That's an interesting concept about the warranty. The core credit is part of the warranty program and is not meant to represent extra profit to the dealer. As I mentioned in a previous post, this is written in the warranty to the homeowner. The dealer is responsible for fulfilling the terms of that warranty.

As far as the problem with the hot water, the service board has no electronics in it so its functions are not dependent on return water temperature. If you do decide to leave the service board in place and forget about the manager, your next oil bill will be a real doozy.

As far as what the dealer wants to charge you for a replacement manager, you'll have to draw your own conclusion about integrity.

If you still have the manager here's something you could try. Move the thermostat lead from terminal THW (top left) to an unused input if there is one, say T3 or T4. On the right hand side move the orange lead from ZHW (second down from the top) to Z3 or Z4. Run the system for hot water. If the problem follows, it's not a manager problem. If the problem went away, it is the manager. If you don't have any unused zones just pick one and remove the existing wires marking them when you do so to avoid confucion.

If you try it post a reply here.