I have a heatpump system. The outside unit is very old and is different manufacturer than the newer (9yr old) inside unit. Outside unit has gone bad. Can I connect a new TRANE Outside unit with SEER of 13 to my 9 year old TRANE inside unit with SEER of 10 which still seems to be working fine:confused:

No especially with heat pumps. The newer units have larger coil surfaces especially the new inside coils. The manufactures do not recommend that the old coils be used with new outside units and as I said before especially with heat pumps.

You will also loose efficiency by using the old inside coils/unit. You are paying for a 13 SEER so you might as well get your moneys worth. Many contractors tell there customers they can just replace the outside unit and on a strait A/C unit you can if you add a expension valve and know what you are doing but not with a heat pump since it depends upon the newer/larger coil to disapate the heat generated by the new outside unit.

If you do just change the outside unit on a strait A/C system you will also loose efficiency.

On heat pumps they are rated as below.

The HSPF (Heating Seasonal Performance Factor) rating is used to identify the heating efficiency of heat pumps: the higher the rating, the less electricity the heat pump uses to heat a given area. 7.7 HSPF Is the minimum standard today

There was a time when you could "get away" with doing this; those days are over. The rated efficiency of the new unit won't be achieved, the service life will be shortened, and the warranty potentially voided.