WIthout doing an analysis of the houses energy needs, neither one may be the correct size! Under ideal conditions, on the coldest design day, the furnace would run constantly. This provides more comfort and efficiency along with longevity. You lose energy with each cold start, and cycles wear things out faster than duration of operations. Replacing a furnace with a similar sized unit is usually going to result in oversizing - oversizing affects comfort, purchase price, and cost of operation.
Not all furnaces are mulitistage, and then it becomes even more important to size it correctly. Each time it starts up, you'll get a shot of cold air as the furnace heats up the ducts and heat exchanger, then when you shut it off, you lose most of that heat which is why running longer is better. Depending on the house, even 35K is likely more than needed. The only time extra capacity is somewhat useful is to rewarm the house after it has cooled off - then, it can recover quicker. Otherwise, excess capacity is just costing money.
No, it would definitely
NOT run constantly, even on the coldest day, even if the heat load at the 99% outside design temp was exactly matched to the furnace output at high fire. Only modulating furnaces with 3:1 (or more) turn down ratios would run constantly on design day, not two-stagers. The 99% design condition persists for but a handful of hours (at most) even on design-day, and by mid-afternoon on that day the heat load is usually under 70% of the design condition heat load. Two-stage furnaces typically run for a fixed amount of time at low fire when a call for heat is initiated before kicking up to high fire, and at high-fire the call for heat would be met it it would cylce off, even on design day. The burns may be long in the cool AM, maybe even for a couple of hours or more in the morning on design day if the temperature is set back overnight, but nowhere
near constant.
It takes a very large or leaky house to have a heat load of even 50KBTU/hr in a Vancouver climate. (
99% outside design temp = 24F/-5C) A medium-sized ~2500' house built to current code minimums on windows and insulation would usually be under 35K in that climate. (My ~2400' + 1500' of semi-conditioned basement 1923 antique with known insulation gaps and antique double-hung single-panes+ storms has a heat load of ~35K at an outside design temp of -15C.) While there is no real efficiency downside to oversizing a hot air furnace by 3x (which is what you'd be doing if you went with an 80KBTU/hr unit), the wind-chill of the higher velocity air of an oversized furnace and more rapid cycling can cut into comfort.
Running longer (and fewer) cycles is generally a GOOD thing, for both comfort and for wear & tear/maintenance on the equipment. If anything the 60K unit is likely to be ~2x oversized for the real load, and you should be looking at something in the ~45K range, but don't make that assumption without doing a heat load calculation.
Even the smallest oil burners out there run ~60-70K, so the sizing of the prior unit won't give much of a clue. But with fuel use measured against weather data between the fill-up dates can put a hard upper limit on what the true heat load is. If your oil supplier stamps a "K-factor" on the billing that's effectively the same information. (I'm not sure if K-factors on billing is customary in Canada, but it is fairly common practice in the US.)
When in doubt go smaller- 999 times out of 1000 that will be the right decision. Real design-condition heat loads are nowhere near what the installed base of furnaces put out, and the average binned-hourly mid-winter heat load in a Vancouver climate is about half the design condition load. If you should be so lucky to find a 2-stager with output EXACTLY your 99% heat load at high-fire you'll be better off avoiding deep setbacks (the recovery times could be pretty long during cold weather), but it would still only rarely have to kick into high-fire if you keep the thermostat at a fixed temp.