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


Suitability for Use


Will this Work in My Home?


Radiant – (in the Floor Tubes) an Emphatic


The FCX is primarily designed for use in radiant heating (in the floor). It is a low mass and low temperature boiler that is designed to fire frequently.  It holds only 4 gallons of water and operates best at its lowest setting of 120° F. This means that operating with mixed outputs of 90° to 100° with return temperatures of 75° to 80° is the most efficient application for the FCX.

In fact, the FCX is the most efficient oil-fired boiler available in the United States.

EPA ENERGY STAR rated 92 to 97%

Listed by the importer at that time Monitor Products, Inc.

Baseboard and other applications

While the FCX works best in low temperature systems, that does not mean that it is necessarily not suitable for other applications. There are two key issues to consider when determining whether any particular boiler is applicable to any particular home.

    1. Is the boiler big enough? (this applies to all boilers)
    2. Can the heat be extracted from the boiler at the lower temperatures a condensing boiler operates at?  This requires examining the whole heating system.

Keep in mind that 99% of all residential systems are not designed.  For example, it was about year 2000 when radiant heat became popular.  Prior to that tube spacing in some cases was three feet and more.  In the next 10 years spacing evolved down to one foot when it was realized closer spacing allowed lower temperatures and lower temperatures saved fuel.  Today eight or nine inch spacing is the most popular.

However, early on when radiant heat was being used, the installer generally just did what he had been doing for the last 30 years.  Where an analysis was done, there was no uniform method that everyone used.  In fact, even today many are applying the same rule of thumbs to in-floor radiant heat that they have for baseboard in regards to zoning and pumping. The point being is there is an extreme variation in installations.  Follow the link to AKwarm, a program developed for this purpose.

The standard baseboard heating system generally operates at the boilers on and off set points range of about 140° to 170°. Some older systems are running at 180° to 200°.  These parameters generally will insure a return water temperature of at least 130° which will prevent condensing in a conventual boiler.  In order to utilize a condensing oil boiler efficiently you must cool the exhaust gases below 115° which is the threshold temperature for starting the condensing of flue gases.  For more details see the discussion on Efficiency and Tuning.

These high temperature ranges may be needed needed to keep the house warm on the coldest day of the year (or they may not be needed).  For sure, they are not necessary for most of the year.  However, they are required for a non-condensing boiler in order not to condense.  A lowering of these temperatures will promote condensing, first in the stack and then in the boiler and will cause them to self-destruct do to the acidity of the condensate (there is a lot of sulfur in our fuel).

Since the key to efficiency in a condensing boiler is low return water temperature, we must design a system to do this.  First of all, the more insulation a home has, and the more and better heat emitters it has, the lower the water temperature that is needed.

In some cases, the situation is not possible to reasonably improve a house enough to accomplish this. A poorly insulated and designed house can require water to be as high as 180 or 200 degrees. If this is your circumstance a condensing boiler is not for you.

However, it has been demonstrated that the well-insulated house with adequate baseboard radiators can successfully utilize 90° to 110°F for the majority of the year.  Additional temperature reducing techniques are discussed in the next section.  During peak heat load times the boiler temperature can be turned up, but efficiency will decrease.  And it is not advisable to run a condensing boiler in non-condensing mode exclusively.  This is all solved using proper controls that automatically control temperatures and are essential to the optimum operation of the FCX, particularly with baseboard systems.  Follow this link ti to the Installation Page Controls.

For further reference, please read the report (2004) by the Brookhaven Institute on using the FCX in a baseboard home with a set-back controller on its mixing valve.  See the references section below.

Baseboard – a Case in Point - My Home

Original Installation:

My baseboard home is 5,000 SF of which 3,000 SF is living space, with 108 feet of baseboard, and 2,000 SF garage and shop which is not directly heated.  It has 2x8 walls with blown in fiberglass and triple pane windows.  Originally it had two Burnham LE’s with a 1.5 gallon nozzle in each.  I reduced this to 0.85 nozzles and still needed only one to run at 40 below. 

FCX Installation:

When I first installed the FCX I left the original system (the LE’s) functional but turned off and valved off as I didn’t know if the FCX would carry the house.

With the FCX I routed the return water from the baseboard through a heat exchanger to preheating my domestic hot water in a salvaged 50-gallon electric hot water heater (electric disconnected).  This further reduced the return water temperature and thereby increased the efficiency of the condensing in the boiler. 

The stack is a four inch single wall PP pipe that was installed in an open chase going through three floors.  Total length is about 35 feet to the tip of the exhaust.  It was created with an open airflow around the stack that essentially made the section in the house a stack-robber.  

Also, the open stack chase of this system produces both a reduction in stack temperature and considerable condensing. When originally installed I could lower my return water temperature to about 100° and my efficiency was about 91% at the boiler with a flue gas temperature of about 150°.  When the measured efficiency with a Bacharach at the boiler is 91%, the measured efficiency at where the stack leaves the living space ranges from 94 to 97%.   The exhaust temperature runs from as low as 80 degrees to about 120° at the top of the boiler cycle.  At 80° degrees efficiency is 97%.

With this initial installation, the boiler ran from 120° to 140° on the coldest days.  The boiler at times is dragged down to an output of 120° with a return of about 110°.  The theoretical maximum for oil gasses to condense is 115 degrees so we are just below threshold for condensing. 

The key here in a baseboard application (or any application) is to further reduce the return water temperature and the stack temperature thereby promoting an increase in condensing combined with lower stack temperatures, with the result of increasing overall efficiency.  You can only do this with a condensing boiler where you have made the problem of condensing the benefit of condensing. 

At this time, I was using a Johnson 419 digital temperature control, set from 120 to 130(140 during coldest weather).  Mixing valve was full open.  Boiler maintained minimum temperature at all times.  The DHW circuit was just a zone.

New Control System:

I have had the Tekmar 402 installed since August of 2018.  The FCX is now cold-start, and the DHW is a separate circuit with its own pump. As of this writing (April 2019), I have noticed the boiler runs less, the motorized mixing valve seems to barely open during the moderate temperatures down to 10°, and the condenser is clean as can be (no soot at all). While the pump runs more, mix temperatures are as low as 90°.  The settings are: boiler design = 130, mix design = 120, DHW = 130.


It has become apparent that with baseboard or any other low mass heat emitters, boiler and mix temperature controls are absolutely essential to optimally control a boiler of any type.  Temperatures as low as 90 degrees are now utilized during moderate Spring and Fall times. The lower the output temperatures, the lower the return temperatures.

Baseboard – the Bottom Line

Baseboard and Energy References

The next link is to a study done by the Brookhaven National Laboratory in 2004 using the FCX with a baseboard home and a motorized set-point controller for the mixing valve.  It truly demonstrates the efficacy of using FCX with baseboard heat even at this early date.  And now in 2019 we have devices that are much more sophisticated, that control boiler temperature, mix temperature, and DHW temperature optimally with in-door feedback.
Brookhaven National Laboratory Baseboard with Reset Control