I May Be Dumb....

[stoveliker]

Excellent write up.
I can say that they do better for better insulated homes (i.e. needing less BTUs); they will be sufficient down to lower temps.

I can direct the vent, and point it straight down when heating (nice warm floor there after a day) to prevent most of the stratification.
(And cooling inlet it blow parallel to the ceiling, as I don't like to sit in cold wind.)

This creates a more even temperature distribution.

 

[JMJ]

What is the size, fuel source, and condition of the existing furnace? Has the thought of a zoned, ducted heat pump been consided for the new bedrooms? In that case the heatpump would be sized for the house (4 ton?) with the existing system as a 2nd stage backup. The benefit of this would be AC for the house. The MIL could still be a separate ,2 head, minisplit system.

What is the planned use for the woodstove? Back up heat for extra cold weather, primary source 24/7 burning?
How often does power go out in this area?

Do you have a sketch of the first floor plan that includes the old chimney chase location?

Current furnace is a 20 year old Rheem, natural gas, 75k BTUs. Seems to work fine and the sticker brags about 96% AFUE, for what that's worth. I was planning mini splits to do the majority of heating/cooler for the entire second floor, getting 5 or 6 total head units. I've never considered a heat pump, I was just trying to avoid ductwork altogether.... I would like to switch to hydronic floor heating in the near-ish future.

Wood stove was supposed to help in the colder months as a "heavy supplemental" source of heat but not quite primary heat source. Power doesn't go out very often, worst was about 2 days without power a few winters ago. Other than that, maybe once a year the power will blip out but come back on shortly.

Attached original blue prints from when the house was built, pretty accurate you what ended up being built. Chimney chase is between the 2 bedrooms behind the hall closet.

 

[JMJ]

I have a 15K BTU and a 12K Fujitsu cold weather mini split, the bigger one downstairs in the small “great room” and the smaller one upstairs in the bedroom. They are about 5 years old now, so they may be superseded by slightly more efficient models, not sure. We got them for shoulder season and AC, and they are perfect for that. (Used to be that in Vermont nobody had air conditioning, and now everyone needs it). Both AC and efficiency for heating in temps above 40 are amazing, quite cheap to run. As it starts to get below 30, they struggle to provide the comfort of the woodstove and we start to feel like making fires. They save us from making countless small fires in shoulder season, on days when we wouldn’t run the stove all day. We have had some brutal heat waves every summer since getting them, and they have saved our butts.

Partly this is the nature of mini splits in a not-superinsulated house to be less comfortable in real cold. The air in the room stratifies. Warm air rises, and the mini split head units are up by the ceiling. That is the temperature they sense and run by. So you can set it for 70, and it’s 70 at the unit, but not 70 in my chair. So it becomes a bit of a moving target to find a temp setting that will provide comfort. At our electric rates, $.22/kwh, the mini splits are cheaper to run than purchased cordwood down to I guess about 30 or 35 degrees (COP 3.5 anyway). Below that and they are more expensive to run than purchased firewood. I’ve been cutting my own firewood for the last years, so I should figure firewood is either more expensive or less expensive, depending on how I feel like valuing my time. Mini splits are cheaper than oil heat down to about 5F and then the expense of running them goes up and they can’t meet demand so well anymore. Before I got the Progress Hybrid I used them to dovetail with the old wood stove, and used them at night, set to 66. They could usually hold 66 in here down to 5F or so. At 0F and below, I shut them off and used strictly wood and oil, both of which are cheaper and more effective, but the mini splits running hard at that temp do a brave job. These units can still provide 12k and 15k of heat respectively all the way down to -15F. But at that temp I need more than 15k of heat anyway, and they are less efficient.

The other thing that happens with mini splits as the temps get below freezing, depending on humidity, is that the outdoor units freeze, so they have to go through a thaw cycle. This has some hit on efficiency, but it also means there is a gap in heating for that 10 or 15 minute period (I’ve never timed it). In real cold, that is something you feel.

Overall, getting the mini splits along with a heat pump hot water heater (which replaced a hot water system run from the oil boiler) has cut our oil bill by $600 or more per year.

Since I got this much better wood stove, I have not used the mini splits at all. It is only just breaking 32F for the first time. Tomorrow I’ll let the stove go cold and run the mini splits — empty the ash pan and clean the glass.

Wow, thanks for the great info. I was looking at mini splits that claim to be efficient down to -22f or something like that. Our weather here this time of year is usually teens for a high and single digits overnight. My hope with the stove was to help out the mini splits. My MIL is perpetually cold, so I figured she could crank hers up and the rest of us can be comfortable in the 64* remainder of the house. Plus, then we have AC for the summers.

 

[stoveliker]

That is for hyperheat minisplits. I don't know what brands do carry them, other than Fujitsu, Mitsubishi, Senville. Maybe Midea. Be aware that not all are hyperheat.

Also be aware that the efficiency (running cost) goes down when you have more heads on a single compressor. The most efficient ones are 1 head on 1 compressor - but buying 6 compressors is nonsense. So you have to carefully check COPs for multihead systems, as most advertised numbers are likely single head performance..

 

[JMJ]

That is for hyperheat minisplits. I don't know what brands do carry them, other than Fujitsu, Mitsubishi, Senville. Maybe Midea. Be aware that not all are hyperheat.

Also be aware that the efficiency (running cost) goes down when you have more heads on a single compressor. The most efficient ones are 1 head on 1 compressor - but buying 6 compressors is nonsense. So you have to carefully check COPs for multihead systems, as most advertised numbers are likely single head performance..

You say a compressor for each head unit is nonsense, I say it's a consideration.... why not?! I was looking at some Senville units, Aura if I remember correctly.

 

[begreen]

I was just process thinking. It's easy on paper. The thought was that if a 10"x 10" trunk duct could be run up the old chimney chase , and the joists ran parallel with the upstairs hallway, then the trunk duct could be run under the hallway floor with take offs for the rooms.

I would like to switch to hydronic floor heating in the near-ish future.

This is the time to do it if that is the plan, even if the boiler is not in place, at least lay the lines. Then make do with the woodstove in the interim. The kids won't freeze if there rooms are cool. My wife had her pet goldfish freeze in her bedroom one winter. She survived. Electric blankets help.

 

[begreen]

You say a compressor for each head unit is nonsense, I say it's a consideration.... why not?! I was looking at some Senville units, Aura if I remember correctly.

I agree. 2-3 heads per unit is a common configuration. Too many moving parts, too much space take up outdoors, a lot more to service, a lot of extra wiring, and more power draw. Outside it would look and sound like a commercial frozen foods plant.

Also consider Daikin Aurora for the minisplits.

 

[John Lehet]

I was looking at mini splits that claim to be efficient down to -22f or something like that.

You have to look at the data tables on the NEEP site. They test all cold weather units at a set of temperatures: 47F, 17, 5, -15 -- and they post the efficiency (COP) at 3 different output ranges at each temp. You can watch the COP drop as the temp drops. If you find a unit where that doesn't happen, let me know. Mine show a COP of 1.66 at max output at -15 (it will be max output at that temp.) That is "efficient," in that it is more than half again as efficient as electric resistance heat But it's not "efficient" in the sense that I want to pay for it at that efficiency (and my electric rate).

I guess the AI data centers are going to be the great equalizer of electric rates. Those of us with high rates won't get the data centers. Those with lower rates will get them, and their rates will go up.

The NEEP link: https://ashp.neep.org/#!/

 

[begreen]

Looking at the 1st floor plan, I see what you mean. Opening up the first bedroom with the chased chimney to the LR would make a nice spot for a stove, but your thoughts make sense too. The heat distribution would be more even if the space over the living room was not so open to the 2nd floor.

 

[JMJ]

I was just process thinking. It's easy on paper. The thought was that if a 10"x 10" trunk duct could be run up the old chimney chase , and the joists ran parallel with the upstairs hallway, then the trunk duct could be run under the hallway floor with take offs for the rooms.

This is the time to do it if that is the plan, even if the boiler is not in place, at least lay the lines. Then make do with the woodstove in the interim. The kids won't freeze if there rooms are cool. My wife had her pet goldfish freeze in her bedroom one winter. She survived. Electric blankets help.

We've been living for the last 2 winters with the thermostat at 62, so I know the kids will survive! We're going to be moving some walls on the first floor once the second floor is finished, so I'm not sure where everything will end up and I wouldn't want to shoot a nail where there shouldn't be one.... plus I have to tear ALL the drywall off the ceilings on the first floor and redo it, so I figured i would just staple up some tubing at that point in time. I can't afford more delays on the MIL moving in....

 

[begreen]

I guess the AI data centers are going to be the great equalizer of electric rates. Those of us with high rates won't get the data centers. Those with lower rates will get them, and their rates will go up.

For the time being, I think we are going to see a lot more small nukes going up adjacent to new data centers.

 

[John Lehet]

And on the up side, my unit at 47F has a COP of 6.06 at minimum output at 47 degrees. At 47 degrees it likely will be running near minimum output. That is just over 6 times as efficient as resistance heat.

Even at my high electric rate, that is $10.75/million BTU, while firewood at $300/cord in an 82% efficient stove is $18.29/million BTU.

That's why at that temp I'll run the mini splits and give the stove and me a break from wood burning.

 

[stoveliker]

About having a single compressor for each head:
Sure, it's fine. But it'll be expensive, and a lot of units outside.

 

[John Lehet]

Also regarding the one to one head to compressor vs multi-head, I'm sure that a single head is definitely better and more efficient. When I did it, Vermont was (and maybe still is) offering an incentive rebate that is on a per-compressor basis. That made the single headed units a no brainer.

 

[kborndale]

I have a comfort aire hyper heat mini split (12000 BTU) in my master bedroom which is upstairs and doesn't get much if any heat from the basement wood stove. The room is spray foamed so good insulation. They claim it can heat down to -22F. I doubt it would do much at that temp but it has kept the room at 68 without struggling when the outside temp was 15F.