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Radiant Floor Heating/High efficiency mod-con & old radiators?


Hi all,

I was so happy when our local utility NStar connected us to the new gas line on our street, am planning to switch from oil to gas for heat and HW, for fuel savings and for more even heating performance. I am also hoping to take advantage of any high efficiency rebates and zero interest loans available in Mass.

Our house was built in 1900, colonial style, 3 floors ~2300 sq ft. Lots of original double hung sash windows with clear glass storms. Very little insulation in the walls, not so great in the attic, and definitely some air infiltration.
We live near Boston, Outdoor Design temp i think is 9 degrees

Current oil fueled boiler is Utica SFH4150WT 175K showing 152,200 BTU/hr on plate.
Current hot water is 50 gallon electric heated.
Boiler is about 20 years old, we used 1350 gallon of oil in past 12 months.
We have 7 large cast iron radiators, 60 ft of cast iron baseboard, and 30 ft of newer fin baseboard.

I thought it would be a snap to move to a high efficiency gas boiler and set about getting 3 quotes before the dirt was settled over the new gas line...
Quote 1 was for a Rinnai E110C combi unit to go with "on demand" HW.
They sent me a Manual J that said my heat load was 68,849 but it lists 2 "rooms" each at 1320 sq ft each? seems like a shortcut estimate listing each floor as a room.

Quote 2
They looked at size of existing boiler and quoted 3 options (all 150K BTUs?) Not sure if they are even planning to do a Manual J
Buderus Logamax Plus GB142 w Megastor 53 gallon IHW
Viessman Vitodens WB200-Ww Megastor 53 gallon IHW
Burnham Alpine ALP150T w Megastor 53 gallon IHW

Quote 3
Spoke to an experienced plumber who has worked in a lot of houses.
He had concerns about the high efficiency mod-cons performing well with the large volume of water. He thinks they would not run at the high efficiency levels in a high mass system, and recommended:
Burnham ES2-5 which has a large cast iron boiler and can be vented out of current chimney (w addition of flue liner) and a Heat-Flo 50 gallon IHW storage.
Also recommends Outdoor Reset. He said we'd have to do a real manual J room by room measures so he could properly size the boiler, but he thinks the ES2 family would be a better fit for our radiators.
Problem is that burner is only 85% rated, which does not qualify for any rebates, AND would not qualify for the 0% loan.

So let's assume that whomever does the proper sizing (I stopped by my library and have been reading Audel HVAC Fundamentals: Volume 1: Heating Systems, Furnaces and Boilers and started my own spreadsheet based on Chaps 3 and 4 !) - in the real world, can the new high efficiency mod con burners be installed and tuned to support high mass? I am pretty sure I want the Indirect HW rather than on demand.

Also, we are hoping to get better performance that we have now, where it takes a while for the radiators to get warm, then they are really hot for a while, then it cools down for an hour or two and then heats up again - thermostat just sits at 66 all days....

Hadn't thought all the details until we got into this process and then i started reading tons of stuff on line and then the Audel books...

We will try to mitigate insulation issues, but have siding on top of asbestos shingles so can't fill walls from the outside, a lot of issues like that...

Just trying to understand the pros and cons of any choices, and muddle my way thru conflicting info.
thanx for any advice or ideas.

A modern high efficiency, condensing boiler with modulating burner and outdoor reset is the only replacement gas-fired boiler that should be considered when replacing any boiler serving old cast iron radiators. To suggest otherwise is outrageous.

Most cast iron radiators were over-sized from day one. This means that they are capable of transferring more heat to any given room they need to. Low average water temperatures then are the norm rather than the exception and given that any condensing boiler will usually start at 86% combustion efficiency and go up from there makes high efficiency condensing boilers a real no-brainer. As the burner modulates on the built-in weather sensitive control, the thermostat is satisfied as before, but rather than the radiators swinging up and down in temperature the boiler maintains a low, continuous water temperature based on an inverse relationship with outdoor air temperature.

In the old days boiler were sized to the now obsolete "IBR" standard, which added horsepower to the boiler to allow it to "pickup" the large pipes, cast iron radiators and considerable water volume of the system in total. With old time thermostats and a fixed burner output sized for the coldest day of the year, this system was remarkably comfortable.

This is a new day. No extra pickup is necessary. The modern condensing boiler anticipates the exact water temperature needed to meet the heat load for the house it serves and if sized properly will rarely shut off. Amazingly, the comfort will go up and the fuel bills go down...dramatically.

Now we have to give the nice folks at Burnham Boiler extra credit for dressing up the cast iron boiler with a built-in bypass and "optional" outdoor reset. We install them for folks that can't afford Burnham's condensing Alpine boiler.

But a cast iron boiler will not condense, missing out on huge savings as the stack temperature remains around 350F while the condensing boiler (built-in outdoor reset) will happily operate with low return water temperatures recovering considerable heat from the condensate and rejecting what little remains through a PVC stack at around 100F. The result is a combustion test like the one we just performed last week on a Viessmann boiler serving cast iron radiators from the 1930's. See attached.

The only way to improve this system would be to add and indirect water heater and shut down that drafty chimney.

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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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