I keep reading lots of horror stories about heat pumps. People are complaining about all of the following: - Enormous energy bills because under some conditions (cold weather?) heat pumps use a lot more energy than expected. - Heat pump equipment and radiators taking too much space in small homes. - Cold homes and/or a requirement to keep windows shut because heat pumps just don't generate enough heat. - Only suitabl…
But, the competence required for heat pump sizing depends on the local climate. If you need more heat than the outside unit can gather, you're stuck with fallback heat, which is usually resistance strips, which is expensive. Some thermostats will call for the fallback heat anytime the requested temperature is 2-3 degrees F above the measured indoor temperature; if you've got a thermostat like that, you've got to be careful how you adjust it, and HVAC installers don't always explain that in ways that the users understand. Lots of people want it a few degrees colder when they're sleeping, but setting that up in the most straightforward way leads to resistance heating in the morning.
You're going to tend to want to windows of conditioned spaces closed, otherwise you're sending your heating budget out of the windows. Heat pumps are usually setup so you can directly regulate the temperature as desired, rather than boilers where it gets so hot that you need to open the windows to balance. If you want to run it with the windows open, you'd need more capacity, and you'd really need to convince your HVAC person. It probably makes more sense to look into a heat recovery ventilation system, which gets fresh air in without losing as much of the heat (or gaining as much heat if you're cooling). This is probably a norms thing.
Insulation helps reduce heating / cooling costs for sure, but if your home is less insulated than the norm, you just need a larger capacity system. That's another place where installer competence comes in. Actually measuring airflow and calculating requirements is unfortunately not very standard, there's a lot of rules of thumb or replacing with similar size units or the neighbor has X so use that. Again, this is workable in mild climates, but if you've got outside temperatures much below freezing, you need a competent installer, as capacity goes down quite a bit as the temperatures go down, and that's also when you need it the most. And, just insulating the heck out of an old home that was built to be drafty can cause issues with retained moisture and mold; it needs to be done carefully and appropriately.