How to Properly Connect an Outdoor Wood Boiler to an Indoor Boiler

In this blog post, you'll find out how to connect two boilers together. This is chapter one of two on how to do the installation of an indoor boiler. Find out how the system works and how to make the best choices during the installation.

 

 

Summary

This guide is divided into two parts. Chapter One focuses on how to properly install a plate exchanger and connect an indoor boiler system the right way. Chapter Two covers the most common issues caused by improper plate exchanger installations and indoor boiler setups.

If you are already experiencing performance or heating problems, it may be helpful to review Chapter Two first, then return to Chapter One for correct installation guidance. For now, this section focuses on best practices for installing an indoor wood boiler correctly.

Outdoor Wood Boiler Installation

When you have an outdoor wood boiler, it needs to be properly connected to the indoor boiler, so both systems operate together seamlessly. While the concept itself is straightforward, correct execution is essential for reliable performance and efficiency.

The Connection

In this setup, the outdoor wood boiler sends heated water to a plate heat exchanger located indoors near the boiler.

 

How Does the Connection System Work in a Wood Boiler?

Now, the reason why we need a plate exchanger is because the outdoor wood boilers are usually under no pressure at all. They're open to the atmosphere.

Why Do I Need to Separate the Two Water Systems?

Indoor boilers are typically pressurized systems operating at approximately 15 to 20 psi, which is why the indoor and outdoor water systems must be kept separate. A common question is why these systems cannot simply be plumbed together. The reason lies in safety and system design.

Indoor boilers are equipped with multiple safety components, including expansion tanks, automatic fill valves, and pressure relief valves. These systems are critical because if a boiler operates without sufficient water, the water can turn to steam. When steam and heat are present, pressure increases rapidly and something must give. Without proper safeguards, a boiler can become extremely dangerous.

This is why pressurized systems rely on dedicated safety controls to manage expansion and pressure. Installing a plate heat exchanger between the indoor and outdoor boiler systems allows heat to be transferred efficiently while keeping the two water systems isolated. For safety, reliability, and proper operation, using a plate heat exchanger is strongly recommended.

 

Water from the outdoor boiler flows into the plate heat exchanger and then exits the exchanger to return back to the wood boiler. The process is straightforward and keeps the system operating efficiently.

On the plate heat exchanger shown here, water enters through the lower port. Plate heat exchangers are internally divided so the two water circuits remain completely separate. In this setup, the outdoor wood boiler water enters through this port and exits through its corresponding outlet, forming one closed loop. The indoor boiler water flows through a separate path, entering through its designated inlet and exiting through its outlet. 

It’s important that these two water streams flow in opposite directions through the exchanger, as counterflow maximizes heat transfer efficiency. When both circuits flow in the same direction, heat transfer is reduced, which can lead to performance issues that some customers experience.

On the plate heat exchanger, shown here, water enters through the lower port. Plate heat exchangers are internally divided so that the two water circuits remain separate. In this case, the outdoor wood boiler water enters through the bottom left port and exits through the corresponding top left port, forming one zone.

The indoor boiler water flows through a separate path, entering through its designated inlet (bottom left port) and exiting through its outlet (top left port). Plate heat exchangers can be installed in any orientation, as long as the two water circuits remain completely separate. It is critical, however, that the indoor and outdoor water flows move in opposite directions through the exchanger, since counterflow provides the most efficient heat transfer while keeping the systems isolated.

Plate Exchangers Dimensions

The plate heat exchangers most commonly used are 5 by 12 inches in size. On our plate exchangers, units with fewer than 50 plates are equipped with one-inch ports, while models with more than 50 plates feature one-and-a-quarter-inch ports on the house side. Some people worry that one-inch ports may restrict flow, which is a reasonable concern. However, for most installations, exchangers under 50 plates perform well with one-inch connections, and larger port sizes are only needed as system demand increases.

In practice, one-inch ports provide sufficient flow for proper heat transfer, and the difference in performance is usually minimal. I’ll explain why this is the case in the next section.

Plate Exchanger Installation Instructions

When connecting a plate heat exchanger, a common question is where it should tie into the system between the supply and return lines. Connecting it to the supply side of the boiler is the worst possible option.

The supply side is designed by the original installer to properly circulate water through the boiler’s zones, hot air systems, domestic hot water tanks, and related components. Interfering with the supply side can disrupt that balance and negatively affect system performance. For this reason, the supply side of the boiler should never be altered.

As for the circulation pumps, whether they are located on the supply or return side of the zone isn’t really critical for this topic. That minor detail does not affect where the plate heat exchanger should be installed.

The best place to connect a plate heat exchanger when tying in a wood boiler is on the return side of the system. In this setup, the return line enters here, is separated, and then routed through the plate heat exchanger before continuing back into the system.

 

The Indoor Boiler

To illustrate how simple this is, consider an indoor boiler operating at approximately 175 degrees. Water leaves the boiler on the supply side and circulates through the system. In our example, a three-zone setup is shown, but the same concept applies whether there are one, two, five, or more zones.

As the water moves through each zone, it gives off heat to the home and returns at a lower temperature. In this case, one zone returns at about 155 degrees, another at approximately 145 degrees, and the third at around 152 degrees.

When you add these return temperatures together and divide by the number of zones, you get an average return temperature of roughly 150 degrees.

So the water's going to be coming back here at 150 degrees, and then coming into this plate exchanger. Now wood boilers are generally running at about 180 degrees, the water coming back is about 150. So in aspects, you've got a delta T (difference between the two temperatures) of 30 degrees. That is a large enough delta T to have a good heat exchange rapidly.

As a result, the water leaving the plate heat exchanger, going back into the water tank, will be roughly to 175 degrees, which brings the system right back to where it started. This is precisely what we want. The indoor boiler can maintain its operating temperature at around 175 degrees, which is well within the normal and proper range for a boiler, therefore not kicking on your conventional heating system.

A common question we get is how the system knows when to circulate. The answer is that circulation is driven by the heating zones. When a thermostat calls for heat, the pump dedicated to that zone turns on and begins circulating water.

If only one zone is calling for heat, the return water in this example comes back at about 155 degrees and flows at approximately six gallons per minute, which is typical for a single zone. When multiple zones are operating, the total flow increases accordingly. In a three-zone system, the maximum combined flow would be about 18 gallons per minute.

Plate heat exchangers with one-inch ports are capable of handling flows of up to approximately 31 gallons per minute. This is why moving to inch-and-a-quarter ports is usually unnecessary. While the manifold does reduce to a smaller diameter at the exchanger, it generally does not create a problem unless the system includes a significantly higher number of zones, where total flow demand becomes much greater.

How Does the Boiler and Aquastat Work?

Basic Temperature Settings

Now, people say, "Well, how does it work?" The boiler has its own aquastat built in, and that's usually set to 180 as a high, and a low of, like, 170. It's a simple little aquastat like this.

 

The aquastats that are mounted on the indoor boilers are usually a little bit more square than a rectangle here, but they're fundamentally the same. They have a little probe that monitors the temperature of the inside of the boiler, and they have a little gauge which tells you the temperature that you would want it set at the high limit, referring to 180 in the case. And then inside, there's a little paddle where it will allow you a 10-degree differential.

On the indoor boiler, you would lower the temperature settings to roughly 160 degrees, with the boiler turning on at about 150 degrees. If the outdoor wood boiler is operating properly, the indoor boiler will never reach those lower temperatures. In that situation, the indoor boiler’s ignition system (oil, propane, or gas) will not fire, because the system temperature remains around 175 degrees, as previously established.

If the wood boiler runs out of fuel or stops producing heat, the system temperature will gradually fall. Once it reaches the 160-degree setpoint, the indoor boiler will automatically fire and take over heating the home. This ensures continuous heat without manual intervention.

Running at slightly lower temperatures does mean the zones may operate longer. For example, if the system normally runs at 175 degrees, a zone might satisfy the thermostat in about 20 minutes. At 160 to 150 degrees, that same zone may take closer to 30 minutes. The space will still reach the desired temperature—it simply takes a bit longer. This is how the transition between heat sources remains seamless.

In practice, the indoor boiler is set to a lower temperature than the outdoor wood boiler and remains idle as long as the wood boiler is supplying heat. Once the wood boiler can no longer maintain temperature, the indoor boiler automatically takes over without interruption.

One additional consideration is during spring and fall, when warmer days may prevent zones from calling for heat. In these conditions, the boiler can cool down naturally. Since no heat is being transferred through the plate exchanger when circulation stops, the boiler’s core temperature may drop, causing the indoor boiler to fire briefly to maintain its minimum setpoint of around 150 to 160 degrees.

Frequent Problems

If that is a problem for you, there's another very simple solution. On the back of a boiler, indoor boilers, there's always a pressure relief valve, the safety systems that I was talking about. You can simply unscrew that pressure relief valve and put a T, put the pressure relief back on one side of the T, and on the other side, you can run small zone and you can tap back in just above the plate exchanger.

In this setup, a simple strap-on aquastat can be installed on the pipe along with a small circulator pump. In most cases, this additional pump is not strictly necessary, but it can be useful in certain situations.

During the spring and fall, when the heating zones are not calling for heat, water is not circulating through the system and the boiler’s core temperature can slowly drop. Instead of allowing the boiler to fire its oil, gas, or propane burner, the aquastat senses the temperature drop and signals the circulator to turn on. The circulator moves water through the plate heat exchanger, pulling heat from the outdoor wood boiler and sending it back to warm the boiler core without engaging the fuel system. This allows continued use of wood heat even during mild weather.

NOTE: This feature is optional and does not need to be installed at the same time as the plate heat exchanger. It is often best to start with just the plate exchanger and see how the system performs. If, on warmer days, the occasional firing of the indoor boiler becomes noticeable or bothersome, this option can be added later. If it does not cause concern, the system can be left as is. The setup remains simple, effective, and easy to maintain.

Trust the ALT Heat Team

In the second chapter, our team will explain the reasons for installing the system this way and the problems that can occur when it isn’t done correctly. We’ll also cover common issues experienced by people who are having trouble understanding why their indoor boilers are no longer keeping up the way they did in the past.

If you want any other information, we'll be glad to help you get through this process seamlessly and easily. Contact us now!