Takes 20 minutes to preheat.

One must preheat the oven to the desired baking temperature. If the oven is turned on and the food immediately put in to bake, it will take longer to bake the first batch than the second and subsequent batches, because the oven need a few minutes to heat up.

In other words, the oven isn't at 350 degrees the instant you turn it on. It takes about 15 minutes for the oven to reach 350 degrees. I hope this helps.

what is your oven if is gas or electric, if is gas easy to preheat by using lower heating element, if electric? that your unit have convection? or not? coz some probelm of the oven is, 1 if the oven is no convection or your oven is single heating in upper element,if your oven have a timer and cut off in 15minute? dont use timer,use a manual timer by it your self,if you want to reach the degrees that you want check your oven if der a lower and upper heating element with convection function,that the one is the most easy way to reach the degree that you want to reach.

Hi,
Check out this tip that I wrote about this problem...

http://www.fixya.com/support/r4511800-oven_problems_electric_oven_heating

heatman101

You have burned out element(s). Lowering the wattage, therefore increasing the time it takes to reach set temp,

That is a long time, but please give more information on what type of oven you have, and how old it is if the following does not solve your problem. First, Make sure you have not covered the bottom of the oven with any type of barrier to keep the oven clean (aluminum foil or inserts), it blocks the vents. If it is an older convection oven, preheat on bake and set the convection after it has preheated. Check the elements. Lift up the bottom panel and do a visual check of the element. Take out the bottom panel and turn the on oven on somewhere near 450-500 degrees. Visually inspect the element and see if there are bright red areas and dull areas. If the element has red hot areas and dark areas, you need a new element. If the element does not get red at all, you need a new element. Electric ovens usually have an upper and lower element, so you should check both before replacing one. The broiler element should be visible without removing anything, just look at the top of the oven after you set to broil. Lastly, make sure the oven door shuts snugly and that any gasket around the door is in good condition. If you don't know if the door is tight enough, just lean up against it to see if it preheats more quickly. The oven also has a temperature sensor, but from your description, that would not be the case because you did not mention trouble maintaining the temperature.The good news is that elements are usually cheap and easy to replace. Try to find your owners manual, it will tell you what part you need. If not, write down as much info on your oven that you can and take the element with you to the parts store. If this is not helpful, leave comments with more information and I will attempt to solve your problem. thank you for your question.

Sadly ours performs this way also. I've always had GE wall ovens and NEVER did they take very long to get up to temperature.
Has anyone else communicated with you about this problem?

if you cant program it for more than the 270 you may have changed it from farienhiet ( bad spelling) to celcius. pushing and holding broil may switch it back.

http://www.applianceaid.com/nuker.html
click the link  Oven temp sensors sense the temperature is the oven cavity and the temp sensor changes resistance, this change in ohm resistance tells the oven control what temperature the oven cavity at and tells the control to shut off the element or turn it back on. The most common temp sensors used today ( *not all ) are approx 1000-1100 ohm resistance at room temperature ( 70�F )
Some ohm readings and there temperature:
Degrees F    -   Resistance
            100 - 1143 ohms
             200 - 1350 ohms
             300 - 1553 ohms
             350 - 1654 ohms
             400 - 1753 ohms
            500 - 1949 ohms


click this link
http://www.applianceaid.com/elecrange.html

sounds like the thermostat may be open and nees to be replaced. have a technician take a look at it.

Cold Stove, you may not be able to. The issue here is that oven manufactures put on their units a fast preheat. However it isn't possible to preheat an oven for 7 to 9 minutes and bake short time bake items properly. Also the oven sensor will respond faster than the absorbation rate of the sheet metal in the oven liner. To preheat an oven properly takes 15 to 20 minutes before putting in the food. I see this will a lot of current ovens. Sounds like a normal format of the control.