Properly Connecting an Outdoor Wood Boiler to an Indoor Furnace

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

There's gonna be a chapter one of how to do the install and then there's gonna be a chapter two of the problems that I see with improperly installed plate exchangers and improperly installs on indoor boilers.

So if you're having problems, go right immediately to chapter two and then come back to chapter one.

But for now, we're gonna be talking about how to do an install correctly on an indoor wood boiler.

Outdoor Wood Boiler Installation

So when you have an outdoor wood boiler it needs to be connected to the indoor boiler so it works seamlessly with the indoor boiler. And it is, again, so elementary.

Connecting and Outdoor Wood Boiler to an Indoor Furnace

 

Connecting and Outdoor Wood Boiler to an Indoor Furnace

So what you're gonna do is the outdoor wood boiler is gonna send water from the outdoor wood boiler to a plate exchanger that is gonna be inside next to the boiler.

 

Outdoor Wood Boiler Connected with an Indoor Plate Exchanger

How does the Connection System work in a Wood Boiler?

Now the reason why we need a plate exchanger is the outdoor wood boilers are usually under no pressure at all. They're open to the atmosphere. They have a field point and they have no pressure.

Why do I need to separate the Two Water Systems?

The boilers inside are generally pressurized systems and they're running around 15 to 20 psi. So we need to separate those two water systems based on that. And people are gonna ask, "Why do I need to separate and why can't I plumb it in?" So many factors actually say you can do it.

There's a lot of safety systems that are installed on the indoor boilers. There's expansion tanks, auto fill valves, and all the safety systems. Because when the boiler runs and it doesn't have enough water, it turns to steam.

And when you have steam and heat, something's gotta expand, something's gotta give. And technically, if you don't have enough water, you can actually make a bomb out of a boiler. That's why they have the auto fill valves, the pressure relief valves, and all the things that accommodate that pressurized system.

So that's why you should. It's very important. And I highly recommend that you do install a plate exchanger between these two water systems.

 

Outdoor Indoor Boiler Connection

So, from the outside boiler, you're gonna come into the plate exchanger, and then out of the plate exchanger, and return back to the wood stove. Basically, it's just that simple.

 

Boiler Split

Now, on the plate exchanger, which is drawn here, I have the water coming in here to the bottom port. And the plate exchangers are basically split this way. So the wood first is gonna come in here and it's gonna come out here. That's the one zone. Then the boiler, where I'm gonna show you, it comes in here and out here.

Plate Exchangers Dimensions

Now the plate exchangers are 5 by 12. One-inch ports are usually what's commonly used. A lot of people say, "Well, the one-inch port is gonna choke me down." And I understand that, too. You can find an inch and quarter plate exchangers. I do not have them myself, but they are up on the internet. I honestly believe they're not that important. And I'll go through that a little bit here in a minute.

Plate Exchanger Installation Instructions

So, anyways, hooking up your plate exchanger. Now, a lot of people say, "Well, where are you gonna hook it up in between your supply and return?" That is absolutely the worst thing you could do. You never wanna touch the supply side of a boiler, because whoever installed your boiler designed it to circulate water through its zones or its hot air systems or its tanks, domestic hot water tanks, boiler mates and things like that. So you don't wanna ever touch the supply side, okay?

Outdoor Wood Boiler Pumps

Now the pumps. If they're on the supply or the return for the zone, it's absolutely irrelevant also. It's not important for us to need to know that.

So, the absolute best place to install a plate exchanger hooking up to a wood boiler in that situation is you're gonna break the return side only. So in this case, the return side is gonna come in right here. You're gonna break this off and you're gonna send water through the plate exchanger that way.

 

The Return Side

The Indoor Boiler

Now, I'll go through how simple this is. So the indoor boiler is running roughly at about 175 degrees. The water comes out of its supply side. In this case, I drew a three-zone system. It could be five, it could be two, it could be one, it doesn't matter. So it's gonna go through and it's gonna come back in this zone at 155. The middle zone is about 145, and the outside zone is about 152 because it's dropping heat as it's going through the zone, hitting through your home.  

So if you take these two and you average 152, 155, 145, you get to 452. You add them up together and you're gonna divide it by 3, which is a number of zones that we have, and you're gonna get an average temperature of 150 degrees.

 

Average Temperature Return

So the water's gonna be coming back here at 150 degrees, and then coming into this plate exchanger. Now the wood boilers are generally running at about 180 degrees, the water coming back is about 150, and you're gonna lose about 20 degrees when it's trying to heat. So this is gonna be about 160. So in aspects, you've got a delta T of 130 or 30 degrees. That is a large enough delta T to have a good heat exchange rapidly.

So the water that's gonna be coming out of this plate exchanger is gonna be in the ballpark of about 175, to right back to where we left off. It's exactly what we want. That means the inner core boiler, this temperature is gonna remain at 175 degrees, and that's okay. That's what exactly what a boiler should be running, or 180.

But most of the outdoor boilers are running at 80. You're gonna lose five or six degrees in the swap of a plate exchanger. So 175, still heating well, and that's just fine. So now it's going back out again, doing it, and then recirculating again.

 

The Temperature Flow

Now people will say, "Well, how does it do it? How does it know to circulate?" Well, the zones are calling for heat, so when the thermostat on the wall calls for heat in that zone, it's gonna turn on the pump dedicated to that zone and circulate it. So, if only one zone is circulating, the water is gonna come back, in this case, at 155 and only heating up at that speed, which is usually 8 to 6 gallons a minute per zone is what the flow is per zone. It's six gpm per zone. So the most you could get out of this, if all 3 zones, in this case, are running, is 18 gallons per minute flow.

These plate exchangers with their 1-inch port can handle up 31. So that's why I don't see much reason for going to the inch and quarter ports. Yes, you're necking down the throttle of this manifold here but it doesn't usually matter, unless you're running multiple zones, in the case the multiplier is six, because that's usually one gallon per minute flow per zone is needed.

How Does the Boiler and Aquastat Work?

Basic Temperature Settings

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

 

Aquastat

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'll allow you a 10-degree differential.

So what you're gonna do is on the boiler inside, you're gonna turn down the boiler to roughly, let's say, 160, turn on at 150. Now, if the wood boiler is doing his job, this boiler will never drop into those ranges. So this boiler's ignition system and heating system, it's oil, propane, gas is never gonna ignite. So because the boiler is always at a temperature of about 175, which we already determined if the wood, for instance, is running and everything is doing its job.

But if the wood burner goes out and it runs out of heat or wood or whatever is going on, this system will slowly drop off, and this 160 will tell this unit to fire its oil, and the boiler will start to heat and it starts heating the house that way.

Yes, losing a couple of degrees in the zone is gonna make the zones run longer. So if it was normally running at 175, the zone, let's say, would hypothetically turn off for 20 minutes to get that room back up to its temperature that it wanted to. But when it's down to 160, 150, it's gonna take about 30 minutes, but it will get there, okay? Just slower. So that's how you do that and that's how you make it seamless.

So basically, you're gonna set the indoor boiler, temperature lower, than the outside boiler and it's staying...and it's gonna maintain. And once the wood boiler runs out of wood, it'll seamlessly turn on this system and keep on going. And that's basically how simple it is.

The only other thing that you might want to consider is during the spring and falls when you get those warm days, these zones aren't firing at all. The boiler will cool down on its own. And since no heat is being swapped out from the plate exchanger from the wood boiler because nothing's circulating, the boiler core temperature will drop and the boiler system will turn on itself to keep its core at 160, 150.

Frequent Problems

If that is a problem for you, there's another very simple solution. I'm just gonna cross this out. 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.

And in this case, you're gonna mount a simple aquastat, that straps on to the pipe, and then you will mount a small pump. This, most of the time is not necessary. So what will happen is if this is spring to fall when no heat is being circulated through the zones, the core temperature will turn on instead of the boiler turning on its oil or fuel source. It will sense that with the aquastat that the core boiler is cooling down. It'll tap the circulator to turn on. It'll circulate through that plate exchanger, getting heat back up, going back in here, heating up the core without turning on its fuel system, and if you wanna use the wood.

This is an option. You don't have to do this at the same time as you're installing your plate. Try out just the plate. And if this starts to bother you on those warmer days when you start to hear that oil fire furnace turn on every blue moon and that's bothering you, which you eventually will, I assure you, then address it. But if it doesn't bother you, leave it alone. It's simple and easy, just the way it is here.

Now I'm gonna go into chapter two and give you the reasons and the problems of not installing it this way and the things that might/will happen, or the people who are having troubles understanding or not understanding why their boilers, indoor boilers, are not keeping up and they used to years ago.

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