I didn't even know there was an advice forum here! lol. I really should come out from the women's forum more often.
This request is not so much for "Christian" advice - though I am a Christian and I need advice, I'm not so sure it's a matter of Christian advice. If that makes any sense at all.
I live in a townhouse. My furnace leaks gas and I need a new one, and I have a ducted system. It's also extremely hot where I live, and this heat is supposed to be the new normal for this area. I want to install a/c. Apparently, a ducted furnace with traditional a/c will cost an extra $100 (or more) per month during May through the first half of September. That's a lot every month. But I hear heat pump split a/c would be cheaper to run. Another option is to install a regular furnace with separate wall units for each room that sort of runs like a mini heat pump.
Any thoughts on what I should do?
Plus, since our units are townhouses, they are supposed to be completely separated from each other. But my unit gets second hand marijuana smoke from my neighbour. Any ideas what's up with that?
Putting in central air conditioning can be relatively cheap if you already have the ducting in place, and arguably makes more sense than fitting a whole new system. That said central air, like central heating, gives a clue as to what it does in the name. If you don't want to be cooling your entire house you'll need some form of room-based thermostat and an ability to restrict the flow of cool air to individual rooms. At a stroke the system becomes more complex than "cool air, pump cool air into the duct".
If you add central air to your house I would expect it to increase the value when you come to resell it, but that doesn't help you if you'll struggle to pay the upfront cost. Window units can be a real hassle to fit and remove every year (unless they are hugely well insulated, or you live somewhere with very mild winters, you'll want to take them out in the winter or they'll cause a lot of heat loss) but they are relatively cheap and you can limit the cooling to the rooms you're in at the time.
I find $100/month a surprising figure to run air conditioning. I try and be reasonably conservative with mine so as to keep the price sensible but during the hotter parts of summer I've usually got a 10,000BTU window unit running for most of the day, then at least one smaller window unit (I think the one in the bedroom is 6,000BTU) running all night. If it's really hot my wife and I sometimes struggle to sleep so one of us will sleep in the guest bedroom and run another window unit. If I'm working in the study for a time we'll sometimes have three window units running. That very seldom adds more than $40/month to our electricity bill. If you live somewhere like Arizona then it will cost more, but also if you live in a townhouse and are joined on to neighbors on either side you'd expect to get at least some heat insulation from the fact those walls aren't exposed to the outside.
If you're getting smoke from your neighbor's house it's probably worth getting someone to check the ducting when they come to do whatever work is needed. If one of you is ducting warm air to the other it could turn out that your neighbor is paying for some of your heat, but at least if you shut that duct off you'll be spared the secondhand MJ smoke as a benefit that comes with paying a bit more for your own heat.
Just to pick up on what Josiah said about the power draw of window units. You do need to consider the total power draw but a lot of what is said about them gives the impression they draw far more power than they actually draw. The unit I have in my study is rated at 8,000BTU and when it's running it draws something in the region of 4-5amps. On a 20amp circuit that gives a lot of slack of other things so it really doesn't need to be on its own circuit. The thing to be aware of is that when the compressor turns on the current spikes briefly - with just the fan running it draws about half an amp, when the compressor fires up it spikes to about 10amps for a split second, then drops back to 4-5amps. Typically a breaker won't trip in response to a very brief spike but if you do have two units on the same circuit and they both spike at the same time it might trip a breaker, especially if you've got other things on the circuit. Be conscious of other appliances on the same circuit that might also cause brief spikes - although the chance of the spikes occurring at the same time is fairly slim you don't want to find you wake up in a bath of sweat to find the air conditioners have stopped working because they tripped the breaker, and you have to find the panel and turn it back on in the dark. Anything that performs cooling (air conditioners, fridges, freezers etc) will cause spikes, as will things like laser printers. You can run an air conditioner on a suitably rated extension cord (just don't be using the ones you get for $1.99 at the dollar store, go for one rated for 10A or better) but the trailing cables get to be a nuisance as well as a trip hazard.