I had to learn the difficult way that just because you have an actual heating, ventilation and A/C program that is perfectly sized for your home, does not mean that it will efficiently heat or cool it… Then you may be able to get the size and power right in theory, however there is another sizable factor that determines whether or not your lake house will be heated or cooled efficiently and adequately.
Your lake house needs to be weather-proofed; this simply means, among other things, that you need to have good insulation, and when you have good insulation, the thermal energy that your oil furnace produces will stay in your home, rather than slip through thin and poorly insulated walls! Also, if you have an attic that is not adequately insulated, you are losing a whole lot of frosty air that is escaping through the roof, causing your air conditioner to work harder.
The better the insulation in your home, the more entirely the cool or hot air that you are trying to generate with your heating, ventilation and A/C program will be contained, instead of slipping away. There is another way the conditioned or hot air can escape your lake house and cause serious energy inefficiency with your whole heating, ventilation and A/C system. There might be holes, cracks, or slits around your doors or windows that are letting frosty or hot air escape. So, if you are busy noticing that your energy bills are rather high, and yet you are not achieving your desired temperature absolutely easily, you may need to weatherproof those areas of your lake house to ensure that they are properly sealed. Again, properly sizing your heating, ventilation and A/C program is important for energy efficiency, however if one hot or frosty air is constantly escaping your home, your temperature control program can’t help you.