Backup Heat Source for Minisplit System
Hi, I recently had two Fujitsu Halcyon ductless wall units installed on the first floor of of our two-story 1800 sq ft house (Climate Zone 5). It’s looking like these units will handle 95% of the space heating needs in the entire house — they keep the downstairs at 70F and the upstairs at around 65F (with the three upstairs bedroom doors left open). The only small snag in this plan is that the youngest member of our family sleeps in one of these upstairs bedrooms from 7 PM – 7 AM with the door closed and the temp in that room can drop to around 60F or so during this time. So long-term, we will still need a little supplemental heat in at least that bedroom (let’s say all three bedrooms to be safe) just to take the chill out and raise the temp 5F or so as needed. Our mini-splits also only work down to -5F, so a backup heat source would be wise for rare cold snaps.
Of course, we do have a central heating system, and this is a hydronic radiator system supplied by a natural gas boiler. Downstairs the radiators are cast-iron baseboards. Upstairs the radiators are severely oversized upright cast-irons (5 feet long by 2 feet tall radiators in each of the 100-150 sq ft bedrooms). The hydronic system has two zones, but one zone is just for the first-floor bedroom/bathroom, and the other zone is for the entire rest of the house — on the first-floor: kitchen, living room, dining room; on the second-floor: three bedrooms and one bathroom. The gas boiler is the last fossil fuel appliance in the house. I eventually want to replace it with something electric — my goal is to have no fossil fuel appliances inside. We currently have a 7.7 kW solar array that produces 10,000 kwh a year. We plan to add maybe 1-2 kW to this a few years down the road, since the total annual electric usage for our entirely electrified house would be in the 12,000-13,000 kwh range. Given all of this, my question is:
Should I replace our natural gas boiler with an electric boiler? Or should I get rid of the boiler and hydronic system entirely and just put in electric baseboard radiators, room by room, throughout the house? Or some hybrid combination of these two options?
The heat load for the whole house at our 99% design temp of 11F is around 30,000 btu/h. If I was going to go with an electric boiler, I was thinking of something in the 9-10 kW range (with an output of around 30,000 btu/h), like an Argo or Electro. Something like this:
If I was going to rip out the boiler and old hydronic system entirely, I’d still prefer the new individually-zoned electric baseboards to themselves be hydronic, so I was thinking of something like Cadet Softheat hydronic baseboards:
But what are the pros and cons of each of these two strategies? Right now, if I want to get a 5F heat bump in the upstairs bedrooms, I have to turn the gas boiler on for 20-30 minutes. I imagine this is partly because the bedrooms are on the same zone as most of the first floor, so the boiler first has to send that 180F water through the dining room, kitchen, and living room before it winds its way upstairs. So would individually zoned baseboards like Cadets in the upstairs bedrooms be significantly more efficient? Since the only need for supplemental heat is in those upstairs bedrooms, is my more-or-less single-zone hydronic system just a hopelessly inefficient way of achieving this, regardless of boiler type?
At the same time, I imagine there are some risks and downsides to just ripping out the old radiators and abandoning a hydronic system in the walls that’s been there for almost a hundred years (the house was built in 1925). New mouse superhighways along those formerly 180F and now cold-to-the-touch hydronic pipelines? New moisture/mold issues in the walls now that we’re suddenly not heating and drying out their insides? Inherent problems with just abandoning a bunch of pipes in the walls and letting them rust there? I guess the more general question I’m asking here is does this 1925 house as a whole system need its hydronic heating system for better or worse in order to remain healthy as a whole system?
Any advice would be much appreciated. Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part