Fifth port HRVs have recirc built in. The fifth port substitutes house air for fresh air and switches between the two with a single controller. Defrost is faster and easier as well. I can't remember seeing a fifth port ERV, probably because they don't need defrosting.

Lbear, minisplits and concrete floors are surprisingly compatible in AC mode. Last summer, there was almost no temp difference between the living area directly under my mini and the downstairs bath, 35 feet away, around the corner and behind a door that was seldom open. This was without the recirc system. Again, architecture matters as much as hvac design. Cold air in a big open room hits the floor and rolls to the door by convection. Conduction takes it under the door.

Upstairs was a different story. I need circulation up there, but there should be enough mass to approach it as a trickle charge problem, a la radiant heat, and the cooling load is perfectly even. This is a house in complete shade when the leaves come out.

On heat, the mini has kept the downstairs at 64 or better even after five sunless days. Convection keeps the upstairs about 10 degrees cooler. This is with a two-ton heat pump in 1600 SF house. The radiant floors are next. Looking forward to asking the long suffering missus, "how warm do you want to be?"

Posted By ICFHybrid on 27 Dec 2012 08:20 PM

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?

I have a Venmar EKO 1.5 HRV, and it has both fresh air and recirculate modes as well as splitting time between the two. In the fresh air mode, the flow rate can be set to approx. 70 cfm (33 l/s) or 140 cfm (66 l/s), but in the recirculate mode, the higher flow rate is always selected. It accomplishes these various options using two ECM fans and one movable door. There is a picture and some details at http://www.residentialenergylaboratory.com/rel_energy_use_hrv.html.

I use the recirculate mode, either part time or full time to try to even out interior temperature differences due to intentional passive solar gain at the south end of the house. The 140 cfm helps, but it still leaves significant temperature differences. In the recirculate mode, the HRV pulls air in from both ends of the sealed crawl space that has four passive vents to the first (and only) floor of the house, and then the outlet from the HRV is tied to the outlet of the hot-air furnace.

I augment the temperature redistribution with the "circulation" mode on the hot air furnace that runs a higher flow rate blower intermittently. Even with both the furnace blower and the HRV fans operating, there is still some temperature difference between the south and north ends of the house.

As an aside, jonr has mentioned balancing the HRV flows to avoid pressure differentials across the house shell. One way to do that is to balance the temperature drops/gains for the fresh and stale air flows. I have attempted to do that with results presented in the appendix of the link given above.

Sorry, ICFHybrid, I didn't see your question about exhausting the great room. The great room gets most of the passive solar heat gain in the house. The circ system was meant to limit overheating by moving it through the house. Turns out that mass is more than equal to the job in Pa, although April will be the key test. (I am still in shade in October) The minisplit was another "surprise" but a happy one. We are more comfortable than the folks around us who turn down their propane furnaces to 60. (Still working on true, radiant comfort.) The economics are compelling given a $1,500 capital cost and 7-cent electricity.

Kind of disappointing that most of HVAC designers have very little knowledge about energy build homes. I checked around and called around but nobody knew how to properly do a HVAC design for an ICF home.

The one quote I did get was $6,000 to do the HVAC calcs.   Absurd!

LG now makes ductless mini splits that are more aesthetically pleasing. Some have cherry finishes or mirrored finishes. The one below looks like a piece of art hanging on the wall but it's the actual indoor unit and it blows air from the sides and top.

There are also mini-split heads that can go in the attic, so only a grate like a typical AC vent is visible. They cost quite a bit more than the wall units, though.

avoid pressure differentials across the house shell. One way to do that is to balance the temperature drops/gains for the fresh and stale air flows.

I would think that actually measuring the pressure differential between each room and outdoors would be much more accurate.

How do mini splits counter the pressure differentials like that of a typical central HVAC unit? Since ducted systems have the main supply ducts and then the return ducts, how would a mini split address this issue?

The intake and the exhaust in a minisplit cassette are only a few inches apart. There really isn't a "pressure differential". They can blow air to the corner of a normal sized room and that's about it. Apart from that, something else has to move the air from room to room.

Posted By ICFHybrid on 03 Jan 2013 12:46 AM
The intake and the exhaust in a minisplit cassette are only a few inches apart. There really isn't a "pressure differential". They can blow air to the corner of a normal sized room and that's about it. Apart from that, something else has to move the air from room to room.

I am looking to install a ceiling fan and there will be an ERV duct at the wall/stair area, approximately 30 feet from the mini split wall unit. This is in a very open area with a vaulted ceiling.

I assume there is a way to hide those refrigerant lines that go to the outdoor unit by running them within the EPS foam on the outside of an ICF wall, foaming it and then stuccoing over it?

Some of the retrofit pictures I've seen make the line runs look gaudy on the outside. They should at least paint them house color instead of having this white plastic cover running down the wall to the unit. 

Do you have a mini split? Any pics of the install into the ICF wall?

There's not much to see. We installed them early on so all you do is cut channel in the foam to the desired location and hang a piece of the future sheetrock, then attach the minisplit unit over that. Later, the sheetrockers just come up to the edge of the sheetorck and mud around it. Same plan over wood framing where the linesets can run within the framing like regular plumbing.

Posted By jonr on 02 Jan 2013 05:17 PM

avoid pressure differentials across the house shell. One way to do that is to balance the temperature drops/gains for the fresh and stale air flows.

I would think that actually measuring the pressure differential between each room and outdoors would be much more accurate.

Matching the temperature drops/gains across the heat exchanger assures a match of the mass flows across the unit.
delta_H = m_dot x C_p x delta_T
Since delta_H is a conserved quantity, then,
delta_H_fresh = delta_H_stale
m_dot_fresh x C_p_fresh x delta_T_fresh = m_dot_stale x C_p_stale x delta_T_stale
Assuming the C_p_fresh = C_p_stale, then
m_dot_fresh / m_dot_stale = delta_T_stale / delta_T_fresh
So match the temperature drops and you match the mass flows. Match the mass flows and the average pressure differential is zero since there is no net mass flow in or out of the house.

This should be a more precise measurement than trying to measure inside-to-outside pressure differentials in the presence of winds, etc.

the average pressure differential is matched

Balancing the average is worth doing, but it doesn't mean that individual rooms don't have severe problems. Perhaps if all the doors are open.

> LG now makes ductless mini splits that are more aesthetically pleasing.

I think that concealed air handlers are much better looking that anything hanging from the wall - for example, a Mitsubishi PLA-A**-BA

Posted By jonr on 03 Jan 2013 10:18 AM
Balancing the average is worth doing, but it doesn't mean that individual rooms don't have severe problems. Perhaps if all the doors are open.

On modern homes (built in the last few years), the gap at the bottom of the interior doors is 1" (25 mm) or greater, so that the doors do not need to be open to balance the pressures. If the full output of an HRV supplying 140 cfm were directed into a single room with a 1" gap at the bottom of a 30" wide door, the air pressure drop between that room and the rest of the house would be about 6.8 Pa = 0.03 inches of water. In practice, the full HRV output would not be directed into a single room, so the pressure differences are even much smaller since the pressure drop scales with the square of the air flow rate.

So balancing the temperature drop and gain between the fresh and stale airstreams at the HRV heat exchanger should guarantee that the HRV is not introducing a significant pressure drop between the interior and exterior of the house whether interior doors are open or closed.

Assuming that you don't have a ducted hot air furnace or AC. Clearly that isn't always true.

I find 1" door gaps obnoxious and would expect transfer ducts in a quality house. Over sized transfer ducts to closable rooms with anything that effects pressures (eg, clothes dryer, etc).

Posted By jonr on 03 Jan 2013 01:50 PM
Assuming that you don't have a ducted hot air furnace or AC. Clearly that isn't always true.

A ducted heating or A/C system should have outlets and return ducting in each room, so it should take care of itself in terms of pressure drops between rooms.

The object of balancing the HRV flows is to make sure that it is not introducing a pressure imbalance across the air barrier of the house. An overall balanced HRV flow of, let us say, 20 cfm to each of 7 rooms in a modern house with jump ducts or 1" gaps under the doors simply will not produce significant pressure drops across the air barrier of the house. If this 20 cfm is added to the heater or A/C airflow, it will not make a significant difference to the pressure differential from what the heater or A/C is already producing. Show me the numbers if you are finding a different result from the one that I already gave.

A ducted heating or A/C system should have outlets and return ducting in each room, so it should take care of itself in terms of pressure drops between rooms.

That's the point - it doesn't "take care of itself". You should balance them with a pressure sensor. Or use transfer ducts (in place of or in addition to returns) to alleviate the differential pressure issue.

We agree on the HRV only, largish door gap, no ducted heating/cooling case.

Balancing the heating and A/C system is a separate issue from balancing the HRV system flows. My post only gave information on how to balance an HRV. There is no way to use the HRV to take care of imbalances in the heating or A/C systems.