Hi, this is called delayed ignition which raw gas is let into the combustion chamber and ignited a little to late. which causes this explosion. I have had my eye brows and eye lashes burned completely off do to this. Johnstone is a very popular brand and have been in the controls of this field for many years? Lets solve this problem. When this condition happens, more then likely the burners are clogged somewhat and not all of the raw gas is igniting when it should be. If you have a good strong flash light or drop light and a shop vac would be helpful, I can walk you through some things to do to solve this. On a unit with the electronic ignition module, whether it be a module box or a circuit board they basicly all work the same. Find where the pilot tube runs to the flame tip on the unit and along side you should have a porcelain rod that has a metal tip on it that has to be resting in the flame at all times. Take some steel wool, not sand paper and give this tip a good cleaning. This is the flame sensor that tells the module that the pilot is established and to open the main gas valve (mgv). On the burners, you may have 2, 3, 4 and so on depending on the BTU rating of the unit. Before I go to far, make sure that the stat is off and you can unplug the furnace, also if you can. And, if you remove the panel to get to the burners and all goes dark, you will have a safety door switch on the perimeter of the unit that you will need to tape down good. This will restore power to unit as you want the panel to be off when you start and check it again. Take one burner section out at a time to clean. Just raise up the back of the burner by gently cocking it up by grasping it and moving it forward to clear the brass orifice. Slide it out and turn it over. Invert it to let any old rust fall out. If the holes looked in good shape, no large burned holes in them, if you have a wire brush give them a good cleaning. Depending on the age of this unit, will depend on how dirty it will be. Take the burner and slip it back over the orifice and let the back rest in the slot. It is important that they are in the right position. Move it around and you will feel it go into place. Look with your light if you have a problem. Sometimes they can be stubburn, but don't get upset. After you have cleaned each one and the flame sensor, you will be ready to start it back up. Make sure the stat is off. Restore power. If you have a helper that can turn on the stat for you while you watch, this will be helpful. With panel off, and your face back some, set the stat to call for heat. The inducer motor should start, close the pressure switch and the pilot will start to ignite. Once the flame sensor senses that the flame is on, the main gas valve will open to fill the 1st burner from front to back and cross over from back to front nice and smoothly. After the dust settles, you should have nice strong blue flames with a yellow tip. This is what causes delayed ignition 99% of the time, plugged burners, dirty or faulty flame sensor. Yes, keep you're face out of the burner section when you go to fire it up, LoL. When this happened to me, I knew I was in for it as it was a roof top pack unit about 20 years old.I removed panel. Jumped it out to start the heat process and this unit had the old standing pilot which is always on.As soon as I heard the gas valve open and could smell it very well, I knew it was to late as the compartment had filled very quick and the burners did not light but it exploded knocking me back and burning my hair, eye lashes, brows you name it. Learned a lessen the hard way. My boss took one look and said, delayed ignition with a big grin?? Anyway my friend, this will solve this problem if you are able to do this. Food for thought to save you $$$$ and time, you can call your local gas company free of charge and just mention you believe you smell gas at your heater and it booms when you turn it on. They will be there very fast to check the unit for this. This is a free service to us all, and really you do have a gas leak? Just food for thought. Please let me know how this turns out for you. I know this will solve this problem. Keep me posted. Sincerely, Shastalaker7