by Allison Bailes —
You know that old saying, “No good deed goes unpunished”? It applies to a lot of homes with respect to ventilation. If you simply bring outdoor air into the house, you pay for it…in more than one way. The outdoor air you bring in is unconditioned so you have to heat it or cool it. If you’re in a humid climate, you may have to dehumidify it. If you’re in a dry climate, you may need to humidify it.
Depending on how you introduce the ventilation air into the house, you may have cool drafts in winter or warm air blowing on you in summer. Then there’s the issue of ventilation causing moisture to get into building cavities, where it can wet the building materials, cause decay, and grow mold. And if you want to increase the ventilation rate, you get punished even more.
The most common types of whole-house ventilation systems use the exhaust-only or supply-only strategies. Exhaust-only ventilation uses fans that are already in the house—bath fans and range hood—with controls to modulate the rate or runtime to provide the required amount of ventilation air. Supply-only ventilation may use dedicated fans that blow air into the house, the blower in the heating and cooling system, or a ventilating dehumidifier. Both strategies can create problems with comfort, energy use, or moisture if you try to increase the amount of ventilation air too much.
But there are ways to ventilate a house at higher rates and not have problems. You just need to use a different strategy. Let’s take a look at two ways you can do this.
Balanced ventilation with heat and moisture recovery
The best way to bring outdoor air into a home is to do it with a balanced ventilation system that has heat recovery (everywhere) and moisture recovery (most homes). You probably want an energy recovery ventilator (ERV) that does both heat and moisture recovery rather than a heat recovery ventilator (HRV) that does only heat recovery, but that’s the subject of a future article. Here let’s just focus on the ventilation.
Bringing outdoor air into the house dilutes indoor air pollutants. By doing so continuously, you can keep those pollutants at a low level. Carbon dioxide levels are a good measure of how much air exchange you’re getting. But, as mentioned above, bringing in outdoor air comes at a cost. With heat and moisture recovery, though, that cost is much lower. (From here on, I’m going to use ERV to stand for both ERV and HRV. If you’re one of those weirdos who uses HRVs, you can make a mental substitution when you read ERV.)
An ERV doesn’t let your expensive, conditioned air just leave the house. Just as you’re required to turn in your shoes before you leave the bowling alley, an ERV makes outgoing air turn in its heat and moisture in winter. And just as Buster Scruggs had to deposit his guns—including the señorita pistols—when he entered the saloon, the fresh air entering the ERV in summer deposits its heat and moisture into the outgoing air.
Those heat and moisture exchanges eliminate or reduce a couple of the costs you have to pay for ventilation. By recovering heat and moisture, you have to do less conditioning of the ventilation air once it’s inside. The heat and moisture exchange also brings the ventilation air close to indoor conditions, so comfort problems are less likely. The equal air flows of exhaust air and outdoor air reduce the likelihood of moisture causing trouble in the building enclosure.
In short, if you want more ventilation air in your house without having to suffer the ventilation penalty, an energy recovery ventilator can do that for you. Just make sure to get one that has a high efficiency of transferring heat and moisture, electronically commutated fan motors, and a MERV-13 filter on the incoming air side. (Some of the brands we like are Broan, Fantech, Panasonic, Renewaire,* and Zehnder*.)
With such a ventilation system, you may be able to crank it up to one air change per hour, which is about three times the recommended rate. Because of the space needed and upfront costs, going higher than that is hard to do, but you can certainly get more equivalent air changes per hour, ACHe, by combining ventilation and filtration.
Balanced ventilation combined with a heat pump for recovery
Another way to get more ventilation air into your house is to use a ventilation system that has a built-in heat pump. I’ll call it a heat pump ventilator. Instead of a heat exchanger core as the ERV has, this device uses the heat pump to transfer heat between the air streams. Currently, you have two options in North America for this kind of ventilator: the CERV-2 by Build Equinox and the PentaCare V12 by Minotair.
The heat pump gives this device the ability to do heating, cooling, and dehumidification. They don’t provide much heating and cooling capacity since their main purpose is to provide clean air. The CERV-2 has a heating and cooling capacity of about 3,000 BTU per hour. The PentaCare V12 provides about 6,000 BTU per hour of heating capacity and 11,000 BTU per hour of cooling capacity.
The CERV-2 heat pump ventilator by Build Equinox
The heat pump in a heat pump ventilator is like an ERV core in that it can introduce outdoor air into the house without that air being at outdoor conditions. The difference, though, is that the heat pump can heat or cool the air all the way to the indoor temperature and beyond. In fact, in small apartments and condos, these systems could provide all the heating and cooling you need, but let’s stick to the ventilation and indoor air quality aspects here.
Another difference between an ERV and a heat pump ventilator is that heat pump ventilators also recirculate air in the house to provide more filtration, heating, cooling, or dehumidification. This combination of ventilation and filtration with high-MERV filters can provide another level of cleaning the air. Both are important.
The heat pump ventilator eliminates some of the penalties associated with ventilating at higher rates: comfort and durability. But unlike the passive heat exchanger in an ERV, the heat pump does use energy. The heat pump ventilator, as a result, will cost a bit more to run than an efficient ERV but you should see lower ventilation costs than with exhaust-only or supply-only ventilation systems.
Supply-only ventilation with a ventilating dehumidifier
In hot, humid climates, your best bet is probably a dehumidifier. When outdoor dew points are 75 to 80 degrees Fahrenheit for many months and you even use the air conditioner in January, this method will allow you to ventilate at higher rates without the humidity. This requires a ducted dehumidifier that pulls some air from the conditioned space and some air from outdoors. Ultra-Aire** is the brand we like.
With the proper controls, a ventilating dehumidifier can run with or without the compressor on for dehumidification. With the compressor off, you can ventilate without dehumidifying. You could also put an electronic damper and controls on the outdoor air duct to dehumidify without ventilating.
All three of the methods described above allow you to crank up the ventilation with impunity. OK, maybe you don’t get complete impunity, but you’ll certainly get a lot less “punity.”
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.
* Disclosures: Renewaire and Zehnder have provided one each of their ERVs to Energy Vanguard.