I have a 850 sq ft home just North of Kenora Ontario (P9N0E7), it’s pretty well insulated compared to most in the area with 2x6 walls and tri-pane glass. There are is a ridiculously large lakeside (east) window (72” X 120” two fixed to casement panels) and a not so great sliding door (double pane glass and leaks air) to the deck (also east side).

It is currently heated with an oil fired forced air furnace (Lennox lowboy at 80000 btu (80% efficient I’m told so really 62000 btu delivered to the house at best). The furnace is oversized resulting in frequent short cycles in the coldest weather further undermining its efficiency.

There is no central air, just a room a/c (mounted in the wall under that huge east facing window)...I believe its rated at 8000 btu and with all the doors open to circulate air can cool the place and dehumidify it on the hottest days (95 f or so at worst with 80% humidity).

The thought of filling the oil tank (1100 liters at $1.20/liter) sends shivers up my spine, not to mention the damn furnace breaks down regularly costing me an average of $400/yr to service/clean/massage etc).

So... I have read and read and read some more till I have come to the conclusion that my next heating system will be the Mitsubishi Mr Slim that will heat and cool the house. I plan on mounting the head in the family room (the one with the sliding door to the deck).

What I would like to do is install an electric furnace (which I already have (used one)) in place of the oil burner and then replace the blower motor in the electric furnace with a variable speed DC unit to both circulate the warm air during the heating months and cool air during the summer and serve as backup /supplemental heat either when the Mr Slim can’t produce sufficient btu (below -25c) or if the Mr Slim goes off line.

What I need to know is... given that the current oil furnace is too big, how do I know which Mr Slim model (if any) can handle the heat load?

Thanks.

Posted By ICFHybrid on 08 Jan 2013 09:38 AM
What is the capacity of your electric furnace?

20KW

Below about 0F, it will have to carry most of the load in your plan. How many weeks a year is it consistently below 0F at your place?

Not sure but I would think at least 8 (I'm in Kenora Ontario Canada postal code is P9N0E7)

Without having better details on your structure and insulation, the safe estimate is between 30K BTU and 40K BTU of heating capacity needed on that place. That means a Mr. Slim of about 36K BTU capacity, but during peak cold periods you will be almost entirely on the electric furnace. If it's a 10 kW, then it might be a little skinny.

Your best plan is to put oil in the oil tank and keep it for those peak lows, but hard to say how running it less like that will affect your yearly maintenance costs.

I'm OUT OF THE OIL altogether I hate it and its stupid expensive

If there are improvements that could be made in the structure, insulation, air sealing, doors and windows, then you might be able to move the heating needs below 30K BTU which would help settle the issue more.

Yes, I will be blowing in insulation to R60 in 1/2 the roof (only half is available (its R30 now) the east side has a sloped ceiling with R30ish), I will be replacing that crappy sliding door and every winter the big window especially but all others as well get the plastic shrink wrap on the inside; thats the best I can do for now...One day I would like to strip off the vinyl siding and add 2" of foam to the exterior before I re-side it.

Posted By ICFHybrid on 08 Jan 2013 08:07 PM
You might not like the oil and the maintenance expense, but 8 weeks of running straight electric will be expensive, too.

10kW X 24 hrs X 56 days = 13,500 kWh. Looks like you are paying 9 or 10 cents there in Kenora, so that runs you about $1200. What was oil costing you for those two cold months?

Can't believe you don't have gas service there.  Doesn't the Trans-Canada run right by you?