Furnace runs but will not blow heat. if i replace the batteries it blows heat for a sort time then cuts off

https://www.youtube.com/watch?v=XrxV6I8C4yY

https://www.youtube.com/watch?v=pJzP1jzYPqg
AUTO ensures that the blower will run only when the furnace heats the air. Your furnace may be blowing cold air because the filter is too dirty. A dirty air filter blocks airflow over the furnace's heat exchanger, causing it to overheat. ... Do this: Turn off your furnace at the thermostat, and check the furnace filter.
https://www.coolray.com/help-guides/how-to-troubleshoot-a-furnace-blowing-cold-air

Maybe you are setting it wrong

A slow blow fuse is needed for a circuit that has a strong surge of current at switch on but runs at a much lower current. This type of fuse is used less common these days and it would be a very good electrical retailer that would stock such things.

If I needed one I would order several from an electrical/electronic trade outlet or a good electronic constructor supplier, though a replacement could be sourced as a spare part for your furnace or from the stock of the sort of outlet that are general suppliers of heating and plumbing spares. .

I would order several because these fuses rarely fail unless there is a fault and it is unlikely the replacement fuse will last very long. You can assume if the new fuse does fail there is a fault needing the attention of an engineer.

As to what size/value is needed, you will need to look at the markings on the original or look in the manual for the furnace.

Furnace Troubleshooting & Repairs ' HomeTips

www.hometips.com/repair-fix/furnace-problems.html

Nov 25, 2015 - If your heater doesn't heat or works poorly, this furnace repair guide will help you troubleshoot and fix typical furnace problems with step-by-step advice. ... If your furnace's motor runs but the blower doesn't move air, the belt that ..... a tune upbut couldn't really find any thing wrong . last night and today when ...

Furnace Not Blowing Air? A Furnace Troubleshooting Guide

asm-air.com/heating/furnace-not-blowing-air-troubleshooting/

Dec 1, 2014 - A furnace works by a very simple principle, it uses some type of fuel ...This series of heated coils (called a heat exchanger) has a fan ... Today we will address the most common causes of a furnace not blowing air, and what you can do tofix it. ... It seems like this would be a no-brainer, but I can't tell you how ...

Simple Furnace Fixes ' The Family Handyman

The AC unit outdoors must be running and rejecting heat by its air flowing through it the 2 copper lines are the refrigerant lines, the fatter one should be "beer can cold" if not its low on refrigerant or not running.

If the sensor knocks the unit off and the the fan keeps blowing without a flame ,the limit (the sensor is a limit comptroller that turns off flame @ a set temp and allows the fan to run until the heat exchanger cools to @ set temp) has out lived its lifetime and needs to be replaced.the button is a manual reset that once reset would allow the unit to continue to run flame and blower at the same time after having been set once, if it is an ongoing problem and needs to be reset more than once then replace the limit control (sensor) in kind. (With the same mode and set-points for the limits)

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

http://www.nordyne.com/Literature/707716c.pdf
http://www.nordyne.com/Literature/359a.pdf
The diagrams above may not be exactly what you are needing. But they will be close. Usually the diagram for the furnace is on the inside of the blower fan door. Two things come to mind. It seems like your limit switch(s) are cutting out, then in, then out, etc and may have failed totally. These are in the system to prevent the temperature in the furnace from getting too high. Why they are cutting in and out may be due to the lack of sufficient air passing through the furnace. This can be caused by, 1. the fan speed is too slow, or 2. the air filter is needing to be changed. I am not sure how proficient you are with electrical troubleshooting. I would (turn off all power to furnace) find the limit switches (usually at the top of the heating elements) and check to see (with an ohm meter) if the switches are allowing the meter's current to flow through them. Your meter should read near zero, if not, then I would replace the switch with an exact one. If you are proficient with electrical troubleshooting, I would place the furnace in the "call for heat" position, with the power on, (be careful, this stuff can hurt) and I would see if I have voltage at the heating elements, and I would check with my voltmeter across the two leads on each limit switch if there is voltage present. If there is voltage present, then the switch is probably ****.
Depending on the furnace you have, you might have a number of switches that control the heating elements, these units are usually time delay and if they fail, you should replace them with the same delay time. I hope this helps.

Sure sounds like the t-stat is bad having to throw the switch on and off to get it to work. Try to find a White Rogers brand they are the best.

I would suspect you have a problem in the defrost circuit of your heat pump. there are a couple of things you can try. first just run the system on emergency heat and see what happens. you could have a defrost board that is shorting when it kicks into defrost. it sends a signal back to the furnace so it will come on during defrost. if you do not blow a fuse as long as you are running in emergency heat, i would suspect the defrost control board. also you could have a dead short in the wire that runs from the "w" terminal on furnace control board, out to the heat pump. keep in mind that any of the wires could be the cause and i am just tying to give you the most common things. your furnace will still run if you unhook all the low voltage wiring out at the heat pump. if you do this and stop blowing fuses. you know it is something on that wiring. goodl luck.