First cold morning of the year, and our new-to-us '05 XC70 (175,000 miles) is barely putting out any heat. There's a little if I turn the knobs to the highest setting, but not much. Thermostat on the dash seems to be where it should be. Any thoughts?

Are all the servo motors/flapper doors working?

With the engine off and key in position II, change the temperature setting from cold to hot, hot to cold and also switch the defrost/panel/floor settings. You should be able to hear the doors opening and closing. It could be that the door that controls the air temperature is broken in the closed position, not allowing air in from over the heater core.

I recently had this condition in my S40. None of them were working. Turned out to be a blown fuse for the rain sensing wipers (apparently the automatic climate control systems takes signals from all manner of things). So it might be worth checking all your fuses too.

Good luck,
Will

Let me add one more possibility. Clogged heater core. If that happens to be the case (hopefully not!), try flushing it first as replacement will be expensive.

Not to refute your answer, but what would clog a heater core?
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Keeping it running is better than buying new

Somewhat unusual problem, but it does happen. Just read that clogged heater cores are a common problem with Jeeps.

Corrosion is the probable cause. If you buy a used car you usually don't know what the previous owner might have put in the cooling system. Using water or stop leak for example. Or no coolant changes which might cause a corrosion buildup.

https://www.ericthecarguy.com/faq/solving-automotive-hvac-problems/part-2-heating-system/clogged-heater-core

I had a clogged heater core on my Dakota pickup several years ago. Probably due to not flushing the cooling system. Flushing it in the opposite direction of the normal flow got it working well again. (A bunch of whiteish setiment came out.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

Same thing with my Passat. Reverse flush is the ticket! Could be worse, at least it hasn’t leaked...:)
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Will I buy another Volvo??? We'll see....

Thanks! Any hints about how to flush a heater core?

Disconnect both heater hoses at their quick disconnects on the engine side of the firewall. Run water through the heater core in both directions with a garden hose. Caution - don't connect the garden hose in such a way that it is perfectly water tight as it will subject the heater core to full house pressure which is probably more that the core can handle.

Reference:

http://www.v70xc.com/0/resources/how-to/pdf/heat_exchanger_removing.pdf
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

Thanks! Is this a DIY?

https://www.youtube.com/watch?v=37u_oQ0Q0Gc
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Keeping it running is better than buying new

So that explains why I had so much trouble getting that darned brake light switch outa there!

Also, he doesn't mention it, but if you take a couple pieces of scrap rubber or plastic hose you can blow the majority of the coolant out of the core and into a container. That way when you tilt it downward while sliding it out it won't drop so much coolant out onto the floorboard (or the plastic bag that's covering the floor board).
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

Several things: Do you have an IR thermometer that you can point at the top coolant hose?

The car's thermometer may be broken and stuck open. Engine will never get to hot and take a long time to warm up. Do not relay on the dash gauge.

If the coolant overflow bottle is empty, not enough coolant in the system to make it to the cabin.

Depending on which system you have, manual or electronic, you may have a loose/mis routed cable or a broken vent damper.

Have you ever changed the cabin filter?

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Keeping it running is better than buying new

I have not changed the cabin filter; would that also affect the A/C? AC blows pretty cold. I guess I have some exploring to do.