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 140° F. This means that operating with mixed outputs of 100° to 120° with return temperatures of 80° to 100° 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 Energystar rated 92 to 97%


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 or not a 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 requuires examining the whole heating system.

Keep in mind that 99% of all residential systems have not and now are not designed.  The installer generally just does what he has been doing for the last 30 years.  Where an analysis is done, there is no uniform method that everyone uses.  In fact, many are applying the same rule of thumbs to in-floor radiant heat that they have for baseboard.  The point being is there is an extreme variation in installations.  See link below 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°.  These parameters generally will insure a return water temperature of at least 130° which will prevent condensing in a convential boiler.  In order to utilize a condensing oil boiler efficiently you must cool the exhaust gases below 115° which is the threshold temperature for condensing the flue gases.  For a mor detailed Discyssion see the discussion on Efficiency and Tuning.

These temperature ranges are suppose to keep the house warm on the coldest day of the year.  However, they are not necessary for most of the year, but are necessarily required for a non-condensing boiler.  And 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 moer heat emmitters it has the lower the water temperature that is needed.  Other techniques will be discussed shortly.

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 190 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 120° to 130°F for the majority of the year.  Aditional temperature reducing techniques ar siscussed in the next section.  During peak heat load times the boiler temperature can be turned up but efficiency will decrease somewhat.  And it is not advisable to run a condensing boiler in the non-condensing mode exclusively.  For further reference, please read the report 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

I installed the FCX in my baseboard home of about 5,000 SF of which about 3,000 SF is living space, with 2,000 SF garage and shop.  It has 2x8 walls with blown in fiberglass and triple pane windows.  It is two stories plus a full basement (which is not directly heated).  The stack was installed in an existing chase that had to be opened for access at the top and bottom.  This in turn created an open airflow around the stack that essentially made this 30-foot section in the house a stack-robber.  The boiler runs at the minimum temperature setting (a max of 140 degrees). On the coldest days the boiler at times is dragged down to an output of 120 degrees 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.  My efficiency here is about 91%.

At times I can lower my return water temperature to below 100.  I accomplish this by taking the return water from the baseboard through a heat exchanger and preheating my domestic hot water.  The heat exchanger is connected to a salvaged 50-gallon electric hot water heater (electric disconnected).  By routing the water through the heat exchanger I further reduce its temperature and thereby increase the efficiency of the condensing in the boiler.  The pump is set to run only when the boiler fires. 

Also, the open stack of this system produces both a reduction in stack temperature and considerable condensing. When the measured efficiency with a Bacharach at the boiler is about 91% , the measured efficiency at where the stack leaves the living space 94%.   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%.  See OVERVIEW General Discussion of Efficiency.

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 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. 

Another option is placing a heating coil in your HRV to temper the incoming air.  Routing the return water through a unit heater would also help.


Baseboard – the Bottom Line

Is the boiler sufficient to heat you home? You may be able to improve your insulation or windows and make it so.

Do you have adequate baseboard (both size and length), to utilize a lower water temperature? You may be able to add more.

Can you add any return water reduction techniques such as perheating domestic water, stack robbers, heating air at the HRV, or combining ground source water system.

Do you have an auxiliary heat source that can help in the coldest months?

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 this boiler with baseboard heat in certain circumstances.
Brookhaven National Laboratory Baseboard with Reset Control

The Alaska Business Science Network ABSN has a downloadable program called AkWarm that can help you evaluate the heating needs of your home.

Additionally, ABSN’s new “Green Book” will list the FCX as the only oil-fired boiler to meet their requirements.