There is something deeply satisfying about firing up your wood boiler on a December morning and feeling warm air blast through your vents instead of just radiators. That is the magic of installing a hot water coil for furnace systems. In this guide we will help you to understand the “How-To’s” of sizing, installing and attaching a hydronic coil so that you can start heating your whole house using your outdoor wood boiler with existing forced air or water baseboard systems.
A water to air heat exchanger also sits in the plenum of your furnace and heats the air that is distributed through ducts. Make the installation correctly, and you’ll have reliable warmth from wood heat without installing an entirely new HVAC system. Mess it up and you’ve just got bad heat transfer or cracked heat exchangers, or no heat at all when you need it most.
Understanding hot water coils and their function
Hot water coils, sometimes called hydronic coils or plenum heaters, are the bridge between your outdoor wood boiler and your forced air system. They are comprised of banks of metal tubing with aluminum fins that sit behind the bumper, providing more surface area for heat dissipation.
With hot water from your boiler running through the coil, the furnace blower forces air over those fins. The air draws heat from the water and spreads it through your ductwork to each room in your home. It’s the same principle as a car radiator, just inverted: instead of cooling hot water with air, you are heating cold air with hot water. In the home, 3‑row or 4‑row coils are typically employed because they offer sufficient airside surface to reach a good heat exchange without creating excess back pressure on the blower.
What tools and materials are needed for installation?
You will also need standard HVAC tools and plumbing equipment to get this job done just the way you want. A hole saw on a drill takes care of any penetrations in the plenum, and then the coil is held in place with sheet metal screws.
1‑inch copper or PEX tubing (or whatever size of pipe your design specifies) plus the fittings and connectors for those plumbing connections. Don’t forget insulation on any exposed runs, and an air vent or manual bleeder valve for the high point on your lines. You will also need a fan‑center relay or aquastat to control when the blower runs, plus a thermometer to verify your output temperatures during testing. And, having everything set-up before starting a task declutters the job and minimizes frustration.
Select an EPA‑certified outdoor wood boiler or a high efficiency indoor wood burning stove with a water heating option to meet your hot water needs and amount of heat usageif adding or replacing one as part of this project. These regulations apply to new wood‑burning appliances, not to the design details of your ductwork or coil installation.
Safety precautions before starting the installation
Kill the power to your furnace at the breaker before you touch anything. Forced air systems have several electrical elements and pain will follow regardless of any efforts to work on them while they‘re alive.
If you have a gas furnace, turn off the gas supply valve, too. You'll be working inside the plenum near the heat exchanger and playing with gas lines or connections while the feeders are hot can be pretty dangerous. Ensure that your worksite is properly ventilated, in particular when soldering copper connections. Wear safety glasses when cutting, and gloves to protect your hands from the metal edges. This attention to detail helps you avoid accidents and stay on schedule.
Step‑by‑step guide to installing hot water coils
Start by measuring your supply plenum to confirm the coil will fit. Your plenum is the metal box’s actual location, and it sits right on top of your furnace where the heated air starts to travel through the supply duct.
Cut a hole in the plenum for access, as large as is necessary to be able to slide the coil inside. Most coils mount horizontally with the water connections extending through the plenum wall. Place the coil where air can pass through the fins without significant hindrance. Use sheet metal screws to attach it, and be sure to make it level and after that secure. The coil should be a tight fit and little to no opening around the edges so there is not bypass air. The coil is installed; seal any gaps around the coil with foil tape or high‑temperature sealant, allowing the heat exchanger air flow to be straight through, not bypassing it.
How do you connect the hot water supply and return lines?
Counter‑flow plumbing delivers the best performance. Counter‑flow the coil so the hot boiler water enters on the leaving‑air side of the coil and leaves from the entering‑air side in order to gain maximum heat transfer.
To keep up with flow, run 1‑inch copper or PEX (or whatever size pipe your load calculation requires). Size your circulator and piping for the BTU rating of the coil, and desired temperature drop; for most residential coils this is a mid‑single or high‑double digit GPM flow rate. For future maintenance, install shut‑off valves on both lines, and on the highest point of the piping include an automatic air vent or manual bleeder. In New Installations Air pockets are a number one enemy when it comes to poor heat transfer. Coat all thread connections with pipe thread sealant, or Teflon tape and support piping adequately to avoid coil fitting stress. And insulate exposed lines that travel through unheated spaces to reduce heat loss.
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Testing the system for leaks and proper functionality
Once everything is connected, slowly bring your boiler up to temperature and start circulating water through the coil. Inspect all connections and fittings for leaks while pressure is being built up in the system.
With the furnace blower on, check the temperature of air entering and exiting the coil with a thermometer. You should see a dramatic increase in temperature difference across the coil, typically on the order of 30-50°F depending on your water temperature and airflow. Look out for abnormal sounds that could mean there’s trapped air or the pump isn’t functioning properly. Leave the system running for a few hours with normal use to ensure everything is stable. If not, then bleed air vents and again check water flow rate and boiler temps.
Common installation mistakes to avoid
The biggest mistake is mounting the coil in the return plenum instead of the supply. When your backup gas or oil burner fires, it can overheat and crack your furnace heat exchanger if the coil is blocking or overly heating the return air path.
Undersized piping kills performance no matter how good your coil is. Do not try to save money by using 3/4‑inch lines when the system needs larger pipe; the cost difference is small and the performance impact is large. Skipping the air bleeder valve is another shortcut that costs you later. You will spend hours troubleshooting low heat output when the real problem is a simple air pocket at the top of the coil. And never forget to insulate your supply and return lines. Uninsulated pipes lose valuable heat before it ever reaches your living space.
Maintenance tips for hot water coils
Change your furnace filter regularly during heating season, often monthly in heavy use. A clogged filter obstructs airflow across the coil, which prevents heat transfer from taking place and causes your blower to strive harder than it should.
It is, therefore, a good idea to check the coil fins every year and straighten any that got bent when it was installed or cleaned. Even minor fin damage blocks airflow and drops efficiency. Occasionally check your water connections for signs of corrosion or weeping, and ensure that the air bleeder is not obstructed. Should you see a reduction in system performance, your water for signs of sediment or scale accumulation inside the coil tubing which could inhibit flow.
How do you troubleshoot common issues with hot water coils?
If you’re not getting heat, make the easy checks first. If you are sure that the furnace filter is not clogged, then make sure the water has a chance to actually flow through the coil by checking whether you feel a difference in temperature between supply and return.
One of the more typical reasons for lackluster performance is air that has been trapped in the coil. Continue bleeding the vents until you have a flow of water without bubbles. If one side of the coil is warm and the other cold, then you either have an airlock or a flow restriction. You can also limit the heat output with low boiler water temperature, so check to make sure your fire is fully lit and that your system’s temperatures are where they should be. Unusual sounds in the coil section usually signify pump cavitation or too much air in the lines. Deal with these problems early on so they don’t cause damage or leave you without heat.
Conclusion and final thoughts on installation
Installing a hot water coil for furnace systems opens up forced air heating to anyone running an outdoor wood boiler. The installation is a doable weekend project if you’re handy enough with basic tools and plumbing, and the payoff comes right away.
No more sacrificing forced air systems or radiant heat when you can enjoy the best of both. Right sizing, installation in the proper location in the supply plenum and adequate piping and flow, along with good numbering-that is what separates a system that works perfectly from one that makes you scream all winter. Install it patiently, according to the principles we reviewed, and test thoroughly before you button up. The time you spend in advance, is saved hours of troubleshooting and guarantees steady comfort when it counts the most.
For quality water‑to‑air coils, installation supplies, and expert guidance on integrating your outdoor wood boiler with forced air heating, visit us at OutdoorBoiler.com. We have been helping homeowners and installers build efficient dual‑fuel systems for years, and we are here to make sure yours delivers reliable warmth all season long.
