Looks like I will have some difficulty running duct work into 1 or 2 rooms. Is it OK to install mini-split units in those rooms to make up for the lack of central HVAC duct work? It is a very open vaulted ceiling design so there is a lot of air circulation in those rooms.

I see that they make units that run off of one exterior compressor unit but it can run 2 interior wall units.

If you have a well-insulated and well-sealed home, is it necessary to provide a heating or cooling source in every room? We are having overnight temperatures near freezing and I am providing comfortable heat in my unfinished place with one mini-split in the basement and one on the main floor. Nothing on the second floor. The place is more than warm enough to keep the drywall activities on the drying path.

no it isn't necessary to provide a heating or cooling source in every room. That is one of the lesser known outcomes of building a superinsulated home, and the reason minisplits work as heating systems for these houses.

I did not know that in a super insulated home you don't need duct work in every room. This will help with the design of the HVAC systems.

So even an airtight home that has mini-splits still must have a separate ERV/HRV setup, correct?

Are you having issues with running ducts perpendicular to your concrete floor decking? Most types of insulated concrete floor decking (Lite-Deck, Insul-Deck, etc) have at least 2" of foam underneath the concrete support ribs. You could remove a 16" to 24" wide x 2" deep section of foam from the bottom of the insulated decking then have an HVAC sheet metal shop fabricate a rectangular duct to fit in this space with transitions on the ends to adapt back to round duct. This would definitely be less expensive than installing and maintaining a separate HVAC system for just one or two rooms.

A 6" round metal duct has a cross sectional area of ~28 in sq. A 16" x 2" rectangular duct has 32 in sq of cross sectional area but more frictional losses than an equivalent 32 in sq round duct.

Below is a field duct sizing chart (google it for printable pdf options). It shows that a 6" round metal duct is rated for 85 CFM air flow. In the lower left is a size chart for 2-1/2" x 14" rectangular duct (35 sq in) and is rated for 70 CFM. To get 85 CFM from a 2" rectangular duct would like require it to be ~20" wide (40 sq in).

http://www.scribd.com/doc/42157897/...zing-Chart

So even an airtight home that has mini-splits still must have a separate ERV/HRV setup, correct?
yes, one is heat, one is verntilation. In a tight house you need mechanical verntilation

I did not know that in a super insulated home you don't need duct work in every room.

I depends on the particulars (mostly outdoor temp and window area), but such rooms can be 10F or more colder than other rooms unless you force some ventilation to the heated areas of the house.

"I depends on the particulars (mostly outdoor temp and window area), but such rooms can be 10F or more colder than other rooms unless you force some ventilation to the rest of the house."

In a truly superinsulated house, I doubt that you'd ever see a 10 F temperature difference between the coldest room and an adjacent area where heat is introduced. You'd need an awful lot of window area and perhaps insulation in the walls separating that remote room from the rest of the house to see a big difference. In our house, the upper level has two zones, one being for the two bedrooms on one end. We like the bedrooms cooler for sleeping, so we have to keep the BR doors closed all the time, and then we've never seen the BR more than 5 degrees below that on the rest of the floor. In winter, we just leave the thermostat for the BR zone set down in the 50s, but it never gets anywhere near that. If we leave the BR doors open, the BR temps rise to within a degree of what it is in the adjacent heated area. Two years ago, when the inside of the new house was being finished, we used a small woodstove on the lower level for heating the house. With the stairwell door and all other doors left open, the warm air distributed itself fairly well through the house. I never saw the upstairs cooler than a degree or two below that in a remote area of the lower level.

, I doubt that you'd ever see a 10 F temperature difference between the coldest room and an adjacent area where heat is introduced.

It's correct even for common cases and with super insulation; the calculations aren't hard. It does vary greatly with the details - window area, outside vs inside wall/ceiling area, etc. In some cases, 2F.

Getting a reasonable heating balance with a mini-split is a bit easier to do (and calculate) from a heating perspective than a cooling point of view. It doesn't take a lot of solar gain from windows to blow peak temps out of the comfort zone on a room that would be just fine from a heat loss point of view without it's own heating supply.

Leaving the doors of the high-gain isolated rooms open to the actively cooled area helps significantly, but that's not always an option- it depends on how the room is being used.

It is not too difficult to use hrv/erv ductwork to circulate air as well as exchange it. You need either a fifth port ventilator or a second fan for continuous circulation. In the second case, you also need a 120V power relay to cut the second fan off when the ERV/HRV goes on by timer or humidistat, plus a wye connector and one way damper. With 6 inch duct, it pretty much has to be continuous circulation at low speed unless the the house is tiny. But an ecm fan is roughly equivalent to a 60 watt light bulb.

I'd like to see testing of how ERV/HRV systems without matched supply and return in each closed room effects infiltration (and ventilation). Certainly pressurizing or depressurizing a room is conceptually bad and it has been shown to have huge effects with forced air furnaces.

I agree that leaving the door open mostly fixes this and the heat distribution issue (in very well insulated homes).

Architecture matters. In my case, I am drawing air from four ducts in a great room that accounts for half of the house's square footage, and distributing it to the other half. Since the remainder is largely upstairs, open doors and an open stairwell handle the return. At any rate, 100-120 cfm isn't creating much vacuum, or sound or convective cooling if you're sitting by a register. Continuous circulation works very well as long as you don't have rooms with extraordinary conditioning demands.

At a 5F delta-T between the ventilation air supplied and the room air you're only moving about 1BTU per cubic foot. The whole 100cfm (6000 cubic feet per hour) would still be only deliver a half-ton cooling at a 5F delta. Of course that automatically doubles to one ton when the delta-T reaches 10F.

A doored off room with an unshaded west facing window could easily see cooling loads bigger than what the ventilation air alone could handle when in full afternoon sun. But with open doors AND whatever cfm the ventilation system provides it could even things up substantially. It really depends on the peak cooling load of the room and it's ventilation flow (convected & mechanically driven.) The individual room cooling load can easily be several times the room's peak heating load, due to the solar gains through windows (even in high-R houses with fairly low-SHGC windows.)

10 cfm pulled from a closed room has to come from somewhere. If not under the door, then perhaps (or more likely partially) through the exterior walls. At 0F outside, that's up to 750 btu/hr or like leaving a 200 watt light on.

Sounds like transfer/jump ducts are a good idea if you have only a HRV supply or return to a room where the door will be closed.

Regarding solar gain in closed rooms, I agree - ducts and zoning are needed for good temperature control.

Jump ducts or grills etc are a given for any ducted system to have any control over where that air is coming from/going-to. Even fairly tight houses/rooms will leak air if pressurized/depressurised. At typical HRV flow rates (rather than AC or heating flow) door cuts can work, provided it isn't too close to the isolated supply or exhaust duct.

Sleeping adult humans are worth ~250BTU/hr, and that sometimes makes a difference when adjusting the heating temperature balance with ventilation systems in high-R homes, (but not so much for most homes.)

You need either a fifth port ventilator or a second fan for continuous circulation

Don't some heat recovery units have recirculation built in as an option?

A doored off room with an unshaded west facing window could easily see cooling loads bigger than what the ventilation air alone could handle when in full afternoon sun

Really, the best plan remains to be not letting it in in the first place. The backup plan is to keep the door open, followed by passive ventilation and active ventilation as a last resort.

In my case, I am drawing air from four ducts in a great room that accounts for half of the house's square footage, and distributing it to the other half

So, you are exhausting the Great Room? Why didn't you SUPPLY the Great Room and exhaust it elsewhere?

Any recommendations on a competent mechanical engineer or at least someone professional who can do a proper heating/cooling calc on a home and design a proper system?

Oho, now you are asking for something. You could probably get this on the internet, but to expect to find it in just any jurisdiction would be ........difficult. What you need is some hippies who love residential yet have to do commercial to make ends meet.