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Radiant Floor Heating/Mixing old radiators and radiant floors



I've asked some other places but not gotten a clear answer on the relative efficiency of mixing radiators (either existing cast iron or possibly replacement Runtals) with radiant floors (under wood and tile).  We have a 1938 brick colonial (1500sf) and are putting on an addition of about 800sf.  We can't easily replace the radiators with radiant flooring in the existing house, but want to consider it for the addition.

Will this have any significant impact on overall efficiency and the monthly gas bill?  I understand the floors need to be mixed down to a lower temperature which seems like it would be wasting the energy used to heat it... or am I missing something.  This will be with a new high efficiency condensing boiler.

I guess a second question is what is an estimated added cost to run 2 of 4 zones at the lower temp in terms of piping, controls, mixing valves, etc.?

Thanks for any help you can provide.

Though we design hydronic based heating systems all over N. America, our local market in Minneapolis, is dominated by older, architecturally significant homes, that started life with steam or gravity coal-fired boilers. Of course all have since been converted to natural gas, but many still are fairly simple systems, often controlled with one thermostat on the main floor.

A good part of our local design/build hydronic business is designing new heating systems for these older homes using existing radiators with new controls and mixing radiation as you suggest. We regularly mix radiant floors, ceilings, walls, cast iron baseboard, standing radiators and wall-hung with many, many, European style, steel panel radiators of various designs. Naturally this is not for the novice and careful water temperature and flow control is required.

The whole thing starts with an ACCA Manual 'J' room-by-room heat load analysis. From this information an experienced designer may specify radiation and controls for each room or floor. The design water temperature delivered to each radiant panel or radiator is dictated by the design temperature of the room, the heat load therein and the rated output of the radiation chosen.

The more zones, the more the cost.

Generally, the lower the design water temperature, the lower the operating cost. This is especially true if condensing boilers or heat pumps are used to heat the water.

Most important is the proper sizing of the boiler and the avoidance of short or "micro" zones that might require the boiler to fire often but not present an adequate load. This condition is common and leads to short-cycling of the boiler (even a modulating/condensing boiler.  

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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

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