Does anyone know about residential air-to-air heat exchangers and how hard it would be to retrofit one of these into a home built in the early 1900s? I think its a great idea from what I understand, I just don't know the applicability for older homes.

I personally have no experience with these systems, but I know that if you have a forced air system, you should be able to do this. If you aren't using forced air, then this really isn't the applicable technology.

I know this is an old topic but I thought I would add a little more.
The whole impetus behind the air exchanger was the requirement of the building code to completely seal up the house and keep air from moving through when it wanted to. The exchanger is a controlled way to do so and the heat exchangers are designed to try and recapture the warmth before the air exits and to temper the cold air coming in during the winter.

In an old house from the 1900's, unless they gutted the house and totally sealed it up, you are already naturally exchanging the air more often then you would with one of these units.

So in addition to the type of hvac system you or don't have, how tight the house is would be another major factor to consider.

vlstaele, while you are right that air exchangers are used in well sealed houses to ensure that there is an exchange of air and to prevent the air from becoming stale, the real benefit of a heat exchanger is energy. You are right that sealing up a house is the first step in this process and to skip that step would reduce the efficiency and benefit from a heat exchanger.

I think its important for anyone new to the sustainable home movement to know what to start with. The basics are to create the best foundation for the high efficiency technologies. In this case, sealing up the home in order to increase the efficiency and benefits of a high performance HVAC system. Another example would be to reduce electrical loads by using LEDs (or CFLs), high efficiency appliances and a whole house switch before installing a solar PV or wind turbine. In most cases the reduce part of the triangle, as it were, is the most important. Reduce your demand, your losses and your inefficiencies before adding the technology.