My home (1100sqft) is passive solar and built on a slab. Currently, we heat with a propane space heater (wall mounted) which we have to supplement with electric space heaters when it's very cold. We cool with a window A/C unit. I have been considering installing a electric radiant floor system (under laminate flooring) for heat and keeping the window unit for cooling, but the thought of a mini split keeps coming up. Any advice?

Location is key. Your ZIP?

35744. Northeast Alabama

A mini-split will reduce energy usage.

jon is right depending on the lay-out. A right-sized mini-split heat pump will cost the least to operate.

How much does electricity cost there?

11.12¢/kWh

If I go with a mini-split, then I have to decide whether to go with one zone in the main part of the house or put a head in each room. I would like to use one head and circulate the air around the house.

If you're able to heat the main space with a propane space heater, a single wall-coil type mini-split head would be able to manage that space.

If you can install the ducts completely inside the pressure and thermal boundary of the house, a ducted mini-split might be the "right" solution for distributing the heat to the remote rooms. Your 99% outside design temp is about +20F, and it's unlikely that the amount of heat needed in those rooms @ 20F is big enough to warrant their own ductless head. If you put a ductless head in every room you'll have issues with minimum modulated output of the compressor being too high to avoid cycling on/off at low efficiency during your average winter temps. Getting to the right solution all starts with calculating your actual heat loads on a room-by-room basis.

How many doored-off rooms need the supplemental heat, and are they close to one another?

How air-tight is the house?

Insulation levels?

Window types?

If you right-size the mini-split(s) your average heating efficiency would be nearly 4x that of radiant floor resistance heat (it would use only 1/4 of the power), and at 11-12 cents/kwh it would be dramatically cheaper than propane (unless you have 50 cent propane or something.) Assuming 85% efficiency on the propane heater you get about 78,000 BTU/gallon into the house. A pretty-good right sized mini-split would deliver about 13-14,000 BTU/kwh on average, a crummier one maybe 10,000 BTU/hr, so you're looking at 5.5-8 kwh to be the heating equivalent of a gallon of propane. Buck-a-gallon propane is 15-25% more expensive to heat with than electricity using a second or third rate mini-split, and nearly twice as expensive as using a latest-greatest version.

Even a fairly leaky 1100' house would have a heat load at 20F of less than 25,000 BTU/hr, most 2x4 houses that are reasonably tight and not high-R insulation would have a heat load under 15,000 BTU/hr. The minimum modulated output of a multi-zone mini-split at 47F is about 6000, BTU/hr, but your whole-house load @ 47F could easily be less than that. You're probably looking at a 3/4 ton or 1-ton head for the zone currently heated with propane, and a separate (not multi-split) dedicated 3/4 ton mini-ducted version for the rest, or possibly a single 1 ton or 1.5 ton mini-ducted version for the whole shebang.

Is comfort your goal in warming the floor? Been there, suffered that in a passive solar house in Pa in the winter before I got the hydronic radiant heat going. We have a two-ton minisplit heat pump, but expecting it to heat all that concrete was like spitting in the ocean. We now keep the slab at 75 and bump it to 80 during cold snaps. Unbelievable difference.

It's too late now if your foundation guys dug trenches for beams, framed the perimeter, and applied rebar, sand, plastic and concrete. Heating concrete and the soil under it isn't practical. (Or wise in cooling climates. An earth-coupled slab lowers cooling bills; thermal peak under the concrete will lag the seasons by a month or more.)

A minisplit plus strategic radiant ceilings would be my choice. Flip a switch in the living areas and you can have the top couple of mm of concrete toasty warm. Turn them off and go to bed. A minisplit works very well for cooling. The cold air sinks to the floor; the slab carries the chill through the house.

Finally, window treatments are key with aggressive glazing. We have cellular shades for light control plus nuclear winter shades (foil-faced bubble wrap glued to patio roller shades.) Again, huge difference.

thanks for all the input. You are all giving me a lot to think about.

my goal is not warm toes, but rather to equalize the temp throughout the whole house. I probably should have stated this from the beginning, but money is definitely an issue.

So, what I am thinking after Dana1's comment is to install a homemade ventilation system to circulate the warm air that I already have (flexible duct and a inline fan installed in the attic). I feel confident that I can pull that off for +/- $200.

How does that sound?

Moving room-temp air around with ducts or fans is a pretty lousy and inefficient way to go due to the wind chill and extremely poor heating efficiency. You have to move a LOT of air at a temperature difference of 5F or less to heat a space (say 73F air into a 68F room), so much air that the draft makes the occupants uncomfortable. That only works in fairly high-R houses.

The output temps of a mini-split (ducted or wall-blobs) are well north of 100F, so they would need only ~10% of the air volumes to heat the space, and even if you were in path of the air it's more like a warm summer breeze than the wind-chill you'd get with a fan or ducted 73F air.

Replace/displace the propane burner with a 1-head minisplit. If you have some mid-winter fill-ups with the exact fill up dates & quantities, with a ZIP code (for weather history and outside design temps) it's possible to figure out what the design heat load is of that zone to be able to properly size the mini-split. (It'll probably be a 3/4 tonner, but I'd rather prove it with the napkin math than just guess.)

If that heats the house most of the time, it's not a crime or very expensive to use resistance heating for the temperature balancing if some of the doored-off rooms need it.

As a mostly-DIY installation with an hour or two of refrigerant tech time to charge and test the system it's possible to put in a high HSPF high SEER 3/4 ton Mitsubishi or Fujitsu for around $2000-2500. A full turn-key installation by a pro should still be under $3500.

In your climate if reasonably sized to the load it will run a seasonal average COP of about 4 or more in heating mode, call it ~13,000 BTU/kwh (that's low-balling it with an as-used HSPF of 13- you should do better than that.) So that would be 1,000,000/13,000= 77 kwh per MMBTU. At 11.12 cents/kwh that comes to $8.56/MMBTU.

Propane has a source-fuel heat content of about 91,600 BTU/gallon, so burned in an 80% efficiency wall furnace or space heater that's 0.8 x 91,600= 73,280 MMBTU/gallon net heat into the house. Normalizing to MMBTU that's 1,000,000/73,280= 13.6 galllons per MMBTU.

So, heating with a better-class mini-split would be like heating with $8.56/13.6gallons= 63 cents per gallon propane in your current equipment.

What are you paying for propane?

According to EIA data, propane in AL is currently averaging about $2.60/gallon:

https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=W_EPLLPA_PRS_SAL_DPG&f=W

So, the operating cost of the mini-split would be only 25% of the cost of running the propane burner (a 75% savings on operating cost!) If you're burning through 500 gallons of propane per heating season at $2.60 that's ~$1300/year, and delivering the same heat with a mini-split would run you $325/year, a $975/year net savings.

So a mini-split would likely pay for itself in just heating-season savings in about 3 years or less, not counting the savings during the cooling season (does it ever get hot in Alabama? Not really, I s'pose... :-) )

Do your own napkin math, maybe get some quotes on something decent like a Fujitsu -9RLS3 or Mitsubishi -FH09NA, either of which can deliver a LOT of heat at decent efficiency even at +10F outdoor temps, and have cooling season SEERs north of 25.

Don't attempt a DIY installation of a mini-split without reading up on it. There are no rocket scientists wasting their careers as mini-split installers, but there are a lot of details to get right. Most people don't want to own the tools & learn the skills to charge & test the system, but the rest is fairly straightforward for those with reasonable electrical & carpentry skills.

One strategy for dealing with cold rooms at the other end of the house is to turn the heat of the mini-split up to 75F or more, and leave the doors open when it's going to be cold. This isn't quite as efficient as just letting it modulate at a more reasonable temp, but it's still a fraction of the operating cost of a propane wall furnace.