Both an HSA and an FSA can pay for an eligible water filter with a Letter of Medical Necessity — so the choice comes down to how each account behaves. The short rule: use an HSA for larger systems you save toward, and an FSA to spend a balance before it expires.
Quick decision
HSA for big or future purchases; FSA to beat the year-end deadline. Both require a Letter of Medical Necessity. If you have both accounts, spend the expiring FSA first and reserve the HSA for the larger system.
| Factor | HSA | FSA |
|---|---|---|
| Funds roll over? | Yes, indefinitely | Often expire Dec 31 |
| Account is yours? | Yes (portable) | Tied to employer |
| Requires HDHP? | Yes | No |
| Best for | Larger whole-house systems | Spending a balance now |
| Needs LMN for a filter? | Yes | Yes |
Because HSA funds roll over and the account is yours for life, it is ideal for a whole-house system that costs more than one year of contributions — save up, then buy. You also get the triple tax advantage (pre-tax in, tax-free growth, tax-free qualified withdrawals). See Are water filters HSA eligible?
If you have an FSA balance that disappears on December 31, a qualifying filter is one of the best ways to use it — you convert "use it or lose it" money into a durable home upgrade. The catch is timing: get the Letter of Medical Necessity and complete the purchase before the deadline.
Have an HSA and an FSA (or a limited-purpose FSA)? Spend the expiring FSA money first on the qualified portion, then use the HSA for the remainder or for future replacements. Either way the rule is the same — the filter needs a Letter of Medical Necessity. See the full account comparison on our main guide.
You can only contribute to an HSA if you are enrolled in a qualifying high-deductible health plan. An FSA has no such requirement and is offered through an employer. So your eligibility to fund each account may decide the question before preferences do — if you do not have an HDHP, an FSA (or HRA) is your route. Existing HSA balances remain usable for an eligible filter even in a year you cannot contribute.
If you are buying a one-time whole-house system and have both accounts, the simplest plan is: drain any expiring FSA balance into the purchase first (so none is forfeited), then cover the rest from the HSA, which keeps growing for future replacements. If you only have one account, use it — both work with the same Letter of Medical Necessity.
Both accounts have annual contribution limits set by the IRS that change each year and differ for individual vs family coverage (HSAs add a catch-up amount at 55+). Because a whole-house system can exceed a single year's FSA limit, savers often lean on an HSA across two years. Check the current year's figures in IRS Publication 969 before planning a large purchase.
SpringWell's TrueMed checkout issues the Letter of Medical Necessity and accepts your HSA or FSA card.
Shop eligible systems →Use an HSA for larger systems you save toward (funds roll over) and an FSA to spend a balance before it expires. Both require a Letter of Medical Necessity.
Often yes, in sequence. Spend an expiring FSA balance first, then use the HSA for the rest. Document the qualified portion.
Yes. The filter is not automatically eligible under either account; a licensed provider must document the medical need.
