One of the problems with building a tightly sealed house is that a lot of things we take for granted in a regular house suddenly become difficult when your main ventilation system runs at under 100cfm. A dryer typically blows 150-200 cfm when it's running. This means that it's going to be sucking cold air in through the HRV. On a really cold day, this could cause serious trouble for the HRV—the exchange plate could frost over. But more than that, it's (ironically) blowing warm air out of the house, while at the same time sucking cold air into the house. Again, on a cold day, really not what you want.
Range hoods cause similar trouble—they want to push air out of the house at >100cfm, and the air they are pushing out is generally warm air from the conditioned airspace, which must be replaced with cold air from outside. This seems like a minor issue until you consider that, aside from insulation, one of the main reasons that a Passivhaus has such a low energy budget is that you aren't heating large quantities of outside air as it leaks in through your drafty building envelope. So when you turn on these vents, your undersized Passivhaus heating system may be unable to keep up.
An additional complication is that if you have any appliances in the house that burn any sort of fuel, you are going to be creating a relative vacuum outside of the those appliances, and that might draw combustion products into the interior airspace that ought to be going up a chimney. We already had to tearfully let go of my 30,000 BTU wok ring dreams (actually, Andrea was remarkably dry-eyed) because of combustion products that couldn't be readily ventilated. No gas stove either. But externally ventilated gas heaters are very popular in tight homes, because they can be very efficient. Marc had a wood stove in his house in New Hampshire (although that wasn't a Passivhaus). Anything like this is going to be a potential hazard if you have exhaust fans running separately from your HRV.
Fortunately, we already gave up on a gas heater and decided to go with a heat pump instead. So we don't have to worry about that. But lots of exhaust vents are still something we have to avoid.
What a lot of Passivhaus people do is to set up drying rooms in their houses. This isn't a bad idea—it can be as low-tech as an indoor clothesline, or as high-tech as an enclosed space with a dehumidifier and/or a heater, plus some kind of exhaust fan that exhausts into the living space. We don't really want to dedicate a special room to this task, but we could certainly set up drying racks in the utility room and the mudroom on laundry days, and I suspect we will.
However, on a practical level, there will be times when we will want a dryer, either because we are drying more clothes than we have space for, or we are in a hurry, or whatever. Plus, for resale purposes, not having a dryer is kind of a non-starter. So I did a little research, which I thought I would share here.
The cheapest product I could find is an LG condensing dryer. This works the same way a regular electric dryer does: there's a heating element that heats the clothes to drive the moisture out, and a vent. Where it differs is that instead of leading outside, the vent leads into a condenser system which condenses the moisture out of the air, filters out the lint (sort of, according to some reviewers), and dumps it down the drain.
This would certainly work, and work well, but there are two problems with it. First, it turns out that it consumes more energy than a plain old electric dryer. When you count the cost of heating the replacement air, it's probably a wash, but this is definitely not a win. The second problem is that it cools the condenser with cold water from your tap, which it dumps down the drain. I get the impression that it's not a lot of water, but there are still some problems. Some people love this device, and some hate it. The ones who hate it often talk about problems they've had with leakage and pump failures. I suspect that the Rube Goldberg nature of the condenser has something to do with this.
So I did a little more research, with the help of Green Building Advisor. Actually, a lot of what I learned came from reading GBA, and I recommend this article highly if you want to drill a little deeper than the presentation I'm offering here. GBA talked about a Bosch system called Ecologixx that's called a "heat pump dryer."
When I was reading the Amazon product page for the LG dryer, I just assumed that it had a fan and a dehumidifier, which seemed like it ought to be more efficient than a heating element, but GBA cured me of that presumption. However, the Bosch Ecologixx series of dryer products do in fact work pretty much the way I had hoped the LG would. Most of these products don't seem to be available in the U.S., but the Bosch Axxis dryer is available, and it's actually pretty reasonably priced—about $200 more than the LG. Not everybody loves it, but it looks like a win in theory.
The bottom line is that I feel pretty good about not putting in an outside vent for the dryer. I don't love that this means we have to get rid of our Kenmore dryer, which has been a friend to us for many years, but I suspect that Freecycle will help us to find a good home for it.
Ordinary houses breathe through leaky joints and poor seals, losing heat and wasting energy. But our house won't leak, so we'll use a heat recovery ventilator (HRV) to admit fresh air and expel stale air, transferring heat from one stream to the other.