How do I wire a relay in to stop humidification during a call for cooling also how do I wire a cooling system up to a old furnace using a fan centre the old furnace only has a fan limit

In both cases: Heatpump or conventional?

2nd case: one or two speed blower? How is the blower controlled? In the old days, the system was heat only single speed. As part of the upgrade a two speed blower was installed.

I believe he is trying to bypass the circuit board and all the safetys. Not good.

I'm just wondering if it's one of those relics where you have a summer/winter switch that controls the fan and the motor HP is like 1/3 instead of 1/2 and no G terminal. AC or humidifiers were not even thought of before the time.

Even this isn't quite enough: How to Install and Wire the Honeywell L4046B combination Furnace Fan Limit Switch Control: a guide to the fan limit switch installation and electrical connections - Honeywell L4064B Combination Fan and Limit Switch

When a two speed fan is involved.

In reality, you need to look at:
1. Is the fan on (Note 1)?
2. Is there a call for heat?
Then enable humidifier

This is tough when there is no low/high speed fan to begin with and it's added.

Usually there is a third pole that can be used/added on fan centers for EAC's and humidifiers.

I forgot about this method as described here:

http://www.ontimemall.com/aprilaire-...ers-manual.pdf

You can use a current sensing relay to determine whether the fan is on. It's also easy to determine if in heat mode and fan is on.

24 VAC Power is between C&W if you have a call for heat, but fan may not be running.

If you put a current sensing relay on the low speed of the fan combined with the internal humidistat, you should be fine.

There is lots of little nuances that can cause grief.