Hot Water Recirculating Pump Without a Return Line
A crossover valve under the furthest sink, and no new pipe anywhere. With one honest side effect.
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A crossover valve at the furthest fixture lets the cold supply act as the return path, so a recirculating pump works with no new pipe at all. The honest side effect: for a few minutes after the pump runs, the cold tap at that fixture delivers warm water. For most houses that is a good trade.
Dedicated return lines exist in houses that were plumbed for recirculation, which mostly means newer construction and high-end remodels. Everyone else has a hot line and a cold line, and the question is whether you can get recirculation without opening walls.
You can, and the mechanism is simple enough to explain in a paragraph.
How a crossover system works
- A pump sits at the water heater, pushing hot water out into the hot line.
- A thermostatic crossover valve is fitted under the sink furthest from the heater, bridging the hot and cold lines.
- While the water arriving in the hot line at that fixture is cool, the valve is open, and the water passes across into the cold line and returns to the heater through it.
- When the water arriving is hot, the valve closes. The loop stops, and the hot line is now full of hot water all the way to the far fixture.
- Open the tap and hot water is there immediately, because it never left.
That is the whole system: a pump, a valve, and no new pipe. The valve is thermostatic and mechanical, so there is nothing to wire at the fixture end.
The side effect, in full
Because the cold line is doing the returning, some of the cold line gets warm. Immediately after the pump has run, the cold tap at the far fixture delivers tepid water for a short while before true cold arrives.
| Situation | Verdict |
|---|---|
| Far bathroom sink, used for hand washing | Nobody notices or cares |
| Shower at the far fixture | Not an issue — you want it warm |
| Kitchen cold tap used for drinking water | This is the one that bothers people |
| Ice maker or filtered water on the cold line | Worth thinking about before fitting |
| Running on a timer only in the morning | Minimal — the effect fades within minutes |
Crossover versus a dedicated return
| Crossover | Dedicated return | |
|---|---|---|
| New pipe required | None | A full return run to the heater |
| Install time | A couple of hours | A day or more, often with drywall work |
| Cold tap side effect | Yes, at the crossover fixture | None |
| Energy efficiency | Slightly worse — some cold line is heated | Better — purpose-built loop |
| Works in an existing house | Almost always | Only if walls can be opened |
| Pump duty | Slightly higher | Lower |
When to re-pipe instead
- You are already opening walls for a remodel. Then a dedicated return is nearly free and strictly better.
- Cold drinking water at the kitchen tap is non-negotiable and the kitchen is the furthest fixture.
- The house is very large with multiple distant branches, where one crossover point will not serve them all.
- You are building new. There is no reason not to plumb a return line in new construction.
Or let the heater do it
If you are replacing the water heater anyway, some units handle this internally. The Rinnai RSC199iN's ThermaCirc360 is designed to recirculate either through a dedicated return line or through the existing cold supply where no return exists — the same crossover principle, managed by the heater's own controller. The Navien NPE-A2 series does something similar with NaviCirc.
What the two routes look like
Pick 1 — Recirculation with no return line
Grundfos UP15-10SU7P Comfort System
The comfort-system pump that made under-sink recirculation normal: 25 watts, a built-in timer and a line cord, in a stainless body rated to 150°F.
| Power | 25 W |
|---|---|
| Voltage | 115 V, 0.23 A, single phase |
| Motor | 1/25 HP |
| Flow range | 0 - 6.3 GPM |
| Head range | 0 - 5.1 ft |
| Maximum pressure | 145 psi |
| Temperature range | 36 - 150°F |
| Body | Stainless steel |
| Connections | 3/4 in female NPT inlet, 3/4 in male NPT outlet |
| Controls | Built-in timer and line cord |
What is good
- 25 watts is less than the light bulb it is usually compared to
- Built-in timer means it only runs when you want hot water, not all day
- Stainless body is the right choice on the potable side
- Line cord: it plugs in rather than needing a wired connection
What is not
- 5.1 ft of head is modest — long or tall runs need a bigger pump
- A crossover-valve install puts warm water in the cold line at the far fixture
- 150°F maximum rules out tanks set higher for Legionella control
Skip it if: Skip it if your tank is set above 150°F for scald-control-plus-mixing-valve reasons. The pump's temperature limit is a hard limit, and a comfort pump cooked by 160°F water is a warranty conversation you will lose.
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Specifications above from Supply House listing, Grundfos 99452459 UP15-10SU7P; Grundfos Comfort System and Alpha pump data. We have not tested this unit ourselves.
Pick 2 — Recirculation without a return line
Rinnai SE+ RSC199iN
The pick when you want hot water fast at a far bathroom and have no dedicated return line: ThermaCirc360 recirculates through the cold line instead, and the 15-year heat exchanger warranty is the longest here.
| Fuel | Natural gas |
|---|---|
| Gas input | 199,000 BTU/h |
| Maximum flow | 11 GPM |
| Efficiency | 0.95 UEF |
| Installation | Indoor only |
| Recirculation | ThermaCirc360 — works with or without a dedicated return line |
| Warranty | 15-year heat exchanger, 5-year parts, 1-year labor |
What is good
- Built-in recirculation means no separate pump, timer or control to buy and wire
- ThermaCirc360 can use the cold supply as the return, which is the difference between a half-day job and a re-pipe
- 15-year heat exchanger warranty
- 0.95 UEF keeps it inside most utility rebate thresholds
What is not
- Indoor installation only — no outdoor variant of this exact model
- Natural gas only in this model number
- Crossover recirculation through the cold line puts warm water in the cold tap for a few seconds
Skip it if: Skip it if you are plumbing a new build where a dedicated return line is going in anyway. You would be paying for the integrated pump twice over, and a plain condensing unit plus a separate pump is cheaper and easier to service.
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Specifications above from Rinnai RSC199iN product page. We have not tested this unit ourselves.
Installing a crossover system
Identify the furthest fixture
Not the furthest in floor plan distance but the one with the longest run of pipe and the worst wait. Time them with a stopwatch if it is not obvious.
Fit the crossover valve under that sink
It connects between the hot and cold supply stops. Thermostatic and mechanical, so no wiring at this end.
Fit the pump at the heater
Usually on the hot outlet. The Grundfos Comfort System has a line cord, so it needs a nearby outlet rather than a wired connection.
Set the schedule
Morning and evening windows covering when the household actually showers. Do not leave it running continuously.
Check the temperature rating against your tank
The Grundfos pump is rated to 150°F. If you store hotter than that for bacteria control, you need a different pump.
Live with it for a week, then adjust
Most people shorten the schedule once they discover how quickly the loop charges. Shorter windows mean less loss.
Procedure: Fit crossover recirculation with no return line
What goes in with it
A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.
- Flush kit
The annual descale is not optional on a tankless unit, and you need a pump, hoses and a bucket.
- Descaler solution
What actually dissolves the scale once the kit is hooked up.
- Recirculation pump
Fixes the cold-water delay that makes new tankless owners call a plumber.