WHAT IS A HIGH LIMIT SWITCH? https://www.newcentralairconditioner.com/ The high limit switch in your gas furnace is a small part with two very important jobs. This switch plays a role in the normal operation of your furnace. Turning the fan motor on and off is the high limit switch’s main job. This is the role it plays in every cycle of your furnace’s normal operation. But that’s not the only thing that the high limit switch does. It’s also safety device that helps prevent your furnace from overheating. The high limit switch consists of a long temperature-sensing probe attached to a metal mounting plate. From the mounting plate, the switch is wired to the blower fan motor and to the gas valve. The switch is generally located just past the heat exchanger. This is so that it can detect the temperature of the supply air. In other words, the air that’s about to be blown into your home.

Thanks for the input. Is it possible to quantify "great"? For instance, my downstairs single stage system is set so that the compressor kicks on when indoor temperature is 1 degree higher than setpoint. So my downstairs (at least, at the location of my thermostat) pretty much stays between 75.0 and 76.0 degrees all day long. (I have a temperature logger to confirm this.) Understanding that proper installation and ductwork are absolutely critical, is there any ballpark expectation for how well the zoned modulating systems perform in terms of how closely they maintain the temperature of each individual zone relative to the zone's setpoint? Or is the best that can be said is that they generally do a good job?

Sizing can be less of an issue when you have multiple units. Because each unit is controlled independently, provided of course the unit itself is not over sized for the area in which it serves resulting in excessive short cycling. In your case 3 ton plus 2 ton isn't necessarily equal to 5 tons of cooling... unless you drive those thermostats really low. By having two systems with total capacity of 5 tons (which, most of the time this would be oversized for a 1700 - 1800 sq ft structure) has a few advantages. 1. You have the ability to use one system only during low heat days or set the opposing unit higher. The unit set higher only runs if load conditions warrant.... meaning if it's not hot, the thermostat controlling the unit does not call... the unit can not run without a call... 2. Utility bills can be much less... because you rarely use both systems at the same time. You run the bedroom system when you sleep, the other when you're awake. If you have a home office in one the bedrooms you may need to rethink things a bit. 3. Extra capacity for when it's hot out. When it's hot you have the capacity to cool the home. (even while entertaining guests --- additional people add heat) Remember heat waves come, heat waves go... a so call 'properly sized' AC system is essentially properly sized roughly 10% of the time... especially if that system is a single speed AC system. Alot of people I come across like to sleep cool. So for that reason I doubt I would change the configuration of the space. Once you realize what you have, the choices you make are mostly budget type concerns. Essentially a set up like this runs like a 2 stage system. This kind of subject is above the pay grade of many... choose your contractor wisely. I service the Katy, Texas area.

I paid in TX $11,700 for a 5-ton 2-stage Trane XR17 heat pump system in Jan 2022, replacing an existing system (not Trane) so no ductwork or replacement linset involved. Sounds high for a 2 stage system, Dadoes. During that time frame I could have probably provided a Bosch Inverter for probably pretty close to that. Uniquely it has an option to configure to a 5 ton or 4 ton. So if it was determined later that 4 ton wasn't enough it could be reconfigured to 5 ton. There was trouble getting these Inverters around that time frame, but I installed one in February 2022 for a customer I had that had been waiting for one to become available. But now? Prices are up again so now it would be marginally higher. That said the inverter will pay you back, not to mention run circles around a two speed HP. Compared to the old unit, the new unit you have is probably more efficient so you are likely saving some cheddar any way. It just could have been better. Just replaced a Trane XR with a dead compressor (Derailed). Manufactured 2015. Had an allied compressor in it. Running it now like a boss... err I mean Bosch! I will probably do a video on that one. So much fun. I love my job. My You Tube Channel here, subscribe so you don't miss this one. I service the Katy, Texas area.