Air-Source Heat Pump VS Oil/Baseboard heat.

I've got a 900 sqft in-law apt with baseboard heat + central AC. The current AC unit is 10 seer, oversized, and runs with flex duct through a cathedral ceiling. The system is about 13 years old and now there is a very slow leak somewhere, which dropped the pressure and caused the coil to freeze up. I figure one charge now to get my parents through the hot months and then upgrade the system in the Fall. In the Winter I get major ice dams over this part of the roof. I'm sure its a combination of recessed lights, the penetrations of the duct work, and shoddy sealing of the envelope. I was planning to attack this problem this year by replacing the cans with track lighting and covering the AC vents during the Fall/Winter months, but now I'm thinking of abandoning/sealing off the duct work and installing a dual-zone mini split system. This way I won't have the penetrations in the cathedral ceiling (maybe even blow insulation into the ducts?) and hopefully make it a little more comfortable for the folks.

My other hope if I go this route is to install a heat pump system instead of straight AC. For a majority of the shoulder season the main house stays pretty warm with the help of the wood insert, but the in-law is being heated from the boiler (90% eff system with outdoor setback). My thought is it may be more efficient/cheaper to use the heat pump during mild weather than the boiler.

HSPF of heat pump is 8.4
KW/h cost is $.18
Boiler is 89%+ eff
heating oil is $3.50+ (and rising)

I'm sure the system eff. of the oil boiler is closer to low 80%. Are there other factors to consider? At what outdoor temp will it make better sense to turn off the heat pump and use the boiler? From my understanding heat pumps are only efficient when its warm.

The more efficient the better. I replaced a 1991 unit with a 2007 (Trane, outside compressor, inside airhandler). I'd say I went from an usable outside temp of 18° to about 12°, the difference was operating costs, and warmer discharge temps during mid 20's and warmer outside temps. Also paid a little more for a system that uses the newest version of refrigerant, not sure if that is an issue anymore.

I didn't get the ultra high efficiency but the next lower model. As you get closer to ultra high efficiency the price gets ultra high as well, and it still won't work in ultra cold temps.
Plan on some electrical work, possibly a larger breaker panel if yours is already filled.

+1 Mini-splits are the way to go.

I figure at an outdoor temp below 30F the boiler is going to be running ( to heat the rest of the house when the wood stove isn't running) no matter what, so that's what I'm planning my cut-off point. Its great to know I could do it if needed but from what I understand there's less a diminishing return the colder it gets.

How is the heat? Is the indoor unit's air temp differential dependent on the outdoor air temp?

I'm going to get the most efficient I can, but as this is really only used for AC during a max 2 month during the Summer, and for perhapse supplemental heat during the shoulder seasons, I may be limited to what makes financial sense. Its probably going to be any rebates that sway my decision. Word on the street from the installers I know is Sanyo is the best (or at least has been very good for a long time), and the new dc inverter technology is the nutz. I will look very closely at the Fujitsu, which I understand has a lot of the same parts as Sanyo. Noise is my 2nd concern.

Still, I get all goose-pimply thinking about 10 seer to 22 seer!

Mine will blow 102° when it's in the 50's
80 or so in the lower 20's, and drops even quicker after that unless the resistance heaters kick, which never happens because the fireplace insert is blowing 190°+ by that point.

It has to run for a longer time to satisfy the thermostat setting as it gets colder out. You will also have the keep the snow cleared for air circulation.

If I didn't burn wood, I would have never replaced the heat pump, I would have went with propane.

I think you're on the right track if you absolutely need A/C in the summer.

I don't need it but my parents don't have a basement refuge like us. Well, let's just say I'm not interested in sharing the man cave with my mom.

Are the newer mini-splits programable for certain temperature shut-offs, or to intergrate with a backup heat source?

On a BTU to BTU basis, 18 cent/kWh electricity is even with $6/gal oil in a straight resistance heater setup. Conversely, the COP of the HP need to be better than 6/3.5=1.7 in order for it to be cheaper than oil. I think a mini-split will beat that above 15°F, pretty conservatively. My conventional Goodman would do that well down to 20°F or so (SEER=15, HSPF = 8.75, with the defrost controller set properly).

Tabulated COP numbers for HPs versus temp can be misleading for conventional HPs as they exclude defrosting, which can shave 0.5-1 point off the COP below 40°F. Generally, different HP running the same refrigerant will have similar COP vs temp (running a 100% duty cycle). IIRC, the higher COP of the mini-splits is due to demand defrost controls and variable speed compressor motors.

Does your utility have a discounted elec rate for winter heating customers? I got elec 50% off for the first few years I had my HP.

There's a 25% rebate on delivery rate.

For those who have made the switch, did you find it easier/cheaper to use individual compressor/air handler units or did you go the dual indoor and single outdoor units? The current cooling needs are only about 18k max, where as the heat load is closer to 40k. Its one open room about 550 sqft with a bedroom/bath of about 300 sqft. It seems like you don't really save much money going this route however.

i installed 2 fujitsu mini splits. 1 in each house in different climates. i think mini splits are the bomb. get an inverter model, and size it larger than what you think you will need, for heat. what i didnt really wrap my head around on my first one is how much the btu's are reduced when the temperature plunges. i installed a 9k, when i should have installed a 12k. but none the less, the unit is awesome. this one is in my vacation home that is heated by electric baseboard. i have noticed a huge reduction in electric this year vs last with it.

i am really looking at a mitsubishi for my next one. they have some kind of hyper heat, and supposedly will heat in the negative degrees. i think its either running the compressor at like 125% for 15 mins or so, or some type of heater on the thing to help out.. supposed to be the bomb.

my understanding is the dual room option is not quite as efficent as 2 singles.

noise from the fan is not an issue unless you feel the need to run it on high all the time.

Another idea you could look at is a ground source heat pump (geothermal) system for both residences. Initial installation cost would be a lot more, but you can currently get 30% back in federal tax credits. If you're going to be in the house for a while payback is reasonable over oil (7 years not unreasonable to expect).

Hi,
You can compare the costs with this calculator:
http://www.builditsolar.com/References/Calculators/Fuels/FuelCompare.htm

To convert the HSPF to a COP (Coefficient of Performance), divide it by 3.412 -- this gives 2.46, so the efficiency is 246% (or so they say.

If I put your numbers in the calculator, it comes out that with the heat pump you would be paying $2.14 per 100K BTU of heat produced and with the oil burner $2.77 per 100K BTU produced. But, the carbon emissions for the heat pump are a bit higher than the oil burner.

It might be worth finding out what kind of climate the HSPF is based on -- if its based on a milder winter than what you have, your HSPF will be worse.

Gary

Another calculator I ran across:

http://205.254.135.24/neic/experts/heatcalc.xls

It allows you to correct HSPF for different locations for a heat pump with electrical resistance backup. You can use the various defaults (the calculator provides links to where they got the default info) or provide your own.

Scott