Oh my. At least this time we're in a commercial application where twinning might actually make sense. In this case we have a large room served by a single ductwork system with (2) 5 ton air conditioners. Those 2 air conditioner coils sit above 3 furnaces with 90,000 Btu/h input, 95% efficiency, and 3 ton capable blowers.
Can you guess what the problem might be? That's right, airflow. Amazingly, I'm told that they didn't have a problem with the air conditioners freezing up. The furnaces have not been so lucky. Because of that 10 inch tall shared plenum above all the furnaces and the blocking plates and AC coils and everything else going on here, the heat is having a little trouble actually getting out of the furnaces and inevitably, 2 of the 3 go off on high temperature limit within minutes of firing up.
So, what to do? This is obviously not going to work in its current state, but how do we get to a reliable, sustainable solution without breaking the bank? After all, these furnaces aren't even 2 years old yet. Like so many times, it starts with figuring out what we actually need to get out of this system to satisfy the heating and cooling needs of the building.
I won't go into the gory details of what is needed to correct this problem, but I can tell you that it is neither cheap nor easy. We can't just move to using two of the furnaces because they can't drive the cooling need for the space so they'll need to be replaced with larger units. The coils are hooked up to very old air conditioners using R22 that has become very expensive. In this instance the recommendation would be to replace both the furnaces and the air conditioners to make it a sustainable and efficient solution. Unfortunately in this case, it will be paying twice to have it done right because it wasn't done right the first time.
I don't believe this is a case of a contractor intentionally trying to put together a system that doesn't work - I believe it is a case of a contractor trying to put together a system for as little money as possible without knowing how much heating or cooling was really required for the space and unfortunately the customer is the one that ends up paying for it - literally - in the end.