As of December 2019 the FCX will no longer be available in the U.S.



Installation – Revised 2019


The FCX was introduced in Alaska in 2005 and was installed at CCHRC for testing and evaluation.  Even though the actual testing and comparisons were not made because they found their methods were not sufficient, they have been quite happy with it’s performance the last 14 years.

Condensing boilers are not new technology in Europe.  In some countries it is required to install condensing boilers by law, and in others their efficiency regulations are so stringent that there is no other choice.  In Europe there are many brands and models available in natural gas and oil.  In the US there were only a few choices even in natural gas because we have benefited from low fuel prices and efficiency has not been a governing factor.  Now every manufacturer offers an option for natural gas.  No one would consider not using one unless high water temperatures are needed.

And in 2005 no one at all had experience let alone expertise with condensing oil-fired boilers.  There was not anyone that even understood the principles involved.  We have now compiled a knowledge base that helps us understand the fundamentals of low temperature heating.

By its very nature the FCX being a condensing boiler, runs cooler, is very quiet, is attractive to look at and is certified for zero clearance to combustibles for all parts, even the stack.  While the installation manual for the FCX is very comprehensive in showing the installer most of the available installation options, it provides little discussion in advising which choices to make.  The purpose of this page is the carry the conversation further and discuss the “whys and wherefores” of an installation.  In order to make informed decisions on installations you must know the advantages and disadvantages of each option.  Here we will try to provide some guidance.  We will be examining the principle involved, and technical decisions to make regarding stacks, venting, plumbing, handling the condensate, heat emitters, controls, and location.  At the end of this document I will give a summary of “Best Practices” discussed.  I will also try to provide links to other pages and sections that cover some technical issues in more depth.

Making the correct choices here will absolutely affect the overall efficiency of the boiler.


See the “Overview” for an in-depth discussion on the principles of condensing boilers and how they differ from conventional boilers.  This will explain the dynamics of how lowering the flue gas temperature increases efficiency and the lower the better.  So how is this done?

Return Water Temperatures

The primary way the flue gas temperature is reduced is to reduce the temperature of the return water from the heat emitter.  A condensing boiler runs most efficiently when the temperature of the return water is as low as possible, because the cooler water is, the more effectively flue gasses will be cooled.  This is why the FCX is a natural fit to in-floor radiant heat where the return water temperatures are about 80 degrees or less.  Other methods are also available to further enhance this or create this effect when alternate heat emitters are used.  See the Suitability page for a discussion on alternate heat emitters.

A good example of controlling return water temperatures for maximum efficiency is as follows:  Suppose you have a supply of 100 degrees and a return of 90 degrees.  Now decrease the flow by one half with a return of 80 degrees.  Both scenarios essentially release the same amount of heat so, which is the most efficient.  In a condensing system the cooling effect of the 80-degree water produces more condensate and lowers flue gas temperatures that increases efficiency.  Secondly, if you reduce the pump speed to one half, you reduce the power requirement of the pump to one eighth.  I.E. the power requirement is inversely related to the cube of the pump speed.  So, do not over pump.  It costs more in electricity and decreases efficiency. (See the Pumping/VFD section on the page). 

Another method to reduce return water temperature is to add another heat exchanger to the return of the of the heat emitter.  This could be a water to air exchanger located in your HRV system or a water to water heat exchanger that preheats your domestic.  Combining return or supply water from a combined geothermal system would have incredible effects.  Stack robbers can also be used and can be made from plastic. 

This also means do not do anything that tempers the return water with the supply water, such as tempering circuits, 4-way mixing valves, injection loops, hydraulic separation tanks or any other device that tempers the return water. These will drastically reduce the efficiency of the FCX.  Further discussion on optimizing pump speeds are discussed later in this web page under Recommendations.

You are limited only by your imagination.

Fuel Lines

Single Line Installations

Single line Installations are not recommended, as they are probably the most susceptible to air being pulled into the lines.  This can be particularly troublesome from the turbulence induced while refueling.  Even when the tank is nearby, air can accumulate and after several weeks a shutdown can occur.

Two Line Installations

While better than one line systems, they have some drawbacks. If a flame out occurs it may be caused from an air leak in the suction line, pump, or solenoid valve.  This might not be easy to diagnose, if it very slow and only intermittently causes a flame out.  Additionally, long runs of cold fuel can induce cavitation in the fuel that can be have the same affect as air in the lines.  The cost of trouble shooting this problem will probably exceed the cost of its solution.

Tiger Loops – the Solution

The phrase Tiger Loop has become the generic name of a group of devices that are fuel de-aerators. The device is placed near the boiler with supply and return lines running to the pump. One line only runs to the tank. Any air introduced into the system is bled off and released by the device during the circulation process. Because this device has a clear top, you can actually see if there is any air being introduced into the system. It virtually eliminates any issues with air in the lines and shutdowns because of this.  A service call to troubleshoot air problems will probably cost more than the de-aerator.  The Tiger Loop Ultra has a built-in fuel filter and is recommended.

Please note that flameouts can happen on refueling even with a Tiger Loop. Filling small tanks that are nearly empty can induce so much air the de-aerator cannot handle it all.  Most servicers recommend shutting down the boiler while refueling, especially in older tanks that may have a lot of debris in them.

Stacks and Venting


The burners in the FCX do not have a draft induced burn, they are pressure burners that must have sealed pipes of appropriate materials for the temperature, flue gasses and the acidic content of the condensate.  However, because of the low stack temperatures, installation is significantly simplified.  The material used is polypropylene (PP) which has a continuous operating temperature rating of 230 degrees (see the Centrotherm Catalog in Downloads and Links).


The concentric system consists of an inner pipe that exhausts the flue gasses and a surrounding pipe. The space between the surrounding pipe and the inner pipe brings the combustion air to the burner from outside the building. This is called a balanced flue and is primarily recommended.  The straight-out side-wall short length method is not appropriate in our climate.  A minimum of at least six feet is recommended in order to sufficiently heat the incoming combustion air. 

These systems can either have a vertical or horizontal termination and have been tested by Riello to over 30 feet in length.  We have installed them at over 40 feet in length.  Even at these lengths the second lowest primary air setting on the Riello burner is used.  A maximum length has not been determined.

Single Wall

The single-wall system is a sealed flue pipe that exits either through the roof or the sidewall. In this case the combustion air is usually taken from the boiler room using a cold air trap to supply.  

Where single wall pipes go through unheated trussed air spaces they should be insulated.  They should extend no more than 30 inches above the roof uninsulated.  They are not subject to the required heights of draft induced chimneys.

This system is only allowed where the FCX is isolated from the building to be heated and can not be influenced by changing air pressures.  This does not mean a separate boiler room, a garage or a crawl space.

Where single wall pipes go through unheated trussed air spaces they should be insulated.  They should extend no more than 30 inches above the roof uninsulated.  They are not subject to the required heights of draft induced chimneys.


This combines the features of the concentric and single wall.  At least six feet of concentric pipe is necessary attached to the FCX to preheat the air.  A special fitting is available that splits the concentric pipe into two single wall pipes.  Therefore, a separate pipe can be routed to a different location for combustion air creating a balanced system.  However, a total concentric system will do a better job of preheating the incoming air.

Special Consideration

Occasionally a circumstance will arise that doesn’t quite fit the above methods.  There are enough options available to adapt to almost any situation given the principles are observed.

Negative Pressures

There can be many unpredictable issues with venting when a boiler is installed in a modern tightly constructed home. One of these is the formation of negative pressure in the structure. A negative pressure exists when the pressure in the room with the boiler is less than elsewhere in the house. The rest of the house will tend to draw air from the boiler room if a balanced flue is not used.

As an example, consider a boiler located on the bottom floor of a two-story structure with a single pipe sidewall exhaust exit and cold air trap intake to the room. The pressure is always less on the bottom floor even if there is a separate air intake. Because the FCX has no damper, when the boiler shuts down the single wall flue becomes a secondary air intake drawing the moisture laden acidic air back through the burner, with resultant deterioration of the components.  This is referred to as back-drafting.  This is not just supposition but has happened necessitating retrofitting to a balanced flue. The same thing happened with a single-story home when the wood stove was installed.

However, negative pressures do not influence balanced flues as the exhaust out and the air in are sealed. Also, when you combine a tight house with an HRV system, bathroom and kitchen venting, and a wood stove, adverse results have been noted due to the changes in air flow and air pressure. This is virtually impossible to predict. The most susceptible are single wall side exits. As stated above, even with a dedicated combustion air entry you may still get negative pressures. The bottom line is that negative pressures are not acceptable. To avoid this, you MUST install a balanced system if your FCX is not totally isolated from the heated structure.

The exhaust gases are moisture laden and are somewhat acidic. With a sidewall exit, hoar frost can accumulate on the siding and can cause stains. This does not happen in all cases, but only where there is dead air. Over-hanging eves can exacerbate the problem. This may or may not be an issue and you may not care, but it needs to be considered. Generally, this has not been an issue.

Summary and Recommendations for Stacks

Use a balanced system for all flues. Use single wall with separate air intake only when there is no chance of negative pressures existing. Keep in mind, boiler rooms and cold air intakes will NOT prevent negative pressures.


Traps and Neutralization

Because this is a condensing boiler it produces water that must be disposed of.  Depending on the application, it can produce up to 5 gallon per day.  This water is slightly acidic (PH 4 to 5) and needs kept away from materials that may be affected.  However, in a residence the condensate does not have to be neutralized as it has been shown normal wastewater adequately neutralizes it.  Since the condensate is produced from exhaust gasses it must be separated from the stack by a water trap.  A simple deep trap about 8 inches can be constructed from ABS fittings.  Assembly of all parts should be made to facilitate cleaning and maintenance.  Rubber couplings are needed. Handling the Condensate


In the event a convenient drain cannot be located near the boiler or if your condensate requires batching (as in the case of an elevated structure where trickling of the condensate is not permitted), a condensate pump is necessary.  These are readily available.  Make sure the one you use is rated for acidic condensate.


Water Connections - Basic

The FCX contains both a mixed and a hot connection. The mixed connection is part of an internal circuit that has a built-in pump and mixing valve. This is a plug and play circuit and is ready to hook to your manifolds. The hot circuit requires an additional pump and can be used with a baseboard circuit, a unit heater, and a water maker (indirect DHW tank) for domestic use.  Note the secondary circuit is not a domestic coil, and only allows for extraction of water from the hottest part of the boiler and separate return that does not interfere with the cool return of the mixed circuit.  The FCX can produce two temperatures of water directly.

Heating Circuits Requiring Different Temperatures

If your structure has both radiant and baseboard it is recommended that an additional external mixing valve be used with a separate pump.  This is because the baseboard can also run reduced water temperatures most of the year.  The supply for the secondary is the hot circuit.  The return from both of these should be plumbed to the mixed return.  The DHW circuit is always from the hot supply to the hot return.  Note that the diagrams in the manual are incorrect on this point.

Check Valves

The use of check valves or other active controls in each circuit (such as zone valves) is absolutely critical to the correct operation of this boiler. Because there are two internal water circuits in the FCX, water movement in one will induce a sympathetic or back flow in the other. All the diagrams in the manuals show these check valves and this is the reason.

Condensing in the Primary

There has been a tendency for some installers to want to up size the pump or add additional pumps either in parallel or series.  This is not advised.  Most installers are using “rule of thumbs” that are outdated or apply to baseboard systems and way oversize everything.  “We’ve done this for 40 years and it works”, or “the more pumps the better”.  The problem with this is that too much thru-put with the combination of low temperature water can cause condensing in the primary welded steel heat exchanger.  This is a very common scenario when combined with a large structure, high mass emitters such as a concrete slab, and setback thermostats (don’t use).  In fact, this is also a common situation with conventional boilers.  The use of smaller boilers regardless of type requires controls that prevent this.  Note that condensing where it shouldn’t be is not covered under anybody’s warranties. The pump within the FCX is adequately sized to remove all the heat that is produced.


ECM Variable Speed Pumps

While the existing pump can distribute all the heat the boiler provides (note that “old school thought tells you if you have a big house with a small boiler you need a bigger pump – not so), there are better ways. It is highly recommended that an EMC variable speed pump be used for optimizing the pumping on the heating circuits.  Depending on application use a ∆P or ∆T pump.

The ∆P pump’s speed is altered to provide constant flow through each zone regardless of the number of zones open.  The Grundfos Alpha is a good choice if this is the goal.

In most cases, I recommend a variable EMC ΔT pump for the heating circuit, as it is most appropriate for getting the lowest return water temperatures.  It adjusts its speed to insure a set temperature drop is achieved between the supply and return waters.

In both ∆P and ∆T, all zones or groups of manifolds must be balanced or unusual results will occur depending on which have calls for heat.

The existing constant speed pump can be removed and used on the domestic circuit. If there are zone valves, you can use the IFC that comes with the EMC pump on the DHW circuit pump.  They are the same size.  Make sure it is not put in upside down.

For a system that has individual pumps for each circuit, I would also use ΔT pumps.  In the case of multiple external pumps using the internal mixing valve, you can just remove the internal pump and replace it with a straight pipe between the flanges.  This insures a good consistency on return water temperatures.

Injection Pumping & Hydrualic Separation

There are many ways to go wrong here.  Incorrectly used loops can temper the return water and pumping the boiler supply greater than the primary can effect the same thing.  These can be used with the proper configuration and pumps with the ability to adjust speed based on temperature but should be avoided if possible because they over complicate the system.


The FCX is a “Plug and Play” boiler in that it can be plumbed directly to the heating circuit and the DHW circuit, can be connected directly to power, and the room thermostat directly to it also.  It will maintain temperature and the pump will start and stop controlled by the thermostat.  When used with the BS series of DHW's an additional pump for this can be switched by line voltage through the DHW aquastat or by 24 volt relay.

Of course, most homes are not so basic as having one zone.  This scenario also does not allow you to cold start FCX when there is call for heat and where it cools down when there is no call for heat.  The goal is to optimize the potential of the FCX.

Temperature Control

It has been proven that controlling water temperatures to the absolute minimum needed for heating comfort maximizes efficiency.

We now (2019) have specific recommendations.  This is based on extensive use of the FCX with varying control systems with different heat emitters and a multitude of plumbing configurations.

The Tekmar 400 series of controls have been chosen because it has been shown to provide the most efficient utilization of the operating characteristics of the FCX.  It provides outdoor reset with indoor feedback and insures optimal efficiency.  This is coupled with either a Tekmar or ESBE/Danfoss mixing valve actuator (the mixing valve is an ESBE VRG or 3MG model).  This system guarantees the optimal operation of the FCX.

The Taco switching relays and zone controllers have also been used in combination with the Johnson 419 and 421 digital temperature controllers.  They provide cold-start and pump control but are not optimum based on our current knowledge and observations. Caution should be exercised to keep the core temperature of the primary welded steel heat exchanger above the condensing point.  These controls do not do this.  The Tekmar’s 400 series of controllers provide boiler protection through modulating the mixing valve.

While only the addition of digital temperature is needed to more accurately set boiler temperatures, they do not control temperature based on actual needs.  Controlling core temperature and mix temperature based on heat demand is absolutely essential to monitoring and controlling the operation of the system to optimum efficiency.

The Technical Page of this website gives detailed drawings of how to do all of this.

General Discussion of Other Control Devices

Balancing Valves

Highly recommended.  They should be used in all radiant applications.  When the characteristics of the system contain divergent sets of manifolds (some close / some faraway) it is best to use multiple pumps rather than try to balance multiple zones.  Then you only have to balance each set separately.

Mixing Valve Control

The simplest thing to do is manually set this valve, but this is not optimum control for comfort or efficiency.  In super insulated houses being built today with radiant high mass floors, it seems that relatively low water temperatures (90 to 100 degrees or so), can comfortably heat homes throughout the heating cycle.  Again, this is optimum to set the valve with temperature feed-back control.

Basic outside reset mixing controls can also be installed that do not have the indoor feedback, but here you are at the mercy of the installer on how it is set, and it does not self-adjust.  If you can’t make these adjustments yourself, do not use them. The Tekmar 400 series of controllers are the only ones that do this.  Your installer does not want call-backs, so he may set them higher than needed.

Bottom Line on Controls

After having used both types of controls - hot boiler vs. cold start, and manual digital boiler temperature with manual mix control there is no question which is better:

Other Controls

Summer/Winter switch on the boiler

This causes great confusion at times.  The only thing this switch does is disable the internal pump.  It does this by interrupting 120V to the pump, not the pump relay.  All of this is diagrammed in the manual.  The entire reason for this switch is speculative.  However, it does eliminate any possibility of the internal pump running if that is desired.


The safety on the left is the high-water temperature and on the right is the high-stack temperature.  These are both fixed at 230 degrees.  When they trip they do not pop out, so listen for a click when resetting.  There are numerous reasons why the high water can trip, but the high stack is critical.  If this trips there is something wrong that needs fixed immediately. The system is probably so dirty it can no longer absorb the heat from the flue gasses.

Some boilers come with controls that purge the heat from the boiler when there is no call for heat into an emitter such as the indirect DHW tank.  The stated purpose is to increase efficiency.  However, considering the temperature differences between the lower temperature FCX’s core of 120° min and DHW temperatures of 120° to 130°, and considering it is a low mass boiler of only 4 gal of water, it is a very small gain. If you have a high mass cast iron boiler (20+ gal of water at 180 degrees), you will gain some efficiency. Also, keep in mind that you may overheat the DWH to unsafe temperatures.

Priority for DHW has been found not to be necessary, as the FCX makes two temperatures simultaneously.

Programable Thermostats

Unless they are integrated into the control system, I do not recommend using them, or if needed for energy ratings, turn them off.  Using set backs can produce confusing results.


While everything regarding the FCX is zero clearance including the stack, having room to work is essential.  Please leave open about 12" on the left side of the boiler for inspection purposes.  Also allow enough room behind the boiler to remove the plastic stack and condensate plumbing also for inspection and cleaning purposes.

The boiler should be mounted on an elevated platform of at least 8 inches even if code does not requiry it.  This facilitates the installation and cleaning of the condensate drains. Where code requires it such as a garage, a standard 14" stand will do.

As stated before the FCX is small, quiet running, and attractive.  However, think twice before putting it into a living space.  Keep in mind it is a piece of machinery.  It uses fuel oil and may be filled with anti-freeze and produces the by-products of combustion.  By their very nature they are not as clean as a refrigerator.  Boilers require cleaning and maintenance.  Accidents happen. 

Cleanliness During Construction

Because this boiler is a low temperature boiler it is extremely sensitive to sheetrock dust, paint over-spray, and fumes. Many hours of cleaning will be saved if the boiler is isolated from this debris. The use of balanced flue venting eliminates this issue.