Backup Heat for Mini Splits
Hi GBA Experts/Community,
We live in split entry home in Southern NH (about 2000sf living space including lower level). Our primary heat source is/was oil fired boiler. Few years ago we added Mitsubishi Hyper Heat units to all rooms on upper level. Lower level would certainly get cold during the heat pump use upstairs (drops to 55F or so) until we fire up boiler that helps heating up lower level just by radiating heat from boiler itself and pipe distribution. We would use combination of heat pump and oil boiler (use heat pump from Sep-Nov and then from March-May and boiler for Dec and Feb). However, we have just learned that our oil tank is leaking (replacement $3500) and our boiler is 33 years old and probably on it’s last leg (replacement about $7000).
All that being said, I am trying to find out what would be the best option to get rid of oil tanks and boiler and provide supplement heat keeping replacement costs mentioned above? Adding resistance baseboard would also require extensive work to rip old and install new baseboards as well as upgrading breaker panel.
I am thinking propane boiler option would be cheaper as I would not have to pay $3500 for oil tanks and if I only rely on heat pumps for the entire winter running propane boiler occasionally would not be too expansive.
Lastly, I would need to add another head to lower level which I have capacity on one of the outdoor units. Currently I have (3) 6k heads connected to 24k outdoor unit so I could add another 6k unit, although this would require adding branch unit since only 3 units can be connected directly to the outdoor unit.
We also have (2) 9k units in our large living area and dinning room (includes kitchen and hallway) with 2 separate outdoor units.
Thank you for all of the help and guidance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
This might be hard to answer, but what was the oil usage before the heat pumps?
You might be able to fit an electric boiler. It’ll depend on your electric service
Hi Paul,
We have (2) 275 gallon tanks and I would fill them up in summer (roughly 450 gallons fill) and most of the time it would last me the entire heating season. Sometimes I would add 100 gallons in March just to make sure I don't run out but would never end up using entire 100 gallons. This past winter I only added 200 gallons because oil price was high.
Using New Haven's heating degree days, it looks like your heat loss is around 25,000 btu, so you're sized decently, just have poor distribution, which is typical of multi-split set ups. I think you can pretty easily make up the difference with electric baseboard or an electric boiler in the lower level. Electric boilers are cheap to buy, expensive to run so fit the backup role well. Could you fit a 4.5kw unit? They come even smaller too.
Is the branch box substantially cheaper than a separate unit? Back in 2021 when I got a single head priced out, it was about $3k installed. I can't imagine a branch box is much less.
Which 24k outdoor unit do you have?
We have reasonably cheap electricity here so a heat pump tends to be lowest cost, oil and propane are about 3x the cost so ditching the burner is a pretty easy decision. This does depend on what your local energy costs are.
For backup I'm with Paul with a dedicated one-to-one unit. Generally the cheapest tends to be a wall mount in the basement.
Since ducting tends to add a lot of cost, I think a even a slim ducted unit without any ducting mounted to the basement ceiling will work just as well as a wall mount and give you the option to add ducting down the road.
The idea here is to overheat the basement a bit and use that to heat the main floor. This might feel like a big energy cost but when it is 0F outside keeping the basement at 75F vs 60F is not all that much and makes a world of difference in main floor comfort. Think of it as poor man's heated floors. You can always insulate the foundation down the road to reduce operating costs further.
I have MXZ-3C24NAHZ2. Adding another head or slim ducted unit would be an option (if the system can support) and also adding a branch box.
Our electricity cost in NH are $0.24 (total cost) currently.
Adding a separate head with outdoor unit would increase my total number of outdoor units to 4 which I am not sure I want to do. Maybe I could replace one of my 9k outdoor units with 18k and then add another 9k head or slim duct in the lower level.
Lower level aka basement is finished and has rigid foam insulation on the foundation wall, so you are right it would not take much to heat it up to even 65 or 70 would not take that much.
My concern for backup heat is for the entire house and not just in the case if the units can't provide enough heat during coldest months, but also when we lose power. House is wired for generator and we do have portable generator that we can use to power up the boiler and give us heat when we lose power.
I'm almost certain but not 100% that the MXZ-3C24NAHZ2 can't take a branch box.
If you want to use that unit, the best bet is to remove one of the 6k wall mounts and install either a PEAD or a SEZ slim ducted unit in the basement. These systems allow indoor units up to 130% of nameplate capacity, so in case of the 3c24, you can have two 6k heads and a 18k ducted unit.
The 3c24 uses about 4000W on max, if your generator can run it, there is no need to add complication of a separate heat system especially since your electricity costs are reasonable the rest of the time.
Seem like an oil tank should cost less than that quote. Shop around. Propane will cost you more than oil to operate.
I called around and that seems to be going price to remove the old and install new tank.
While I’m sure this has been considered (and success would be highly dependent on you building’s internal configuration) but a ceiling fan for destratification might be worth a shot. We got a pretty good looking 80” fan a year or so ago that’s allowed us to limp along using an older 18 kbtu/hr heat pump heating a poorly insulated 1400 ft2 house in 6B. Otherwise I’d go with electric resistance, it doesn’t sound like the lower level is that frequently occupied, so perhaps you could fit a smaller wall fan coil unit (more responsive than other options and easy to only use when you need them) without the panel upgrade or replacing the existing baseboards.
My second option (especially if power outages are a concern or the electrical precludes electric resistance) would be one of those freestanding propane stove heaters thingies that look like wood stoves. Again for simplicity and responsiveness to variable occupancy.