[54regcab]

Static pressure is what determines airflow through the coil, not physical size. Could be that your coil has 30 years worth of dirt in it and needs to be removed and cleaned. Rather than cleaning an old mismatched coil, it may make more sense to replace it with one that matches the outdoor AC unit.

 

[54regcab]

Sure that it's newer than the coil?

 

[54regcab]

37c/67f temp rise isn't unusual for an older furnace. If you are changing the furnace you will need to improve airflow and/or downsize the furnace. Newer furnaces run closer to 50f rise. The good news most furnaces are grossly oversized so downsizing shouldn't be an issue.

 

[54regcab]

With a 67f temp rise you are only within 8f of max. Plenum limit switch could be getting to 210f due to radiant heat from the heat exchanger. Are the burners staying lit the whole hour your house is warming from 18c to 21.5c. A furnace tripping limit will allow he blower to run the whole time blowing warm air and the homeowner may not realize the burners are cycling. Watch it for 10-15 minutes and verify the burners do not shut off. Clock the gas meter to verify furnace is burning the correct amount of gas. In US measurements seconds for the 1cuft dial to turn divided by 3600 is your btu/hr in thousands. 3600/70k BTU = 51 seconds for the 1cuft dial to turn one revolution.

 

[54regcab]

AC coils can "look clean" but be clogged deep inside the fins. Especially possible is somebody has recently "cleaned the coil" without pulling it. Static pressure test is the only way to know for sure.

Leave the gas pressure ALONE unless you have the proper equipment and expertise to set it correctly. You can easily cause more harm than good by setting it wrong. Clocking the meter is just a quick check to see if fuel consumption is close to spec, it does not verify correct gas pressure.

 

[54regcab]

The coils don't look that bad considering thier age, I think your problem is elsewhere. What were the results of your gas meter clocking?