Why is the heat in my car only hot when I’m accelerating?

You may have a "vacuum leak". But first things first. Check the fluid level of the coolant reservoir, not the radiator (unless you know how). It's much safer to check the coolant reservoir because it's not under pressure and won't burn yourself if you make a boo boo.

The coolant reservoir is a plastic container mounted somewhere under the hood and is easily accessible. Your owner's manual will indicate exactly where it is located. In the morning, or while the engine is cold, look at the level of coolant inside the reservoir. The reservoir will have a line near the bottom for "cold", and another line closer to the top for "hot". If the level is below the "cold" line, or there is no coolant at all in the reservoir, then the system is low on coolant. Add antifreeze to the "hot" or "full" line near the top of the reservoir. If you add too much, don't worry, it won't hurt. After driving the car normally for a day, check it again the following morning and add antifreeze if the level is below the "cold" line. If you have to add coolant the second time around you definitely have a low coolant situation.

The reservoir will only add a small amount of coolant to the radiator at a time. But this way you don't have to open the radiator cap and deal with having to get the cap back on correctly or burning yourself. Any technician can easily check the coolant level for you. Whether or not this fixes your problem you first need to check and make sure you have enough coolant. Otherwise, you could damage the engine and have worse problems.

Also, the coolant reservoir's cap is always made of plastic and can be removed by hand without any tools. It is never hot or under pressure. So it's kind of "idiot proof" 🙂

If it turns out to be a vacuum leak, then a technician can diagnosis it for you. But generally the symptoms you have are visa versa. In other words, the heat would go away on acceleration, as opposed to gaining heat on acceleration. 🙂

Does the fan work? Does it blow any air at all? If not this is why it only gets warm while the car is moving. Check the temperature gauge. If it is running hot or above normal, then check the coolant level. If the level is very low, then check the water pump for leaking after you refill the radiator. Radiator full and stays that way? Check the heater fan. Fan works? Might be a clogged heater core.

i would have your water pump checked, because if the pump isnt pumping as much coolant as it should be at idle, it might not be pushing enough coolant through the heater core which is why you might not have heat, but when the engine spins faster it finally pumps enough coolant to produce warm air out of the heater

you might have a bad heater control valve, the reason you don't get hot air in the morning is because the coolant needs to heat up but it doesn't take more than 15 min

You have air in the system probably due to a leak make sure your coolant is full in radiator not overflow and check for a leak.

make sure the coolant is full,also listen for a vacuum leak