Hot water recirculation eliminates the wait for hot water at distant fixtures. Instead of letting cold water run for 30-90 seconds before hot arrives, a small pump keeps the loop primed so hot water shows up in 1-5 seconds. The basic concept is simple; the implementation choices matter a lot.
The problem recirculation solves
Without recirculation, water sitting in the pipe between the water heater and a distant fixture cools. When you open the tap, that cold pipe water has to drain before fresh hot water arrives from the heater. In a typical 2,500 sq ft home, this is 30-60 seconds and 1-3 gallons of wasted cold water per draw.
Recirculation keeps the loop continuously primed with hot water, so the wait drops to a few seconds.
Two architectures
1. Dedicated return line (best, but requires plumbing)
A third pipe runs from the farthest fixture back to the water heater. A pump (either external Grundfos / Taco or built into the heater) circulates hot water continuously through this loop. The pipe is dedicated — separate from the cold and hot supply lines.
- Best for: new construction. Easy to add during framing
- Cost in new build: $400-800 extra plumbing
- Cost in retrofit: $1,500-3,500 — need to fish a new line through existing walls
- Performance: hot water at all fixtures in 1-3 seconds. The clean architecture
2. Crossover valve (retrofit-friendly)
A thermostatic crossover valve installed at the farthest fixture allows hot water to flow through the cold-water line back to the heater when loop temperature drops. The cold line briefly carries warm water; when hot water arrives back at the unit, the crossover closes.
- Best for: retrofits without a return line
- Cost: $300-700 (pump + crossover + install)
- Performance: hot water in 5-15 seconds — slightly slower than dedicated return
- Tradeoff: the cold-water line carries warm water occasionally. Most homeowners don't notice; some find the "cold tap not immediately cold" annoying
Pump options
Built-in pump (in the water heater)
Some tankless units include a recirculation pump inside the cabinet. The two examples:
- Rinnai Sensei RX — built-in pump + dedicated return-line port + Circ-Logic learning firmware. Premium option
- Navien NPE-A2 — built-in pump + 0.5-gallon buffer tank + HotButton on-demand support
Built-in is cleaner — no external pump installation, no separate electrical, integrated controls. About $300-500 more than the equivalent non-pump model.
External pump
- Grundfos UP15 — the standard. About $300 for the pump kit
- Taco 006 — competitor; similar price/performance
- Install: inline on the return line or on the hot-water output. 120V circuit needed
External pump works with any water heater. Choose this when retrofitting recirculation onto an existing tank water heater or onto a tankless without built-in pump capability.
Control schemes — how often the pump runs
Always-on (continuous)
Pump runs 24/7. Maximum convenience, maximum energy cost. The burner cycles frequently to maintain loop temperature. Energy cost: $20-50/month additional gas.
Time clock / schedule
Pump runs only during set windows (e.g., 6-8 AM, 5-7 PM). Saves substantial energy vs always-on. Energy cost: $8-18/month additional. Setup via timer outlet, smart plug, or the heater's app.
Aqua-stat / temperature-controlled
Pump runs when loop temperature drops below a setpoint. Stops when loop is hot. Better than fixed schedule for irregular households.
On-demand (push button)
Wireless button at the farthest fixture. Press before turning on the hot tap; pump runs for 90 seconds. Maximum efficiency. Examples: Navien HotButton, Watts Premier on-demand. Energy cost: $1-3/month when used.
Learning / adaptive
The pump learns your usage pattern over 2-4 weeks and runs only when it predicts a draw. Most sophisticated option. Rinnai Circ-Logic is the best example. Energy cost: $5-12/month additional.
Does it pay back?
Strictly on energy/water saved: no. The pump uses electricity; the burner cycles more often. Net energy cost is higher than no recirculation.
The payback is in convenience and water saved (cold water that would have gone down the drain waiting for hot). A typical household saves 12,000-30,000 gallons of water per year with recirculation — that's $50-150 on the water bill in metered cities, plus reduced sewer charges.
Net: pump cost roughly offset by water savings; you get the convenience of instant hot water effectively for free if you use a smart control scheme (on-demand or learning).
When to install
- New construction — always (incremental cost during framing is small)
- Frequent complaints about hot-water wait time
- Distant fixtures (kitchen 40+ feet from heater, master bath at the other end of the house)
- You're already replacing the water heater AND upgrading to a model with built-in pump (Sensei RX or NPE-A2)
When to skip
- Small home (under 1,500 sq ft) — wait times are short anyway
- Tight budget — pure energy cost goes up
- Water heater is in the right location (within 25 ft of the main bathrooms)
Related guides
Bottom line
For new construction: install a dedicated return line and pick a tankless with a built-in pump (Sensei RX or NPE-A2). For retrofits: crossover valve + external pump, controlled by schedule or on-demand. Avoid always-on schedules — they double your gas bill on the recirc side without adding meaningful convenience.