Winter is coming and I’m new to these older vans. Here’s a weird one. When on “heat” mode, low/no air comes from the vents regardless of fan speed.

-AC blows cold and responds to fan speed
-Vent blows warm and responds to fan speed on “vent” mode
-Defrost works well
-van runs cool on temp gauge (never over 1/4 up)
-heater hoses hot
-heater core replaced/system flushed a year ago (previous owner)
-upper radiator hose cool/empty feeling
-coolant topped off
-new radiator cap
-recent water pump (previous owner)

I’m thinking thermostat, but shouldn’t the vents be blowing air at me in heat mode?

In "heat" setting you will only get air flow from the floor/under dash openings.
To get warm or hot air from the dash vents you would need to swap to vent and push the temp control to hot.

Is there air flow from the underdash/ floor area in "heat"?

Thanks for the quick response Catfish! That's helpful to know that air doesn't come from the upper vents in "heat" mode. Yes I believe there is air flow coming from below in heat mode. So now it's just a matter of figuring out why the van runs cold. I will swap the thermostat this weekend.

Hope it helped.
We currently have three GM vans. All of which are "cold natured" as the saying goes. They all seem to take their sweet time warming up from a cold start even with a proper working T-stat.

I know pretty much nothing about Chevy vans or technology, but I have a few generic suggestions. First, if your van has a clutch fan, check if the clutch has failed in the "on" mode and is making the fan spin at all times. This can cool the radiator fluid too much and make warmup times longer. Second, reverse flush your heater core. You will be surprised the gunk that can build up in there. Third, verify the temperature blend door is moving properly and the seals around the door are in good condition.

Good suggestion on the fan clutch, but that is not the case here. I drove it a little longer today and the upper radiator hose got hot, so the thermostat is certainly working. Warm air comes out strong from lower vents, but it’s not as hot as every other car I’ve owned. Perhaps it really is just cold blooded. A Cardboard radiator blocker might be necessary to survive a MN winter!

Maybe an air bubble in the heater core?

Are both hoses to the heater core hot?

Or.... get one of those fancy laser temperature things and make sure the whole core is getting hot and not just one path through it.

on my '89 G30 there was basically no heat coming out of the vents. pulled the blower van housing off and could feel the core getting hot. Turned out the PO didn't install a thermostat when they replaced the heater core. The temp gauge was also sitting flat the whole time so replaced the sending unit and installed a new thermostat and everything is working fine now.

Updating this thread while I'm here! I put cardboard in front of the radiator (but behind the grille because she's classy) and it does help. It takes quite a long drive to get hot air out of it though, I rarely bother with the heat since most of my trips are 10 minutes or less this time of year. I still plan on changing thermostat in the spring. It's too damn cold for wrenchin' currently.

There should be a "mixer" door that regulates the mixture of hot/cold air to the vents. Also , does the van have a shut off valve for the heater core? Does it have full range of motion?

The mixer door works, when on heat mode I can switch from hot to cold and the temp changes and it feels like there's full open/close. There's a shut off valve for the heater lines going to the rear heater. That valve works but leaks. Lines going into and out of the rear heater core both get hot, but the blower doesn't work, so I just leave the shut off valve closed. Getting that rear blower assembly to work is on my spring project list!