurizing)
3) Separate ERV and dehumidifier but shared ductwork with HVAC system. Not ideal since static pressure ratings on these smaller units are not the same as your main air handler it will cause uneven distribution and underperformance.
4) Some combination of bath fans, vent hood, and a ventilating dehumidifier.
You do not want an HRV. ERV's are better in 90% of cases. The only times they are worse is if you have consistent freezing air temperatures that can cause the media core to freeze. This was a problem with early ERV's but they have added pre-heaters and other mechanisms to address this in recent years. An ERV will regulate the humidity of the outdoor air you are brining in closer to that of the indoor air which you are sending out. So if outdoor humidity is higher than indoor, the ERV will drop outdoor air humidity before it gets in your house. Same is true if outdoor RH < indoor RH. And if outdoor RH = indoor RH, then either an ERV or HRV could work but might as well go with ERV because there is no chance throughout the entire year indoor RH = outdoor RH. ERV cores cost a little bit more than HRV's but not much more.
matthew25,
In the PNW, mechanical ventilation alone won't control indoor humidity in the brief shoulder seasons, and in summer - but most people have their windows open, So I'm not sure de-humidification is necessary.
The PNW is the one region where HRVs are recommended over ERVs, as you are always trying to reduce indoor humidity, and the outside air is always dryer. What typically favours using ERVs here is their price and availability.
If you will have all HVAC equipment in the crawlspace will it be part of the conditioned envelope?
Most ducted dehumidifiers have just one speed, but some can be set to run the fan continuously and only dehumidify as needed. My Santa Fe Advance 90 only uses 50 watts when the fan is running without the condenser operating. I don't have it ducted currently, it's just in freestanding mode in my encapsulated crawl space.
Had I not gotten an in-wall Santa Fe MD33 for the main living space I'd look more into attaching either the Advance or another dehumidifier to ductwork, even if just a basic return in one spot and a supply in another. I would recommend a ducted system over putting a dehumidifier like the MD33 into the living space due to noise. The Advance is higher capacity and I never hear it.
Relative humidity vs absolutely humidity--ah ha! Thank you for the explanation. Here's my understanding. For Seattle, peak relative humidity is ~80% in December with an average temperature of 47 F. But that is only 6.759 g/m3. If I heat the fresh air up to 68 F, the relative humidity becomes ~39%. So, I am getting some dehumidification from an HRV by exchanging indoor air at ~60%+ for outdoor air at ~39% in the winter. In the summer, my heat pump will provide some dehumidification when running in A/C (but not a lot because we don't have a lot of cooling load). To your point, the shoulder seasons could be an issue as the A/C is off and the temperature differential is not sufficient to have the HRV dehumidify.
One other question--don't most modern heat pumps have a dehumidification mode, and can you set that to a target humidity level? We'll be using a Mitsubishi hyper heat air handler, and I'm thinking that might provide all the dehumidification I need on this house.
tdh2106,
Ah - here is where my broad-stroke advice might not be that useful for your individual circumstances.
Around here most people are still able to get by opening windows in the summer for cooling, rather than using AC. The indoor humidity hovers in the low 60s, but that doesn't affect much when the temperatures are the same inside and out.
If you are going to rely on AC and mechanical ventilation in the summer - which as you say may become more and more necessary as wildfire smoke pervades your area - indoor RH may be more crucial to control, and Matthew may be right saying that some de-humidification may be necessary. Whether the heat-pump will be adequate to supply that I don't know. Hopefully some other posters will chime in.
Take a look at the CERV2 by BuildEquinox: https://www.buildequinox.com/cerv2/
It's not cheap -- I think around $6K. But it's a "magic box" that can vent or circulate and has a heat pump that can heat or cool. It has indoor sensors for temperature, humidity, CO2 and VOC, and outdoor sensors for temperature and humidity, and it uses them to control whether it heats or cools and whether it vents or circulates. When it's venting, and heating or cooling it uses the vented air for the heat pump so it's energy-recovery. If it's not venting and heating or cooling it uses outdoor air for the heat pump.
If you want a system that always does the right thing this is the model to follow.