HVAC and 2-level Heating/Cooling

[redpoint5]

I would go with ducts for the simple fact you want more than 1 room to be heated. In my situation the ductwork is already installed, so might as well use it.

There should be no reason a conventional heat pump couldn't achieve the efficiency of a mini split because they operate on the same principles and mechanics.

Originally Posted by hayden55

Registers and dampers aren't the same thing so hopefully you know i mean adjusting the dampers on the main legs attached to your central air unit's body. It has these wafers like a throttle body that adjust air. Adjusting registers is the last thing you do as they aren't a great way or efficient way to reduce air flow but helpful at the end after adjusting the whole leg if certain rooms need more or less cfm.

The variable speed blower would be interesting for when the unit isn't heating or cooling but when it is you want to go back to default cfm or your efficiency will take a hit and more than likely your furnace will just overheat and go into thermal protection mode in the winter. The mini splits with an inverter can run at variable capacity so they can match the fan speed and be just fine (i wish we could do that though lol)

Govee has some wireless temp/humdity sensors on amazon. Just as accurate on both as the expensive ones. You can connect 10 on wifi and they datalog every hour. If you can put them all at the same height and away from registers so you get a good measurement on room temps it seems like it would make measuring a lot easier.

Yup so the mini splits have a coefficient of performance while the resistance heaters and natural gas heaters have efficiency.
NG furnaces with a metal exhaust pipe : 78% efficient
NG furnaces with pvc exhaust : 82%+
electric resistive: 33%
Air source heat pumps: cop based on temps and delta T
But basically they use 1 unit of energy to do 3 units of work. So they can be a lot more than 3x more efficient.
So they might have a cop on average of 2.5 in the winter and 4 in the summer, so lets say 325% efficient on average.
The big thing to worry about though is cop is variable with temp so it comes into play when you lose heat because its too cold outside. Some work down to -5 or -20 or -40 depending on unit and thats a massive delta T to a 70 degree inside temp in the winter and they will basically have a cop of like 1 in those conditions so you will need back up heat for those 4 days of the year we get winter storms. Still personally looking into a central air handler mini split unit that will get me the performance i need with my natural gas fire place as a backup.
But in recent years they are finally getting more efficient and usable. A mitsubishi central 3 ton unit has a seer rating of 19, but the new itty bitty 6k unit they just came out with has a seer rating of 33. So it looks like the 33 seer unit has a peak cop of 9.7 which is insane.
The only issue with the ductless units is they suck at removing humidity from the air so i personally would stay ducted.

The Govee is what I bought. At this moment it's 62 degrees and 52% humidity downstairs while 70 degrees and 45% humidity upstairs.

I've got 2 direct vent NG fireplaces, so that can keep the house warm when a heat pump isn't efficient. That's what we used a couple years ago during the ice storm that knocked out power for a week.

[hayden55]

correct a traditional heat pump is the same thing as a mini split. The mini split is just also reversible and the traditional isn't. The technical term for both of them is an air source heat pump like how geothermal is actually called a ground source heat pump.
But you can just compare your summer cooling bills by seer rating if you plan to run both in default mode.
The old ac unit is just older tech so 10 seer vs 18 seer is probably a good one to go on. Winter is trickier.
But I did compare the 30k unit and the 36k unit with the inverter and variable output capacity and the minimum cooling outputs. The cop on the 30k with the 121 W fan was 4.29 vs the 36k with the 431 W fan at 4.41. So it looks like our question about messing with fan speed has been answered. Never mess with fan speed leave it alone.

They do multi head ductless systems so you just run cooling lines to each head and if you do a fresh build it can simplify the build and lessen cost.
But correct, ducts are premium air. If you already have them i would never downgrade to ductless. The main con to a ducted system is if they run through a non conditioned space like a hot attic you can lose a lot of heat or cooling as the ambient temp steals your btus on your duct runs. I have a bunch of basically free insulation though so im currently burying mine with the rest of my attic in R-40+ lol
Good enough for me.

[redpoint5]

Originally Posted by hayden55

But I did compare the 30k unit and the 36k unit with the inverter and variable output capacity and the minimum cooling outputs. The cop on the 30k with the 121 W fan was 4.29 vs the 36k with the 431 W fan at 4.41. So it looks like our question about messing with fan speed has been answered. Never mess with fan speed leave it alone.

I don't follow what you mean. The more expensive, supposedly more efficient system has a 3x+ more power hungry fan. Are those maximum values, minimum, or average? Does the cop take into account the blower fan power consumption?

The main con to a ducted system is if they run through a non conditioned space like a hot attic you can lose a lot of heat or cooling as the ambient temp steals your btus on your duct runs.

All the upstairs registers are in the floor, and all the downstairs registers are in the ceiling. The only thing in the attic is the upstairs return which is the ceiling. There's 4 registers fed through an unconditioned space in the basement that are insulated.

[hayden55]

Yeah confusing thats for most people in one story houses with non conditioned attics getting duct losses. But upon further review it looks like the 36k setup is a slightly tuned up 30k setup. Looks like the compressor is bumped up a bit but the fan cfms are up quite a bit. The fan adds a lot to the effectiveness. Yup it looks like on those pdfs for the system combos the cop is calculated off of system consumption.
And yeah that looks to be the minimum output's cop for both systems for cooling.

[redpoint5]

Here's my HVAC in an unconditioned basement room. Wall on left is concrete and wall on right is the conditioned rooms.

[hayden55]

i see a metal exhaust pipe so its a 78% efficient unit. Hmmm.... i don't see dampers. Keep taking pictures of the ducts until we see something like a wingnut or a handle sticking off the side.

Also, i forgot to mention if you don't care about looks you can take the registers off the vents on the down stairs and it will have a similar effect.

[Piotrsko]

The expensive quality installers use dampers in the ducts, the cheapo guys dont. Figure they shave $100 parts and couple hours labor not putting dampers in. People aren't smart enough to know or care. There are aftermarket balloon style remote controlled ones if you insist

Huge fan in the airhandler, only ran full bore once on a hot day rapid cool. Blew the insulated drapes open. Most times you need a tell tale to see the interior fan on, cant hear it run.

Oddly, sometimes it is pushing relatively cool air heating the house

[hayden55]

Originally Posted by Piotrsko

The expensive quality installers use dampers in the ducts, the cheapo guys dont. Figure they shave $100 parts and couple hours labor not putting dampers in. People aren't smart enough to know or care. There are aftermarket balloon style remote controlled ones if you insist

Huge fan in the airhandler, only ran full bore once on a hot day rapid cool. Blew the insulated drapes open. Most times you need a tell tale to see the interior fan on, cant hear it run.

Oddly, sometimes it is pushing relatively cool air heating the house

Yep. See it a lot. Well if you don't see main dampers you will need to add some either way new system or not. Track your legs and get a feel where they go and spend $200 bucks on some dampers and add them in one weekend.
Realistically if the upper house is one run you just need one damper

[redpoint5]

Originally Posted by hayden55

Yep. See it a lot. Well if you don't see main dampers you will need to add some either way new system or not. Track your legs and get a feel where they go and spend $200 bucks on some dampers and add them in one weekend.
Realistically if the upper house is one run you just need one damper

What's the difference though besides where in the system receives back pressure?

I'd need to control both top and bottom ducts because I need more cooling upstairs in the summer than downstairs. In fact, if there was no cooling downstairs it would still remain cooler than upstairs. Same with heating. If I diverted all heating downstairs, upstairs would still end up being warmer.

[hayden55]

Originally Posted by redpoint5

What's the difference though besides where in the system receives back pressure?

I'd need to control both top and bottom ducts because I need more cooling upstairs in the summer than downstairs. In fact, if there was no cooling downstairs it would still remain cooler than upstairs. Same with heating. If I diverted all heating downstairs, upstairs would still end up being warmer.

Its not on or off. You can go halfway. Mine are all at varying light touches of percents. But every leg on mine has a damper.
So for you it sounds like putting in a damper on the main leg to the 2nd floor would solve some of your problems (while the unit is running and shortly after, but the two floors have different heat and cooling loads) if not most of them. If you put it in an easy spot to get two you just mark where you want it for summer and winter with a paint pen and adjust it twice a year.

The return air doesn't just grab air that comes out of the duct and return it its mostly there to grab stale air so it might grab a little bit less air and cause a slightly higher pressure up there which would probably not even cause a noticeable pressure difference besides the efficiency loss the system will see.
Blocking it at the main leg will be more direct and noticeable right away.