Quick Answer: If you own a tankless water heater, skip the recirculating pump aisle at Home Depot — most of what’s stocked there, including the Watts 500800 and the Grundfos Comfort System (UP15-10SU7P), is explicitly built for tank water heaters and is not rated for tankless use. What you actually want is an on-demand (push-button) pump, like the Rinnai Control-R Wireless Demand Recirculation Kit if you already own a Rinnai unit, or the brand-agnostic Metlund/Taco D’MAND system or Chilipepper CP9000 for any other tankless heater. On-demand pumps push 1–3 GPM in a short burst — enough to clear a tankless unit’s roughly 0.4–0.5 GPM firing threshold — instead of running continuously at the low flow a tank-style pump is designed for.
Search “recirculating pump for tankless water heater” and most of what ranks is written for tank-heater owners, with tankless mentioned as an afterthought or not at all. That’s a problem, because the two appliance types need genuinely different pump architecture — and the two most commonly recommended pumps in generic buying guides, the Watts 500800 and the Grundfos Comfort System, are both explicitly documented by their own manufacturers as tank-only. This guide sorts out which pumps actually work with a tankless unit, and why the wrong kind either does nothing or shortens the life of your heater.
The one rule that explains this whole category
A tankless water heater needs roughly 0.4 to 0.5 GPM of flow to trip its internal sensor and fire the burner or element. A standard tank-style recirculation pump is designed to sip water slowly and continuously — flow that can sit right at or under that threshold.
That single mismatch is why the pump aisle splits into two incompatible product families:
- Dedicated-loop / aquastat-timer pumps (Watts 500800, Grundfos Comfort System) are built to keep a storage tank topped up by circulating water slowly and constantly through a physical return line. They’re excellent products — for a tank heater. Point one at a tankless unit and you get either a burner that never fires, or a unit that short-cycles trying to chase a flow rate that keeps dipping under its activation point.
- On-demand (D’MAND-style) pumps (Rinnai Control-R, Metlund/Taco D’MAND, Chilipepper) push water at 1–3 GPM — comfortably above the tankless firing threshold — for a short 30- to 90-second burst triggered by a push button, app, or motion sensor, then shut off completely. This is the architecture every major tankless manufacturer’s own accessory line uses.
The confusion is understandable: both product families are called “recirculating pumps,” they’re often sold in the same store aisle, and the box art rarely spells out which water heater type they’re engineered for. The Home Depot and Watts product listings do disclose it, though — you just have to read the fine print most buyers skip.
Our top picks at a glance
| Model | Type | Best for | Return line needed? | Control | Price |
|---|---|---|---|---|---|
| Rinnai Control-R (RWMKT01P) | On-demand | Rinnai tankless owners | No (thermal bypass valve) | Push button + app | ~$500 MSRP |
| Metlund/Taco D'MAND (00R) | On-demand | Any tankless brand | No | Push button / motion | ~$350–$750 |
| Chilipepper CP9000 | On-demand | Budget retrofit | No | Push button | ~$200–$250 MSRP |
| Watts 500800 | Aquastat/timer | Tank heaters only — not tankless | Yes | Timer | ~$199 |
| Grundfos Comfort System (UP15-10SU7P) | Aquastat/timer | Tank heaters only — not tankless | No (comfort valve) | Timer | ~$728 |
1. Rinnai Control-R Wireless Demand Recirculation Kit (RWMKT01P) — Best for Rinnai Owners
Rinnai Control-R Wireless Demand Recirculation Kit
- Rinnai's own accessory line, sold through Home Depot and plumbing wholesalers, built specifically to pair with Rinnai tankless units.
- Uses a Grundfos pump under the hood, paired with a wireless push button and app control rather than a hardwired switch.
- Works in homes without a dedicated return line via a thermal bypass valve installed at the farthest fixture from the heater.
- Because it's an on-demand system controlled by push button, it satisfies Rinnai's own warranty language for recirculation-connected units.
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If you already own a Rinnai tankless water heater, this is the pump to buy first, before you even look at third-party alternatives. It’s engineered by the same company that built your heater, it’s explicitly listed as a warranty-safe control method, and the app/push-button combination is more polished than most aftermarket kits. The tradeoff is the MSRP — it runs close to $500, roughly double the Chilipepper below, for the convenience of a matched system and app control.
2. Metlund/Taco D’MAND System (00R) — Best Universal On-Demand Retrofit
Metlund (ACT) / Taco D'MAND Hot Water Recirculation System
- The system most frequently named by tankless installers as the go-to on-demand pump regardless of heater brand.
- Available in push-button and motion-sensor trigger variants, so you can retrofit it without touching existing plumbing beyond the pump and a check valve.
- Uses the existing cold-water line as a temporary return path, then a check valve isolates the two lines again once the cycle finishes.
- Price spans a wide range — roughly $350 for a basic push-button kit up to $750 for motion-sensor and multi-fixture configurations — so match the kit to how many bathrooms you're actually solving for.
This is the safe default if you don’t own a Rinnai, or if you’d rather not pay for brand-matched app control. It’s the same on-demand architecture as the Rinnai Control-R — push a button, the pump runs for well under two minutes, hot water arrives, then everything shuts off — just sold as a universal kit that works across Navien, Rheem, Takagi, Noritz, and every other tankless brand covered elsewhere on this site.
3. Chilipepper CP9000 — Best Budget On-Demand Pump
Chilipepper CP9000 Hot Water Recirculation Pump
- The least expensive genuinely tankless-compatible pump in this roundup, at roughly half the price of the Rinnai kit.
- Rated to pump approximately 2.5 to 3 GPM per the manufacturer, comfortably above the 0.4–0.5 GPM most tankless units need to fire.
- A built-in temperature sensor shuts the pump off automatically once hot water reaches the tap, so there's no timer to babysit.
- Installs under the fixture farthest from the heater, cutting into the hot and cold lines there rather than at the water heater itself.
If cost is the deciding factor, this is the pump to buy. It gives up the app control and OEM branding of the Rinnai kit, but the underlying mechanism — push a button, pull hot water forward at above-threshold flow, shut off on temperature — is the same physics that makes the pricier options work.
The two popular pumps to skip if you have a tankless unit
Watts 500800 Instant Hot Water Recirculating System
- Watts' own product documentation states this system is not designed for tankless water heaters.
- It's built to loop water at low, continuous flow through a storage tank — the opposite of what a tankless unit's activation sensor needs.
- A genuinely good pump for a conventional tank heater, and one of the most frequently recommended pumps in generic (non-tankless) buying guides — which is exactly how tankless owners end up buying the wrong product.
Grundfos Comfort System (UP15-10SU7P)
- Home Depot's own product Q&A for this pump states plainly that it is for tank water heaters only and will not work with tankless units.
- It's a well-built, well-reviewed stainless-steel circulator — just engineered around a continuous timer-driven loop rather than an on-demand burst.
- If you're switching from a tank to a tankless unit and already own this pump, plan to replace it rather than reuse it; it's not a matter of reconfiguring settings.
Both of these are legitimate, well-made products from established plumbing brands — they simply aren’t built for the appliance this article is about. We’re naming them specifically because they rank near the top of most general “best recirculating pump” searches, and neither product page hides its tank-only design, but almost nobody reads that far before adding to cart.
Why the warranty question isn’t about the pump brand
Rinnai’s own limited warranty documentation is unusually clear on this point: a recirculation system must be controlled through an aquastat/thermostat, timer, or an on-demand system to keep the extended heat-exchanger warranty in place. All three control types are acceptable — what actually voids coverage is an undocumented or improperly controlled loop, not the mere presence of a pump. That said, other manufacturers write their warranty terms differently, and the practical reason to default to on-demand hardware for a tankless unit isn’t warranty risk so much as function: only an on-demand pump reliably clears the 0.4–0.5 GPM activation threshold in a short burst instead of drifting under it during a slow continuous loop. Confirm your specific model’s language before you buy — see our tankless water heater maintenance guide for the other manufacturer-specific rules that catch owners off guard.
How to choose
- You own a Rinnai tankless unit: the Rinnai Control-R (RWMKT01P) is the OEM match and the safest warranty answer.
- You own any other brand — Navien, Rheem, Takagi, Noritz, Stiebel Eltron: the Metlund/Taco D’MAND system is the proven universal on-demand pick.
- Budget is the deciding factor: the Chilipepper CP9000 gets you the same on-demand physics for roughly half the Rinnai kit’s price.
- You already own a Navien NPE-series unit: check first — Navien’s ComfortFlow models ship with recirculation built in, so an add-on pump may be redundant. See our best tankless water heater guide for which models include it standard.
- You’re shopping generic “recirculating pump” results and see the Watts 500800 or Grundfos Comfort System recommended: keep scrolling — those are correct answers for a tank heater, not for yours.
- You have a dedicated return line already plumbed in from a previous system: a continuous aquastat/timer pump can still work, but confirm with your tankless manufacturer that continuous low-flow circulation won’t trip false-firing or short-cycling before you install one.
The bottom line
The best recirculating pump for a tankless water heater is whichever on-demand, push-button model matches your situation — the Rinnai Control-R if you own a Rinnai unit, the Metlund/Taco D’MAND system for any other brand, or the budget-friendly Chilipepper CP9000 if cost is the deciding factor. What all three share is the ability to push 1–3 GPM in a short burst, comfortably above the roughly 0.4–0.5 GPM most tankless units need to fire. What you should actively avoid is the pump aisle’s more familiar names built for tank heaters: Watts itself states the 500800 is not designed for tankless units, and Home Depot’s own product page confirms the Grundfos Comfort System (UP15-10SU7P) is for tank water heaters only. Buy for the appliance you actually own, not the one the generic buying guide assumed you had.