A mini-split does not bring in air, so you don't lose humidity in the winter.
If you get a new furnace, consider one that draws its air from outside. If you have the common design where combustion air draws from the inside air, and sends gasses up the chimney, it creates a vacuum in the house. Dry winter outside air comes in through the cracks, and humidity goes up the chimney.
While some mini-splits work below zero F, they are not very efficient. A furnace will make less load on your new generator than a big heat pump would.[/QUOTE]
The
combustion air draw isn't enough to significantly depressurize the house. For a gas burner it's about 1 cfm per 2400 BTU/hr, so even a 100K furnace is drawing less than 50 cfm, which isn't much, given the low duty cycle of the house. But even 15% duct losses from a crappy duct system adds up to 150cfm of potential infiltration drive.
The dry winter air is from:
A: an air-leaky house
B: imbalanced or leaky ducts, creating air-handler driven pressure differences to drive infiltraion
C: excessive active ventilation (does the system have a "fresh air" intake on it somewhere?"
D: all of the above.
A current code-minimum house in the northeast needs to blower-test at less than 3 air exchanges per hour at 50 pascals pressure (3ACH/50) to meet IRC 2015 or newer code. Most existing homes in New England (even many antiques) would pass that test, or at least get close, but if the house seems extra-dry in winter it probably doesn't pass, unless the drieness is due to excessive active ventilation, or air-handler driven air infiltration from duct system design/implementation defects.
An Energy Star duct system needs to have room to room pressure differences of not more than 3 pascals (0.012" water column) at all air handler speeds, under all conditions, room doors open or closed. The total duct leakage may not exceed 4cfm per 100 square feet of conditioned space at a duct pressure of 25 pascals. Most existing duct systems will fail one or both of those key features. (Energy Star ducts have other parameters to meet too, but those are the parameters most related to dry air issues.)
We haven't been happy with the furnace for the main level, as it seems to take a while for it to catch-up once we turn it on.
A
right sized furnace
should "...take a while to catch up..." from a deep set back. The question is whether it loses ground at some temperature that is likely to be encountered more than 25 hours per season. ASHRAE recommends only a 1.4x oversize factor for the load at the 99th percentile temperature bin (only 87 hours in a typical year are colder than the 99% outside design temp.) Once you're at 2x oversizing and up the recovery ramps are brief, but the temperature maintanance cycles become so short that rooms on the far end of the duct runs or rooms with somewhat different heat loss characteristics such as bonus rooms over garages or sun-rooms, etc get short shrift, and are never comfortable. At 3x oversizing and up even the main rooms end up with a hot-flash followed by the long chill during maintenance burns in cold weather. For comfort you really
WANT it to be running 70% duty cycle or higher when it's at the 99% outside design temperature or colder.
I'm thinking that with the new better siding, and new layer of insulation, that maybe that problem will not be an issue. The contractor plans on having an HVAC guy come out and do the calculations to figure out if the furnace and AC can handle the changes on the main level, and what size mini-split to use for the upstairs.
HVAC contractors tend to do a lousy job on right-sizing, and typically end up at 2-3x oversizing, out of the totally human anxiety about UNDERsizing and having to deal with the irate client at 5AM on the coldest night of the year. It's better to have an engineer or RESNET rater, somebody who makes their living & reputation on the accuracy of their numbers make those calculations, not somebody whose bread & butter is installing and maintaining systems.
My question is around if we should do a mini-split, or just have a new(second zone) system put in for the upper level, and just have them run vents and, all the work they would do for that. If our walls are being opened, is it better to just have them do a 2nd zone..versus a mini-split to handle the upstairs. Our current heating is furnace, and it uses propane. I'm thinking that when you go to mini-split, it ends up being a heat-pump for heating, which is never the same feel as a furnace. Reading some other threads, I know that propane isn't as efficient as other forms of heat. No option for natural gas in the area.
Mini-splits are great when right-sized for the loads, but over-rated if oversized. They are modulating heat pumps, but don't have an infinite turn down ratio. Anything more than 1.5x oversizing would usually cause it to cycle on/off during a large fraction of the season rather than cruising along modulating it's outpu up/down with load. Done right they run almost continuously, idling along nearly silently at low cfm except during truly cold weather. When you have firmed up Manual-J heating & cooling load numbers, a good place to sort out which mini-splits or heat pumps can handle those loads is to
search the NEEP website.
From an operational cost point of view, in most locations it's cheaper to run a mini-split than with condensing propane furnace. In fact, replacing the furnace with a right sized cold climate heat pump (or a ducted mini-split) might be the right thing to do for comfort. Nate Adams is a former insulation contractor in Cleveland OH who has made a business and career out of fixing comfort problems in existing houses. More often that not, arriving at the "comfort" destination involves air sealing & insulating the house combined with DOWN SIZING the HVAC. His
house whisperer blog has several HVAC and comfort discussion videos & free chapters from his book, and his business website has a number of well documented
case study examples of real projects, and what was done to get it right. Since you're in the middle of major house hacking & insulation, it's worth reviewing some of those.
Since you have a heating history on the place it's possible to
estimate your whole house heat load and oversizing factor of the "before upgrades" picture using fuel use and heating degree data, essentially using the existing furnace as the measuring instrument. That's a fairly quick & easy starting point for getting a handle on where it will be "after upgrades".
Very often the total heat load can go DOWN after adding a second floor, if the second floor is tight and code-min or better, and some effort was applied to air sealing & insulating the lower level (including the basement/foundation walls). Heat loss is largely a function of the total surface area- the second floor may be doubling the wall area, but the attic/ roof area remains about the same. If the prior attic was an air-leaky R25 and the new one is super tight R49 the roof losses and infiltration losses go way down.
Whatever you do for the new HVAC upstairs, do NOT install ducts & air handlers in the attic above the insulation. While that's a bad enough idea even in the south, it's an efficiency and ice-dam formation disaster in New England/New York and similar climates. Yet somehow HVAC contractors still seem to run AC ducts in the attic, creating a cascade of cold ( and condensation/frost inside the ducts) all winter, and lower system efficiency during the summer. Keeping all the ducts & air handlers inside the pressure and insulation boundary of the house reduces the comfort & efficiency hits from leaks, and reduces the parasitic losses.
