BTW what is OP and COP?

OP== "original poster", or "orgiginal post", the person or post that started the discussion thread

COP=="coefficient of performance", which is the ratio of energy output to the energy used to operate the system. A resistance element heater has a COP of 1.0 it puts all the operating energy into the building/room as heat. A heat pump with a COP of 2.8 puts 2.8 times as much heat into the building than the amount of energy it took to run the compressors & blowers.

At $3.80 oil in a 75% boiler you get 0.75 x 138000 BTUs= 103,500 BTUs into the room for $3.80

Assuming a COP of 2.5 in a mini-split and 20 cent/kwh, you get 2.5 x 3412 BTU= 8530BTU for 20 cents. It takes 103500/8530=12.1kwh of power to equal the heat output of a gallon of oil, so heating with the mini-split @ 20cents/kwh is equvaltent to 12.1 x $0.20 =$2.42/gallon oil, which is more that a 1/3 reduction in heating cost.

We were figuring a 16x 20 room with R30 batts roof& floor, call it 640 square feet of surface at ~U0.05 ish for a design condition heat loss of about 1800BTU/hr, to which we add 2350 or so previously estimated for the glass, and then about 400 square feet of exterior wall at ~U0.1 for another 2200BTU/hr, order of magnitude we're looking at ~7500BTU/hr for a 10F design condition heat load. If you keep the room constantly 70F, that'll be something like (7500 x 24 hours)/(70-10)=3000BTU per heating degree day, so times ~6000 heating degree days it would take 18MBTU/year.

With the mini-split it would cost something like...

$0.20 x 18MBTU/8530= $422/year

to keep it wicked-toasty in there all fall/winter/spring. (I guess my prior $400/yer SWAG-example based on that other project wasn't that far from the mark, despite a different climate & rate, eh?)

With oil it would take...

$3.80 x 18MBTU/103500=$661/year. (about 170-175 gallons)

In the oil situation you could probably save 20% off that by setting the temp down to 60F when not in use- call it $500 /130 gallons per year.

Mind you these are intentional overestimates, since there will be very real solar gains through the east & west facing glass, and since it's reducing the heat load on the main house by the reduced amount of exterior surface, the real numbers added to the total whole-house heating bill will be more like $250(mini-split) or $400(oil) above your heating bill if keeping it as unheates space.

If it costs you $2500 to install a new baseboard zone, and $3500 (after rebates) to install a 1-ton minisplit, the $1000 difference in upfront costs is easily paid back over time, but if the delta is $2000 you'd have to rationalize it another way. (The air-conditioned comfort and dehumidification counts for something.) But if there's a main zone of the house that is open enough to be heated with a split-system, going for a 2-ton multi with 2 heads would likely come in around $5000 after rebates, but it could take a huge bite out of your oil use, with a quicker return on investment. If oil heads north of $4 and electricity stays under 25 cents in the next 5-10 years it could be QUITE the investment.

At 20 cents/kwh it's not a slam-dunk in the same way that it is in places where electricity is 8-12 cents (and there are many parts of the US where that's the case), but I'm not expecting $2.42 oil any time soon, are you? (At 12 cent electricity it's like heating with oil for under $1.50/gallon, so if it knocked 500 gallons/year off the heating bill you'd get a 2-3 year payback.)

How many gallons/year are you currently burning in the boiler, and are you heating your hot water with it too?

Posted By padrino01803 on 23 Aug 2011 12:03 PM
money is tight. I'd like to use the rm yr rnd.
My concern was is it feaseable given the amount of
glass and being on the north side w/ west exposure
Am I throwing $ away to condition it yr rnd?
If there's a way that I could use it all the time
cost to run would be my 1st concern.

Train on the tracks:
Are you throwing away money to condition a room year round? rather subjective, but wouldn't one be throwing away money to build a room they cant use?
Cost to run first concern? Definately price geo. Will have lowest cost to run though not necessarily lowest install cost, won't know 'til you price it.

Using Dana's calcs the geo console would cost about a third less than minisplit to run.

A different heating strategy for the whole house would make more sense.

Joe

"A different heating strategy for the whole house would make more sense."

That's the long and short of it.

You could probably get the smallest-in-line 9000BTU Mr. Slim installed for about $2.5-3K after rebates, and it would take care of the addition, but not much else. (Internet pricing on 'em is ~$1400, and you might be able to cut installation costs with some amount of DIY, but IIRC it's not legal in MA to commission it yourself, and it takes some amount of training to fully test it, even if the installation itself is fairly straightforward.)

But $3.50 oil burned in an aging high-mass boiler there's a mid & long term incentive to find an alternative solution for the whole house. A 2-2.5 ton multi-split would cost less than a best-in-class oil boiler and save more money. Geo would cost more and save more, but whether that makes more sense depends on a lot of factors, not the least of which is the level of subsidy available. The fact that fact that the state of MA will spot you $25k interest-free for 7 years for installing high-efficiency heating systems, and the feds are currently picking up 30% of the tab for geo may be a sufficient driver in your case:

A decent geo system would cut your heating bills in half- if, say you're currently burning 800 gallons of oil at an average cost of $3.50, that's $2800/year that then drops to $1400 year. The $1400/year savings over 7 years adds up to $9800 (after taxes!).

Example 1: If a geo system costs $27K up front, and the feds knock that back to $18K, which you borrow interest-free from the state, your real cost after 7 years for the higher-efficiency geo is ($18000 - $9800 = ) $8200, or about the same as a multi-split of somewhat lower efficiency.

Example 2: If the geo costs $21K up front, knocked back to $14K by the federal subsidy, your real cost after 7 years is ($14000 - $9800 = ) $4200, which is less than the installed cost of a 2 ton mini-split (single-head.)

Since MA won't lend you the money for a mini/multi-split and only kicks in $500, that's cash out of pocket up-front rather than stretched out over 7 years.

But until you have some quotes on the table you can't really tell where the break-points are. If you tell us how much oil you actually burn per year it's possible to put an upper bound on the actual system sizing, which may have a bearing on likely cost. MA has pretty good homeowner subsidies for retrofit weatherization & insulation too, and it may be possible to down-size the system by lowering the load too, which will also bring down system cost.

Can someone recommend a reputable contractor in Burlington area?

Thanx