Attored

What is your location?

You may get marginal better comfort and perhaps small savings with the mod. I do think it's important on the mods that they be able to modulate both up and down based on demand, not just up.

IMO

The 58MVC is a very nice furnace when paired with the Infinity controller. You will be happy with it if the contractor is able to get it.

The 59MN7 has a stepper motor to control the gas valve. This an additional moving part which may be prone to failure. You can argue it is less reliable, but there is no history on this. You get a 10 parts warranty from Carrier, and you could purchase a 10 year labor also from Carrier in case you have any concerns.

Thanks for the responses. I'm in a pretty mild area (nights barely hit freezing), but building construction from 60's is not well sealed. Long L-shaped one story, will have a single furnace with two zones.

How do I know whether it can modulate down as well as up (the 59MN7)? It would have the Infinity controllers for either furnace.

I would think the 59MN7 can modulate up and down. The 58MVC does this with three stages (low, med, high). The decisions are made by the Infinity controller, so I assume both furnaces would operate in the same manner.

This would be a good question for the contractor.