Balanced ventilation without existing ductwork

We have a new 1200 sqft home in upper VT. It is airtight, so bath fans struggle to pull out moist air. If we crack a window, it’s fine. My wife hates cold air blowing on her after a hot shower. We heat with mini split heat pumps, have a heat pump water heater in basement, so it keeps RH around 40%. Upstairs RH is 42.5%
Moisture is not an issue, it is drawing fresh air into home that is an issue. It seems every article I read says to hook up HRV or ERV to existing ductwork. We have none. What are my options? I feel a one location in/out exchange for air would work, what do you think? Suggested systems?
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Panasonic used to make a spot ERV that could directly replace a bath fan and worked great for smaller places.
Since it is no longer made, you can replace the bath fan with one of the smaller condo sized ERV units. Something like this would fit between trusses at 24 OC:
https://reversomatic.com/products/rerv-80-mini-s
You can reuse the existing bath exhaust as the outside stale air outlet and run a duct to a new wall calp for your fresh air feed to the ERV. You can get tandem vent kits for these if you want to re-use the existing bath fan vent hole for both.
You would run the stale air pickup with a 90 deg bend straight down to a standard ceiling grill and I would run the fresh air supply with a length of flex to above the intake of your mini split. This helps to condition the fresh air feed plus does a better job of distributing it through the house. Make sure neither the stale air pickup or the unit is above your shower as that is too much direct moisture for these.
You would have to build a small cubby out of rigid insulation over the ERV in the attic air sealed to the drywall bellow and cover the bottom with an access panel that lets you access the ERV filter from bellow.
mtsoxfan,
Supplying balanced ventilation is just one aim of installing an ERV. The other - and more important one - is improving indoor air quality. If you aren't somehow supplying fresh air to the bedrooms, I would measure CO2 levels there at night. That may help determine where the supply air should go.
Like Malcolm said, air quality is a concern in a house that tight without mechanical ventilation. I'm also surprised that the bath fan can't eventually catch up with moisture. My house is also small at 1350 sq ft and very tight with a blower door test that came in at 0.61 ach50. My bath fans move more air with a window open than closed, but still pull enough air to keep the steam down in the bathroom with all windows closed. It would be very hard to seal a house enough to prevent a bath fan from working, which makes me think there might be something wrong with the setup (e.g. undersized fan, restrictive duct work, frozen backdraft damper). In VT, 40+% RH is high for January. I'm in NY and I try to keep it down in the low 30's this time of year to avoid excess condensation on the windows.
A single point exhaust/supply ducted HRV would be a major improvement, although supplies in each bedroom would be nice if you sleep with the doors closed. For the exhaust, I would suggest that it be located in the kitchen to remove cooking smells, but the bathroom is fine if that's the main concern. Exhaust vent(s) can be located at floor level. The supply should go high on the wall so that a cold draft isn't noticeable and it should be centrally located if there is only going to be one. Make sure you located the HRV/ERV in conditioned air space and as far away from living quarters as possible as it will make some noise.
Balanced ventilation systems, ideally, have a central unit and dedicated ducting. Connecting them to the heating system ductwork is not as good but it's ok.
Lunos makes two kinds of point-source ventilation. One has two fans housed together, meant for bathrooms and other small rooms. The other has two separate through-wall fans that are meant to be mounted on opposite sides of larger rooms.
I appreciate all the ideas so far.
To clarify, the bathroom vents work, but they reach their potential when window open. Same goes for running stove vent and dryer. They work, but not as good as when window cracked. I have not run the door blower test, just don't feel I need to. If the fans are weaker when windows are closed, I have an airtight home. It is a brand new home, all sill plate cracks sealed, outlet boxes, attic access door etc. It wasn't until I discovered an issue that I looked into ventilation. My wife gets bloody nose from to dry of house, and at 40%RH, she is fine. There is some condensation on windows, not a lot. A quick wipe with towel takes care of that. I'm not opposed to cutting RH some, but again, the wife...
I was hoping for a one location air exchanger to supply some fresh air for all ventilation needs. . I will look into the Lunos, and any other suggestions. Between the wood stove with outside fresh air vent, and water heater heat pump, humidity stays pretty well in check, summer and winter.
mtsoxfan,
It’s important to make the distinction between ventilation air and make-up air or that used for combustion. Balanced ventilation won’t help with the latter two.
https://www.greenbuildingadvisor.com/article/rethinking-the-air-inside-our-homes
Thanks Malcom, I could only read some before it cut me off. I only need makeup air. Wood stove has its own vent,.
I check out Lunos, e2 and it looked great. Until it said the wall must be at least 300mm, or 11 or so inches. Still looking...
Could you just put some blocking to increase the depth of the wall? Like a block of wood, decorative stone, drywall, etc?
When remodeling my home and making it much tighter(yes, I did install an HRV) I was still getting back drafting at times with my wood stove if a bath fan and the dryer were on at the same time. The wood stove came with a 4" proximity fresh air duct. To help the back draft issue I enlarged the duct to 6" and installed a manual damper that I can adjust airflow. This would not work if your fresh air goes directly into the firebox. I figured what better place to have make up (cold) air coming into the house but right at a hot wood stove.