Try installing an incandescent light bulb in series with the heater.
Start with a 100 W bulb and see if that works. Measure the AC voltage across the heater.

 

What voltage rating? Or is this from some surplus outlet?

150uF motor run capacitor

 

150uF motor run capacitor

Although they say "run capacitor", they also say "starting", and they have 1.6% on the case, which I think is duty-cycle, indicating that they are really just start capacitors.
They also look like electrolytic types which are used only for starting.
Run capacitors are typically film/oil type, which are much larger.

 

Try installing an incandescent light bulb in series with the heater.
Start with a 100 W bulb and see if that works. Measure the AC voltage across the heater.

You need 5Ω in series. A 120V 100W lamp has an operating resistance of 144Ω, so will be about 20Ω when cold. A 24V 100W lamp might be close.

 

You need 5Ω in series. A 120V 100W lamp has an operating resistance of 144Ω, so will be about 20Ω when cold. A 24V 100W lamp might be close.

I am aware of the resistance change with temperature. One would have to experiment with the wattage of the lamp to see what works. I expect that the lamp may just glow red.

 

I am aware of the resistance change with temperature. One would have to experiment with the wattage of the lamp to see what works. I expect that the lamp may just glow red.

Red heat is 900K to 1000K: filament lamps are almost PTAT, and Normally run at 2700K to 3400K, so the resistance would be about a third of normal operating resistance, 48Ω for the 100W 120V lamp.

 

Red heat is 900K to 1000K: filament lamps are almost PTAT, and Normally run at 2700K to 3400K, so the resistance would be about a third of normal operating resistance, 48Ω for the 100W 120V lamp.

We need about 5Ω @ 100W.
I don't know where you can find a 24V 100W lamp.
Put two 12V 50W lamps in series.

 

We need about 5Ω @ 100W.

My calculations give 5Ω @ 80W (20V x 4A).

 

An "AUTOTRANSFORMER" connection of a regular transformer is an excellent scheme. Of course, it is often better because the boost winding current rating is greater, since the boost winding carries the total load current.

 

I don't know where you can find a 24V 100W lamp.

commercial vehicle headlight,
https://www.ringautomotive.com/en/product/R454
projector lamp.
https://medlamps.co.uk/osram-hlx-64638-24v-100w-g6-35/

 

150uF motor run capacitor

That one is a fancy Chinese cap that comes with a number of differing denominations but looks too small for being permanently powered. The most informative link I found is the following:
https://jennyli168.en.made-in-china...citor-CD60-150UF-250V-AC-Start-Capacitor.html
(This one shows dual faston contacts for both terminals.)

Here it is denominated explicitly as "starting capacitor" - as well as others looking similar. I would not leave this unsupervised while trying to boil water ;-(