Age-. The general rule in the industry is that once an A/C unit reaches 10-15 years, the idea of replacing the unit is usually a much better investment than putting money into an older system. If your system is at this threshold, it's usually wise to begin expecting, not wondering, when the system will fail.
You can keep the money in the TSP after you leave your federal job to benefit from the low fees, or you can roll over the money into an IRA or a new employer's 401(k), 403(b) or 457.
Even if you have a shorter time horizon, you will forgo significant savings opportunities by cashing out your TSP. For example, if you are 45, your $20,000 will grow to $53,855 in 17 years. Keep in mind that even if you really need the money, you may be better off borrowing from your TSP account.