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Suggestions for best heating/AC strategy: central or minisplits?

Rita Friedman | Posted in Mechanicals on

Just wanted to ask for your advice:

My house was flooded during hurricane Sandy (we live on LI). We currently have System 2000 – oil, stacked – for all of our heating and hot water needs. We only installed it in 2010. It instantly reduced our oil usage by 35%. Existing central air system is very, very old. We cannot only change the condenser, we need to change air handler as well. Our dilemma is finding an optimal solution in our situation. We used to have 2 zone heat (main level and 2nd floor). We are being advised to change main level to 2 zones since baseboards will be installed. We still need to make a decision whether we should replace existing damaged central air system with new one (1 stage, 2 stage, SEER?) or just abandon air ducts and go for minisplits? The house is approx. 1050 sq ft on the main level and 700 sq ft on the second floor. The main level is completely gutted at this point. There are only 2 of us living in the house.

Also, we plan to remodel the kitchen – partition wall between the kitchen and dining room was removed along with radiator. We will close off kitchen door and relocate it to the dining room. This creates a slight issue with heating.

Thanks for your input.

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Replies

  1. Keith Gustafson | | #1

    Do you have gas in the street? Only propane is more expensive than oil

  2. GBA Editor
    Martin Holladay | | #2

    Rita,
    Install minisplits, and use them for both heating and cooling. You don't need a boiler or baseboard radiation.

  3. Rita Friedman | | #3

    Keith, we do have gas line on the street. However, we have already invested into this system2000 - it is much more expensive than other boilers and is very efficient. Our thinking is that we will install mini-splits (we are considering Mitsubishi and Fujitsu) and not restore central air since existing setup for central air is quite bad. It is very difficult to assess efficiency of the proposed solution. Eventually, if this works, we would replace oil with natural gas if prices stay much lower for natural gas (still prefer induction range over gas range).

    Martin, our additional concern is whether min-splits are noisier than central air. Quote that we have for central air replacement (condenser and air handler) is comparable to converting whole house to mini-splits.

    Thanks!

  4. Expert Member
    Carl Seville | | #4

    Rita - I'm with Martin on the mini-splits, I put Mitsubishi ductless models in my house recently and they are incredibly quiet and efficient. My only concern would be if electricity is so expensive vs. oil that the cost to operate for heating would be prohibitive - although this is probably unlikely.

    Since your house is gutted, you have an opportunity to do a top notch job of insulating and air sealing which will duct down on heat loss and gain in the walls. When you have a very tight building, ducts to distribute the heating and cooling to the perimeter of the house is less important than in a leaky home. If certain parts of your house make it difficult for the ductless models to work, you can install ducted models to distribute air to multiple rooms.

    BTW - I grew up on long island (Oceanside) with oil fired steam heat. Haven't been back in many years.

  5. Expert Member
    Dana Dorsett | | #5

    Mini-splits are far QUIETER than ducted air conditioning and heating systems not louder, even when running full blast. And since they spend most of their time modulating at fairly low blower speeds, most of the time they'll be quieter than a refrigerator. During the absolute peak load periods when the blowers are running full-out it'll be louder than your refrigerator, but still quieter than almost any ducted AC system running at full blower speed (assuming you even had a continuously variable speed air handler for the ducts.)

    Better yet, unlike ducted AC, mini-splits don't generate room-to-room pressure differences that drive air-infiltration, which lowers the actual load (!), most of which is latent heat (humidity) in your location. Less infiltraion= lower indoor humidity= lower cooling load. Cooling with ductless is simply more comfortable, with stabler temps, lower humidity, and less noise. On a house that size I couldn't imagine installing a ducted air conditioning system, if it's the same cost as a ductless.

    But size it correctly, eh? Sizing for the heating load only might leave you a bit shy of cooling power, but not by much. Resist the temptation to oversize- at about 1.5x oversizing for the heating load they start losing efficiency during the shoulder season due to on/off cycling and idling losses rather than modulating.

    The System 2000 can be purchased with either natural gas or oil burners, and the conversion is dead-simple. SFAIK you could probably even order up the exact burners used on their gas versions and the modifications to the system would be next to nothing.

    Natural gas and electricity on L.I. are a bit more expensive than the national or regional average, but converting to natural gas would cut your heating bill by roughly half. Where that stands relative to heating with the mini-splits depends on what your actual gas and electricity rates, but heating with mini-splits WILL be cheaper than an oil-fired System 2000, but may be pretty much a wash compared to gas- it depends, and we'd need more information to run the crude financials, such as:

    Do you have neighbors heating with gas who would share a mid-winter gas billing in formation, so we can estimate the delivered costs per therm (or ccf, if that's how it's billed)? IIRC it's about $1.50/therm these days on L.I., but I could be way off in either direction. What is the delivered $/kwh on your electricity billing (the whole bill divided by the number of kwh billed.) My best guesstimate is 20 cents, but it's been awhile, could be pretty wrong on that too.

    Assuming my guesses are right, with buck-fifty gas you're getting 100,000 BTUs for $1.50, and with $4 oil you're getting 138,000 BTU/s for $4, which is ~$2.90 per 100,000, almost exactly twice the cost of gas. They both run about 86% efficiency, so you get only 86,000BTU of heat into the house per therm, ther rest went up the flue. Normalizing the price to $/100,000 BTU of useful heat, that's ($2.90/ 0;.86=) $3.37 / 100,000 BTU for oil, ($1.50/0.86=) $1.75 / 100,000BTUs for gas, the cost to beat.

    In a L.I. climate average efficiency of a mini-split over the heating season would be a coefficient of performance of between 3.0-3.5, which means for every kwh of electricity you use you'll get at least 3 kwh out as heat (on average- it's less when it's very cold, and more when it's mild.) To run the conversion, a kwh is 3412 BTU, and you get 3x that for every kwh in, so for every 20 cents you get (3 x 3412=) 10,215 BTU of useful heat. Normalizing that to $/100,000 BTU of useful heat, that's (100,000/10,215=) 9.79 kwh, which at 20 cents/ kwh costs (9.78 x $0.20=) $1.96, a bit more than heating with gas, but WELL under heating with oil.

    Mind you, that was the conservative estimate. If you sized and optimized the mini-splits perfectly for heating you'd run an average COP of about 3.5, and the costs would be ($1. 96 x 3.0/3.5=) $1.68, a hair cheaper than gas.

    During the shoulder seasons when the outdoor temps are 40F and higher the mini-splits will run SUPER efficiently, in the COP=4+ range, and would surely beat a gas-fired System 2K on operating cost. During 15F and colder weather the mini-splits would be running a COP of 2.8 or less, and be somewhat more expensive to operate, but on L.I. it's warmer than 15F for 99% of the winter (+15F is the 99% outside heating design temp for most of L.I.)

    An induction range is actually more source-fuel efficient than a gas range, and doesn't have the same indoor air pollution issues as a gas range does, so go for it!

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