We have an enormous house - 6400sf or so - built into a hillside so about 3500sf is the first floor and 2900sf is a walkout lower level. Right now we have three separate HVAC units - traditional ducted gas furnaces with regular old DX air conditioning on each. The problem is they're configured in the worst way possible; that is to say each unit handles a vertical "slice" of the house, east - center - west. That means to get the upstairs sufficiently cooled, the downstairs, which is partially earth sheltered thanks to being a walk out lower level, ends up being much cooler than the upstairs. We also end up with quite a temperature gradient across the upper floor of the house since the thermostat has to be in one room and the AC is cooling a bunch of rooms. The different A/C units sometimes end up fighting one another - it's impossible to keep the cooled air from one unit away from the thermostat for another unit so sometimes one part of the house will be comfortable while another is warm.
My question is: would a mini-split type system make sense for a house this big? I did see that they sell 9 zone systems on some of the shaky direct selling web sites. That would be enough to handle the entire upper floor of the house with one wall unit per room. Would it make sense to install a 9-zone system on the upper floor of the house, see if we like it, and if we do, install another similar system on the downstairs?
It looks like all the mini-splits are heat pump units that could also heat in the winter. My thought is to leave the existing furnaces in place and use them for base load heat then the mini-splits could regulate each room's temp in case they were too cold. Does that make sense?
I've got a couple HVAC contractors I can ask about this, but I would like to have some education before involving them. I'm also interested in DIY-ing it; what are people's experiences with installing these systems?
Interested in this as my system while zoned ok could be better if it had more separation of the zones.
RossD
MegaDork
6/12/18 8:37 a.m.
Look into a multiple thermostat situation for a single furnace, but just have three sets of these t-stats. They can use different criteria for 'voting' for on/off of the furnace or A/C. That way one room doesn't control the whole zone. Each thermostat can have a voice.
As for the three furnaces, usually different exposers constitute a different zone, ergo a single zone furnace. Obviously most houses don't need to have multiple units so usually one is fine. I wouldn't go through the trouble of removing the the units quite yet.
The mini splits will give you more control but then you will see the units in the space, unless they are the ducted fan coil type. Now you have a more units that can go wrong. If one in five units will fail, you now are getting enough of them to break into that probability.
If you are having wide variations in temperatures, you might want to look into a variable flow refrigerant system or a packaged Water source heat pumps (read both as: expensive). That way if one side of your house is cold and one side is hot, the heat removed from the hot side can be use in the cold side, and vice versa.
T.J.
MegaDork
6/12/18 9:14 a.m.
Seems like it would've made more sense for the builder to have each existing unit supply a whole floor instead of 1/3 of three floors. Trying to change that after the fact seems like a big mess since it would mean re-routing or reconfiguring the duct work.
I am interested in mini splits as well, but I am only considering a single zone set up.
T.J. said:
Seems like it would've made more sense for the builder to have each existing unit supply a whole floor instead of 1/3 of three floors. Trying to change that after the fact seems like a big mess since it would mean re-routing or reconfiguring the duct work.
I am interested in mini splits as well, but I am only considering a single zone set up.
Sure. But the house was built in 1954 then added onto in the 90s, so lots of things that made sense then don't make sense now. I'd love to have the furnaces supplying a floor each instead of the vertical stack split.
Forced-air systems only work if there's a path for the air to go from register, through your living space, to the return. If you have rooms that are too hot in summer, you need to make sure that they get more airflow. Either leave the doors open for those rooms, or punch a cold-air-return through the wall next to the door. If you're already leaving the doors open and it's still not regulating, try a fan-assisted register to pull more air through that space.
You can also upgrade to ecobee 3 or ecobee 4 thermostats with the remote-temperature-sensors so that each HVAC system knows more than just how warm one spot is. It can run the system fan longer to equalize temperature within it's zone, or run the A/C or heat to bring overall zone temps in-line with what you want.
