Dura Heat GFA40 Propane Space Heater

Let's say you have something like one of these, a Dura Heat GFA40 propane space heater:

and you want to repair it.

First you remove the 4 screws that hold the bottom plate of the base in and see this.

and this. You have access to a nice PC board with connectors. That's about where the fun ends because to get to the burner plate to access the thermocouple and over-temperature safety switch, you have to:

1. remove 3 screws to remove the rear screen (easy)
2. remove the motor
• remove the (4) screws that hold the motor base, accessible from within the base. The ones in the 10, 4, and 8 o'clock positions, as viewed from the base, are easy. The one in the 2 o'clock position is behind the switch mechanism. I ended up loosening everything so I could get to that screw straight-on so I could loosen it correctly. That  meant removing the 4 screws that hold on the base,  the 6 screws that fasten to the standoffs between the inner and out tubes, the machine screw and nut that hold the capacitor, the 4 screws (accessible through the label under the knob) that hold the valve to the base, and the valve knob. That make everything loosey-goosey enough that I could loosen the last screw for the motor mount and extract the motor and fan blade.

From there I had good access to the burner plate and could remove the thermocouple, over-temp safety switch, and igniter post as needed.

Now I know I could have just tested between the blue wire (next to the yellow one, not the one to the capacitor) and the bare silver earth wire. First for continuity to confirm the safety switch was closed, then for voltage as I heated the tip of the thermocouple.

But I was sure the themocouple was bad because the heater ignited fine and burned until I released the knob and hence the TC bypass. This was after I repositioned the heater that I had trouble. The first run was perfect - it ignited and ran on the first try. Now I'll get the propane tank and test again with an eye on the relay and the yellow lead to the control valve.

Summary

My takeaway from this exercise is that this unit is not particularly serviceable, due to the singular lack of accessibility to that one motor mount screw. The component layout could have been a little better, but it's generally good, and not a mess of dodgy soldering and wire splices. Virtually all of the screws were the short black sheet metal screws. Exceptions were the machine screw and nut that secures the capacitor, and the small silver screws for the safety switch.

Recap

I spent more time troubleshooting the heater and diagramming the PC board (next post).

When I back-probe CN4 the blue lead shows the thermocouple (TC) generates at around 20mV or more after several seconds. The yellow lead which is connected to the blue TC lead via the relay contacts may only show only 4-5mA. I did not see a matching differential voltage across the leads (?!). The resistance across the pins (with cable disconnected) is consistently 200mΩ. The relay's spec for contact resistance is ≤ 100mΩ. I removed and inspected the spring contacts from the connector housing - both looked good.

I resoldered a number of pins, sometimes removing the old solder, not that any solder joints were looking problematic. The heater is still very sensitive to tapping on the base, or removal of the meter probe from the connector. The relay coil is getting 11.9Vdc. So I suspect the relay could be a little better, but the unit is usable as is, with an occasional tap here and there.