By it’s 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.  There is very limited experience with condensing boilers, but keep in mind, this is not new technology in Europe where in some countries it is required to install condensing boilers, and many brands and models are available.  But they are fairly new to the USA where we have benefited from low fuel prices for sometime.

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 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.  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 winter we hope to test on at CCHRC.

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, 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 shut down 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 cavitations 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 phrase Tiger Loop has become generic to 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.


Stacks and Venting (revised July 2013)


Because of the low stack temperatures, installations of stacks are significantly simplified. However, sealed stacks of materials approved for the temperature and flue gasses are required. The primary material used is polypropylene, which is a type of plastic and is readily available from the dealer (see Centrotherm Catalog in Downloads and Links). Stainless steel is not recommended as it is extremely expensive and note, all stainless steels are not created equal. Some are totally unsuitable. Also, plastic has a much longer life.

Stack Types


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 conducts the combustion air to the burner from outside the building. This is called a balanced flue.

Single Wall

The single-wall system is a flue pipe that exits either through the roof or the sidewall. In this case the combustion air isusually taken from the boiler room using a cold air trap. A separate pipe can be used for combustion air creating a balanced system, but the concentric system will do a better job of preheating the incoming air.

Concentric vs. Single Wall

The determination of which to use will be governed by the configuration of your stack arrangement. In some configurations you will not have a choice because of the airflow/pressure and/or other physical considerations.

Sidewall Exit

The simplest installation using the “straight out the side of the house option” is not recommended because there is less heat exchange from the stack to the inside air or combustion air. This results in reduced efficiency due to not adequately preheating the combustion air. A vertical and horizontal distance combined of at least 6 feet is recommended.

Physical Considerations

The choice of using a vertical stack in a multistory house is good for several reasons. For one it is less expensive. An additional advantage of the vertical stack is that additional condensing will occur in the stack. This adds to the overall efficiency in two ways, the reduced stack temperature and the actual condensing. Greater efficiency will be obtained if the stack is not in a chase but open to the room air so better heat transfer is accorded. A difference of 3% has been measured in a 20 ft. difference in stack height. Remember, the stack is approved for zero clearance. It can also be painted since it is polypropylene. For the roof penetration, a standard 4" rubber drain water vent seal is all that is needed. In the case of a retrofit, the new stack can be sleeved inside a 6" or greater existing stack. Putting in a concentric stack that is 35 feet tall would be cost prohibitive. If concentric is necessary other systems are available (see Centrotherm’s Catalog. There are systems to line existing chimneys and use the space between as an entrance for the combustion air. Exterior uninsulated stacks are not recommended and could cause safety issues in extreme temperatures. Too much cooling can occur. Also, stacks passing through uninsulated trussed roofs should be insulated, and the stack outside should be no longer than 30 inches. Also, we have currently sleeved an outside metalbestos stack with plastic. The owner has observed no problems through the three winters in use.

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 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. 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. And again, 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 you are not exiting at the highest point in the structure. To date there has been no problems with tall single stacks exiting through the roof. The tall stack tends to balance the house as a whole.


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.

Summary and Recommendations for Stacks

Use a balanced (concentric preferred) system for all sidewall exits. Use single wall for tall structures or buildings where no chance of negative pressures exist. Keep in mind, sealed boiler rooms and cold air intakes will NOT prevent negative pressures.



Traps and Neutralization

Because this is a condensing boiler it produces water that has to 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 has to be separated from the stack by a water trap.  A simple deep trap about 8 inches can be constructed from ABS fittings. In any stack over 3 feet above the outlet (like going out the rim joist) a stack drain is required. It stems from the fact that the condensate that forms in the stack seems to be of a more concentrated nature than that formed in the condenser, and the longer the stack the more the condensate.  It is not desirable to have this material drain back into the condenser. The draining of the stack is accomplished by means of a condensate tee.  See Diagram.

Assembly of all parts should be made so as to facilitate cleaning and maintenance.  Rubber couplings are advised.


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.




Water Connections - Primary and Secondary

The boiler contains both a mixed and a hot connection for use. 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 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.


External and Combined Circuits

If for some reason you install an external mixed circuit, and/or multiple baseboard combined with radiant, fed from the “hot” port, there are two options for plumbing the return, through the hot return or the mixed return. The “hot” return port enters at the top of the condenser just before its return to the primary heat exchanger so the hotter return temperature will not contaminate the cooler returned water from the mixed circuit.  But when you have only the baseboard active you can return the water through the mixed return and get some benefit from condensing.  The use of 3-way zone valves would allow this.  If the temperature on the secondary return is cool (from a radiant circuit), be sure to plumb the return through the mixed return port. The reason is that the mixed return port enters the condenser at the bottom, thereby best utilizing the cooling effect of the water.  Again, 3-way zone valves can be used if there is a wide variation of return temperatures. This does not agree with the generic diagrams in FCX’s installation manual but is in fact the proper way to do it. Most installers do not see the need for external mixed circuits.  When combining radiant heating with some baseboard the installer needs to use his judgment based on expected return temperatures.


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 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.  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 is not covered under anybody’s warranties.


Variable Frequency Drive (VFD) Pumps

While the existing pump is capable of distributing all the heat the boiler provides, there are better ways. It is highly recommended that a VFD pump be used for optimizing the pumping on the heating circuits.  Depending on application use a ∆P or ∆T pump.

In some cases pump speed can be automatically altered, by measuring the pressure differentials to produce the same flow regardless of the number of zones open.  This is called a VFD ∆P pump, and the Grundfos Alpha is a goog choice.

 In most cases, I recommend the Taco Bumble Bee ΔT pump for the heating circuit, as it is most appropriate for getting the lowest return water temperatures. The existing constant speed pump can be removed  and used on the domestic circuit. If there are zone valves, don't use the IFC that comes with the Bumble Bee.  It can be used in the DHW circuit pump.  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, you can just remove the internal pump and replace it with a straight pipe between the flanges in this case.  This insures a good consistency on return water temperatures.  The TACO SR series of controllers work well with this.


Injection Pumping

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 affect the same thing. Consult with me before you do this. There are no installers experenced with this.



The FCX can also be wired for cold start (it cools down when there is no call for heat).

The Honeywell controller on the older Heat Wise burners comes from the factory with TT jumpered for running in hot start mode (the boiler maintains temperature at all times). So to cold start the TT controlled option is available with third party products. to control pumps and boiler.

The Riello burner does not have a TT control.  To make it a "cold start" system you need to interrupt power to the boiler.  On radiant systems, I use the Taco SR501 to 506 switching relays depending on the number of pumps.  On base board systems where temperature control is more likely needed the Tekmar 402 series is great.

Also, I recommend the use of a Johnson A419 ABC (120V) controller be put in series with the existing aquastat.  The existing aquastat would then serve as a secondary high temperature safety.  The 419 is more accurate, the differential can be adjusted, and it also displays boiler core temperature.

In general there are sophicated that controls can take over complete control of all functions of the boiler including prioritization, heat purging, variable temperatures, automated mixing settings, and cold starting. The FCX can be used with any of these options, if properly installed and set.


Temperature Control

Boiler temperature outside reset controls are not generally needed on radiant floor heating.  The reason is boiler temperature controls are made primarily to reduce the temperature on boilers when high temperatures are not needed.  The FCX is already a low temperature boiler.  Further reducing the boilers core temperature can cause condensing in the primary heat exchanger that is simply welded steel.  This would drastically reduce the life of the boiler and of course void the warranty.  Generally in radiant applications the boiler works quite well at its lowest setting even for heating domestic water.  Only if higher temperatures are needed and totally automatic operation is required should you consider this option.  In new installations the need for high water temperatures should not be “designed in”.


Mixing Valve Control

Outside reset mixing controls can also be installed.  However, they are mostly not warranted.  In the radiant houses being built today with radiant high mass floors, it seems that relatively low water temperatures (100 degrees or so), can comfortably heat homes throughout the heating cycle.

If this is considered desirable, Paxton Corporation and others make set point controllers that bolt directly to the mixing valve.  The valve is an ESBE 3-way valve.  The controller monitors outside temperature, the supply, and the return temperatures, and adjusts the valve in accordance with your programming.


Cold Start vs. Hot Start - Discussion

It is generally desirable to wire the boiler for cold start, as it does take energy to maintain temperature.  However, it is questionable whether much savings is realized over a hot start in certain circumstances.  If you are heating domestic water with an indirect DHW tank and the domestic is a major use, the boiler will probably fire many times a day to keep up with demand.  For instance, if there are showers in the morning, laundry is during the day, cooking 3 meals a day, and showers or dishwashing at night, demand is pretty much all day.  The FCX is well insulated, contains only four gallons of water, and has very little stack loss.

However, if there are many long times when there are calls for heat, the cold-start method is the way to go.  The easiest way to accomplish this is with switching relays or zone valve controllers.  Taco makes a variety of these devices.  In this case the device activates the TT on the burner control. 

The internal pump is operated by the set of wires at the back of the boiler that use dry contacts (the relay that operates the pump is built in).  If your controller outputs 120V to the pump, the simplest solution is to put a RIB relay on the rear of the boiler rather than rewire the pump.

Summer/Winter switch on the boiler:  Causes great confusion at times.  The only thing this switch does is disable the internal pump.  It does this by interrupting the 120V to the pump, not the pump relay.  All of this is diagrammed in the manual but it is counter-intuitive to me.  The entire reason for this switch is speculative.  However it does eliminate any possibility of the internal pump running if that is desired.



There are two reasons for purging.  Some boilers come with controls that purge the heat from the boiler 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 122° and DHW temperatures of 120° to 130° it is a very small gain.

The second reason is more technical.  The boiler is a low mass boiler of 4 gal of water.  On shutdown, when running the FCX at higher temperatures the water over temperature safety may trip.  What causes this is the residual heat in the steel of the boiler (no pumps running) can over heat the water in the boiler causing a shutdown. This is not a problem with the current models.



In large homes, if all the zones turn on at once, as in a morning reset, the boiler temperature can actually be dragged down below the hot water heaters set point for a period of time. During this time the boiler will not be hot enough to make domestic water, and may actually cool it off. To avoid this a simple relay can shut the zones or the pump off when there is a call for domestic hot water.  The above mentioned controls can have this option built in.



While everything regarding the FCX is zero clearance including the stack, having room to work is advisable.  Please leave open about 8" on the left side of the boiler for inspection purposes.  Also allow enough room behind the boiler to remove the plastic stack also for inspection purposes.

The boiler should be mounted on an elevated platform of at least 2 x 8 construction.  This facilitates the installation of the condensate neutralizer/trap.  See the DIAGRAMS section of my web site on one way to build one, or see how it's set up at the Woodway where a condensate pump is used.

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 antifreeze, and produces the by-products of combustion.  By there very nature they are not as clean as a refrigerator.  Boilers require cleaning and maintenance.  Accidents happen.  Neither do I advise the boiler be put into the same sealed utility room any appliance that affects air flow.  It needs its own constant air supply.  Interactions between appliances that move air such as HRVs, wood stoves, vent fans, dryers, etc. are not always predictable.  A small closet is not as forgiving as a large garage or basement.


Cleanliness During Construction

Because this boiler is a low temperature boiler it is extremely sensitive to dust, sheetrock dust, paint over-spray, and fumes. Many hours of cleaning will be saved if the boiler is isolated from this debris. If this is not possible, an external filter can be made from a shop-vac filter, and attached with length of dryer hose. See Diagram.