How to Install a Water-to-Air and Water-to-Water Heat Exchanger
Posted by SEBASTIAN ZOTA
A heat exchanger, sometimes called a hydro coil, lets your outdoor wood furnace heat the air inside your home without mixing wood boiler water with your household air supply.
We’ll share where to install a water-to-air heat exchanger in your plenum, how to size one for your furnace, and how a water-to-water heat exchanger connects to your existing system.
How to Install a Water-to-Air Heat Exchanger
A water-to-air heat exchanger mounts inside the plenum of your existing hot air furnace. Where exactly it goes depends on a few things:
- Whether you have central AC
- What type of furnace you're working with
- How close the unit sits to your furnace's overheat sensor
Where to Install a Heat Exchanger in the Plenum
Most hot air furnaces have a return side and a supply side, with the plenum sitting between the furnace and the ductwork. Air gets pulled in through the return, pushed through the furnace fan, reheated, and sent back out through the supply.
If you have central AC, you likely have an A-coil mounted in the plenum already. Mount your heat exchanger at least a foot above the A-coil. Getting closer than that risks freezing the water-to-air heat exchanger, which can burst it and ruin it.
If you don't have an A-coil, you can mount the heat exchanger anywhere in the plenum. Right on top of the furnace works fine.
Mount a Heat Exchanger on the Supply Side, Not the Return
You can technically mount a heat exchanger on the return side, but don't. Here's why:
- Return ducts are usually smaller, longer, and harder to work with than supply ducts.
- A heat exchanger on the return reheats air before it reaches the furnace's overheat sensor, so the sensor never reads "cooled off." That keeps the furnace fan running continuously.
- Reheated return air also runs hotter across the fan bearings, which can overheat the fan and cause it to cycle on and off repeatedly.
Mount on the supply side instead.
Keep the Heat Exchanger Away from the Overheat Sensor
Furnaces that run on oil, gas, natural gas, or propane have a sensor (often a Honeywell unit or similar, usually mounted on the side of the furnace) that keeps the fan running until the air has cooled down enough after each heating cycle.
If your heat exchanger sits too close to that sensor, it can prevent the sensor from ever reading "cooled off." The result is a fan that never shuts off. Mount the heat exchanger with some clearance from any sensor housing to avoid this.
How to Size a Heat Exchanger for a Furnace
Getting the right size heat exchanger depends on a few factors:
- Plenum size. Furnace manufacturers size the plenum to maximize CFM (cubic feet per minute of air movement) through the house. A heat exchanger that's undersized will restrict airflow and not maximize heat efficiency. One that's oversized can restrict airflow more than needed.
- Furnace fan speed. Most furnace fans run at two or three speeds, and many aren't set to their highest speed out of the box.
- Household heating load. The exchanger needs to move enough heat to keep pace with what your home actually needs on the coldest days.
There's no universal size that fits every setup. Check your furnace's specs and your heat exchanger's BTU rating before buying, and when in doubt, give us a call. We can help match a heat exchanger to your furnace and plenum.
Heat Exchanger Reducing Airflow? Check the Fan Speed
Adding a heat exchanger does create some resistance in the plenum, and that can slow airflow through the house. In most cases this isn't a real problem. But if you notice weaker airflow after installing one:
- Pull the panel off your furnace and look for the fan speed dip switches or wiring.
- Bumping the fan up a speed setting usually compensates for the added resistance.
- If you're not comfortable with the wiring, call an electrician or heating tech.
Install the heat exchanger first, then only adjust fan speed if you actually notice weaker airflow. Most installs never need it.
Water-to-Water Heat Exchanger Installation
If a water-to-air heat exchanger heats the air in your house, a water-to-water exchanger does the same job for anything else running on hot water, whether it’s your domestic hot water tank, a pool, or a hydronic loop.
Installing one comes down to two things:
- Understanding how the plate exchanger keeps your boiler water separate from what you're heating
- Matching the hookup to the specific system on the other end.
What a Plate Exchanger Does
A plate heat exchanger is a water-to-water heat exchanger used with outdoor wood furnaces. Hot water from the wood furnace runs through one side of the plates. Water from your home's system, whether that's your domestic hot water, a pool, or a hydronic heating loop, runs through the other side. Heat transfers between the two without the water ever mixing.
This matters for outdoor wood boilers specifically, since it keeps boiler water (which isn't potable) completely separate from your household water supply.
How to Hook Up a Water-to-Water Heat Exchanger
The plate exchanger sits between your outdoor wood furnace's water loop and whichever system you're heating, whether that's your home's indoor boiler, a domestic hot water tank, or a pool. The exact plumbing connections depend on what you're connecting it to.
- Connecting to an indoor boiler: see our guide on connecting an outdoor wood boiler to an indoor boiler.
- Installing a pool heat exchanger: see our complete wood boiler pool installation guide.
- Installing a side-arm style exchanger: see our sidearm heat exchanger installation guide.
Each setup has different plumbing requirements, so match your install to the system you're actually connecting.
Heat Exchanger Installation FAQs
Can I mount a heat exchanger on the return instead of the supply?
You can, but we don't recommend it. It reheats air before it reaches the furnace's overheat sensor, which keeps the fan running continuously and can overheat the fan bearings over time.
How close can a heat exchanger be to an A-coil?
Keep at least a foot of clearance above the A-coil. Closer than that risks freezing and bursting the coil.
Will a heat exchanger reduce airflow in my house?
Slightly, but it's rarely a real problem. If you do notice weaker airflow, bumping up your furnace fan speed usually solves it.
How does a water-to-water heat exchanger work?
A water-to-water heat exchanger uses a stack of thin metal plates to transfer heat between two separate water loops without letting them mix. Hot water from your wood furnace flows through the channels on one side of the plates. Water from the system you're heating, your domestic hot water tank, a pool, or a hydronic loop, flows through the channels on the other side. Heat passes through the metal plates from the hot side to the cooler side, so by the time water reaches the far end of the exchanger, it's picked up heat without ever touching the boiler water.
How does a water-to-air heat exchanger work?
A water-to-air heat exchanger uses a coil of tubing, usually finned copper, mounted inside your furnace plenum. Hot water from your wood furnace runs through the coil. As your furnace fan pushes air across the fins, the air picks up heat from the coil and carries it through your existing ductwork into the house. The water stays sealed inside the coil the whole time, so it never mixes with the air moving through your home.
What's the difference between a water-to-air and a water-to-water heat exchanger?
What matters most when choosing between a water-to-air and a water-to-water heat exchanger is what you're actually trying to heat.
Choose a water-to-air heat exchanger when you're heating the air in your house through an existing forced-air furnace and ductwork. This is the setup for whole-house heating off a wood boiler.
Reach for a water-to-water heat exchanger when you're heating a second water system, your indoor boiler, your domestic hot water tank, a pool, a spa, or a radiant floor loop. Anywhere you need hot water rather than hot air, this is the one you want.
In short: ductwork and forced-air heating call for water-to-air. Showers, pools, and radiant floors call for water-to-water. Many setups run both off the same boiler, one for the house, one for hot water.
