Underground Insulated Pipe for Outdoor Wood Boilers

An outdoor wood boiler heats water outside your house, then that hot water has to travel underground to reach your home, garage, pool, or wherever the heat is going. Bare PEX tubing buried in a trench would lose most of that heat before it ever arrived. An insulated boiler pipe solves that problem.

Our underground insulated pipe bundles supply and return PEX lines together inside a closed-cell foam jacket. The foam traps dead air around the tubing, which cuts heat loss and keeps the water hot on its way to the building. Most runs are buried 18 to 24 inches deep, shallow enough to stay above the water table in most soil conditions.

You'll choose between two PEX types depending on your system:

• Non-barrier PEX is for open-loop, non-pressurized systems. Outdoor wood boilers run open-loop, so this is what about 9 out of 10 systems will need.
• Oxygen barrier PEX is for pressurized systems. It blocks oxygen from diffusing through the pipe wall into the water, protecting steel and cast iron parts elsewhere in your system, like pumps, heat exchangers, and boiler components, from rust.

Even with ferrous metal parts in the system, an outdoor wood boiler will still need non-barrier PEX in most cases, since it's the open-loop setup that determines which pipe you need, not the metal alone. If you're unsure which one your setup needs, give us a call and we'll help you figure it out.

What Else Your Underground Boiler Pipe Needs

Once your underground line is buried and connected, a few other parts of the system come into play. You'll need PEX fittings to make the connections at the boiler and at the house, and a circulator pump to actually move water through the run. 

On the receiving end, a water-to-air heat exchanger distributes that heat once it reaches your furnace, or a plate heat exchanger if you have an indoor boiler or a domestic water tank. Water treatment keeps the water itself from rusting out your pipes and pump along the way. 

Underground Boiler Pipe FAQs

Should boiler pipes be insulated underground?

Yes. An uninsulated line buried outside will lose heat to the surrounding soil before the water reaches its destination, especially over longer runs or in cold climates. Insulated pipe keeps that heat where it belongs, inside the water.

How deep should an insulated boiler pipe be buried?

Most installations run 18 to 24 inches deep. That's typically enough to stay below the frost line in most regions while staying above the water table. Check your local frost depth if you're in an area with especially cold winters.

Can I use regular pipe wrap insulation instead of pre-insulated PEX?

Not for an underground run. Pipe wrap insulation isn't designed to handle direct burial or ground moisture, and it will break down and lose its insulating value over time once buried. Pre-insulated PEX is built with a waterproof, closed-cell foam jacket specifically for underground use.

Do I need oxygen barrier PEX or non-barrier PEX?

It comes down to pressurized versus non-pressurized systems. Outdoor wood boilers run non-pressurized, open-loop systems, so about 9 out of 10 will need non-barrier PEX. Oxygen barrier PEX is built for pressurized, closed-loop systems, which is less common in this setup unless you're tying into an indoor boiler or another pressurized loop.

How do I run PEX pipe from an outdoor wood boiler to my house?

The insulated PEX bundle runs from the boiler's trim-out kit, through a trench, to where it enters your home's mechanical room or connects to a heat exchanger. The trench should slope slightly toward the boiler to help with drainage, and the pipe should be laid on a stable base rather than directly on rocky or uneven soil.

What size PEX pipe do I need for an outdoor wood boiler?

1 inch pipe is the industry standard and what we recommend for most setups. If you have a very long run, typically 500 feet or more, 1-1/4 inch pipe helps maintain flow over that distance. Check your boiler's specs or give us a call if you're not sure which size fits your run.