Pairing split systems with ERVs
A short preface: I am a “leftover” student at Auburn University’s Rural Studio in Newbern, AL. Prior to being an architecture student at Auburn, I worked full-time doing commercial electrical for about 3 years and masonry for about a year which has given me a great introduction to construction (and then 4 years as a Marine infantryman basically prepared me to be a homeless alcoholic and gave me a really dark sense of humor). I graduated in May, but my team and I are finishing up our thesis project here in Hale county which is a ~900 sf, 2-bedroom “20K” house (version 23) with a focus on aging-in-place and especially how that relates to accessibility. As you might imagine, a vast majority of the homes in rural Alabama are inaccessible to someone unsteady on their feet while over half of the households here have someone living with a disability.
This relates to ductless mini-splits and ERVs because we are also considering financial accessibility in addition to physical accessibility, and how operation costs affect affordability.
At this point, we’ve got a raised slab (with R6 sub-slab Rockwool ComfortBoard 80 mineral wool insulation below a 6 mil vapor barrier) , 2×6 walls (adv. framed, anticipating R23 Rockwool ComfortBatt), trusses, metal roof, and OSB sheathing (R15 at the ceiling between trusses, R23 across the top of the truss bottom chords to limit thermal bridging), and ZIP sheathing on the walls (taped across top plates above and to the embedded termite shield below to create a continuous air barrier). We’re basically a ridge cap and a front door away from being dried-in. We are currently installing electrical and plumbing systems in preparation for drywall in a few weeks.
We really want to get the ERV right, and I believe the air conditioning system and the ventilation system must be considered as a pair instead of as divorced systems. The mini-split will probable be ceiling 0r wall mounted and would likely be ductless. The hope was that we could extract stale air from the bedrooms and return fresh air into the main central space, where the split system will temper the air to be drawn into the bedrooms (see attachment). Does that seem like an effective option? One of the issues of this placement is that the trusses run vertically in the drawing, so that duct would be in the unconditioned attic.
I realize the system will only work as well as it is understood and maintained, and our intent is to help the homeowner understand what the system is doing and what must be done to make it continue to function effectively.
Tips, tricks, and recommendations?
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I can tell you that pulling X CFM from the interior of house into a closed door bedroom and pushing X CFM of outside air into a closed door bedroom are not at all equivalent in terms of CO2 reduction. The former may require more than 2X the airflow (say 40 CFM/person). But disregard if the intention is to keep bedroom doors open. If not, you have a heating/cooling problem that won't be fixed with ERV air movement.
Jed,
Trying too cool a place with doors with a central unit means uncomfortable bedrooms. The only way to make that work is with open doors, which might not be for everybody.
A better solution would be to nudge the washer and dryer over a bit and install a vertical ducted mini split against the wall there (they are about 8", looks like there is enough space).
You can than put a return air grill on the bottom right into the living space and feed all the rooms from there with minimal ducting. This would mean that nothing would need to be mounted into the attic.
Looking at the simple layout and short runs, you can probably get away with flex duct for most of the supply runs.
Since the blower on the mini splits runs all the time, you can connect up the ERV fresh air supply to the return of the unit, this way the air will be tampered and distributed by the split as it runs. This would save you having to run the supply ducting for the ERV.
Since there is only one bath, you can put a larger stale air pickup for the ERV there and eliminate the bath fan. Another pickup through the wall into the living space is all the ERV ducting is all you would need.
You don't get as much control over the air distribution with this setup vs a fully ducted setup, but simpler to install.
Also, looks like you show a french door fridge right beside a wall. The doors on those typically need to open more then 90 deg to be able to remove the drawers. Check with your fridge specs, but most likely you need to move it something like 12" to 18" away from the wall.
Akos,
We have the space along the wall in the bathroom, and we actually have considered the option of dropping the ceiling in the bathroom to run ducting from the mini-split to the bedrooms. I certainly believe we should design the system as if the doors were all closed, and I think placing that mini-split either there at the side of the washer and dryer or above the ceiling would give us flexibility in where to run those ducts to better serve the bedrooms. The problem with dropping the ceiling is that we are planning on using a compact water heater above the dryer and it may not fit with the desired clearance (have had bad performance outcomes with tankless in our location). But we will have to do either exposed ducting or some encasement to create a chase.
Either way, I think it would be wise to keep the ducting in the conditioned space and avoid running through the air barrier (ceiling drywall) whenever possible. We can take a closer look at this and I'll post a plan of a few options.
Is there an issue with using the ERV to exhaust very humid air from the bathroom?
Also, if the blower doesn't run continually (I'll have to check whatever split system manufacturer we go with on that), would the ERV have enough power to push the necessary volume of air through the mini-split and to the bedrooms if we used supply air from the ERV to feed the mini-split? I suppose that is something we would have to calculate.
You shouldn't need to drop the entire ceiling to fit in the ductwork.
With 8' ceiling and 80" doors you have about 12" above the door.
Working with a vertial mounted mini split. Ducted units have about 30" wide outlet, roughly speaking you can split it up as 8"x10" side elbow to feed the MBR and living space, 8"x8" eblow for the kitchen and 8"x6" side elbow for the bathroom and 2nd bedroom.
All that ducting should fit into a 12"x10" bulkhead above the door. You would need a long throw diffuser for the kitchen to avoid short circuiting as the return is right bellow it.
If the blower is not running the fresh air form the ERV would go out through the return grill into the living space as it is the path of least resistance. Very little of it would make it to the bedrooms.
If this is the case, you might need to interlock the fan of the mini split with the ERV. Something like a Fujitsu 9RLFCD blower is 80W on high, so on low it should not consume that much electricity.
There is no issues with the ERV exhausting moist air from the bathroom. On boost, most have around 50% latent recovery, so most of the moisture will go outside. The bit that gets recovered will eventually be removed by the mini split.
An ERV will not dehumidify the place. In hot/humid area all ventilation system introduce outside moisture to the house, an ERV just brings in less.
To control humidity you need something active to remove the moisture, most mini splits have a dehumidify option that works is most cases. It might not be enough during the shoulder season when there is little need for AC, really depends on the occupants if a separate dehumidifier is justified.
> Since the blower on the mini splits runs all the time,
I'm not convinced this is true under very low or no load conditions. If it is, the lack of latent removal is an even bigger problem (they already have a problem with low fan turn-down ratios). In any case, build a spot to house a dehumidifier (drain, power, airflow).
I have a 5"x12" false ceiling beam/lintel between two walls/rooms that looks fine. Something similar could be a duct chase (ie, you don't have to put ducts in the attic). Looks about like this. Or maybe there is a way of running a duct above the bathroom door.
Most mini-splits can be set to run the blower all the time, but that makes for lousier latent load cooling, and the speed is user-settable.
>"...I believe the air conditioning system and the ventilation system must be considered as a pair instead of as divorced systems. "
The notion that the ventilation is best married to the heating or cooling is generally false:
Heating or cooling varies by the hour and season, whereas ventilation is needed whenever the house is occupied, independent of the heating or cooling load. These are good grounds for a divorce, even MORE so in high-R/low load houses that can be heated & cooled with a single mini-split.
The splits I've worked with (LG and Pioneer) run the blower continually whenever the unit is put into heat or cool mode. This is pretty much the same thing as I've seen when travelling overseas. I know some units can be configured to shut the blower off, doesn't seem to be the norm.
Running the blower all the time is bad for moisture removal, this is especially true with older non-inverter units. You can feel the puff of humid air each time the compressor cycles.
The Fujitsu (ductless) I just had installed only runs when cooling is active. It's not blowing air most of the time.
I think we could do something similar to what you linked to with a space to run the ductwork. It might help to separate the bathroom from the laundry spatially anyways.
A dehumidifier would probably be necessary, if for no other reason than it is Alabama and the humidity seems to be 90% all day every day. Would the ERV be able to do both functions, or does that have to be a separate system?
Here’s what I’ve got right now from the suggestions. I think it will work well, I’ve just got to get in touch with Mitsubishi and find dimensions for the ducted split. When we drop the ceiling, we’re going to clad the bottom of the truss with taped zip panels so that the drywall contractor won’t have to come back after we frame. Most everything will be below the air barrier.
Jed,
That is a pretty good plan. From the mechanical side, make sure there is an access panel about 6" larger than the mini split in case it needs to be serviced, good to have extra clearance where the refrigerant lines and electrical connections are.
Keep in mind the Mitsubishi slim ducted unit has a pretty low static pressure blower (~0.25" water), check your duct design and your intake filter size to make sure the unit's blower can supply enough flow.
The drawing doesn't specify the mini-split returns. It appears to be a single common return located in the bath rather than ducted from each room. With the room doors closed/open it's going to dramatically affect the static pressures on mini-split, and upset the balance on the ERV. You may be able to design in sufficient jump-ducts to the bathroom to use it as the return plenum, but with the Mitsubishi KDs you don't have much static pressure to work with. I really don't know how the ERV can stay balanced in that configuration though, even with jump ducts for the mini-duct cassette returns.
Making the bathroom as the common-return plenum will also distribute humidity and odors immediately to every corner of the house, which isn't exactly what most people are looking for. If the bath had it's own return register or jump duct to a different return plenum those effects are diminished.
Unless you can show that something else will clearly work, for each closed off room, use dedicated heating/cooling supply AND return ducts . A separate supply OR return for the ERV may work - it depends on expected closed door time, how tight the interior doors are and how you feel about building to the limit in terms of room pressurization. While 3 pascals will not have much effect on mini-split or ERV balance, 0 pascals is better (less air in/ex-filtration, better wall health). So dedicated supply and return for the ERV is the safe bet.