Will a single head do it and what about the basement?
George_7224612
| Posted in General Questions on
The house will be a 1500 sq ft single story with full, unfinished basement in CZ 4 near Annapolis Md. I’d appreciate opinions on whether a 1 ton mini split will do the HVAC job. There are just two of us in the house and we seldom close doors. I propose to locate the head in the center of the great room wall aimed at the two bedrooms on the opposite side, see attachment.
Basement will have 2″ EPS under the slab and 2 ½” polyiso on the walls. Manual J (attached) loads are pretty low, so I’m wondering what to do to condition the space.
Thanks for any assistance.
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Replies
Hello? *tink, tink* taps glass. Anybody out there?
George,
The single head will do an OK job heating, your bedrooms would be a bit colder than the main rooms, most people find that to be pleasant.
The bathrooms would be too cold, something like an electric resistance underfloor heat or a panel heater would be needed. You generally don't want to let your bathroom get too cold as it can increase chance of mold, also cold bathrooms are just plain uncomfortable.
Cooling wise, with the doors open, you'll probably be OK. The bedrooms would still be warmer than the main space (which is what you don't want), so you might need to crank the AC before going to bed. With the doors closed the bedrooms would get too hot fairly quickly. This is why overall a ducted unit is better.
You can probably put a low static ducted head in the celing of your mudroom and with a couple of ducts feed all 3 bedrooms and living space. With such low load house you would not need large ducts, you can probably squeeze most of it into a bulkhead above your kitchen cabinets.
You can partially share the ducting with the ventillation, which is a good thing to add for any well sealed house.
Allison Bailes performed your Manual J? Now that is awesome!
Building inspector would probably require heat in the baths, and maybe the mudroom, since there is plumbing
As a side note if you opened the bath to the mudroom instead of the bedroom, it would give you a guest bath/powder room, and a shorter path when you come home after being stuck in traffic after drinking a big gulp
Since there's an unfinished basement, I'd suspend a mini-ducted unit below the floor, which would be FAR easier to install and maintain--and modify, if necessary--then a bulkhead or ceiling installation. It also makes the location flexible, and allows for a more central cold air return. A right-sized unit could include the basement conditioning, too.
You might consider double pocket doors into the bedrooms to facilitate air movement. An addition in our previous house had no heat upstairs, but double pocket doors left open to the rest of the house made a big difference in heat transfer. Pocket doors help a lot with furniture placement issues.
I would look into a Fujitsu mid-static unit for this situation. You could place it in the basement below the living room so it is in a central location and this will allow you to get individual supplies to not only the bedrooms but also the bedrooms. You could probably get away with a central return. I would also add a supply and return in the basement. I just had one of these installed and I am pretty blown away by how quiet it is. While we have not had any real cold temps yet, I have been able to use it to raise the interior temp from 64 to 70 degrees on a not yet completed build (air-tight, drywall up) with mid-50's outdoor temps. The only way to know it is on is stand under the supply register or look at the t-stat.
I also have a slim-duct that I am using to condition a wide open 1st floor. Nice to have just one supply register as opposed to a wall mount, but the sound is noticeable.
Speaking of pocket doors - you might want to consider swapping out the swing door in Bed 3 for a pocket and move it towards the front of the house. This will allow you to add a closet where the swing door is.
I will also recommend a slim duct solution given the insulated basement.
The 1 ton ARU12RLF will do 17,400 BTUh with a HSPF of 11.5. The mid static version actually has a higher HSPF of 11.7. It does this by upping the CFM from 383 to 500. I think the standard version makes more sense in your situation because the moisture removal is significantly better on the standard version due to the lower fan speed. The lower CFM will also make a central return much more feasible.
A 3/4 ton Fujitsu slim duct ARU9RLF will produce a bit more than 16,000 BTUh at you design temps with a rated HSPF of 12.2. But it might actually come up a bit short on the cooling side. The 1 ton also has better moisture removal due to a higher cooling BTU/cfm ratio.
I am also curious why the cooling loads for each floor don't add up to the total house? The heating loads do add as expected. I am not super familiar with manual J for cooling loads.
Total House P1 - Equipment Total Load (Sen+Lat) 9933 Btuh
Basement P2 - Equipment Total Load (Sen+Lat) 948 Btuh
First Floor P3 - Equipment Total Load (Sen+Lat) 10032 Btuh
I agree the Bath rooms 1 & 2 are very likely to be cold with a single mini so far away, the walk in closets will be even colder.
Are you sure there is room to open the door to bath#3 with a queen bed in the room?
If the door for bedroom #3 is 3’0” the door will open very close to the bath wall and could be a pinch point.
Without a closet some code enforcement, loan officers, realtors and buyers may not consider it a bedroom.
From a resale point of view there should be one bathtub some ware.
Do your plans include a HRV system?
Consider moving the radon vent to a more central location.
In most cases a full basement will stay well above freezing without any equipment. It is a different story if this is a walk out with big windows and will be used a living space.
I think you have a great plan.
Walta
George,
My house is about 1,500 square feet with a full basement (we also have 10 foot ceilings). My heat loss figures are nearly identical to yours but I am in CZ 6.
Our singular, one ton Fujitsu mini-split is our only source of heat for the entire house. We had no issues heating our bathrooms or even rooms 30 feet away. During the summer, the bedrooms and bathrooms were a couple of degrees warmer with doors closed but it was never an issue.
Our floor plan is bit more open than yours however and we do have a balanced ventilation system. (13 cfm from each port probably isn't doing much though...)
I think your plan might work- just be prepared so install some baseboard heat or supplemental heat to the distant rooms if needed. Another approach would be install a bath fan in an interior wall as a kind of jump duct. This would bring air through the wall. Seems like GBA had an article on this novel idea recently.
Thanks for your recommendations, everyone. I was looking for some input before I talk to the HVAC people. They have no clue about high performance houses.
I neglected to say that the bathrooms will have electric heaters and that the house will have an ERV. We'll have ceiling fans in the bedrooms and living room. We do prefer cool bedrooms. I think that a single head will work. However, I was also thinking about a ducted unit in the basement which would allow some conditioning down there as well. I'm not sure it's needed as the current uninsulated basement seldom falls below 50˚F, but some dehumidification would be advantageous.
Walt, your comments about bedroom 3 are well founded and will be addressed. The plan I posted is an earlier one while waiting for the latest, but is close enough to final to talk about HVAC. Bedroom 3 will have a Murphy bed for occasional guests, but will be used mostly as our computer room. We're a pair of seniors so the place has a bunch of ADA features and is designed to house us til we croak. Bathtub? Real estate agents? Somebody else can worry about that stuff. :-)
Matt, thanks for the specific recommendation. That's the kind of stuff I need. Was hoping Dana Dorsett would see this and put in his 2¢, too. Dunno why the cooling loads don't add up to the whole house. That info may be in some of the Manual J that I didn't post. Energy Vanguard supplies a lot of information and I included just what I thought would help make sense of the loads.
Are you going to have Energy Vanguard do the system design as well after picking a general direction?
You have a pretty straight forward layout to duct. Just keep the velocities around 400FPM and you will be good. Flex duct is fine if it is pulled tight, mostly straight, and upsized if needed. Remember, just because it is ducted, doesn't mean it needs to be fully ducted. Just getting supplies into the bedrooms would be significant improvement over a single head.
A single central return where you have the mini split head located would probably work. You can size it for a MERV 13 filter.
https://www.phius.org/NAPHC2018/Think%20Little%20Slide%20Deck%20-%20NAPHC%202018.pdf
Do you have the room by room loads?
I'm pretty sure that with a decent duct design the AOU/ARU12RLFCD would be the "right" mini-ducted solution for your house, but the 12RGLX might be easier to design for, and would also be pretty good:
https://ashp.neep.org/#!/product/25312
https://ashp.neep.org/#!/product/25349
The lower cfm will provide slightly more stable room temps and comfort in shoulder seasons compared to an ARU12RGLX married to the same AOU12RLFC compressor unit. Both can modulate down to 3100 BTU/hr out, but the lower cfm of the ARU12RLF would provide somewhat better latent cooling.