Is it more efficient for me to heat my home with my ductless mini split or my old boiler?

Submitted by CatoFriedman t3_zm5og5 in massachusetts

I just had a ductless mini split system installed in my 1907 home. We also have an old boiler. Maybe 30-35 years old. I used my mini split for heat for almost all of November and was shocked by the electric bill. This month I am using the oil boiler and am wondering if this will be more affordable. I feel frustrated that I cannot figure out which is more efficient to use and am hoping someone more knowledgeable than me might be able to help. Here is some information that I hope is helpful:

  1. My mini split system is a Gree 23 SEER 24,000 BTU Heat Pump Condensing Unit and Air Handler. It has a 10.5 HSPF rating.

  2. My boiler is a very old System 2000. There is some information on the front of it with four different models, but I am unsure which model I have, those ratings are as follows: EK-1 - Oil GPH Input .85, BTUH output 102k, EK-1 - input 1.00, output 120k, EK-1 DV, input .85, output 102k, EK-1 DC input 1.00, output 120k.

  3. Oil is $4.09 per gallon

  4. Last month, I used 409 kWh, and was charged $200.04. This equates to an effective rate of $.4927/kWh.

Please help me reddit! Let me know if you have any questions and thanks so much for your help.

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UniWheel t1_j0960l3 wrote

You mini split is probably cheaper for maintaining temperature most of the time.

Your boiler may be cheaper for raising the temperature, or on the days when it's coldest out, which is when the mini-split's effective efficiency (of "pumping" rather than "creating" heat) drops from almost 400% down towards 200% and may no longer overcome the inefficiency of the original electric generation (which in MA is largely from burning fuel)

Either way key is also going to be using only the heat you absolutely need in the parts of the house where you need it. Though do be careful if you close part of a building off entirely to consider the risks of pipes freezing - including heating pipes. Hot water heating systems often control the return, actually, so if they freeze they'll happily send huge quantities of heated water into the break.

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wkomorow t1_j09j1hw wrote

Great advice. I leave my main thermostat at 61 in the day and 59 at night. My living room splits (where I spend most time during the day) is set at 65 during the day and off at night. My bedroom split is off during the day and set at 63 at night. With humidifiers, it is quite comfortable. I added a zigbee temp sensor in the basement to warn me if the temp in the basement goes before 40 at night.

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UniWheel t1_j0c8jnb wrote

I'm personally trying to avoid running the heat pumps (mini-splits) during the coldest part of the 24-hour cycle, because the colder it is outside, the more energy required to produce the same amount of heat.

So I've been turning them off when I go to bed, and then trying not to turn them back on again until mid/late morning when it's had a chance to warm up outside or at least the sun is on the compressor. It's close enough to the bedroom window that I can just hear it in the silence at night, and it's the sound of money being spent.

Many would argue that you shouldn't adjust the setting on heat pumps at all, but just leave them. I don't quite agree with that, because the same heat is more or less expensive during the colder/warmer parts of the outside day, and my home cools slowly enough that I can meaningfully average those.

But I would argue against turning up heat pumps "for comfort" during a particular part of the day, especially if that's a time when it tends to be cold out, like first thing in the morning - so for someone who's leaving the house to go to work or school, trying to wake up to a warm space is fairly expensive.

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wkomorow t1_j0cckbu wrote

Thanks, I just got my monthly gas bill $86 for furnance and indirect water heater. It was $207 last year this time. Heat pumps really help. My electric went up to $68, it was $47 last year this time, and last month it was $18. I am on a city electric pool, so my rate is .09 per kw, which helps There wasn't a lot of really sunny days, so my panels could only do so much. Snow covered them for a day even though it was sunny after the last storm. I may just set my main thermostat to 61 and keep it there and not run the splits in the bedroom. My bedroom is on the second floor of a cape so it accumulates heat from the first floor. Having a warm mist humidifier really makes a big difference. I also put down the shades and close the drapes before sunset and open after sunrise. (They are automated, so I really don't do much.) And I keep the door closed on rooms I am not in.

I appreciate the advise, Jan is always the real test.

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UniWheel t1_j0d2bn3 wrote

>I am on a city electric pool, so my rate is .09 per kw

Damn you are lucky :-)

With those rates, I'd use the heat pumps exclusively.

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wkomorow t1_j0d6kcx wrote

Don't know what happens Jan 2024, but Pittsfield contracted with Dynegy Electric Services a fixed rate at $0.09603 per kWh for all rate classes for 36 months (January 2021 to January 2024). The mayor deserves a lot of credit for this deal. Great Barrington just went wit Dynergy and their rate is $.22. Salem is locked in at $.11 to the end of 2024.

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No-Bug3247 t1_j174mf0 wrote

Just kidding but who are you people with 63F? My wife won't be comfortable until the house is at 75F!

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wkomorow t1_j187is1 wrote

At night, I can not sleep if it is warm. During the day, I wear a pair of chunky socks over my socks and typically a chunky sweater or sweatshirt. I hate hot weather. In the summer, I garden before 9 AM because I am worthless once the temp gets just south of 80. I run the air conditioning a lot in the summer. With the splits and solar (which I own not lease), it costs me nothing for the air condition. Normally my electric bill shows a credit from Mar-Nov.

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modernhomeowner t1_j09ms4g wrote

Check to see exactly which gree you have. In Massachusetts, roughly half your heating load is for when the temperature is above 27°, half below. So if I look, the only Gree 24k I found that fits the 23seer, 10.5 HSPF is their multi+plus unit, which at 27° has a COP of 1.76 at full capacity. They don't list their partial capacities, assuming your home doesn't need the full BTU load, efficiency may be higher, something like 2.6 COP.

If we assume your oil boiler has poor efficiency, let's guess and say 65%, oil has a BTU of 90,000 adjusted for efficiency. divided by 3412 btu per kWh of electricity, divided by the COP (we're guessing at the 2.6 since we don't know the load or the efficiency of the gree at a lower capacity), times your electric rate of $.482, we get $4.89. Oil would have to be more than $4.89 per gallon to make you want to use your heat pump all year. Certainly below 27°, you'd want to use oil. Now, if your heat pump needs to pump out it's full capacity at 27°, the COP is 1.76, using that same formula above, Oil would have to be more than $7.22 a gallon for you to want to use electricity. So certainly, you'd want to use oil if your heat pump is operating at high speeds.

I should make note that when I say "you'd want to use oil" I mean from a financial standpoint - I always get one or two comments about environmental impact. And of course, I am only guessing at your oil boiler's efficiency, good ones are in the high 80's, you mentioned an old one, so I went with 65%, Same with the COP assuming your home doesn't need it's full 24,000 btu heating load, if it does, you'll be closer to that higher break even point for oil, meaning you'll want to be using oil, certainly with temperatures below around 30°. Check with gree and your specific unit to see if they have better numbers for you, and if you can get an energy meter on your unit to see how many kw your unit pulls to see how close to capacity it is running at any given point.

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SynbiosVyse t1_j09tw0m wrote

I think 65% efficiency for an old boiler is even a very low estimate. Boilers have been at least 80% for a very long time.

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modernhomeowner t1_j09uy5g wrote

Well, a higher efficiency makes oil an even cheaper proposition over electricity. The boiler itself may be rates at 80% but other system inefficiencies also take place, it only operates at maximum efficiency right after cleaning and gradually gets worse. I've pretty much calculated my system at 72% and that's with a 12 year old boiler. But if this person gets 72% or 80%, you can recalculate the math and it's going to result in higher equivalent oil prices, meaning you'd want to use oil even more often then electricity.

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SynbiosVyse t1_j09zlt1 wrote

Yeah I agree. I made a similar post a few days ago saying my gas boiler seemed more economical to run than my heat pump after some anecdotal testing and I got eaten alive.

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modernhomeowner t1_j0a09jp wrote

Gas as in Natural Gas? Yes, that's much cheaper if you have the same electric rate as the OP. Much cheaper! Assuming again a bad efficiency of 80%, a 2.6COP, natural gas would need to be under $4.35 a therm, which Eversource is $2.51. if you have a 92% efficiency, you'd be at $5 equivalent, so natural gas would be half the cost of the heat pump.

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Kspoonie t1_j0batjr wrote

OP has a System 2000 which is the only high efficiency oil boiler I know off. I’ve seen them upwards of 90%, even old ones. He’s probably better off getting a System 2000 Qualified technician to tune it up and run that. Pricey unit for sure.

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Dismal_Ad_9603 t1_j09kki9 wrote

If you’re not paying a decent rate per kw/h you’re going to pay a lot for electricity. Just did something similar with Mitsubishi’s ended up paying over $500 (NGrid pricing) for 4 weeks of service. At these current temps I feel like the efficiency has dropped off. Currently (no pun intended) researching alternative electric suppliers for the coming months but will likely be using the oil as sparingly as possible with lowered set temps and using my fireplace. Bought wood last spring long before that went up too.

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modernhomeowner t1_j09neoq wrote

Certainly have to lock in providers sooner rather than later. You should lock in before the winter months. I did last year for 9.5¢ when National Grid was 14¢, and this September for 12.69¢ after national grid announced their 33.8¢. Ambit has a plan for 23¢, saves you 10¢ at least, and I don't think there is an early termination fee. Inspire has some good rates, cheaper I think but I can't get them to show up right now, also no termination fee. Constellation has even lower rates, but they have long contracts with big cancellation fees.

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Dismal_Ad_9603 t1_j09p7dj wrote

Thanks, just got the minis installed in October wasn’t sure what to expect in terms of usage, so November was an experiment, the fact that the minis can provide heat was not the initial intent for installing them. I was expecting to use them for late fall and early spring for warm ups. Another factor in the electric bill was the added fees for supporting electric cars and a few other things that are percentages of the electricity charge which I feel are totally ridiculous and don’t feel that I should be paying for ( but can’t opt out of).

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CatoFriedman OP t1_j0a6v5m wrote

ModernHomeOwner, when you say lock in rates, are you talking about the alternative energy suppliers? Like this, https://www.massenergyrates.com/massachusetts-electricity-suppliers? I have not done this but feel like it is an obvious way to save money? Is that true? I have solar panels as well, would signing up affect my solar benefits, if you know? Also, thanks again for your help.

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modernhomeowner t1_j0b2v6h wrote

In MA it does not affect net metering, you still get your summer credits based off your utility's rates at that time. But when you buy back your energy, you get it at the third party rate (hopefully lower). Note these suppliers sometimes have lower starting rates and raise them after your contract. Sometimes this isn't the case, but if it is that's why it's good to be hunting around for cheaper rates before your contract is up and why it's nice to not have a termination fee.

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SilverRoseBlade t1_j0b0cqv wrote

If you have National Grid, they doubled their Basic Service charge to ~0.33 cents when it was ~0.18 before. Trust me I got a giant sticker shock when Nov’s bill came in.

And Eversource is raising their rates too soon I think someone in r/Boston mentioned a while back. It sucks but oil could be cheaper if you do the math. Sadly I’m stuck with electrical in my building.

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Lucybruin t1_j09y0br wrote

Try comparing your electric bill with some of your neighbors because there’s probably went up a lot too, because of the increases in electric cost alone

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viomoo t1_j0a23nb wrote

I did the mini split for the past 30 days and just got a bill of $800. Granted, this is because constellation completely messed up and we ended up on the National grid rate, but still ended up using 1700 kWh of electric (so even at a sensible rate the electric would be over $400).

Oil is coming down in price and is at under $4 a gallon now, so I’m thinking it will be cheaper to go that route.

I’m in MA as well if that matters!

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SynbiosVyse t1_j0exvk3 wrote

1700 kwh is a ton. Are you running electric strips on the heat pump? You should probably turn those off and run gas/oil if you have the option.

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viomoo t1_j0gh5ed wrote

5 different mini splits and running 24/7. Small child in the house so it is at 68 degs constantly. I have switched over to oil now though! I was ‘testing’ to see just how much power they would use. Turns out it’s a lot!

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SynbiosVyse t1_j0ghm4u wrote

What do you mean 5 mini-splits? Are there 5 registers (head units) to 1 compressor?

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syphax t1_j0a55w5 wrote

Who is your electric utility? 49c per kWh is very expensive, even for MA.

Also, how big is your house? Have you invested in airsealing, insulation, etc.? Basic insulation can have an incredibly good ROI.

Look at both the supply and demand side!

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CatoFriedman OP t1_j0aau8c wrote

National Grid. I might need to sign up with one of these alt providers though, like through this - https://www.massenergyrates.com/massachusetts-electricity-suppliers.

1300 sq ft house. Just added a lot of insulation for cheap using mass save. Mostly in the attic. Also have solar panels so that helps a lot. I went into this winter with a $300 credit towards electricity from producing more than i used during the summer. Thats going quick though!

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ZaphodG t1_j0aosmv wrote

What about windows and doors? Have you found all the air leaks with an IR camera and stopped them?

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CatoFriedman OP t1_j0bl3s8 wrote

No, might need to try that. Thanks

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ZaphodG t1_j0bnnm5 wrote

A few cans of foam and some weatherstripping could make a big difference

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snagleradio78 t1_j09attw wrote

Mini for sure. My electric bill is high but still way cheaper than paying for the oil worth the boiler. Also. Mini heats more efficiently

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noodle-face t1_j09ppio wrote

How much space is the mini split having to heat? We have 4 in ourn1500sqft house and the prices haven't been insane yet. That said it's still in the 30s

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what_comes_after_q t1_j0abllc wrote

Short answer is the mini split.

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Long answer is it depends on a number of variables like temp and humidity, but heat pumps generally cheaper to run than a gas boiler down to low single digits. It's cheaper than oil even lower than that. So heat pumps are cheaper to run for most of the temperature range that we get here in Massachusetts. There will be a few times during the year where gas or maybe even oil will be cheaper, but those days will be uncommon.

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DunkinRadio t1_j0babse wrote

Not quite the same situation, but I just did a back of the envelope calculation for my house after switching to a heat pump from a 96% efficiency gas furnace.

Comparing last years bills to this year (avg temp was almost exactly the same) I found: Using the therms to kWh equivalence, we used about 1/3 of the energy as last year to heat the house. However, electricity is about 3.8x the cost of gas for the same amount of energy, so my total cost went up some (but not an outrageous amount). I'm willing to pay somewhat more if I'm reducing my carbon footprint, so the next step is to figure out how much NG is used to generate a kWh of electricity. If I'm not reducing carbon emissions enough (or at all) to justify the extra cost, I'll go back to using the gas furnace.

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