High performance buildings need to be air tight and have a heat recovery system – HRV or ERV – to efficiently ventilate, but to ratchet the performance up a notch, ditch the bath and kitchen range exhaust fans too. After all, every penetration in the building envelope means a leakier house. Air leakage is bad for durability, makes us uncomfortable and cost money. Hence the saying “Seal tight, ventilate right!”
What the Good Doctor Says
To quote Dr. Joe Lstiburek in one of his latest missives, “With a single point exhaust you need about three times the flow rate compared to supply air ducted to a central air handler that distributes the air an provides mixing to get similar ventilation “effectiveness”… To get exhaust only ventilation to work minimally well you need to provide mixing and distribution with the house central air handler, and even then you don’t get to filter the air before you bring it in, you don’t get to pick where the air comes from and you certainly don’t get to precondition it.”
He goes on to say “Blowing is better than sucking. Sucking and blowing at the same time is better than blowing. And sucking and blowing at the same time with mixing is better still. When you add spot intermittent sucking at bathrooms and kitchens things are fabulous. If you then add energy recovery you are over the top. But if the only option is sucking over nothing, then suck. Sucking still sucks, but it is better than nothing. But remember you need to suck a lot and sucking a lot has its own problems.”
Rubber Hits the Road
Passive Home builder Ed Marion recently built a high performance home with an airleakage rate of 0.75ACH50 and he installed a fully ducted ERV in the house for ventilation. Given the low loads in this super efficient house, there was no conventional forced air system installed. So he installed a fully ducted ERV to distribute the fresh air throughout the house. Part of Mr. Marion’s strategy for getting a really air tight home was to eliminate as many envelope penetrations as possible. Cleverly, Marion used this as an opportunity to get greater value from his ERV by eliminating superfluous ventilation systems like bath exhaust fans and kitchen ranges. It meant that the home couldn’t have a gas range, but with the high tech induction cook tops, it really increases the performance of the house significantly.
Bath exhaust fans typically are over sized and the timer doesn’t stay on long enough for the wet walls of the shower and bath mat to dry, which is the goal of a bath fan; to throw out excess humidity. The other issue with sucking only ventilation means that outside air is being pulled through cracks in the building envelope. If exhaust ventilation only then stratification happens prolonging drying times whereas the supply or “blowing” mixes the air in the room promoting drying.
Fly in the Grill
The municipal inspector asked that a grease filter be installed on the ERV Intake and that the ventilation supply and return ducts be fireproof, which they were. The problem was there was no commercially available grease filter for that small an intake grill. In the end, some creative assembling took place and the client now has a grease filter.
All this to say that Mr. Marion clearly understands that to maximise the value of an ERV, we need to make the house as tight as possible and in so doing, provide the best fresh air distribution system that delivers a precise dose of fresh air for each occupant. It just makes sense and it can be done!