System decision

[sleek98]

I am in the stage of building a lake house that I need to decide on a system style.

I have a 2 story with walkout basement. 2x6 walls, zip system r19 walls and r38 ceiling insulation. Main floor is basically wide open with a small closet and pantry in the middle
Of the back wall that I can run ductwork in from the basement to the top floor.

Running a calc it’s showing I need a 2-2.5 ton ac system and 37k btu heat. I was planning on doing a heat pump.

House is 2,700 sq feet. 900 a level. I was not very happy with my first 2 story house using one system. As the upstairs was always a solid 3-4 degrees warmer. But it was 2x4 walls.

Is there a downside to doing a traditional hvac on the main level and running a two headed mini split up stairs?

There will be 2 bedrooms, 2 bathrooms and a laundry room on the top floor. So im not sure how well the bathrooms will be cooled with a mini split being in the room with no vents in there.

How much of the closet will the supply and return ductwork take up? How horrible would it be to put a system in the attic?

 

[danski0224]

Attic is the worst place for ductwork and/or equipment, unless the roof is spray foamed and the attic is then conditioned space.

Temperature differences between floors is almost always a ductwork problem, as in ****** design.

 

[PoorUB]

It can be a pain to balance out a two story home. I get the idea of mini splits but I am not a fan of then in new construction unless you go with an air handler style and duct.

You can deal with the imbalance with ductwork. Put a full size returns on both floors. Each large enough to satisfy the ar requirements of the furnace and put balancing dampers in both so you can adjust the return air flow. Put the second floor return near the ceiling to pull off heat in the summer.
Make certain your supply ductwork is large enough. I generally figure out what size is required, then go up one size. You can always close down registers. It is difficult to increase duct size afterwards.

 

[thammel]

I would do two systems and have both air handlers in the basement. (nothing in the attic unless it is conditioned space) Mini-splits are great for open areas, but I would run ductwork to each room. Heat pumps make a lot of sense, but I would go with a hybrid system with gas or propane back up heat.

 

[reader2580]

Why not zoning with a single system? I had zoning for my first house and it worked great. I wanted zoning for current house, but I was adding ductwork to an existing house and the HVAC company said there was not enough room for zoning.

Two systems seems overkill for the load.

 

[sleek98]

Thanks guys. Wasn’t sure if the “easy” way throwing a unit in the attic was too overkill.

I at least have a good idea on what to ask for.

 

[Fav Onefour]

A properly set up two zone would be simpler.
You are lucky to be on top of the subject with a new build. Now is the time to design and do it right.
My main home is an open vault ceiling to the second floor rooms. It is a nice open space. The second floor was a magnet for heat. I opted for zoning the system and it wasn't an easy retrofit. The end result has been fantastic.

I have another home with forced air setup on the two lower floors and boiler heat up top. I know it seems backward, but that was the setup. It's a 100+ year old house and retrofit from basement was not feasible without some major gut work. Mini split setup would have needed at least four head units and that would have been a compromise. I went with a SDHV setup in the attic for the upper floor. It works well, but it's in the attic. I'm not a huge fan of that setup for maintenance purposes. Three systems in one house is three times the work.

BTW, I would advise doing the setup without mini splits if you are doing a new build. You will have less work and headache down the road. I get the love for those things, but there is a dirty little secret. Quite a few are notorious for lasting just past the warranty.

 

[sleek98]

Does anyone have an idea of duct sizing for a 900 sq foot floor? I know it’s pretty general question but looking to see if I have enough room in the bathroom to get duct work upstairs. I have about a 18-20” space to run it through.

 

[PoorUB]

With the information provided You will need about a 10x16 duct. I would recommend an over sized return duct too. so at least double that.

Not knowing the situation, but you need to make room for duct if you want a properly running, comfortable system.

I generally sized residential duct for 1 CFM for 1 sqft of floor space and I would be over sized.

 

[sleek98]

Thanks. I am pretty confident I can get the ducts in there if they don’t have to be wider than 20”

 

[danski0224]

Does anyone have an idea of duct sizing for a 900 sq foot floor? I know it’s pretty general question but looking to see if I have enough room in the bathroom to get duct work upstairs. I have about a 18-20” space to run it through.

You can buy a "single use" license for HVAC-Calc for about $50.

Then you can do a room by room load calculation.

Then you can use the duct sizing utility that's part of the program.

There is a demo on their site.

If you want to do it yourself.

Otherwise, I would suggest paying someone to do a load calculation, duct design and equipment selection. Given the blueprints and specs, this could likely be done for under $1k. Then you will have a set of good drawings for the HVAC. There are companies that do just this- so no conflict of interest with selling HVAC services. Some towns near me require this (the load calc, duct layout and equipment selection) as part of the permit process.

On average, half of the utility dollars spent annually are for heating and cooling. This is an ongoing cost for as long as you live there. It is NEVER cheaper to do it better the second time. Never.

It is foolish to cheap out and rely on answers from random strangers on an internet forum.

 

[sleek98]

Thanks. I have someone coming on Tuesday to spec everything out. I was just trying to see if it was even possible to get the temp difference between floors down to a degree or two or if the consensus was to just go two systems, or hey your going to need a 3’x3’ run for ducting etc.

 

[75gmck25]

Since this is a lake house (primarily summer use?), what is your priority for good temperature balance between floors - heat or cooling?

I have my A/C system in my conditioned attic and I cool two floors of about 650 sq ft each. It works great in hot weather because it cools better on the 2nd floor than the 1st (great for sleeping), but the cool air naturally drops down to the lower floor, so it balances out.

I have hydronic heat, so I don't have to worry about HVAC air flow/imbalance during cooler weather. I'm not sure how well it would work if I tried to heat my house using a system in the attic. Probably not that well.

 

[sleek98]

Cooling would be my preference. We go down in the winter but not that often. I usually keep the house at 62 all winter so the drywall doesn’t crack. I didn’t really think about putting the unit upstairs in the laundry room on the top floor and just force cool air down. That’s actually a good idea.