Hi and welcome to FixYa. I am Kelly. I don't have the answer to your knob problem other than to contact a Holmes Service Center. However, this particular heater was the subject of a recall (see link below.) Maybe you can get lucky and get a whole new heater vs the knobs should you have a unit with a product code listed at the bottom of this post..
http://www.holmesoilfilledheaterrecall.com
If you are in possession of a recalled model with a listed Production Code, contact us at (800) 306-2471 to confirm your eligibility and register for a replacement heater.
There is a 4 digit production code between 2 horizontal lines about 1/3 of the way down the product label.
These are the codes elligible for replacement:
Acceptable Production Codes
2305
0106
2405
0206
2505
0306
2605
0406
2705
0506
2805
0606
2905
0706
3005
0806
3105
0906
3205
1006
3305
1106
3405
1206
3505
1306
3605
1406
3705
1506
3805
1606
3905
1706
4005
1806
4105
1906
4205
2006
4305
2106
4405
2206
4505
2306
4605
2406
4705
2506
4805
2606
4905
2706
5005
2806
5105
2906
5205
3006
3106
3206
3306
3406
3506
3606
3706
3806
3906
Thanks for choosing FixYa.
Kelly
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