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Installing Your Outdoor Wood Furnace

   At Pineview Woodstoves, we offer complete installation, including delivery and trenching. We deliver and set the unit with our hoop trailer. The customer is responsible for preparing the place where the unit is to be set. Cement blocks, paving stones, or a small slab can be pressed into service as a pad on which to set the unit. Just make sure it's level. Unit foot print information is available upon request. We hire a third party with a power trencher to do the trenching, and we lay the line. We can connect your outdoor wood boiler to almost any existing heating system, including forced air, radiant in floor heat, radiators, or hydronic base boards. We can also connect to your hot tub, swimming pool, or domestic water heater. Contact us for a free installation estimate.

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I. General Installation Information - Before You Start

   A. Pump Placement - Back of Boiler vs. In Your Building

   B. Minimum Water Flow Rates

   C. Air Eliminators (Air Bleeders/vents)

   D. Order of Operations - Should Your Lines Go First to Your Water Heater or Your Heating System?

   E. Mix Valves

II.Heat Loss Calculation - Size Your Outdoor Wood Boiler

   A. Wall Heat Loss Calculations

   B. Window Heat Loss Calculations

   C. Door Heat Loss Calculations

   D. Ceiling Heat Loss Calculations

   E. Floor Heat Loss Calculations

   F. Air Leaks

III. Pipe and Pump Sizing - What Size of Pump Does Your Outdoor Wood Furnace Need?

   A. Choosing the Right Size of Pipe

   B. Calculating Pressure Drop

   C. Sizing the Pump

IV. Heating Your Domestic Hot Water

   A. Plumbing In Your Plate Exchanger

V. Illustrations

   A. Air Handler Installation Illustration

   B. Unit Heater Installation Diagram

   C. Backup Electric Boiler Installation Diagram (Turn on Manually)

   D. Backup Electric Boiler Installation Diagram (Automated)

   E. Backup Boiler in Pressurized System Diagram

   F. Domestic Hot Water Heating With Plate Exchanger Diagram

   G. Flushing Your Plate Exchanger - Diagram

   H. Domestic Water Heating - Side Arm Diagram

   I. Radiator in a Forced Air Furnace Diagram

   J. Radiator in a Forced Air Furnace + Domestic Water Heating Diagram

   K. Workshop Heating - Radiant Floor Heat & Fan/Coil Unit Heater Diagram

   L. Slab Heating - Injection Mixing - Diagram

   M. Slab Heating - Thermostatic 3 Way Mixing Valve - Diagram

   N. Staple-up Radiant Floor Heat with Slab Heating & Domestic Water Heating

VI. Glossary of Outdoor Wood Boiler Installation Terms

Before You Start

This outdoor wood boiler installation guide is intended to be exactly what it is, a guide. Always be certain your installation abides by the local codes and regulations of the governing bodies for your location. If you are unsure of anything presented in this guide don’t hesitate to contact your local dealer or the manufacturer for further assistance.

General Practices

Pump Placement

In most cases, the best place for the pump is in the weather proof enclosure at the outdoor furnace. Is your outdoor furnace higher or lower than where you need to send the main supply line to your building? If the bottom of the outdoor furnace is lower than the entry point of the supply line to the building, the pump should always be located in the weather proof enclosure at the outdoor furnace. If the bottom of the furnace is higher than the entry point of the supply line to the building, then the best place for the pump is most often in the weather proof enclosure at the outdoor furnace. In this case you may also locate the pump in the building being heated if the layout falls within the following criteria. In an open system, you have to be sure to maintain as much pressure as possible on the intake of the circulation pump. Any piping on the intake side of the pump creates a certain amount of pressure drop. A simple guide for typical systems is if you have less than 7’ of drop per 100’ of supply piping to the potential pump location in the building, the pump should ideally be at the outdoor furnace. If the drop is more than 7’ per 100’, the pump could be effectively located in the building. Take note that, when in a building, the pump is ALWAYS located in the hot supply line and ALWAYS at the earliest possible location in building. Remember! ALWAYS install isolation valves on both sides of a circulation pump. Pumps won’t last forever and if you need to service one you don’t want to have to drain down a lot of piping in order to remove/repair the pump.

Minimum Flow Rates

An outdoor furnace has a required minimum flow rate that must be circulating at all times. This minimum flow rate keeps the fluid from “stratifying”. The hottest fluid, being less dense, rises to the highest point in the water jacket. Without sufficient flow this fluid is heated past the safety limit setting on the furnace and often the high limit switch will cut the power until the temperature has cooled sufficiently. The minimum flow rate ensures that the fluid in the furnace is properly mixed for a relatively even temperature throughout the water jacket. This allows the controls to sense an accurate fluid temperature and provides the best heat transfer and distribution to the connected buildings. The amount of flow will depend on the model of furnace. The HeatMaster SS “G-Series” furnace minimum flow rates are listed here. G100 - 8 gpm G200 - 14 gpm G400 - 30 gpm A rule of thumb is to target a 20-30 degree fahrenheit temperature drop (also called the ”Delta T”) across the furnace at it’s maximum heat output. To maintain a 20 degree drop a furnace with a rated output of 100,000 BTU’s per hour would require 10 gpm. To calculate this use the flowing formula. GPM = BTU / Delta T / 500 Where: GPM = The water flow required in US Gallons per minute BTU = Furnace maximum output in BTU’s per hour. Delta T = The water temperature drop desired. Typically between 20 and 30 F. for an outdoor furnace. 500 = This is a constant number for water. if you are using a glycol mixture use 470 for a 50/50 mix. Be sure to properly size the piping and pumps to deliver the required minimum flow for the furnace. If the total flow supplying your buildings does not meet the requirements it is necessary to pipe a by-pass loop at the back of the furnace. Essentially this involves installing an additional pump which pulls water from a hot supply connection and returns it directly to a cold return connection. This pump and pipe must be sized to deliver enough flow to bring the total flow rate of all the circuits up to the minimum flow rate. For information on sizing pumps and piping please refer to the “Pump Sizing” Section in this guide. An example of a by-pass loop is shown below.

Air Vents (or air eliminators)

Automatic and Manual Air Vents are two typical types used. Air is always an enemy in any hydronic heating system, but even more so in an open system. The location of the air eliminators in the heating system is critical in how effective, or hindering, they may be. A properly placed air vent should allow for quick and simple removal of the air upon initial commissioning of the system and for easy inspection or service at a later date. Typically an air vent is located wherever the fluid in the system flows horizontally and then turns to go down. At that point use a tee in place of an elbow and install the air vent on the top of the tee. Should an air vent ever be installed on the intake side of a pump? If the pump is located at the outdoor furnace then there should be no need for an air vent on the intake of the pump. The piping should be simply routed from the connection at the furnace down or horizontally into the pump. If the pump is in the building it should be positioned so that, if at all possible, there are no air entrapment points in the piping before the pump. If this cannot be avoided, then an air vent may be installed at the air entrapment point on the intake side of the pump if the location of the vent is at least two feet lower than the water level in the outdoor furnace. This vent should ALWAYS be a manual type vent and only be opened to release air when the pump is OFF. If this vent is opened when the pump is on, it may draw air in through the vent and add to the air problems in your system.

Order of Operations

When servicing more than one heating load in a system, the order you supply each demand is quite important. The reason for this is that, after supplying each load in a primary/secondary or series piping system, the temperature of the heating fluid in the primary loop will drop. When a heating system is designed it is important that this temperature drop be taken into account in order for each component in the system to meet its demand. The typical order is as follows:

   1) Domestic water heat exchanger. This may be a brazed plate heat exchanger, a shell & coil heat exchanger, or an indirect hot water tank. Typical design temperature required is 160 F. to 180 F.

   2) Hot water baseboards. Finned copper tube design. Typical design temperature required is 140 F. to 180 F.

3) Radiator or Fan/Coil Unit. A radiator installed in the plenum of a forced air furnace or a fan unit with a radiator built in. Typical design temperature required is 140 F. to 180 F.

   4) Staple-up floor heating. A floor heat system that is hung with clips or transfer plates to the underside of a floor, in a wall, or even a ceiling. In this method the piping radiates its heat through the air surrounding the piping and then into the room through the floor, wall, or ceiling. Aluminum heat transfer plates may also be used in this method to boost the performance in high heat loss areas. Typical design temperature required is 120 F. to 160 F.

   5) Pools or Hot Tubs. A special stainless steel or titanium heat exchanger may be used to heat the pool or hot tub water. Typical design temperature required is 120 F. to 180 F.

   6) Embedded Floor Heating. A piping system embedded in a concrete floor such as a basement, garage, or workshop. A floor covered with a topping pour of gyp-crete or concrete would also fall into this category. Typical design temperature required is 80 F. to 130 F.

   7) Snow Melt. A piping system designed to melt and evaporate snow and ice from outdoor areas such a sidewalks, driveways, or decks. This piping may be embedded in concrete or hung in staple up fashion depending on the application. Typical design temperature required is 40 F. to 80 F.

When designed properly, this enables the maximum amount of heat to be extracted from a minimal amount of flow from the outdoor furnace. Less piping, smaller piping, smaller pumps and lower heat loss. That translates into money saved in both initial set-up and long term operating costs.

Mixing - Supplying Low Temperature Water From a High Temperature Boiler

     If we look at the last two items in the Order of Operations list above we see that the water temperature required for slab heating a basement, workshop, or snow melt area is significantly lower than what we generate from our outdoor furnace. We need to cool that water down before we send it into the slab. One way to do this is to take heat off the water in other areas before we supply the floor as laid out in the Order of Operations. But what if those heat loads are satisfied and are not taking any, or enough, heat off the water? We need to be sure the water temperature going to these slabs is carefully controlled or several problems can result. A slab of concrete is basically a HUGE storage tank that slowly releases it’s heat to the area around it. If we have floor heat in our workshop and our thermostat calls for heat and our pump starts feeding 160 F. water into our slab, what will happen? Very little, for awhile. Concrete is heavy and it takes a long time to warm that mass up even a few degrees. The conventional thermostat may call for heat for an hour or so before the floor has warmed up and heated the room to the point where the thermostat is satisfied. Now what? The thermostat turns off and the cycle repeats itself, right? Wrong. If we have been feeding 160 F. water into our slab for an hour we will now have a LOT of heat stored in the concrete that will continue to radiate into the room until the slab cools off. This can cause the temperature to overshoot our thermostat set point by several degrees making the room uncomfortably hot. Floor heat does not only warm up the air in the room but everything in the room as well. These objects, and the building structure itself, act as another heat storage mass. These objects slowly release their heat to the room as the building cools down and this can keep the temperature above the thermostat set point for another period of time. All this time the slab has given up it’s heat to the building as well as loosing some to the ground. Now our thermostat calls for heat again but the floor has been off for so long that it has lost a significant amount of temperature and it will have to run for a lengthy period of time to start contributing heat to the room. In the mean time the building continues to loose heat and may actually drop slightly below the the thermostat set point causing things to get a little cool in the room. Now the cycle repeats itself. This is only one of the adverse effects of supplying water that is too hot to a floor. Floor coverings may also be damaged as a result of this excessive temperature. Hardwood floors can dry out, shrink, and crack. Carpets can loosen and concrete can crack. People’s feet will get too warm causing sweating and fatigue. Needless to say it is very important to control the water temperature going into a floor. Can you control the temperature by just slowing the flow by closing a valve a little? The water will come out of the floor cool but it causes uneven heating across the floor. The first part of the loop will be excessively hot and the last part of the loop may not be hot enough. Controlling the fluid flow is not nearly as effective as controlling the temperature. We need to keep the flow up to the proper rate to provide even distribution of the heat across the floor and proper conduction from the water through the pipe. There are several ways to accomplish this, two methods we will look at are using Thermostatic 3 Way Mixing Valves or Injection Mixing.

Thermostatic 3 Way Mixing Valves

Thermostatic 3 Way Mixing Valves are basically what they sound like. A valve with three ports, Hot, Cold, and Mix. Use the “Slab Heating - Mixing Valve” illustration to follow along with this description. Most valves are adjustable from 80 to 150 F. by turning the “head” of the valve. The Hot port is teed into your primary loop coming from your outdoor furnace. The Mix port goes to your floor heat pump and then to your supply manifold feeding the floor. The return manifold from the floor gets teed back into the primary loop down stream of the first tee. The Cold port on the valve gets teed in-between the return manifold and the tee going back into the primary loop. These valves work excellent for basements, garages, and smaller workshops as they are designed for fairly low flow. Once you need more than 4 or 5 gpm you should look at injection mixing.

Injection Mixing

Injection Mixing is a technique that works beautifully for any system from a house to an industrial building. The basic costs tend to be higher for this type of system but there are many added benefits. Use the “Shop Heating - Injection Mixing” illustration to follow along with this description. The primary loop is circulated by the pump at the outdoor furnace and the injection loop is teed into it. The floor heat loop is circulated by a second pump. The injection pump pulls high temperature water off the primary loop and blends it into the floor heat loop. The injection pump is controlled by an injection mixing controller which speeds up or slows down the pump to maintain the desired water temperature in the floor heat loop. When the room thermostat calls for heat it activates the injection controller. In the illustration you see the controller sensor on the pipe downstream of the floor heat pump. There is also a sensor on the primary loop pipe just before the first injection tee. The controller is programmed to supply either a constant water temperature to the floor loop or an Outdoor Reset temperature which changes depending on the outdoor air temperature. Most controller manufacturers allow you to use a standard wet rotor circulating pump up to a certain horse power as the injection pump. This is very handy as they are often the same pumps used in the rest of the system. A rule of thumb for sizing injection pumps is that they need to supply approximately 1/3 of the flow rate of the floor heat pump in a typical concrete floor application with a primary loop temperature of 160 to 180 F. If your floor heat loop is circulating at 9 gpm your injection pump would need to provide 3 gpm at 160 to 180 F.. The injection pump pushes the 3 gpm of high temperature water into the floor loop and displaces 3 gpm of cold return water back in the primary loop. This cold water gets mixed with the high temperature water in the primary loop and is pumped back to the outdoor furnace to be reheated. The primary loop must be circulating at a flow rate high enough that you have an acceptable water temperature returning to your outdoor furnace.

Heat Loss Calculations

In order to determine the size of the outdoor furnace, supply piping, and pump, a heat loss calculation should be done for each building to be serviced. To be precise, these calculations should be done by trained technicians, but for rough calculations, a simplified method is shown here.

   To start you need to know some basic information about your building and climate conditions.


     - R-values of walls, ceiling, floor, windows, and doors.

     - Area of the above items in square feet.

     - Quality of construction (How drafty is the building?)


     - The outdoor “design” temperature for the building location. This number can usually be found by obtaining local weather data for your area on the internet.

   Let’s use an example to illustrate this calculation.


     Gary would like to install an outdoor furnace to heat his house, attached car garage, and work shop. He needs to know the heat load of his buildings in order to decide what size of furnace to purchase.

   Starting with the Work Shop:

     Gary’s shop is 40’ x 60’ with an 18’ high ceiling. The walls are insulated to an R-20 value and the ceiling to R-40. He heats the shop with radiant floor heat and has insulated under the slab to an R-5 value. He has double pane windows rated at approximately R-2 and his doors are about R-10. Gary lives near Minneapolis, MN. where the outdoor design temperature is roughly -16 F. and he would like to keep his shop about 65 F.

     Wall Area: 200’ perimeter x 18’ high = 3600 square feet

     Windows: 3 Windows at 4’ x 6’ each = 72 square feet

     Main Door: 1 at 3’ x 7’ = 21 square feet

     Overhead Door: 1 at 16’ x 16’ = 256 square feet

     Ceiling: 40’ x 60’ = 2400 square feet

     Floor Area: 40’ x 60’ = 2400 square feet


     Formula is:

     Q = A x Delta T x U


      Q = Heat Loss in BTU’s / hour

     A = Surface Area Delta T = The difference between the desired indoor temperature ( in degrees F.) and the outdoor design   temperature (in degrees F.)

     U = 1 divided by the R-Value of the wall, ceiling, floor, window, or door..

Wall Calculation

U = 1 divided by 20 (his wall R-Value)
U = .05
A = Wall area - window and door area
A = 3600 - (72+21+256)
A = 3251
Delta T = Desired indoor temperature - Outdoor Design Temperature
Delta T = 65 - (-16)
Delta T = 81
Q = U x A x Delta T
Q = 3251 x 81 x .05
Q = 13166
Wall Heat Loss = 13166 BTU’s per hour

Window Calculation

U = 1 divided by 2 (his window R-Value, approximately R-1 per pane of glass)
U = .5
A = Window area

A = 72
Delta T = Same as the wall
Delta T = 81
Q = U x A x Delta T
Q = 72 x 81 x .5
Q = 2916
Window Heat Loss = 2916 BTU’s per hour

Door Calculation

U = 1 divided by 10 (his door R-Value)
U = .1
A = Door Area (Overhead Door + Man Door)
A = 277
Delta T = Same as the wall
Delta T = 81
Q = U x A x Delta T
Q = 277 x 811 x .1
Q = 2244
Door Heat Loss = 2244 BTU’s per hour

Ceiling Calculation

U = 1 divided by 40 (his ceiling R-Value)
U = .025
A = Ceiling Area (40’x 60’)
A = 2400
Delta T = Same as the wall
Delta T = 81

Q = U x A x Delta T
Q = .025 x 2400 x 81
Q = 4860
Ceiling Heat Loss = 4860 BTU’s per hour

Floor Calculation

U = 1 divided by 10 (his insulation R-Value under his floor)
U = .1
A = Floor Area (40’ x 60’)
A = 2400
Delta T:
Ground temperature is fairly constant in most areas at about 45 F. Slab
temperature for a shop like this should be about 77 F at outdoor design
temperatures. Water table levels and soil types can change the floor heat loss
dramatically. In this case we will assume Gary has a water table at roughly 8 ft
below the floor and has heavy clay soil. If the level were to be much lower and soil
type gravel or sand, divide the Q value by 2 for your Total Floor Heat Loss.
Delta T = 77 (slab temperature) - 45 (ground temperature)
Delta T = 32

Q = U x A x Delta T
Q = .1 x 2400 x 32
Q = 7680
Floor Heat Loss = 7680 BTU’s Hour

Infiltration (Building air leaks)

Gary’s shop is well built with vapor barrier in the walls and good seals on the doors and windows. His shop may exchange about half of its air volume every hour. In a poorly built/maintained shop that number can easily double or triple. To calculate how much heat he is loosing through infiltration we use this formula:

Q = (V / 60) x IR x Delta T x 1.068
Q = Heat loss in BTU’s per hour
V = Building Air Volume (length x width x height)
IR = Infiltration Rate
Delta T = The difference between the desired indoor temperature ( in degrees F.)
and the outdoor design temperature (in degrees F.)

Gary’s Infiltration Calculation:

V = Shop air volume (60’ x 40’ x 18’)
V = 43200
IR = .5 (Gary’s shop changes half of it’s air every hour)
Delta T = Desired indoor temperature - Outdoor Design Temperature
Delta T = 65 - (-16)
Delta T = 81
Q = (V / 60) x IR x Delta T x 1.068
Q = (43200 / 60) x .5 x 81 x 1.068
Q = 31143
Infiltration Heat Loss = 31143 BTU’s per hour.

Gary’s total shop heat loss is the sum of all the totals:
Walls - 13166
Windows - 2916
Doors - 2244
Ceiling - 4860
Floor - 7680
Infiltration - 31143
Total Shop Heat Loss - 62009 BTU’s per hour at Outdoor Design Temperature.

This calculation changes dramatically based on how the area is heated. Gary’s shop is heated from the floor which keeps the air temperature at the ceiling very close to the air temperature at the floor. If his shop is heated with a radiator and fan unit heater the figures would change considerably. We would loose less heat from the floor but considerably more heat from the walls, ceiling, and overhead door due to the high air temperatures in the upper part of the building. In that case, if the thermostat was set for 65 F the ceiling temperature in this shop could be 75 to 85 F. This factor combined with the added heat loss from the air turbulence created by fans can increase the overall building heat loss by 30 to 70% over the same building with radiant floor heat.

Pipe & Pump Sizing

Properly sized piping and pumps are necessary to supply adequate heat to a building. Once you have completed your building heat loss calculation you can size the pipe and pump to supply the heat. There are a couple pieces of information necessary to do this with success. You will need:

- A pressure drop chart for the piping you want to use
- A pump performance chart from your pump manufacturer

Let’s build on the heat loss calculation we used for Gary’s shop to illustrate this process. Gary needs to run pipe underground from his outdoor furnace to the shop to supply the heat. His outdoor furnace is 80’ from the shop and by the time he gets from the connection area at the back of the furnace to the floor heat manifold area in the shop he will need 100’ of pipe each way. Gary is going to be using insulated Kitec piping to accomplish this task and has acquired this pressure drop chart showing the flow specifications for the pipe.

Outdoor Wood Furnace Pump Sizing

The formula to use here is this:
GPM = BTU / Delta T / 500
GPM = The water flow required in US Gallons per minute
BTU = Building Heat Loss
Delta T = The water temperature drop desired. Typically between 20 and 40 F. for
an outdoor furnace.
500 = This is a constant number for water. if you are using a glycol mixture use
470 for a 50/50 mix.
Gary is targeting a 30 F. Delta T for this circuit which is acceptable for both the
outdoor furnace as well as the radiant floor heating system in his shop.
Gary’s Flow Rate Calculation looks like this:
GPM = BTU / DeltaT / 500
GPM = 62000 / 30 / 500
GPM = 4.13

Gary needs 4.13 gpm to deliver the amount of heat his shop needs at design
conditions and not allow the return water temperature to be more than 30 F. less
than the supply water temperature.

Choosing the Right Size of Pipe

When selecting the size of pipe it is important not to go too small or, in some cases, too big. It is best to target between 2 and 4 feet per second velocity for these primary lines feeding a building. If your velocity is too high it causes excessive friction between the water and the pipe which also increases the size of pump required to deliver the amount of water you need. This higher friction can, in some extreme cases, cause the pipe to erode and wear out. If the pipe is too big your water velocity drops and you may have trouble getting the air out of the system on start up as the water will be moving too slow to purge the air. Looking at the chart, the 1” diameter pipe has a velocity of 1.53 ft/s at 4 gpm. This would still work but it may be a little tough to flush the air out. The 3/4” pipe has a velocity of 2.52 ft/s and would be well suited for the requirements here.

Calculating Pressure Drop

We need to know the total amount of head (or pressure drop) this whole loop will create in order to size a pump. We know Gary needs 100’ of pipe each way to go to and from his shop, so that makes 200’. If we look again at the pipe chart for 3/4” pipe we see that there is 1.28 psi drop for every 100’ of pipe at 4 gpm. If we have 200’ of pipe we have 2.56 psi pressure drop from the pump at the outdoor furnace to the “cold” connection at the outdoor furnace. We need to allow for some friction for fittings and valves in the loop as well so we will add 10% to the pipe loss for a total of 2.82 psi. If we look at the pump chart below you’ll notice that they measure pressure drop in “feet of head”. In order to obtain this unit of measure, take your psi and multiply it by 2.31. Gary has 2.82 psi x 2.31 = 6.5 feet of head.

Sizing the Pump

Now we know what size of pipe we are using and how much water we need to carry so we can start the process of sizing the pump.

Outdoor Wood Furnace Pump Chart

We need a pump that can produce 4.13 gpm at 6.5 feet of head. Now the chart above shows several models of pumps but many of the smaller ones are not designed for this application. We will look at the models 007 and 008. We need to plot the point on the chart where our flow rate intersects our pressure drop in feet of head. On the bottom of the chart is gpm so draw a line straight up from approximately 4 gpm. Now from the left side draw a line horizontally from roughly 6.5 feet of head. Where your two lines intersect is your pump target. In order for the pump to be able to satisfy your demand your pump target point must be under the line shown as the pump curve. If we look at the curve of a 007 pump it can make up to about 11 feet of head at zero flow, and it can move up to 23 gpm at zero head. If we were to need 10 gpm at 10 feet of head the 007 pump would not be able to do it, we are outside the pump’s curve. We need only 4 gpm at 6.5 feet of head so the 007 would easily do the job. We could also use the 008 and have the potential to overcome more head if necessary. When choosing a pump you want to be big enough, but not too big. If you were to use a 0013 on Gary’s loop you would be wasting energy running a bigger motor and possibly pushing our flow rate higher than our safe zone of 4 ft/s. In Gary’s system his actual flow rate will be higher than 4 gpm as the pump will always push as much water as it is able through the loop. As the flow rate increases so does the pressure drop (feet of head) and so here we may actually get 6 or 7 gpm through the loop which only means that our water will come back warmer to the outdoor furnace.


One other thing to keep in mind here is how high you need to lift the water in the piping loop. If your piping goes higher than the water level in the outdoor furnace you need to add one foot of head for every foot your pipe is higher than the water level in the furnace. This is only needed for filling the system as once the pipe is full the weight of the water in the pipe going down offsets the extra push needed to lift the water up. If we had a unit heater up at the ceiling that was 15’ higher than the water level in the furnace we would never get the water up there with our 007 pump. A common misconception is that if your piping goes higher than the expansion vent on your outdoor furnace the water will run out of the top of your expansion vent. This can happen, but is very easy to prevent. If we have a unit heater 15’ higher than the expansion vent on the outdoor furnace we would normally install an air vent at the highest point in the piping where the water turns to go down. If our pump is sized properly we should be able to close the valve on the return line and, with the pump running, open the manual air vent and purge any air that has collected there. If the pump turns off and the air vent is closed the water will “hang” in the system and there will be a negative pressure in all the piping that is higher that the water level in the furnace. If the air vent was then opened air would suck into the vent and allow the water to run back into the furnace. If the furnace was right full, the water would push out of the expansion vent on the furnace.

Domestic Water Heating

Allowing your outdoor furnace to heat your domestic hot water is just one more way of cutting back on your energy costs. These components often pay for themselves faster than any other part of the heating system. Brazed plate, or shell and coil, heat exchangers are compact, safe, and offer very high heat transfer rates. There are a few things to consider before incorporating one of these units into your domestic water circuit. a) What type of fluid do you have in your outdoor furnace loop? If it is straight water, or a non-toxic glycol, you’re in good shape. If you are using any other type of antifreeze (automotive or ethylene based glycol's) or any type of additives that may be harmful for human consumption you need to make some changes. Although heat exchangers are designed to keep your heating fluid and your domestic water separate, a leak is still possible. As unlikely as it is, especially when using an outdoor furnace in an open system, a leak could cause your heating fluid to mix with your domestic water. If you are using the wrong fluid this can cause harm to humans or animals that consume this domestic water. b) Do you have “hard” water? If you have trouble with excessive mineral deposits on your faucets and other plumbing fixtures you may also run into troubles with build up in your plate heat exchanger. The installation diagram shows flushing ports for this purpose but you don’t want to have to do this very often as it does involve extra time and equipment. You may want to look into a filter or water softener to help make this option more user friendly.

Piping a plate heat exchanger for heating domestic water

The plate heat exchanger will normally be the first component in the primary loop after the pump. It is important to mount the heat exchanger so the longest side is vertical to allow the air to escape without trouble. When connecting the piping make sure the heating fluid and the domestic water are flowing opposite directions through the heat exchanger. This is indicated in the diagrams by the arrows on the unit. When possible, allow the heating fluid side to pump up through the plate and the domestic water to flow down. The domestic system operates at a higher pressure and has an easier time flushing the air down and out of the plates. On the domestic side the heat exchanger is piped in series with the hot water tank.

In Operation (See “Plate Flushing Diagram”)

When using your outdoor boiler the ball valves 7A and 7B should be OPEN. Valve 7C, between the two tees, should be CLOSED. This will force the domestic water through the heat exchanger before it enters the hot water tank. When operating properly, the water should leave the heat exchanger at a temperature higher than the hot water tank set temperature for the elements or burner. The hot water tank should not need to fire unless there is no water usage for an extended period of time. In this case the tank will slowly loose its heat to the room and the tank will fire to maintain a desirable temperature and be ready for use at any time. If you need to by-pass the heat exchanger on the domestic side you may close valve 7A or 7B and open valve 7C. Do NOT close both 7A and 7B. This can cause excessive pressure build up in the plate heat exchanger which may lead to premature failure.

Flushing the heat exchanger

When you notice poor temperature performance from the plate heat exchanger it may be caused by excessive scale (mineral deposits) on the plates in the heat exchanger. In this case the domestic side of the unit may be flushed with a de-scaler to remove these deposits. Consult the heat exchanger manufacturer for the proper solution used for this purpose. A small “pony” pump, three short (6’ to 8’) pieces of garden hose, and a 5 gallon pail work well for this project. Some companies also manufacture handy “flushing carts” with all this equipment ready to go.

Flushing the Heat Exchanger

Please refer to “Plate Flushing Diagram”
1 - Before flushing, close ball valves 7A, 7B, & 7C.
2 - Drain the water in the heat exchanger by opening the sediment faucets 5A and
3 - Fill the pail approximately 1/2 full with the approved flushing solution. Thread
one end of a short garden hose onto sediment faucet 5A and another onto 5B.
Attach the opposite end of the hose from 5A to the outlet of the “pony” pump and
the hose from 5B feed into the pail. The third hose attaches to the intake of the
“pony” pump and the other end is submersed into the fluid in the pail.
4 - Open the sediment faucets 5A and 5B. Start the “pony” pump up and let it
circulate the solution through the heat exchanger for the amount of time
recommended by the manufacturer.
5 - Reverse the hoses on sediment faucets 5A and 5B and pump the fluid the
opposite direction through the plate exchanger to purge as much of the scale as
6 - This procedure may need to be repeated a few times to get rid of all the build up.
Once the heat exchanger has been cleaned to satisfaction you need to flush the
cleaning solution from the plate heat exchanger. This must be done carefully to
avoid contaminating your domestic water with your flushing solution.

1 - First close sediment faucets 5A and 5B. The hose attached to sediment faucet 5B
should be routed into an empty pail.
2 - Open sediment faucet 5B and allow any solution to drain into the pail.
3 - Slowly open ball valve 7A on the domestic water line feeding the heat exchanger.
This will flush the de-scaler solution into the pail. Allow this to flush several pails of
water. Be sure to dispose of the flushing solution as per manufacturers instructions.
4 - Close ball valve 7A and sediment faucet 5B. Route the hose from sediment faucet
5A into the pail.
5 - Open sediment faucet 5A, ball valve 7C and 7B. This will flush the heat
exchanger backwards with fresh water. Allow this to flush several pails of water.
6 - Repeat steps 1 through 5 until you are satisfied all the de-scaler solution has
been purged.
7 - Close all valves, remove hoses, and return the ball valves to the desired
operating position. Again, be sure to dispose of the flushing solution as per
manufacturers instructions.


Illustration Parts Reference

Air Handler

Typical air handler unit that might be installed in a garage, workshop, barn, or greenhouse.

Unit Heater

Typical unit heater that might be installed in a garage, workshop, barn, or greenhouse.

outdoor wood boiler unit heater
Backup Electric Boiler (Manual Changeover)

In order to change from using the outdoor furnace to the backup boiler, simply turn three way ball valve on the intake of the primary loop pump to the opposite direction. This will prevent the backup boiler from heating the outdoor furnace. Be sure the outdoor furnace has been properly shut down as indicated in the owner’s manual and that you have adequate glycol in the system to prevent the exterior piping from freezing. If the outdoor furnace is still operating while the three way valve is in the backup boiler position, it may cause the outdoor furnace to overheat and possibly boil over. If the backup boiler is less economical to operate than the water heater, the domestic water heat exchanger should be bypassed as described on page 19 “In Operation” in order to allow the water heater to take care of its own demand. Be sure there is a properly sized pressurized expansion tank installed at the backup boiler to accommodate expansion/contraction in the system. This is very important. If the valves going to the outdoor furnace are closed, the expansion of the fluid must have somewhere to go or there may be a rupture in the system.

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