Alternatively, we got one piped into the HVAC ductwork. It's not as optimal as its own ducting, but it's much easier/cheaper than running a bunch of new ductwork.
That's what we did. Our house was built in 1916, but the weather sealing we've done combined with six people breathing in it led to quite high CO2 levels. We have a traditional ducted air conditioning system in the attic (heat is hot water radiant), and added the ERV there. It made a massive difference.
