What's your water source, city or well? If city, check your plumbing for a pressure reducing valve. Should be just downstream of the main water shutoff for your house. It might be adjustable to get you a higher pressure.
Not sure about running your unit at such a low pressure. Probably no damage, but your output will be agonizingly slow and probably somewhat inefficient. 21 psi is pretty pathetic for pressure if it's from the city.
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