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I was nervous and excited when the residential building codes started to include whole-house ventilation requirements. Based on things I’d read from experts, I knew this was supposed to be important for people living in houses, so I figured these new codes would be a good thing. But adding new stuff to code is always challenging—both for those of us trying to build new, and from the code officials’ side. We do not always interpret the text the same way. (For example, for three code cycles, a local county required kitchen makeup air to be ducted directly into the kitchen—even after the ICC added special language specifically stating the makeup air could enter anywhere connected by permanent openings or ductwork to the kitchen.) So, when the code language came out, I was relieved to see that some systems were simple, inexpensive, and easy to implement.
Exhaust-only ventilation
The simplest are “exhaust-only” systems. We’re just adding a timer switch to a bath fan or two, which turns them on for parts of every hour. Air is blown out of the house, and because houses are not balloons, every cubic foot of exhausted air is replaced with “fresh” outdoor air, which comes in through the building’s air leaks. The switches cost under $50 and should be pared with high-quality fans that are durable and quiet. These types of systems continue to meet IRC requirements.
Yet there are two shortcomings of bath fan–based exhaust-only systems. First, the makeup air could be coming in on a path we don’t want it to take. Second, air quality may not be great when all we are doing is sucking stale air out from one or two locations inside the house (so-called spot ventilation.)
On the first point, it depends on…
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3 Comments
Excellent overview thank you. I have investigated many of the solutions you have mentioned here and have settled on what you are calling inlet-only ventilation. My air handler runs continuously (ECM motor, 80-100 watts / 450cfm / Merv 13 filter) both filtering and supplying fresh air to all rooms all the time. I have a whole house dehumidifier that keeps indoor RH between 50-60 during humid summer days and a central AC that maintains summer indoor temps between 74-76 deg F.
Am I wasting energy...yes, but I don't have to deal with ERV issues (ducting, connections, flows, maintenance) and my CO2 in all rooms never goes above 1200 ppm. I worry less about back-flow down my chimney in summer and make-up air for an always running radon control fan. I also have a trickle vent in my rim joist with a backwards installed dryer check valve that allows more outside air in when multiple fans (bath, dryer, vacuum) are running. CT climate zone 5
This is a great overview.
I would urge caution in concluding that a CERV or Minotair is "more efficient" than an ERV/HRV. If you look at the heat energy recovered per energy consumed for a good HRV unit, you get numbers upwards of 20. (28 for a Zehnder Q450 based on its passivhaus certification numbers). If you ask a "magic box" unit to do the same job, it needs to use a lot more energy because it is doing the heat transfer actively rather than passively. The HRV is only consuming energy to move air.
Of course, what the magic box can do is go beyond that recovery and bring incoming air above the room temperature. To make a fair comparison to that we'd want to look at a combination of an HRV/ERV plus a small separate heat pump, vs. a magic box, with both delivering the same heating and ventilation. I've never seen the magic box manufacturers actually do that comparison, which leads me to believe that they won't look good if you do make that comparison. I'd be delighted to find out that I'm wrong about that, but for now, I think they offer the convenience of advanced capabilities for good control of indoor air, but do not offer as good efficiency as an HRV or ERV.
I'm going through your same story right now! I am also in the DC area, and I also have an older (1956) home that I assumed would be pretty leaky, given the original windows and two fireplaces. But I just got my new AirSense Indoor monitor running, and I was pretty stunned to find the CO2 levels in my living room was at about 1500 ppm! After turning off the AC and opening every window in house, I sat down to research ventilation options.
We are also planning other HVAC work, and it feels like it should probably all be coordinated. In particular, we would like to switch our HVAC to a heat-pump system this year. We've also been considering some kind of radon remediation for our ~3.5 piC/L basement. I'm already a bit overwhelmed trying to learn all the considerations for sizing, configuring, and placement of heat pumps, whether they should be ducted or unducted, and where we could put ducts inside the building envelope, air flow, pressurization, etc.
I'm a smart person with a DIY spirit, but I put my major skill points into biology and programming. I'm pretty out of my depth with all the physics and engineering stuff here, and I don't think I have time for all the learning I would need to do to catch up.
So, I'm thinking I should hire an expert to design a retrofit well-ventilated, energy-efficient HVAC for us. Would it be practical/cost effective to get that kind of help for a "normal" middle-class house (typical MD suburb midcentury brick rambler)? Or does this kind of thing require "fine homebuilding" amounts of money? I'm also not sure exactly what kind of engineer (or other professional) does this, or how I would go about finding a good one. I don't know if you are allowed to make specific endorsements here, but given that you are familiar with the DC area, I would welcome any recommendations for specific companies or people, in addition to any general advice on the topic!
Just in case it matters...we're also about to repipe our hot and cold water lines, replace the gas hot water heater with a heat pump version, and potentially have some electrical work done. None of that necessarily relates directly to the HVAC stuff, but it does seem like it should be at least somewhat coordinated, since it will presumably involve accessing some of the same wall and ceiling/floor spaces.
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