As we gut a Philadelphia row home, we’re also making plans for it to become a frack-free house, a phrase popularized by the architecture firm Bright Common. This translates to: No gas appliances delivering fracked gas from Western Pennsylvania into our home. Everything that used gas will be replaced with electric options.
So far, we’ve happily removed the gas oven / range and the associated gas pipes out of the kitchen. For cooking, we’ve selected an electric stove. And already have other electric appliances to supplement this: toaster oven, microwave, induction cooktop and crockpot.
For heating the house, we realize the row home design that abounds in Philadelphia means we are tucked in between two neighbors. The only heat loss would be from the exterior walls. We’re therefore building out the walls by 4” so that we can pack in insulation. Matus Windows, a local company with a good reputation, will replace the windows. After this, the exterior walls should be draft free.
We’ll still need a heating system. Looking at our options, we’d heard that mini-split ductless systems are the most efficient. So we called in a recommended HVAC company to advise us on this. They were fixed on one brand: Mitsubishi, and that’s what was priced for us, a 20,000 Btu system. They were unable to advise us how much electricity this would use. And we learned that our favorite thermostat, the programmable and self-learning Nest, would not be compatible.
What we did learn was that one outside compressor and 2 inside air handlers would do the job, offering us 2 zones to heat and cool. I’d hoped the compressor could stay in the garage or basement, spaces with the least temperature swings during the summer and winter, but was advised this was against the building codes in our area.
I’ve since learned that 12,000 Btu equals 1 ton. Also, that estimates vary on the area this would “condition”; from 400 – 500 sf, and up to 800 – 1000 sf. Most sites mention ductless mini-splits when discussing indoor cooling, but since we prefer to cool off with ceiling fans and have rarely used air conditioning, our focus is on using the mini-splits for wintertime heating.
Per one site (7 tips to get more from mini split heat pumps), we also learned to
- plan for average lows, not record lows.
- For Philadelphia, the mean minimum is 6.4F, with record low of -11F. We should plan for 6F.
- plan on using space heaters during extreme cold spells.
- We’ve got plenty of these from when we used a gas boiler to ramp up indoor heat in the mornings, and then, like task lighting, used space heaters in the specific rooms with people.
- plan for outdoor compressor to draw air from indoor space.
- This makes so much sense, since the interior air has less temperature fluctuation. The one installer we called advised us this isn’t to code. I’ll have to see what the next company says.
- position interior air handler about 18” off floor
- This too was a surprise. Most mini-split installations I’ve seen are mounted closer to the ceiling. But perhaps that’s because they were designed for cooling the space.
- an actual example of a 3/4 ton (or 9,000 Btu) system suffices for a 1,500 sf home in Massachusetts, and uses about 1,500 kWh per year.
- A system this size (9,000 Btu) should definitely work for our 650 sf row home in Philadelphia. Note that this is much smaller than the size recommended by the first HVAC company (20,000 Btu).
- Assuming it will use proportionately less electricity, we estimate the annual usage to be 650 kWh per year.
- select higher HSPF (heating season performance factor), which is measured in Btu / Wh.
The buying guide from Consumer Reports suggests we also consider noise levels and demand defrost options. The recommended noise level is about 7 decibels. The demand defrost option is to keep icicles from forming on the outdoor compressor fans, which sounds worthwhile.
My initial reason for asking annual electricity usage from the HVAC company was because we’re also planning to install rooftop solar. We wanted to know that the rooftop system would not only suffice for our lighting, electronics, cooking, and hot water needs, but also for our heating and cooling needs.
Chatting with Dara Bortman of Exact Solar (an area residential solar installer), I learned that there were solar mini split systems out there, powered by either DC from the panels or AC from the grid. The system I’ve got my mind set on is the unit by HotSpot, sold as an air conditioner, i.e. cooling needs.
This 35 SEER system cools by using 11,500 Btu per hour, or about 328 W (11,500 / 35). Assuming we might only need cooling for 8 days, about 8 hours per day, the electricity used for the season would be
328 W x (8 days / season) x (8 hours / day) = 20,992 Wh, or 20.9 kWh per cooling season.
Of more interest to us is the heating season. This 10 HSPF system heats by using 13,000 Btu per hour, or about 1300 W (13,000 / 10). Assuming we need heat for 5 months, 8 hours per day, the seasonal electricity usage would be
1300 W x (5 months / season) x (30 days / month) x (8 hours / day) = 1,560,000 Wh = 1560 kWh per season
Note that we had estimated 650 kWh per heating season, based on the Massachusetts example. It could be they like the house a little cooler. Or it could be we don’t really need to run a heating system for 8 hours per day. The other difference is that their 9,000 Btu Fujitsu system has an HSPF of 12.5, whereas the Hotspot’s HSPF is 10.0.
My concerns are
- Noise – the indoor noise level is stated to be 26 dB at the low setting, while 7.6 dB is recommended. Would this feel too loud?
- Placement of compressor – I would like the heat exchanger to be in the basement, not outside in extreme temperatures. If it must be outside, we’re thinking of mounting it over the garage door, with the 3 panels above as a protective awning.
- Circuit load – Could I have the heat going, while making tea or taking a shower on a cold morning? Without blowing a fuse? The stove is rated for 30 Amps, the tankless hot water system at 60 Amps, and the mini-split heating system at 5.3 Amps. All together, about 95 Amps. Well within the 100 Amp panel in the garage.
- Thermostat – It’s unclear how the setback thermostat works for this. I plan to keep the house cool at night, and expect a thermostat to warm up the house before I wake up.