I used a Honeywell Prestige IAQ for years until I got communicating equipment. It can control several stages of heat, cool, ventilation, humidification, and dehumidification.
Ecobee (and other thermostats) can control one accessory. I think that the data logging feature of the Ecobee is valuable.
Some furnaces and air handlers will respond to a fan only signal by using a fan speed that can be adjusted in the unit for low fan.

 

I may be headed toward communicating equipment. Are there standards there, or just different implementations by each vendor?

It is generally a feature of communicating systems to be able to set a desired fan speed? Or are you still limited to temp and humidity and the system chooses fan speed based on it's own calculations?

 

I've replaced my thermostat a few times over the years, most recently with a Honeywell T9 which I generally really like. But something I've noticed is they all have only three modes for Fan control: Auto, On, or Circ. On runs the blower all the time, Auto runs it when heating or cooling are called for, Circ is like On but runs for about 20 minutes per hour.

What is the problem you are hoping to solve with better control?

I'm considering adding an energy recovery ventilator, so it seems like it would be good to have constant air circulation.

Don't do this in MD unless you have a really tight house and want to spend a fortune on dehumidification. What is the problem you are hoping this would solve?

Are there modern thermostats that can be paired with a split system that has a variable speed blower? It seems like it would be healthy and comfortable to have some amount of air circulation going all the time, but perhaps at a fraction of full speed for noise and utility bill purposes.

Carrier Infinity Heat Pumps solve a ton of problems. Reheat dehumidification can do most of the dehumidification heavy lifting allowing dehumidifiers to be backup instead of first and only defense.

Does this exist? What's the feature called in a thermostat or when shopping for a new split system that could do it?

What are the problems you are hoping to solve?

 

Noise and comfort. My existing system is 25 years old and needs to be replaced. It is so loud. Got some feedback from contractor today that it seems like the house was build with a bad duct design that is only supporting 3 tons in terms of possible cfm due to duct sizing, but has a 4 ton unit (that doesn't keep up on hot days) and I have neighbors who have replaced with 5 ton systems for same house design, and other neighbors with 2 systems.

I also have a CO2 monitor and see CO2 easily creeping up over 1000 unless I keep several windows partially open. 5 people live here, 4 bedrooms, 2600 sq feet. The house must be pretty well sealed. When I'm the only one home for a day or so CO2 drops back to 600-700. That's the reason for interest in ventilation.

I think constant (not necessarily high) flow of air through the HVAC would just keep the rooms more comfortable. Also seems to have health benefits, more ACH lowering infectiousness for any airborne viruses family members might have. Kids in school, every bit helps.

With the current system, I've never had a humidity problem other than too low! Rooms in basement, 1st floor, and 2nd floor all stay between 45-55% RH all summer long. In the winter I use a whole home humidifier to keep it from getting uncomfortably dry.

With the current system, I run the circulating fan on full time over night, then circ or full during the day. It's the noise that bothers me and that seems to stem from the returns not being sized sufficiently (and maybe also from the age of the system.)

Goals for being able to control blower speed, when system not calling for heat or a/c:
- Lower noise, preferably silent
- Room air circulation (not getting stale)
- ASHRAE recommended health benefits

Hadn't heard not to use ERV in Maryland. I thought ERV was supposed to help control humidity too (by exchanging some of the humidity from incoming air to the exhaust air across a membrane)

 

Noise and comfort. My existing system is 25 years old and needs to be replaced. It is so loud. Got some feedback from contractor today that it seems like the house was build with a bad duct design that is only supporting 3 tons in terms of possible cfm due to duct sizing, but has a 4 ton unit (that doesn't keep up on hot days) and I have neighbors who have replaced with 5 ton systems for same house design, and other neighbors with 2 systems.

Sounds like you might want to understand your load and duct capacity. If you have 3 ton of duct and 3 ton worst case load, you'll be good.
If you install a 5 stage unit it'll run lower most of the time - you willing to take some risk on both load and duct capacity?
2500 feet if you don't have crazy fenestration load (huge west windows?) the risk is probably mostly shell performance. If the house remains uncomfortable, willing to improve leakage?

I also have a CO2 monitor and see CO2 easily creeping up over 1000 unless I keep several windows partially open. 5 people live here, 4 bedrooms, 2600 sq feet. The house must be pretty well sealed. When I'm the only one home for a day or so CO2 drops back to 600-700. That's the reason for interest in ventilation.

Learn about fresh air strategies here:
https://www.natethehousewhisperer.com/blog/bad-ass-hvac-part-1

I think constant (not necessarily high) flow of air through the HVAC would just keep the rooms more comfortable. Also seems to have health benefits, more ACH lowering infectiousness for any airborne viruses family members might have. Kids in school, every bit helps.

Learn about fresh air strategies here:
https://www.natethehousewhisperer.com/blog/bad-ass-hvac-part-1

With the current system, I've never had a humidity problem other than too low! Rooms in basement, 1st floor, and 2nd floor all stay between 45-55% RH all summer long. In the winter I use a whole home humidifier to keep it from getting uncomfortably dry.

New equipment often doesn't dehumidify as well as the old "inefficient" stuff

With the current system, I run the circulating fan on full time over night, then circ or full during the day. It's the noise that bothers me and that seems to stem from the returns not being sized sufficiently (and maybe also from the age of the system.)

Goals for being able to control blower speed, when system not calling for heat or a/c:
- Lower noise, preferably silent
- Room air circulation (not getting stale)
- ASHRAE recommended health benefits

Hadn't heard not to use ERV in Maryland. I thought ERV was supposed to help control humidity too (by exchanging some of the humidity from incoming air to the exhaust air across a membrane)

When you bring in high moisture content air you will need to remove the moisture. Taking fresh air stream dew point from 90 to 80 is still driving your latent removal issue UP in a 75 degree house.

How we learned this was not fun.

 

If you have a 4 ton system on 3 ton duct work and can not cool the house the problem is not equipment it is duct. My guess is that it originally had a 3 ton system on it and it didn't cool so they went to a 4 ton which as you know didn't help. This is a common problem because people kinda understand the equipment but don't understand the delivery system (duct work). They would understand the equipment better if they knew duct design.

You may be lucky and by changing a couple fittings near the equipment may improve air flow enough to solve most of your issues, or it could be someone designed the duct system with the philosophy of "air is smart, it will figure out where to go". That never works by the way.

If you can find one, a tech that can do a load on your house, then measure the air to each room and compare to what the load says. At that point it can be determined how big of a project fixing it will be.

 

A system with variable speed blower will automatically run at the speed it needs to for each mode of operation, including at a low speed when fan only is being called for. There is no need to set the fan speed from the thermostat - though communicating systems can generally do this.

The only place where the thermostat really needs to interface with a variable speed blower is by lowering the speed when in cooling mode and dehumidification is called for - most high end thermostats can do this.