You are here:

Radiant Floor Heating/Replacing radiant floor boiler


Hi Morgan

Thanks for the wealth of information on your site. It's truly a wonderful job. We have radiant heating in our home of 20 years old. We are considering replacing our boiler with a more efficient one.
I believe our existing boiler is a Slant/Fin GG-75HXBCV. It seems to do a fine job.
If we replace this unit to a more efficient unit can we simply cross over size to size? Are there other issues we need to consider?

Thanks again for a great website!

Badger Boiler ModCon Solution
Badger Boiler ModCon S  
How to choose a boiler

No one should specify any boiler before doing a thorough heat load analysis; this is something I do after I win a contract or receive a consulting fee. Your prospective contractor should produce a sample of his heat load calculations. There is little difference in cost between the various boilers available, but an over-sized boiler (modulation notwithstanding) can affect over-all system efficiency by as much as 50% and can lead to comfort and reliability issues.

All modern high efficiency gas-fired boilers condense AND modulate flame. Condensing boilers recover the latent heat found in the water vapor (smoke up the chimney) to usable hot water used to drive radiant floors, radiators and even fin-tube baseboard. This act of recovering latent heat typically reducing stack temperature by several hundred degrees and allows for a Class IV vent (PVC).

It would seem a boiler that modulates flame (output) would not require such careful sizing. However, just the opposite is true. It is at low fire that much of the fuel savings occur, and in most ModCon boilers that low-fire is determined by the high-fire output, i.e. the "bigger" the boiler the bigger the minimum output. This condition can lead to short cycling (death to system AND thermal efficiency).

As I am a consultant, AND a contractor, people all over North America often ask me; which boiler is best? Most contractors are married to one supplier and advocate one boiler brand (usually a non-condensing, low efficiency cast iron model). As you can see from my website, I have personally installed nearly all ModCons currently available in the USA. The reason for this admittedly odd practice is twofold; first I like to compare ease of installation, maintenance, performance and factory support. Second, no, one, boiler, fits every application and many of the boiler companies have short lines. This is particularly true of the European boiler manufacturers' N. American offerings.

I would also like to determine the type and size of indirect water heater you will need. Since you are considering one of the most efficient and lowest polluting fossil fuel appliances on the planet, it makes sense to put it to good use satisfying what is most likely your second highest fuel bill, domestic hot water.

With a Mod/Con and indirect or "companion water heater" you will be able to permanently close the old fashioned chimney and the open combustion air pipe, saving even more fuel!

If you have an off-the-shelf programmable thermostat it will lend nothing to comfort and little to efficiency when applied to the typical hydronic heating system. They are made for forced air furnaces. You, on the other hand, will benefit both in comfort and efficiency by use of the onboard microprocessor that resets boiler water temperature to outdoor temperature (at least one ModCon has an on-board setback feature with boost mode to catch-up after a cold night). Some ModCons even feature water temperature setback for more fuel saving.

Properly applied, sized, installed and programmed; all of the Mod/Cons will condense and operate at their full potential efficiency (95%+ A.F.U.E). Warranties are very similar and the last thing I consider; as labor will double or triple any replacement part you may need. Certain models have factory extended parts and labor warranties. Extended warranties are available for all models but are a form of insurance. Unfortunately warranties are not reflective of quality, reliability, serviceability or efficient system design.

Annual maintenance by a factory certified technician (such as me) is key to the safe, reliable and efficient performance of any Mod/Con driven heating system. This is not just a matter of knowledge and experience but requires a combustion analyzer among other expensive tools. So ask your contractor about factory training on the model he advocates.

A final word on "Big Box" boilers. Your local hardware/department store or "Web" supplier may "sell" boilers, but they are seldom qualified to design, install or service any of them, much less a modern ModCon. If your local gas supplier sells boilers keep in mind installation may be done by sub-contractors who are - by definition - the low bidder.

Radiant Floor Heating

All Answers

Answers by Expert:

Ask Experts


Morgan M. Audetat


Radiant panel heating design, including floors and ceilings, European wall panels and snow melting for residential and light commercial buildings..............................................................-Master of Steam and Hot Water,City of Minneapolis....................................................--- Master Plumber, State of Minnesota...........


30 years..... Charter Board Member - Radiant Panel Association, former manufacturer of condensing boilers, former national distributor of radiant floor products, son and apprentice of mechanical & plumbing contractor. Current mechanical/plumbing contractor specializing in hydronic based, integrated HVAC systems. Radiant floor heating/cooling. Snow Melting. Condensing Boilers. Indirect water heaters. System design, consulting and technical training world-wide. Licensed contractor, designer, installer and consultant.

B.S. University WI 1981 CONTINUING EDUCATION: Viessmann Condensing boiler and Solar water heating certification 2010, N.D.S.U. Lead Worker RRP certification 2010, Knight condensing boiler certification 2009, Wrightsoft Manual J CAD certification 2009, RPA Designer & Installer Certification 2008, Nate Hydronic/Forced Air Certification 2008, Uponor/Wirsbo advanced design school certification 2007, Buderus Wall-hung certification 2007, Power Limited License (low voltage controls) 40 CE credits 2005, Basic Hydronic Certification IPEX & Northern Alberta Institute of Technology 1998, Charter Board Member - Radiant Panel Association 1994, Residential Off-Electric to Hydronic Conversion Heating School Canadian Hydronics Council 1994, B&G Little Red School House 1993, Rood Utilities (now Auburn Technical Institute) Oil Burner School 1993 ,Tekmar Controls residential and commercial 1993, Division Manager and Advanced Hydronic Seminar Instructor for the first Exclusively Hydronic Radiant Floor Distributor in the USA 1990, Hydronic Radiant Heating Association Workshop participant with Richard c. Bourne, PE spring 1988, Master Plumbing, Hydronic, Solar Course Red Rocks Community College 1987.

Awards and Honors
2009 System Showcase Award - Radiant Panel Association

©2017 All rights reserved.