The furnace and ac are 30 years old and were sized for the house without the addition.

Could I add an air return from the addition to the furnace and add a single duct for new conditioned air/heat?

Any other ideas on how to circulate the air to the addition and remove the existing air?

It's likely not going to work and I can kind of see you don't think it will work either or you wouldn't have said the furnace and AC are 30 years old. A single duct added (steal air) from your current 30 year old AC is going to do what to the rest of the house?

Circulate air? It depends on what you mean by this. Obviously radiant baseboard heat isn't going to circulate air. You could accomplish circulation by ceiling fans. That will just "move air around" but there isn't anything cheaper than that unless you can live with floor fans.

You have ductless AC units, those would circulate air but not much more than a few feet from where the heads are located on the wall or in the ceiling if cassette version.

A traditional HVAC system circulates air by pulling room air into filtered return, then expells this air back into the structure... there is no "fresh air" unless the system is designed for it. There are times of the year in which you may not want fresh air (outside air), so there are controls to limit the amount. This would involve designing a system and then incorporating this into the area of the home you want this system to control.

Because this house is on multiple levels you may want additional controls to be able to operate the areas of the house independantly from other areas (this costs even more). This is called HVAC zoning. HVAC Zoning is an "elegant" solution that due to design and inexperience of those who sell it often leads to regret. It can work, but these kinds of things are tools in the tool bag. They should only be considered under certain scenarios. Certain home designs make some homes inappropriate for HVAC zoning.

In the world of *free* quotes, bids, proposals (whatever you want to call it) there will be someone who will just sell it because they can, without taking into consideration what it is you really want other than to do something for little money, that is not likely to work.

What I mean by work: mild weather, hot weather, cold weather ---- the system works. The load of the structure whatever it is (it varies) due to the load being placed upon it. --- This variation can cause alot of problems in multi-level homes using a single HVAC system to provide comfort to the home.

I service the Katy, Texas area.

Integrating the upstairs HVAC system with the existing system isn't a good option, but it could be if the way-past-its-prime existing A/C and furnace were replaced with new versions sized for the combined load, and the plan allows for installation of the ductwork. A zoned system will enhance comfort.

In the interim, you can improve the air quality on the 2nd level by installing a properly sized energy-recovery or heat recovery ventilator (depending on your climate zone.) That way you'll be bringing fresh air into your home and exhausting indoor pollutants.

It may be possible to add a supply and return. Have you investigated how the new ducts would run and connect to the current supply and return? There is a good chance your current 30 year old system is over sized and could handle the additional space. You need to look at the sizes of the current equipment.

Your 30 year old system will need to be replaced in the near future. One option is to size the heating and cooling so it can handle the additional space. I would recommend creating a zone for the addition.

Another option is install a mini split for the addition. Heating with mini split would cost much less ($600 bill becomes $150 - $200) and you would have an separate zone and humidity control. This would allow you delay the replacement of the 30 year old system and you would not have to spend the extra money on a zone system.

I have a split system in the addition for the AC but it doesn’t have heat. The split systems don’t control any humidity. They advertise they do however they don’t.

Ultimately, I’m trying to get the air moving so it’s filtered through the furnace circulates, dehumidifies or humidifies depending on the time of year. I’m in WI..

I’m trying to see if there are any ways to essentially pull the air from the main floor up and with out or back through the furnace. When I said circulate I mean throughout the house, not just the room.

Thank you for the feedback!

I’ll google the ventilator idea. I am not sure what that is.

I don't know your electric rates, but trying heat a room over an uninsulated garage in Wisconsin is going to be very expensive. You should think of this as an investment with a pay back.

Garage is insulated, but I will be insulated the garage door, and sealing that up better than it was. I will be insulating below the stairs too.
Thank you for the help!

Is the garage heated?

It is not heated.

If the garage is not heated, then the addition is effectively exposed to outdoor temperatures on 5 sides. A heating and cooling load calculation for the room should be done if you are going to be make an significant investment in an HVAC system.

Not sure what you mean by a split system for AC only and why it is not controlling the humidity. Does the room have its own air handler with a condenser outside?

It’s the Mitsubishi, split or ductless AC. There are two units on the wall and two larger components outside. There are different opinions if they dehumidify, but they don’t. I have sensors in the addition and with them on it climbs to over 60%, even if they are in the dehumidifier mode.. these units don’t have the upgraded heat option on them.

Thank you!

This is a similar unit. I just took a screenshot of the one online

The insulated floor over the unconditioned garage is analogous to an insulated floor over a vented (unconditioned) crawl space. The R-value of the insulation is an input for the manual J calcs. Nothing crazy there.

If the mini-split is a heat pump as shown in the OP's screen shot, it should provide heating as well as cooling, albeit that it may not be able to meet the entire heating needs for the living space in cold weather.

I didn’t install the units. However these models are AC only. Heat isn’t an option on this model of them.

The big question, is there anyway to be able to pull the air from the main floor and recirculate or exhaust it?

The air on the main floor goes through the filter, so it’s less dust, allergies, etc. The air in the addition is the same air that is never turned over. There are ceiling fans in every room of the house.

Your wall units should be filtering the air that is circulated through them. If you want to add a stand-alone air filter/purifier there are a number of options for that.

Exhausting air will add load to one or both of your HVAC systems. Without an equivalent flow of make-up air, it will result in operation at negative pressure. ERVs and HRVs are balanced ventilation systems which bring in fresh air and exhaust an equivalent amount of "stale" air. That should improve air quality on the upper level, but it doesn't address your heating or cooling needs.

You pose the question in a simplistic manner that beg's a simple answer. Run a 6"- 8" duct between upper and lower floors at points well away from stairs. Install an inline booster fan. Control fan on/off cycles with a relay. Control relay from furnace blower relay.

I’m trying to see if there are any ways to essentially pull the air from the main floor up and with out or back through the furnace. When I said circulate I mean throughout the house, not just the room.

To circulate you would need a system dedicated for that option. Sure you could put in an inline blower but I doubt that will provide you with any real result, or notice any difference. Inline blowers are rarely used in the professional realm, often they cause more problems than they cure.

As an example you have a filtered return situated on the lower level that feeds a furnace or AH (air handler) of which are "sized" to move the amount of air you want. The ducted supplies run to the room(s) in which you want to put downstairs air into them.

Because you have minisplits in these areas your options are few. Major constuction to do as you say... and probably would be better to just create a separate system like you have for the downstairs area rather than trying to pull air from that area and put it upstairs.

I assume this is due to allergies? If that's the case you got your work cut out for you. No two people are alike when it comes to these kinds of things. It sounds like you're trying to replicate what you have on the main level by just circulating the air?

It is too bad the mini split is AC only. I suppose the previous owner was told a heat pump would not do well in the Wisconsin winters. A heat pump would need back up electric resistance heating but it would still cost much less than full time electric baseboard heating.

It is possible as klem1 explained how you can go about pulling air from the main house and returning it. The question no one on this forum can answer is how well it would work to improve to improve the comfort in the addition.

I suggest you investigate why the humidity in the addition is climbs above 60%. Is there an attic above the addition? If so does it have ventilation? When was the last time the mini splits had preventative maintenance?

Yep, trying to filter the air for allergies, and reduce the baseboard heating costs.

It sounds like it will be an expensive upgrade to accomplish what I’m really looking for.

Thank you for all of the great questions and responses!

It’s a vaulted ceiling, and a small attic over the bathroom. I haven’t but will check for the fan.

I have no idea when the mini splits were PM the last time. I cleaned the filters on them and there was close to an 1/8th inch of dust. I Also replaced the smaller filter on those too.

I think the previous owners did a lot of things to reduce costs during the addition that probably costed them more over the years.

If the AC is cooling the addition adequately but not dehumidifying, then the two minisplits may be oversized. You need a cooling load done also and compare this to the combined capacity of the minisplits. Replacing the old HVAC system with a suitably sized new system with careful attention paid the ducting (supply and return) can only help. Definitely get rid of the resistance baseboard heaters!

Relative humidity tends to be higher on the upper floors of a home due to stack effect; simply stated, moist air is more buoyant than dry air.

Air conditioners and most mini splits will remove moisture, but only when cooling is called for--even at super low compressor speeds (no, "inverter" isn't the solution to every problem including world peace, Ray.) I think the Daikin Quaternity mini split is the exception. It has a dehumidification mode which allows for dehumidification independent of cooling.

If the A/C units are adequately cooling the space, but not dehumidifying the space, one option is to add a stand-alone dehumidifier, but I think the OP would be better off replacing the mini-split A/Cs with heat pumps capable of heating as well as cooling and dehumidifying.

"The big question, is there anyway to be able to pull the air from the main floor and recirculate or exhaust it?

You pose the question in a simplistic manner that beg's a simple answer. Run a 6"- 8" duct between upper and lower floors at points well away from stairs. Install an inline booster fan. Control fan on/off cycles with a relay. Control relay from furnace blower relay.

It sounds like it will be an expensive upgrade to accomplish what I’m really looking for."

How much do you think that duct would cost and how much were you expecting to spend when you asked for a solution?

You don't fix a problem with a multi-zone system by combining the zones.