I have an Ultra 97 gas fired heater made by Magic Chef. I assume its about 15-18 years old.

When the heater kicks on, the only thing that happens is that the blower starts running but the ignitor does not ignite. There are 2 flat cylinder type switches when you open up the door. If you tap on either one of them , then you start to get a clicking sound and the furnace fires up. They are made by Tri Delta Industries which I found out has gone out of business.

I assume these are high and low pressure switches but Im not 100 % sure. Do you think these need to be replaced or do I have a problem somewhere else ?

The previous owner told me I needed to add a 1/4 cup of water into a tube that is on the side of the furnace. Can anyone explain what that is all about ?

Any help would be appreciated

The devices you are describing are indeed pressure switches. The external micro-switches mounted on the diaphragm canister are susceptible to oxidation over time, and eventually can fail to close the circuit when engaged. They are responsible for proving the combustion draft to the ignition control. If you were experiencing this problem at the beginning of a heating season, the suggestion to add water to the condensate trap may bear some merit. At this stage, however, it is more likely that you have an obstructed combustion vent, or that the switch(es) failed. Try this: follow your previous procedure to get the furnace going. After the burners have lit, nudge the wires connected to the switch(es). If the burners immediately drop out, you have a defective switch.