If you've ever looked into a used radiator and seen white scale, etc, this scale/sediment is comprised of various Phosphorous compounds that have chemically bonded with the minerals found in tap water. Most antifreeze manufacturers (Prestone, Zerex, etc) use a lot of Phosphates in their coolants and part of the reason why Volvo Coolant is so expensive ($19 a gallon) is because it's completely Phosphate free. If you're stuck at a gas station and have to top up the cooling system with tap water, you're much less likely to get scale and deposits on the inside of your radiator, engine block and heater core. Any scale or deposits that could accumulate would be based only on the minerals contained in the tap water, and not on Phosphorous from the coolant (absent). Ideally, a 50/50 mixture of Volvo coolant and distilled water is best. Volvo Coolant also includes conditioners to prolong the life of the various water pump seals, hoses, etc, but I don't have all the details on this subject.

Maybe I'm anal about it, but since I usually only service the cooling system once every 3 years, I try to do a good job of it and remove as much of the impurities and scale as possible. Some of you may think of my actions listed below as a bit extreme, but these are my personal cooling system flushing procedures. During all of the steps listed below, the heater control should be in the full hot position. When a Volvo heater control is switched off, it completely stops the flow of coolant through the heater core.

1. Purchase a can of radiator flush. Typically made by Barrs or other companies, most are sold as 10 minute coolant flush products. This can be bought for about $3 at your local auto parts store, Meijer, Walmart, etc. They also make a Heavy Duty engine flush, but I've had better results with the 10 minute flush. The Heavy Duty flush is intended to be in your car for a week or so to sufficiently clean the scale and is designed to work as a passive 'soap' solution rather than an active chemical cleaner that will dissolve the scale.
2. If your radiator cap is of questionable integrity, or it is of the colors Green, White, or Grey, I would strongly advise buying a new Black Radiator Cap from your local Volvo dealership. Read the following post for more info: Volvo cooling system pressure caps.
2. Drain old coolant. I usually park my vehicle facing downhill in order to drain as much coolant as possible from the heater and engine block. Also, there's a block drain that can be unscrewed for additional draining and it's just below the #4 exhaust port. Save the old coolant in containers for recycling, empty laundry detergent bottles work well for this and are stronger than milk jugs.
3. Try to wipe out any sediment from your overflow bottle with a lint free rag. You may find it easier to do this with the overflow bottle removed.
4. Refill the system with distilled water and add the cooling system cleaner (radiator cleaner). "Distilled Water" is available at most grocery stores for about $0.50 a gallon. Note, "purified water" is not distilled water, there is a big difference.
5. Run the car for about a minute, let it cool, and then add more water since most of the air will have come to the top now.
6. Drive the car for 15 minutes or so, keep an eye on the temp gauge and don't let it get more than 2/3 of the way up. You want the entire cooling system to get up to operating temperatures. Turn the blower up if you're fighting overheating. (heater control should be on full hot to circulate the cleaner through the heater core)
7. Park the car and let it sit a few hours. I've let my Pontiac sit overnight to clean some bad scale out of the radiator. The extra time really helps and it gets a chance to dissolve more of the scale.
8. After sitting several hours or overnight (the engine should be completely cold), top up with water as necessary and drive for another 15 minutes. Make sure not to let the coolant temp gauge get more than 2/3 of the way up. Use the blower as needed to fight rising coolant temps.
9. Drain the cooling system. You'll want to save and recycle this fluid since a lot of nasty contaminants and scale are going to come out with the water. Phosphorous kills and its ecological impact is much worse than you're probably aware of.
10. Refill with distilled water, run the engine couple minutes, and drain again.
11. With the system empty, replace your thermostat. There are typically 2 different thermostats that you can buy and you'll want the one specified for your car. My '87 240 uses an 87-degree C thermostat (as specified by my local dealership). The official Volvo thermostats are currently being made by Whaler and you can buy the same thermostat from FCP Groton or eEuroparts.com for $10 rather than the $20 at a Volvo dealership. The only difference between the two thermostats is that the Volvo unit has a Volvo part number stamped next to the words "Whaler 87C" on the brass chassis.
12. With the cooling system still drained, replace your water pump if needed. Due to the heavy loads a 240 puts it's water pump (the alternator and the cooling fan), I replace mine about every 60,000 miles (or 2nd cooling system flush, whichever comes first). This is definitely worthwhile as preventative maintenance, especially if you factor in the cost of the Volvo coolant. The typical life expectancy of a 240 water pump is usually about 60,000 to 100,000 miles before the upper head seal or the shaft seal starts to leak, so it's not like you're wasting money.
13. Again, refill the cooling system with distilled water.
14. Drive car for 10 minutes, turn off, check for leaks at your thermostat and water pump, wait 10 minutes (or however long you need to so you don't get burned), and then drain again.
15. Repeat steps 13 and 14, this is to ensure you've gotten all the chemical cleaner out of the system.
16. Refill the system with an appropriate amount of Volvo coolant (aka: Volvo Green Type-C anti-freeze) and distilled water to make a 50/50 mixture. Be advised, you may have some distilled water sitting up in the block and heater that didn't drain earlier so you should base your mixture on the numbers in your service manual. Add the necessary amount of antifreeze first, and then add distilled water to bring the cooling system up to proper level.
17. Road test. Check for leaks and top up with a 50/50 mixture as necessary. If the cooling system is functioning properly, an overheating car can be brought back to normal operating temps while parked and the engine idling (AC Off).
18. If necessary, add a container of Water Wetter to the cooling system (made by Red Line). This is a chemical additive that helps to break down steam buffers that can form between the engine block and the coolant that's flowing through it. If steam buffers start to form (coolant is actually boiling at the points where it contact the superheated engine passageways), then the car will overheat even faster. Most people probably do not need this in their vehicle unless you race your car, drive with a heavy foot in a hot climate, or are stuck in stop-and-go traffic with your AC running on a very hot day. I personally don't use the product in my vehicle, but I live in Michigan and my driving habits don't merit it.

Shopping list of the above mentioned items:
1. 10-minute cooling system flush (Barrs, etc). About $3.
2. Good quality antifreeze, you get what you pay for. (I buy the true Volvo Coolant for $19 a gallon).
3. 9 or 10 gallons of distilled water, about $0.50 a gallon at your grocery store.
4. Water pump and necessary gaskets.
5. Black Volvo radiator cap if your current cap is any color other than black. (aka: cooling system cap since it's not actually mounted on the radiator)
6. Red Line's product: Water Wetter. (if needed)

I hope this helps and God bless you and your vehicle.
Fitz Fitzgerald.
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'87 Blue 245, NA 231K