Blowing luke warm sometimes and hot others

You need to check the flame sensor. There is a wire that tells the unit when the burner is on. The burner may be coming on and then gets warm and shuts off, then the fan comes on. It could also be the upper limit temp thermostat that is tripping but since you did not post the make and model, it is hard to say

-Sounds like the system is clogged or the on off valve is not open
properly.
-Check the valve and flush the system. It should be good to go.

Good luck

Mai

heater blows cold air idle chrysler 300 Questions & Answers ...

www.fixya.com > Forum > Tags

Find solutions to your heater blows cold air idle chrysler 300 question. ... blow warmair for a few mins then back to luke warm. i replaced thermostat ... HEAT BLOWSCOLD AIR WHEN IDLING ONLY GETS HOT WHEN START DRIVING ... About 15 minutes after driving the diver side vents start blowing straight cold air but the ...

Heater only works when pressing the gas? - DoItYourself.com ...

www.doityourself.com/...repairs.../459521-heater-only-works-when-pres...

Nov 3, 2011 - When I press the gas, the air from the vents is nice and hot. .... So am Idriving a car with a ton of problems, or could the power steering ... Just anotherquestion, when warmed up does the engine temperature ... When the heater is on and I'm not pressing the gas pedal, luke warm air blows from the vents.

Heater blows cool air when stopped and warm air while ...

https://www.reddit.com/.../heater_blows_cool_air_when_stopped_...

RedditFeb 12, 2014 - Heater blows cool air when stopped and warm air while driving 2008 Kia Spectra (self. ... just fine but appears to blow cool air when I was stopped at a stoplight. ... My temp gauge would also be pegged high when at idle / low RPM. ...thermostat only blocks coolant going to the radiator, and the heater core, ...Missing: 300m ‎luke

Why is My Car AC Blowing Hot Air? - BlueDevil Products

gobdp.com/blog/car-ac-blowing-hot-air/

if its electric heat one of the sequencers or elements out

do you have an automatic change over thermostat? this would switch it from heating to cooling.

Check the air filter and clean. If you can, raise the blower speed up to the next speed. Make sure all of the supply and return grills are open. Is the ductwork connected under the home and well insulated? Does the furnace ever stop or keep on running? If it does stop, you may have a bad thermostat.

If you don’t see any frost on the suction line or big line on the furnace
(that goes from the furnace to the outdoor unit) and the filter and coil are clean then you are probably low on refrigerant.

if this has happened since you got hte furnace it is normal
heres how a heter works
themostte tells heater to turn on
heating element turns on
followed in about a minute by the blower

during the peried before the blower kiks on the elements become supper heated
after the blower kicks on it slowly leves off the element temp and air temp coming out

You may have a problem with a temperature safety system.  Before changing anything else, check to see if the unit is clean!!!  Dirt/dust can fool the safety system into an overtemperature status.  Inspect the heat coils for damage and dirt.  Gently clean.  

Sound like air flow problem to me, the inside and the outside. check the inside filter to see if it's clean and clean the outside condenser coils by washing it.