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You must not have seen the secondary heat exchangers fall apart and fill the homes with CO.

I did

The cost of maintenance on a pulse that is almost 20 yrs old doesn't make sense to me. The extra 2 or 3 % in efficiency doesn't cover that cost.

Sorry up front but I can't assume anything

If you brazed the piping with the sensing bulbs attached you may have over heated the sensing bulb and damaged the valve.

If you monitor the suction...

In Tranes defense the reason it comes with a 17 x 25 filter is that is the dimensions of the bottom of the cabinet and if you were to use a bottom return this is the the size filter you would need....

Make sure the aqua stat is on the hot water in line and not the water out line also see if you can set the differencial on the aquastat to at least 20*

Air flow and ducts being too small and or...

Carrier will give tighter and better control/comfort
Trane is less $$$
Whats more important to you?

The possibility of hum operating in cooling mode may exist if you wire it to operate with fan only

Carrier infinity goes to 21 seer very $$$

3.5 tons of air sounds a little light for 2600 sq ft living space in which 400 of that could double as a solar gain experiment. mini split is for you.

Try York

I know it's late but if your still having trouble, you should monitor suction line temp right behind each comp and one at a time shut off each comp and see if the temp rises quickly and if it does...