I have read through the "best practices" literature for installing an HRV unit.  I know the most effective/efficient design places the unit and all of its associated duct work within the conditioned space, with similarly-lengthed runs pulling from and supplying each main living space and bedroom individually, minimize bends, etc.  This obvioulsy just isn't possible for some layouts.  I have seen the alternate methods in which the pull from/feed to ducting is integrated in various ways with the central HVAC system's feed line, return line, or both.  This, by my understanding significantly reduces the unit's efficiency and balancing pressures of the two systems effectively is tricky, at best (and borderline guess work the way the manual made it sound).  That bummed me out because, before I even got to that part of the literature, I had thought about integrating the HRV with the HVAC ducting as being a viable alternative to a secondary independent duct system.

So, with that said, would it be at all "acceptable" to just have a single feed and a single pull line at opposite ends of the main living space (ie. pull line near exterior south wall in a dining room and feed line near exterior north wall in living room).  This is on the assumptions that the HVAC fan is set to continuously circulate air, that these single HRV lines are sized to match flow requirements, and that each bedroom has it's own return air vent.  It seems like this would get fresh air into the house and use the HVAC ducting to distribute it... but without the guess work of balancing pressures between the two systems.  The fact this makes sense in my mind tells me there must be something blatant that I'm missing.

I have been doing the same research as you. I think the reduced efficiency they talk about is because the HVAC fan has to run at the same time.

I was thinking the same as you about 1 supply and 1 return but not having to run the HVAC fan continuouly. My HVAC runs almost all the time anyway, slow and steady.

Chris

would it be at all "acceptable" to just have a single feed and a single pull line at opposite ends of the main living space

Not ideal, but why not? When designing an HRV system, I think it is important to work with the natural airflows within the house. If you have a forced air system, it is obvious that system becomes the natural flow. You would want to make sure that you weren't bringing the fresh air in at a point where the HVAC could quickly force it back out again. Will your HVAC system run all night, continuously bringing fresh air to each of the bedrooms?

I currently have the fan set to run continuously.  It helps balance out the temps between the rooms; and I figure keeping the air circulating through the filter can't be a bad thing from an air quality standpoint.  It's an ECM fan; so there isn't much extra electricity being used.

Well I'm pleasantly surprised at the apparent lack of opposition to this approach.  Seemed too good to be true... I was prepared to get shot down (and learn something along the way).

If you are really bringing in the fresh air and moving it around to the rooms, then the house is getting ventilated and the only downside is the extra electricity being used. Have you measured what that is?

One survey of systems back in 2005, for all types of motors, showed that furnace blowers used about 0.5Watts/CFM of air moved. A small study in Building Science showed that ECM motors in furnace systems didn't really turn in a much better showing, with a range of 0.33 W/CFM to 0.5 W/CFM. If you assume that a modest system moves on the order of 1000-1200 CFM, you are looking at a usage of 10-15kWh per day for running the fans alone, not to mention the HRV usage.

I don't doubt that an ECM fan uses the same energy per CFM as a multi-stage fan strictly looking at the numbers; but I would think it could use less energy per day in a real-world application if both fans are left to continuously run due to it's ability to ramp down.  If you have a two-stage fan with 800CFM for low and 1200 for high, it's going to always be pushing at least 800 CFM.  With an ECM fan, it could ramp down to 400 CFM when the unit isn't actually heating or cooling but air circulation is still desired.  I don't know how realistic those numbers are since I'm not an HVAC guy; but I think the general concept holds true.  I have a 4 ton two stage WF Envision GSHP with Carrier Infinity LGP furnace with ECM fan.  Set to continuous, the air flow is barely audible when the unit is running; but not audible at all when there is no heating/cooling even though I can still feel steady airflow from the vents.  I do know that the proposed set-up in general is not optimal; but does seem viable given the design an accessibility constraints of the house.

I think what you find is that as the ECM fans slow down, you lose some efficiency, so you save some, but not in a linear fashion. You might want to put a clamp on that fan motor circuit to see. If 400 CFM/ton is a rule of thumb, then you are using even more juice than the estimate above.