A few things I learned:
- Yes, you can duct the exhaust of a 90+ efficiency furnace horizontally and have it exit at near ground level. By building code you can't have it be any closer than 3 feet away from any openable window. The exhaust is near room temperature and thus won't rise up a vertical chimney flue, but it doesn't contain very much CO -- mostly the product of combustion is CO2 and H2O.
- My current forced air gas furnace is of a type which i about 50% efficient. If it is rated at 100,000 BTU, about 50,000 BTU of energy is actually heating the house, while the other 50,000 is going out the chimney. Now, say I replace that 100,000 BTU 50% furnace with a 100,000 BTU 90% furnace. That won't actually save me money, why? Because I'll be putting 90,000 BTU worth of heating capacity into my home, where 50,000 was adequate. The furnace will short cycle (turning on and running at full blast for a short time, then turning off) rather than run constantly, which is inefficient and will cause the heating bill to be higher than it ought to be. I'm thinking this is probably what happened at the old apartment where we lived -- they had just installed a new furnace which should have been much more efficient than the one that it replaced, yet we were still paying $350/mo for the cold months even with the thermostat set down to 60 or lower, probably because the unit was improperly sized for the application. More efficient furnaces give you the ability to use a lower capacity unit, which is how you actually realize a fuel savings. So if I go with a 90% furnace, I'll probably only need a 60,000 BTU capacity furnace. If I were concerned about the most brutally cold days, I might be better off with an 80,000 BTU furnace, but it's something of a judgement call. Space heaters or wearing sweaters would be just as good a solution for the coldest days if I went with a 60,000 BTU unit and found that the temperature dropped when the temperature dropped below 0. As I plan to improve the insulation in the home in the future, this would change the thermal loss properties of the home and enable a smaller capacity unit to keep the temperature up on those colder days.
- Turning down your thermostat in the winter will save you energy, but only up until a point. If you set the temperature too low, you can give your furnace essentially the same over-capacity/short-cycling scenario as you get by having too large a unit for the space you're heating. Then you end up being colder than you'd like to be, and your bill is high. This might be part of the problem we had with the apartment we were in last year -- I freaked out when I saw the bill for Oct/November, when we were barely using the furnace at all, and was probably overly stringent with the thermostat setting for the rest of the winter. In reality, the bill for Oct/Nov was probably inflated because it was based off of estimates from previous years with the old furnace, and the sticker shock I got made me try to conserve energy to the point where I picked a sub-optimal strategy which caused the furnace to short cycle. The house was also rather poorly insulated on top of that, and the upstairs was uninhabited and unheated, and the attic windows kept blowing open, and so the heating bills there were bound to be nightmarish in any event, but I probably contributed more to the problem by being overly aggressive with keeping the thermostat set ultra-low.
- There are single- and two-stage burners in modern furnaces. A single stage furnace is either running full burn or off (pilot light, unless it's got electric ignition, in which case it's really off). A multi-stage burner has a low-burn mode and full-burn modes. On low mode, the furnace burns a lot less fuel, and runs at a fraction of its full BTU rating. This saves you money in the mildly cold weather during the heating months. On high mode, it's running at full capacity. It doesn't really need to run at full capacity but when it's really cold out, which in my area (Cleveland, OH) is not that many hours out of the year. So during most of the time of the year when I'm using energy to heat, I will be using low mode and saving a lot of energy.
- Multi-stage is more of a money saver than efficiency rating. Given his choice, the HVAC guy would recommend an 80% two-stage model over a 90% single-stage model. But they do make two-stage 90+% furnaces, so you can have the best of both worlds.
- 90% gives you a few nice things besides just more efficiency, though. They use sealed combustion, which means that the air that they draw in to combine with the fuel in order to burn it comes in from outside the house, goes through the furnace, burns, goes through the heat exchanger, and then out the exhaust, and never mingles with the air in the house. The air in the house comes in through the cold air return, enters the furnace, travels around the heat exchanger, exchanges heat with the hot air that fed the combustion, and blows into the house. Since the sealed combustion draws air from outside the house rather than inside, the air pressure inside the house does not go negative. Thus, you don't get a strong draft in your windows, doors, and miscellaneous cracks in the walls. This means not only does the house not have so much of a draft problem (though you should probably still seal as much as you can), but you can also count on less dust going into the house, since the dirty air carrying dust from outside won't be getting sucked into the living space.
- If you have an air cleaner installed on the intake duct of the furnace for the air that circulates in the living space, it'll help clean out the air in the house, and as long as the windows stay closed you should see a reduction in the amount of dust that accumulates. There's no magic preventative for dust, since everything we do generates small amounts of dust, but it should be a noticeable amount.
- Heat pump vs. AC. A heat pump is basically an AC unit which has a reversible valve which changes the way the freon goes through the system. This adds about $600 to the cost of the unit. I had thought for a moment that if I went 60,000 BTU furance + heat pump, the heat pump instead of AC, the heat pump would be able to help the furnace on the really cold days and combined the two would be able to equal the 80,000 BTU furnace I'm chosing between. But this is not the case -- Due to limitations of physics, freon will only help you exchange heat from the outside to warm the inside of the house down to about 30 deg. F or so. Below that temperature, there isn't enough of a differential between the outside ambient air and the freon in the system to make a meaningful contribution to the heating of the home. There's still plenty of latent heat energy in the outdoors at 0C, as that's a whole 273C above absolute zero, but unless you used something exotic like liquid nitrogen for your refrigerant, you wouldn't be able to suck the heat out of the air at a temperature much below 0C. Obviously, no residential AC units would run off of a nitrogen coolant because of the immense amount of energy that it would take to compress nitrogen to turn it into a liquid, the pressures that the system would need to run at would make maintenance more of an issue as well. Thus, a heat pump provides an assist to the furnace at the same temperature that the low-powered setting of a two-stage furnace will. A 2-stage furnace costs about $200 more than a single-stage furnace. Therefore, a 2-stage furnace is a better way to save energy than a single stage furnace + heat pump. But a smaller capacity furnace + heat pump isn't the answer for the really cold, cold days.