HVAC and 2-level Heating/Cooling
 

My house is heated with forced air natural gas and AC cooled. Downstairs is a walkout basement with 3 bedrooms. Upstairs is the main living area and has 2 bedrooms including the master. House built in 2004.

Downstairs is always about 10 degrees cooler. In the summer, it smells a little musty. Upstairs, the AC struggles to maintain a comfortable 78 degree temperature. In the winter, downstairs will be 60 degrees when it's 70 upstairs.

My single-speed air handler draws 700 watts, so running it 24/7 is out of the question. There is one air return upstairs and one downstairs.

My thought was to block the upstairs return in the winter so hot upstairs air is forced to travel downstairs to reach the furnace, and block the downstairs return in the summer so cool downstairs air is forced to travel upstairs.

I blocked the upstairs return tonight and found the air handler is drawing 60 less watts despite having half the air intake. Air from the register is 10 degrees hotter; blowing 117 instead of 107.

I'm hoping to figure out solutions with existing equipment, but probably the recommended solution would be to have a separate HVAC system for upstairs and downstairs. If I could somehow convert the single-speed blower to variable, that would likely solve a lot of the problems since I could continuously run the blower to level out the temperature and humidity.

Replacing the entire thing has me wondering about a heat pump. AC is a 5-ton 10 SEER. Furnace is likely as inefficient as legal minimum back in the day. What is that, like 80%?

My friend has a house set up like that but with an hald finished basement. The hvac would draw only from the upstairs and was isolated in the basement and the basement was musty and stayed about 30 degrees cooler in the summer with the AC off. I had him remove 2 duct elbows from his hvac so the return air would vent into the basement and the hvac would draw air from the basement floor. During the summer that saves at least $100 per month between the A/C drawing cool basement air and not running a dehumidifier any more.

Making the fan infinite speed is easy (change motor to bldc or add a variable speed controller) , controlling that return air speed/volume cheaply and accurately to reflect changed demand, not so much.

Redpoint you have a lot of issues with that kind of setup in a house. That smell downstairs is mold from lack of humdity control. Concrete is vapor permeable so it lets in moisture. Stay between 30-60% year round. So your top house gets solar gain on the walls and roof year round, your lower house doesn't and even gets the opposite effect. The concrete leaches out heat, a lot in the winter, and not much in the summer, but no solar heat gain since you're below ground. The big heat loss is really in the 3 feet below ground area and below that i think ground temp is more or less 55 degrees year round. Also, houses breathe upwards so heat will always go up.
Look into manual J calculations and see how far off you are right now. Also look into air sealing and blower door test even updating or improving your insulation strategies, and look into a paint on vapor barrier for the basement

I know the new equipment is so good im looking to swap my oversized furnace, 10 seer ac system out to a new air source heat pump one day and then when my whole house is electric and efficient and balanced as i can get it i will swap to solar as i will have the most balanced load with the smallest solar system

I have a cheapo hygrometer/thermometer in the basement logging conditions. I blocked the upstairs return for 1 week and found no difference in temperature or humidity. Since then I've opened the upstairs return since it made no difference (except to increase temperature measured from the registers).

There's no insulation in the boundary separating the ceiling of the basement and the floor of upstairs. I wonder if that could help keep heat downstairs AND attenuate a bit of noise?

Highest priority to me is to install a door since I'd like downstairs to be capable of being a separate living space. Sound travels upstairs as if it's just an adjacent room.

It seems that the musty smell is almost gone during the winter. Humidity has averaged 54%, peaking at 63% during the NYE party and falling back to normal after 2 days.

I have no summer data since I just started recording 10 days ago.

The previous owners hastily painted the concrete portion of the walls, I suspect in an attempt to control humidity. No idea what product they used, but my plan was to more thoroughly seal, though that requires the previous application was a sealer, or removal of that paint first.

I'd like to apply an epoxy to the concrete floors. I might as well do my garage floor at the same time.

My guess is that spring through fall humidity exceeds 60% most of the time, leading to the musty smell. It's somewhat faint, so I don't think it's extremely humid. Perhaps small mitigation strategies will shift it just enough. I want to avoid running a power hungry dehumidifier.

I see references to blowers drawing as little as 60 watts when there's no call for heating or cooling. Perhaps just keeping the air mixing between floors is enough to close the temperature and humidity gap to within acceptable limits.

My hunch is at minimum I have a blower replacement in my future, but since I don't see mention of blower only replacements, more likely is a furnace replacement. Once I'm at that point, replacing the inefficient AC with a heat pump becomes a consideration.

Oh okay so the basement is above ground. Well seal the floor or any surface connected to ground with epoxy waterproofer to start. Also, you want to unblock the upstairs return as it will return all of the hot air sitting up there, and block the return air in the basement, but your humidity levels down there don't allow that. What you want to do is adjust the main air damper to the upstairs and block it to divert more of the air to down stairs. It is more effective.
But yeah a good manual J calculation gives us heating and cooling loads required for the house and on a per room basis. Not using one is like me design shirts based on height and i get two 6'2 people but one is fat a old guy so he ends up in a muscle shirt flexxing man boobs and the other is a 6'2 skinny kid so he ends up in a short dress. lol
The new hvac units are called minisplits and they are air source heat pumps. Super efficient, but e is more expensive than natural gas so on the year it comes out that they are roughly half the cost to run over our stuff from what i looked at. But they can also run at variable load and speed so they can run all day which is what you want. My AC/Heat is double oversized for my house so it hardly runs at all which is also very bad especially for humidity removal in the summer lol. Also, the new ones can run down to -40 on some of the new models so you just have to calculate which one will work for you. Coefficient of performance is also variable with temp so you want one that will work in winter on top of all things with these types.

If I had a variable speed blower I'd have more confidence closing registers without fear of creating too much pressure in the ducting.

I'll see about half-closing the upstairs registers and monitor temp/humidity downstairs.

I am curious to run some numbers to see cost of heating from an HVAC vs natural gas. My baseline understanding is that resistance heating is 3x more expensive than NG, but heatpumps are 3x more efficient, so about even in cost? I'll look at my utility costs and do the math.

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.

Didn't know that about ductless. Thanks.

Went with a central handler because my ducts are in a newly installed expensive wood floor with no easy way to splice in new flooring without refinishing the whole house

Here's a link to some of the new units. Output wise it looks like one of these will finally work for me. Below like 2.2 cop though its more expensive to run electric for me so i would really want some sort of european wood stove (ultra efficient with variable cfm output as well).
https://www.mitsubi****echinfo.ca/si...KA1_202201.pdf
https://www.mitsubi****echinfo.ca/si...NKA_202103.pdf
https://www.mitsubi****echinfo.ca/si...NKA_202103.pdf

Interestingly i did see that the outdoor pump is variable output, but the blower fan indoors is single speed. I wonder how converting to a variable speed indoor fan would go.