Drain Water Heat Recovery in Ontario: How It Works and When It Pays Off

Drain water heat recovery (DWHR) in Ontario homes uses the warmth of shower water flowing down the drain to preheat the cold water heading to your water heater and shower. A DWHR unit is simply a length of copper drain pipe wrapped in a coil of copper supply pipe, installed vertically on the main drain stack, and it typically cuts the energy needed for showers by roughly a quarter to a half while the shower is running (estimate).

It is a simple, durable technology with no moving parts and no electricity use. Whether it pays off depends mostly on how you heat water, how much your household showers, and whether your plumbing layout allows a vertical installation.

How does drain water heat recovery work?

When you shower, warm water at about 35 to 40 °C goes down the drain. In a vertical drain pipe, that water clings to the inside wall in a thin film. A DWHR unit replaces a section of the drain stack with a copper pipe wrapped by a smaller copper coil. Cold incoming water flows through the coil, picks up heat from the drain water and leaves several degrees warmer.

The preheated water then goes to the cold side of the shower, to the water heater, or to both. Supplying both gives the best savings.

Feature	Detail
Moving parts	None
Power needed	None
Typical length	About 75 cm to 180 cm (30 to 72 in)
Pipe sizes	Matches common 3 in and 4 in drain stacks
Rated efficiency	About 30% to 60%+ under standard test conditions
Works for	Simultaneous flows, mainly showers
Lifespan	Similar to copper plumbing, often decades

DWHR units are tested and certified to Canadian standards (CSA B55.1 for efficiency and B55.2 for construction). Look for the rated efficiency on the product listing; higher is better, and longer units generally perform better.

Note: DWHR only recovers heat while hot water is draining at the same time as cold water is flowing in, which is why it works best for showers. Filling a bathtub and draining it later recovers very little.

Which homes are a good fit?

Use this checklist before getting a quote:

Is there a vertical drain stack below the main showers? Most two-storey homes with basements have one. Bungalows with slab floors often do not.
Is there enough straight vertical drop? You need roughly 1 to 2 m (3 to 6 ft) of straight pipe, usually in the basement.
Is the basement ceiling or mechanical area open? Access to the stack makes installation much simpler and cheaper.
Do you shower often? Households of three or more people with daily showers get the most out of it.
How do you heat water? Savings are highest with electric tanks, oil or propane, and lowest with gas tanks.

The Ontario Building Code's energy efficiency requirements for new houses include DWHR in some compliance options, so many recently built Ontario homes already have one. If you are buying new, our guide to buying an energy-efficient new home explains what to ask the builder.

How much does drain water heat recovery save?

Showers often account for a large share of household hot water use. The estimates below assume a family of four, a DWHR unit with a rated efficiency around 45 to 50% connected to both the water heater and the shower, and a resulting reduction of about 15 to 25% in total water heating energy. Prices: about 15¢ per kWh all-in for electricity and 45¢ per m³ for gas.

Water heater	Annual water heating energy (estimate)	Annual DWHR saving (estimate)	Value per year
Electric tank	3,500 to 4,500 kWh	600 to 1,000 kWh	About $90 to $150
Heat pump water heater	1,100 to 1,500 kWh	200 to 350 kWh	About $30 to $55
Gas tank	250 to 350 m³	45 to 80 m³	About $20 to $35
Condensing gas tankless	170 to 240 m³	30 to 55 m³	About $15 to $25

Two further benefits are easy to miss:

More effective capacity. Because incoming water is preheated, a tank's hot water lasts longer during back-to-back showers, and a tankless unit can deliver higher flow in winter.
Winter help in Ontario. Incoming water is coldest in winter, often around 5 to 10 °C, when the preheat makes the most difference.

Our water heater comparison calculator can help you see how DWHR compares with changing your water heater.

What does installation cost?

Item	Typical range (estimate)
DWHR unit	$700 to $1,400
Installation where the stack is accessible	$300 to $900
Installation where finished ceilings must be opened	Add $300 to $1,000+ for drywall and finishing
Typical total	$1,000 to $2,300

Installation involves cutting out a section of the drain stack, inserting the unit vertically with proper support, and connecting the supply coil to the cold water line. It should be done by a licensed plumber, and a plumbing permit may be required by your municipality.

Simple payback

Electric tank: roughly 7 to 25 years depending on cost and use (estimate).
Heat pump water heater: often 25 years or more.
Gas tank or tankless: often 30 years or more on energy savings alone.

With gas water heating, DWHR rarely pays back on energy alone at current gas prices, especially since the federal carbon charge on natural gas was removed in April 2025. It is easiest to justify when installation costs are low, such as during a basement renovation or plumbing replacement, or when you value the extra hot water capacity.

Installation tips

Install it as low as possible on the stack while still catching all shower drains above.
Connect to both the water heater and the shower cold supply for the best savings ("equal flow" configuration).
Keep it vertical. Most units lose much of their performance if installed at an angle or horizontally.
Leave access in any finished ceiling or wall, for inspection.
Plan it with other upgrades. If you are insulating basement walls or rim joists, as described in our basement insulation guide, that is a good time to add DWHR.

Frequently asked questions

Does drain water heat recovery need maintenance?

Very little. Because the drain water clings to the pipe wall and there are no moving parts, units do not clog more than ordinary drain pipe. Hard water scaling inside the supply coil is possible over many years but uncommon.

Can it be installed in a bungalow?

Only if there is enough vertical drop below the shower drain, usually in a basement. Homes on a slab or crawl space often lack the needed height.

Is the water from the drain mixed with my drinking water?

No. The drain water and the fresh water stay in separate copper passages: drain water runs down the inside of the main pipe, and fresh water flows through the coil wrapped around the outside.

Is there a rebate for drain water heat recovery?

The editorial team is not aware of a current provincial rebate specifically for DWHR in existing homes. Check our rebate finder for current programs, since they change.