First diagnose the problem: is the heater core the prime suspect?
Some symptoms of a heater core leak are:
* you are losing coolant when the heater is used.
* you are NOT losing coolant when the heater is NOT used.
* you can smell antifreeze when the heating is on.
* you can see condensed vapour exiting vents when the heating is on.
* your car's footwell carpet is wet.
* windows steam up with soapy-looking film.
Heater core failure can be a major problem because of access difficulties. If your car is a Volvo 850 then the job is not too hard, but I wouldn't recommend trying this instructable unless your car has almost all of the above symptoms (heater core might not be the cause), are a reasonably competent DIY mechanic, and enjoy the challenge of working in inaccessible and unpleasant places to save yourself a large garage bill.
The job might take you approx 4 hours of solid work, largely depending on how much antifreeze leaks out and how much you care about cleaning it off the carpet), and it will be messy.
This Instructable illustrates the repair in a right-hand-drive Volvo 850 1994 GLT 2.5 AUTO. If your Volvo 850 is left hand drive then procedure is similar but you'll be removing the heater core radiator from right hand side instead of left. In either case: you'll be removing heater core from passenger side footwell, which gives you better access than driver side footwell.
All loose removable carpets should be taken out at first sign of coolant leaking onto them, and replaced by absorbent rags or newspaper.
* torx screwdrivers (several sizes) and philips screwdriver.
* dry newspapers
* plastic sheeting/bags
* carpet cleaning kit e.g. wet vacuum cleaner.
If the heater core does need replacing you will also need a new heater core:
* Volvo 9144221 Heater Core/matrix/radiator. An OEM (non-Volvo) part costs circa US$150 or £100).
* two o-rings (3545586S) for installing matrix - you can probably re-use the ones that are already there, but maybe be a good idea to replace whilst you have access.
I replaced my heater core after seeing info at volvospeed.com - you might want to view that page too.
There's more info about Volvo 850 A/c & heating system at volvoclub.org.uk, but I wouldn't use hose clamps on the water pipes to core, because in my car they appear to be solid! So try to stop the pipes at outlet rather than by clamping.