A 3000 sq ft house should have either two HVAC systems or at the very least zones for each floor. Any discussion about this?

The sizes builder's HVAC man is suggesting seem oversized given the amount of insulation you plan to install. Ask to the heating and cooling load calculations.

I would think a 21 SEER AC would be overkill for Canada. What are the average high temperatures in the summer?

Ontario gets pretty hot and very humid, at least southern ontario does. OP didn't mention heat pump, at the very minimum they should be going for that. You won't need to burn any natural gas probably, outside of dec-feb I would think, the heat pump will draw heat from outside, and the newer ones are good down to close to 0c.

I thought SEER was an energy efficiency rating, in which more, is more better-er?

I personally would go as high as I could afford.

You are correct that a higher SEER rating results in more efficiency. But you have you consider the pay back on spending the extra money in order to get this efficiency. In most parts of the US a SEER rating of 16-18 is the best value. You have to live in a part of the country were the summer temperatures are consistently above 100 degrees F and electricity rates are high. Othewise you may never see a pay back during the life of the equipment.

4.5 ton, 110MBtu seems rather large for new tight construction of a 3000sq ft house. One system with perhaps zoning should suffice.

Will you have a 'Smart Meter'?

I would recommend the Carrier Infinity Greenspeed system with either the gas furnace or fan coil unit, up to 20-SEER, 13-HSPF and up to 97.4% efficient gas furnace. Heat pump should be good down to about -20ÃÂC (-4ÃÂF) minimum. You will need the gas for the defrost mode. You will see a payback given your climate.

You could also look into geothermal.

SR

Absolutely re the geothermal. I would have suggested it, but so many people dismiss it out of hand. Where I'm living right now, people have wood fires or pellet stoves pretty much as a matter of course. I think if they bit the bullet and spent more on geo up front, they'd save a bundle in the long run. It gets cold here, and there's no natural gas, just electric, fuel oil or propane.

Even a heat pump, as I mentioned before.

Ontario is not some magical place where it is really cold. Minnesota has areas far colder than where most of the people in Ontario live.

If you have NG and you build tight, geothermal is a ridiculous proposition. NG is super cheap and if you build tight in the relatively moderate Toronto area, your heating will be very cheap. Canada makes codes that are very strict regarding insulation.

Seer 21 is another ridiculous proposition. The a/c cost for a house like this with seer 13 should be less than $100 a year. You can spend $4000 more for seer 21 and save $20 a year - ridiculous...

Lots of places in BC have Geothermal, there are a lot of reasons why people don't consider it 'ridiculous,' there's a whole community/development in Kamloops that's Geothermal. Temps there are relatively high in winter, typically, it's not that much below freezing most of the winter, however, summers are pretty hot with makes Geo more attractive.

NG may not stay this cheap forever, and a lot of people like the idea of reducing their dependence on fossil fuels. People make their own choices, it's not 'ridiculous' to them.

There's no one answer, such discussions tend to have significant regional variations due to power source differences and resource availability.

Much of Canada has hydro power. While not without its own ecological detractors, it's cheap. In the US, more than half of electricity is produced with fossil fuels, and the majority of it uses dirty coal. Coal produced power is also cheap, and also not without significant eco-cost.

Using hydro electricity instead of natural gas for heating does reduce fossil fuel use. Using coal produced electricity instead of a gas furnace doesn't, it just transfers the location where the pollution is produced from your house to the power plant. Around the US, there are some areas with relatively high power rates (I live in one of them) where heat pumps of any kind are WAAY too expensive to consider.

Ridiculous was a strong word. In areas where geo is very common, it can make sense. But to start talk about "absolutely" is a bit strong too.

I don't know but I suspect S. Ontario is not 100% hydro.

In general, it is far cheaper to build tight and insulate well rather than spend the money for geo. The only reason geo gets used in the US as much as it does is the rather significant gift from the government to do it. Which is a ridiculous waste of money on a new build because they just should have insulated better....

Since Canada requires good insulation, the payback for geo is long but if electricity is very cheap and hydro based, then I can see the argument. Then of course the argument against seer 21 is even stronger.