I’m more convinced than ever that DUCTED hvac is not the way
I’m building a two bedroom granny flat. 1120 sf with 800 sf conditioned attic.
in North Texas. the heating load is about 21kbtu on the worst day.
I mentioned in another group that I would be doing FOUR single zone mini splits(1 per bedroom, 1 in living room/kitchen, and 1 for the conditioned attic.
)and everyone was against this. They said a central unit would be much better.
I’ve thought about it a lot and a ducted system makes even less sense.
for information I would most likely be looking at four 9000 btu single zone minisplits, as they can be bought and installed for about $500 each. there are 6k units available but they start at $1400
1. a right sized ducted system can better control humidity – So why is it that every single high performance build I’ve seen in this climate zone using a central ducted system still installs a whole house dehumidifier?
2. the single zone mini splits will short cycle – I think the opposite. when the conditions are right for a single zone to short cycle, you can use one or two to condition the whole house. it should actually be better than a central 24k unit.
3. with a central unit, the whole house is at the same temperature. so if the occupant wants to sleep at 68f, then you have to cool the whole house to 68f. this makes absolutely no sense to me. with the minisplits you can cool a 150 sf bedroom and leave other units off.
4. better filtration. yes, ducted units CAN have better filtration but if you are exchanging the air at the recommended .35ach with an erv you can use the ERV to filter the air just as well as or better than ducted unit, which is the plan.
5. where do you put the thermostat for a ducted unit a house with passive solar heating? the living room and kitchen are south facing and will have passive solar heating via windows. the north side bedrooms will not. so on a sunny winter day the living room is overheating while the bedrooms remain cold, so where do you put the thermostat in this situation?
6. efficiency losses with supply and return ducts
7. the grossness that will accumulate in the ducts over the years and will eventually have to be replaced. at least with a ductless you can see the gunk and clean it.
8. redundancy : if a ducted unit goes out there is no conditioning. with the ductless unit there are other units that can step in and provide relief.
In summary, I just cant see any benefits to ducted central system in a high performance small house
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You are running into one of the more significant conundrums of small efficient houses - there just aren't any small enough minisplits. Individual minisplits for each room are certainly going to provide more and better control, but installing oversized equipment, whether ducted or not, creates other issues. Given the alternatives, I think I would still go with the individual minisplits rather than a ducted system.
One alternative might be to use one medium sized minisplit for the entire first floor, with the head mounted near the center of the house. Use some of the cost savings to better insulate the north facing bedrooms. If the doors to the bedrooms are typically left open during the day in a well-insulated house, they won't run at much of a temperature difference to the main spaces. Passive transfer grilles between the bedrooms and main spaces would also help.
As to some specifics of your questions:
1. There are lots of reports of dehumidifiers being necessary in well-insulated, tight houses. Especially in the shoulder seasons when there is little conditioning necessary, moisture will accumulate inside. An ERV doesn't help, because it just keeps bringing in more moisture from outside whenever the outside absolute humidity is higher than inside. I suspect that is often the case, even in north Texas.
2. My suggestion of skipping the units for the bedrooms goes in your direction. Depending on the design, the attic unit could potentially provide all necessary cooling in low demand conditions.
5. Put the thermostat as close as possible to the center of the house, preferably shaded from any direct sunlight. With passive solar heating, you always get higher temperature differences between rooms. Some people like this. Some don't. Again, leave bedroom doors open and use transfer ducts to minimize differences. If you have to, you could install small fans in the transfer ducts to further minimize these differences. More exterior insulation also reduces these differences.
18btu/sqft is heat load for a well sealed uninsulated masonry building in zone5.
Somewhere a number is off on the heat calc in your warm climate. I could see 2 tons of cooling load if there is excessive unshaded glazing though.
In your climate, bedroom heat loads should be under 10btu/sqft, that means even a 6k wall mount is 3x to 4x oversized, never mind the 9k. You are also installing a small minisplit farm with four outdoor units on the side of the house.
For these smaller structures, the setup that works reasonably well is a single slim ducted unit for the bedrooms and a wallmount for the main living space. ERV connects to ducted unit to feed fresh air to the bedrooms with stale air pickups in the bath and near the kitchen. You can then set the bed ducted unit lower at night to keep the bedrooms cooler.
ERV filtering is not enough. In a radiant heated home, I have a merv10+HPA filter on the outlet of my ERV and still have to run a stand along air filter to keep dust down. The problem is the biggest source of dust in most tight homes is people, our clothes, dryer and fabrics. Filtering on the ERV won't pick any of that up.
With slim ducted units, I put the thermostat in the hallway to the bedrooms. If you over-provision the feed to the bedrooms, you can also do some local zoning by installing a zone damper plus local thermostat. This can reduce the flow to the rooms so if you have un-even solar gain, it can help balance out the system. Key is to adjust the damper so it never fully closes but instead used to modulate the airflow.
With a well sealed return, proper filter frame/filter grill (sealed around air filter) and decent filters, the ducting will be pristine. Dusty ducts come from sloppy installs.
I have also cleaned a number of minisplits over the years, if you think ducts are bad, look a minisplit that has been running for a couple of years. They are also a real pain in the behind to clean, times four and that is a whole day job every couple of years.
Akos, as much as it goes against the crux of my argument, I have to say I dont hate the idea of a ducted unit for the bedrooms and a single unit for the living area ( I could make this a dual zone unit and include the conditioned attic as that head will seldom be used)
what I like about the ducted unit for the bedrooms is that since the erv is intended to dump into the bedrooms, that is where the humidity source will be, so with a ducted unit into the bedrooms I can add a whole home dehu to the same ducting to mitigate humidity.
I can also run the erv to the same ducting (in the return) so it doesn't add significant cost since I would have needed ducting for the erv anyway.
I think I will put the thermostat in the primary bedroom
“#1 So why is it that every single high performance build I’ve seen in this climate zone using a central ducted system still installs a whole house dehumidifier?”
Because much of the 9 months of the year the outdoor temp is very close to the indoor set point temp. This means the HVAC equipment is pretty much not running. The outdoor humidity is very high and the humidity get indoors and people are not comfortable at the normal set point. The options are to lower the set point and be cold or install a dehumidifier.
#2 Is a straw dog argument.
#3 in a tight well insulated home the rooms on the same level are going to almost the same temp.
“#5 where do you put the thermostat for a ducted unit a house with passive solar heating? “
When you have rooms that have drastically different loads as you described. The best answer is to install multiple sensors and use the average reading to control the system.
Note Passive solar is a quaint idea that has failed in real life but not for the lack of money and the efforts of smart people. The people that think it works for them all built tight well insulated homes that work in spite of the big south facing window not because on them.
“#6 efficiency losses with supply and return ducts” If all the ducts are inside the conditioned space there is zero losses.
#7 Cleaning mini split heads is a pain in the what it. The filters in mini slits are a joke. Supply ducts will be clean unless someone runs the system without a filter. Returns are irrelevant as they lead to the filter.
It seems you are the only person in TX smart enough not to fill their attic with HVAC ducts and equipment. Why would you choose to needlessly condition 800 sqf of attic almost certainly with spray foam at a measly R30 or so when you could have R60 on the attic floor for that would lose half the energy and cost 75% less to buy.
Walta
Walta, I agree on all points. We are using ducted systems on most of our projects now, even though they are not very common here in Maine.
walta100,
"When you have rooms that have drastically different loads as you described. The best answer is to install multiple sensors and use the average reading to control the system."
that seems like the worst of all worlds. then some rooms will still be too hot while others will be too cold.
“#6 efficiency losses with supply and return ducts” If all the ducts are inside the conditioned space there is zero losses."
the fan still has to push through potentially 100s of feet of TEL so I assume it will still be less efficient than a ductless unit
"Note Passive solar is a quaint idea that has failed in real life but not for the lack of money and the efforts of smart people. The people that think it works for them all built tight well insulated homes that work in spite of the big south facing window not because on them."
I built my house as a passive solar house (in TX, I know, crazy). I think it works rather well with the exception of our annual weekly freeze). The living area can get to 90f on a sunny winter day which allows us to open windows wide and get the FRESH FRESH outside air.
"It seems you are the only person in TX smart enough not to fill their attic with HVAC ducts and equipment. Why would you choose to needlessly condition 800 sqf of attic almost certainly with spray foam at a measly R30 or so when you could have R60 on the attic floor for that would lose half the energy and cost 75% less to buy."
been there done that with spray foam. never again. this will be flash and batt using reclaimed xps. 10% of the cost of spray foam and much more uniform.
I'm not really sure why we waste our attics on hvac here in Texas but that 800sf is valuable, very inexpensive, conditioned space that can be used for other things
In your post you say a "central unit". Having a central unit does not necessarily mean the system is ducted. There are multizone ductless split systems with a central exterior heat pump and multiple indoor ductless units. Could the people in this other group have been referring to a central ductless system?
No. The amount of hate they have for multi zone mini splits is shocking.
Not shocking but well founded. I have a multi and will be replacing it with a ducted unit in the near future. Why? Bad humidity control, bad room temperature control, noisy operation, low COP. About the only good thing about it is the single outdoor unit.
As for your project, you don't need a head in the attic to condition it. What I would do is mount a stand alone dehumidifier up there and also include a small supply vent from the ducted air handler and a transfer grill to the hallway bellow. This would both condition the space and also help manage house humidity.
In your climate, you can do an unvented conditioned attic with fluffy only if you include a diffusion vent:
https://buildingscience.com/documents/insights/bsi-088-venting-vapor
Sealing the soffit area will still be challenging, a small amount of spray foam there is really the simplest option. Cut and cobble rigid sealed in place with canned foam would also work if you don't mind the elbow grease.