Minisplits vs gas furnace?

We are thinking of replacing our gas furnace with a mini-split system (several indoor units), so we are going from gas to electric.
Venice (CA) is mild compared to the rest of the country with an average winter low above 50 and summer high below 80, with some very occasional outliers. Our gas furnace needs replacing and since I’ve been overproducing solar since 2008 (>$1k balance!), it would make sense to go electric and we even get AC in summer(which we currently don’t have).

My extra gas energy cost for heating in winter is maybe $60-80/month and I am curious what I can expect in winter electric cost using a ductless solution. Much more? Similar? Most of the rest of the year the system won’t typically even be running except on exceptionally hot days.

Has anyone done a similar upgrade and can share the experience? Thanks!

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I can’t answer that question exactly from personal experience but I assume since you currently have a furnace that you have a forced air system. Why not just change the furnace to a heat pump? It’s literally perfect for your climate and would provide both heat and cool using electricity and your existing duct work. Additionally, you won’t have to put indoor head units in all the rooms.

Most people prefer forced air systems over mini splits and they go the opposite way that you’re proposing.

In your climate (I was raised in Monterey so I’m familiar with it) you really don’t need a gas system like you have and a heat pump would work great. In colder climates gas is a much more efficient heat than electricity.

I do have experience with gas water heaters vs electric and maybe you could draw some conclusions from that. My house was all electric. I recently installed propane and switched the water heater. I was spending about 40-50 a month on the electric. With on demand propane I’m spending about $5 a month.

Agreed, I also would not want to waste existing duct work and add all the mini split units. If your layout works, consider a new HP with variable speed blower. I am however building a 350SF accessory building in my back yard for my home office and guest room. In this case I’m using a Mitsubishi ducted mini split.

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Thanks for all the details. These are good suggestions that all still are under consideration. Todd, was you electric water heater resistive or a heat pump?
(Yes, my water heater will remain an on demand gas unit).

Unfortunately the situation is much more complicated. All the existing ductwork would need to be rebuild from scratch anyway (currently asbestos from the mid 50ies, some air outlets are in the wrong place due to a kitchen remodel decades ago, etc.). Also I want strong zoning because only 1/3 of all systems will be used unless we have visitors or temperatures are extreme. So the choice is between

  1. A central heat-pump system with brand new ductwork and the extra associated thermal losses and complicated zoning controls for all air outlets with many thermostats and settings.
  2. A distributed, redundant, more quiet, and more failsafe system with two relatively small and independent outdoor units (on opposite corners of the house) and three indoor units each.

I am really trying to weigh the pros and cons of all possibilities.

That adds a lot of context to the conversation. Here’s something to consider. IF you have asbestos in the duct insulation and you know about it, it’s mandatory that you replace it in California before sale of the house. If you don’t replace it, mandatory reporting to the buyer is required. So it causes an issue further down the road. Replacing it means that you don’t have to disclose that because it was changed out.

As for zoning. You could easily put in dampers if you wanted to when you’re changing out the duct work if you go that route. Or you could use something like Flair Vents (https://flair.co/). Thermal loss on a properly insulated system is very small, but yes it does exist. Interesting that you mention noise since I don’t hear my units at all. I would argue that newer units aren’t noisy.

To add another pro/con. Mini splits do run the risk of refrigerant leaks on the head units. Since the head unit is inside the house that could lead to poisoning. When the unit is contained outdoors it evaporates into the atmosphere.

To answer your question, the water heater I replaced was a heat pump water heater. But to be honest the heat pump sucks on those things. When it was time for the family to shower we had a routine that would put the unit into “high demand” mode which would use mostly elements anyway. The nice thing about on demand gas is that there’s no tank of water to always keep hot, even when you’re not using it.

A lot of people forget that efficiency of heat is always greater with gas. Furthermore, a lot of people who want to “save” the environment by going electric forget that most electricity comes from fossil fuel, even in California where we have one of the most progressive energy systems (California ISO - Supply). That conversion from fossil fuel to electricity then to heat is terrible compared to a clean gas heat (own solar generation is an exception of course).

Lastly, here’s something else to think about. 1 forced air unit vs 2 mini splits. That’s 2x the potential problems and 2x the annual service cost.

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Furthermore, a lot of people who want to “save” the environment by going electric forget that most electricity comes from fossil fuel

This is changing, and, when considering the typical lifespan of a water heater, it is very likely that the majority of electricity generation will be from non-carbon sources by the time someone replacing their water heater today replaces it again. Unless, of course, if Wyoming wins its (ridiculous) legal battles with other states forcing them to buy Wyoming coal.

May I suggest modifying the political language it’s really not relevant.

The reason I included the Cal ISO link is so anyone can see where California is getting their power from, i.e. renewables vs fossil vs nuclear etc. Additionally the OP states he’s in California. California is the most progressive state with renewable energy. As of right now, 7:25am PST California is only using 26% renewable, so my fact based assessment stands. At night, California uses about 90% fossil fuel as there’s no sun to feed the majority of the California renewable grid.

A couple more fun facts:

  • 80% of the US power is still from fossil fuel in 2021
  • In 1980 we actually had 10% renewable power in the grid.
  • We now have 20%. So we have doubled renewable power in 40 years.

And I think you took my comment slightly out of context. The energy conversion ratios to heat a home or water with electricity when that electricity comes from fossil fuel is actually far more harmful to the environment than heating with a clean gas. This is also why I said unless you have solar, which would be the exception.

I disagree. I feel like the statement regarding lawsuits is incredibly relevant. If the political landscape of the country is such that we are going to require utilities to continue to burn coal, then electric generation will continue to be dirty. Given that coal is quickly becoming the least-economical fuel source for electricity generation, the lawsuit is ridiculous because it flies in the face of the charter of every state utility board, whose purpose is to stand up for the ratepayer. Why have state utility boards if other states can dictate how my state generates electricity?

From the same page (probably) that you copied that image from (California ISO - Supply), you will see another graph on batteries. While utility-scale batteries are a very tiny proportion of the grid today, that is likely to grow, addressing the “it’s dark outside, so solar isn’t doing anything” issue.

Lastly, you may have missed that my post was very much forward-looking. I realize that every ISO in this country is majority carbon-fueled today. Many water heaters have lifespans of well over 10 years, and, while I can’t make any fact-based assessments of where we will be in 10 years, I can point to renewables standards for states (see CA here: Codes Display Text. ). Again, this doesn’t say what WILL happen by 2030, but the current legislation is that California will be at 60% renewable by 2030 (within the lifespan of a new water heater installed today).

I will concede that fossil-fuel → electricity → grid → home → heat has terrible efficiency. That was never the basis of my argument.

As I already said, I have been overproducing solar since 2009, but of course since I will be feeding less into the grid using heat pumps instead of gas, the statistics for California still holds for the big picture.

Good discussion! Still looking at all options, of course. :wink:

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@DevOpsTodd, @qrnef , not to divert the conversation, but I did want to point out that Sense is beginning to give us tools to refine these kinds of decisions. The most recent addition to the iOS/Android app (not the web app), gave us a Carbon Intensity (CI) metric that overlays our personal usage with CI of the region grid operator, to enable better tradeoffs:


It’s not perfect:

  • Not everyone lives in a region covered the regional operators, so some people won’t have relevant CI data.
  • Some folks, like me, buy less carbon intensive electricity, either via a utility or CCA (community choice aggregator), so their Sense CI will run higher than their usage.
  • Sense is still sorting out the best way to handle users with solar. For example, using from-grid energy during times of higher CI is “bad”, but does producing lot of “to-grid” solar during the day cancel that out ? Or is it still better to try move your “from grid” usage to periods of lower CI ? Tough to get the “signaling” metric right on what is most helpful.
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Last week I replaced our 15 year old gas water heater (last gas using device on the property) with a hybrid electric one. In my setup it uses about 2kWh of energy per day and I have it setup to be using that only during the day when I have solar power.
Funny story: It took multiple calls to have the gas turned off and despite that still someone showed up to put a tag on the door “for the next person to live in the house” that they would have to call to turn the gas on.
I (also) overproduce with my solar and last week 75% of our electricity usage came from our solar panels, including charging our 2 EV’s during the day.


Yesterday’s bill I pulled 176kWh from the grid and put 1497 kWh into the grid
Currently working on battery storage (enphase) to help offset high rate ToU from the grid plus provide backup power in case of events.
I am very happy with my 5 split units in the house (1600 sq ft) which were more than enough during last winter (southern Cali, so not really cold) for less costs than the central furnace the year before for heating the house. Now I am overproducing with solar, my heating is basically free.
I think since OP is living in CA as well, you would actually save money (just heating wise) with heat pumps vs gas furnace. I also like the fact that you can regulate each room individual much easier than central AC/heating.

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My house had a HVAC system installed in 1991 with probably 75% of the ductwork installed behind walls and under the floor. What I can see is 0% insulated or sealed, so my HVAC runs a lot, especially in the summer. I want to insulate what I can see, but that does nothing for the majority of the system which is probably leaking like a siv.

As for noise, our 2002-ish condenser is crazy loud. Our neighbors replaced their condenser last year with the absolute least efficient model. It is so power hungry, our lights dim when it turns on and it is even louder than our almost 20 year old unit, and it’s brand new. Being a new condenser doesn’t mean it will be power efficient or quiet.

We do have two Mitsubishi mini splits and those are literally whisper quiet. They usually only run at 12w all day, except for when it needs to “crank” to cool down a room, say the kitchen when we have the stove or oven running, or the living room if we’ve just opened the front door for a while. They top out at 1,100w, but I rarely see that. Mainly they stay at mid-400w when actively working. First screenshot is the past week of my kitchen mini split, second is the living room, both monitored by DCM.

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