I currently have an over-sized furnace for my home square footage. I'm happy with the duct's performance, they are quiet.
The furnace is 27 yo and I'd like to replace it with a heat pump and air handler. To keep the project cost down, I would prefer to keep my existing ducts.
I would step down to a 2.5 ton system from the 4 ton furnace - what would I expect to happen if I keep the ducts?
Definitely maybe... Sorry but it depends on whether or not the existing ducting will work with the new unit. So you should take that into consideration when selecting a new air handler. Many units are similar but not identical and so you may have to have an "adapter" fabricated to mate up the new unit with the old ducting.
Why do you think the old unit is oversized? It would be unusual to oversize like that because a larger unit is more expensive and a waste of money if it's larger than the needed size. You should have an HVAC professional evaluate your home and make a size recommendation.
Yes, an air handler will be the same size or smaller than a furnace that has the same heating capacity. The same heating capacity will use the same size blower and the air handler’s heat exchanger is more compact than a burner and hot plate.
Changing ductwork is costly. Unless it’s defective, just have it cleaned. There’s no disadvantage to larger ducts than necessary. If the system capacity is being reduced so much that the exit velocity will be too low to mix with the room air volume there’s something wrong with the chosen capacity.
Installers like to install small high velocity duct hoses because they have a lower material cost and are easy to install. The downside is they consume more energy pushing air through them and sometimes whistle.
I’m not sure why you are persuaded that the furnace is oversized. If your furnace was short-cycling and running less than 2/3 of the time in the coldest weather then reducing the system capacity from 4T to 2.5T will be appropriate. If it ran more than 40 minutes per hour at any point on the coldest days then 2.5T won’t be enough heat.
A heat pump’s nameplate capacity isn’t its capacity in cold weather. Pay close attention to the de-rating curve that applies to a heat pump’s capacity in cold weather, because the system capacity when you need the most heat is the only capacity that matters. Use the derating factor that applies to the coldest weather where you live. If 2.5T has to be derated to 80%, then its 2T heating.
Choose the heat pump size carefully. With variable speed drive the only life cycle cost penalty for being a little oversized is the incremental equipment cost. Conversely having to resizing a heat pump that’s too small is very expensive.
If you aren’t sure about the right size, the existing systems actual fuel consumption will provide a more accurate estimate than any model. The thermostat settings can be combined with the building’s fuel use and the local degree-days to get an accurate actual btu/degree for the building. If you just moved in and none of those figures are available, wait until spring.
One final unfashionable observation. Potential life cycle cost savings almost always assume a comparison to electric resistance baseboard heating. For almost all of the USA there is no life cycle cost saving to be realized by replacing natural gas heat with a heat pump of any type. The exceptions are locations with unusually high natural gas prices and low electric rates that are in the southern US, e.g. Phoenix, AZ.
