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Radiant Floor Heating/wall-to-wall carpeting old 1950's radiant slab


QUESTION: Hi Morgan,
Two indentical tract houses, side by side, with mid-'50's steel pipe in concrete slab radiant heat systems. Our house is 75% vinyl tile directly on concrete/25% wall-to-wall carpet w/pad. The neighbor's house is about 85% w-t-w carpet + heavy throw rugs/15% vinyl. The latter house is using more than twice as much natural gas per month to heat!
My theory is that the carpeting results in heating the dirt under the house rather than the interior to a significant extent. We are in California SF Bay area where climate is moderate.
The latter house has a digital thermostat with ability to set daily time/temp schedules. My belief however is that the very slow time responsiveness of a slab system makes timing virtually useless. Nonetheless, would you have any suggestions as to optimzation of the thermostat to possibly reduce the neighbor's fuel consumption?
I've recommended removing unnecessary throw rugs (specifically the one below the thermostat) and also reducing the flow rate in less-occupied rooms. Would you agree with that?
I notice as well at the temp of the water in that system is much higher (140 vs 100F) though flame settings are similar at the boilers.
I can monitor the gas consumption at the meters day-to-day to see the results of any modifications.
Your thoughts would be welcome.
PS I too am an 'expert', but in the auto repair category (Chrysler repair). Thanks for volunteering here.

ANSWER: This is a great primer for radiant floor heating and thermodynamics rolled into one, real-life experience.

You have answered your own question (perhaps what "expert" really means).

There are 10's of thousands of homes built after WWII with wonderful slab-on-grade radiant floors and plastered radiant ceilings, all over the country. In fact I just looked at a radiant ceiling system in Richfield, MN built in 1946. But I digress.

Most of the homes built on slabs with embedded steel or copper tubing were not insulated well if at all and no one thought to insulate below the slab or around the outside of the building. Fuel was cheap. In snow country you will notice the bare ground a foot or more from the outside edges. The ground itself is a pretty fair insulator but most of the slabs run to daylight and therefore present an un-insulated surface to outdoor temperatures. Depending on the climate, the ground temperature several feet below the surface will also be elevated. Temperature difference is what drives heat flow i.e. fuel bills higher. Radiant slabs can only make things worse since the slab would normally be a quite uncomfortable 60F or so at the perimeter and now is likely in the 130's in your heavily carpeted house.

In your case in particular, all things being equal, we can't blame the lack of insulation, since it is doubtful that any part of either slab is insulated. We have to look to other factors. As you have astutely observed the house with full carpet is operating at a higher average water temperature than the neighboring home, lacking excessive floor coverings. This is not surprising since the radiant floor has to reach a higher temperature in order to overcome the floor coverings and satisfy the thermostat.

Floor coverings can have a huge affect on the output of any radiant floor heating system but more so when the space below the panel is not insulated properly. When consulting on old radiant floor heating systems I often specify insulation be placed at the perimeter of the home at a thickness and depth appropriate for the climate.

In my own recent remodel I attempted to dispel many misconceptions about radiant floor heating and floor coverings using tile, floating engineered bamboo, traditional 3/4" oak and even a bit of the very high R-value cork to demonstrate that many floor coverings may be used if the radiant system is designed properly. In my case the insulation was specified and heat loads low enough to allow for just about any floor covering while still using modest design water temperatures.

Carpet itself is not always the culprit when the radiant floors below them perform poorly, but the pad can easily double or treble the resistance to heat flux (flow) and drive the required operating temperature up.

Digital thermostats with set-back are really made for inherently inefficient forced air furnaces and in nearly every case where they are installed, comfort suffers while fuel bills are hardly affected. In fact, if you set back the thermostat too far (more than 4or so) for less than half a day, you may find the fuel bill going up. This is the equivalent of highway vs. city miles and even worse if the system is pulling a large thermal load/mass. Like a tractor/trailer the larger the load the more energy it takes to change the "speed".

All of the negative effects of setback thermostats are exacerbated by the very nature of controls made for radiant heating.

We try to incorporate weather sensitive controls in all of our hydronic designs. Most radiant floors, walls and ceiling will provide quicker response, more comfort and lower fuel usage if fitted with a control that modifies delivered water temperature in response to changes in the outdoor temperature. All condensing boilers have on-board weather sensitive controls and many also have proper reset, which is usually used to set-back water temperature during sleeping hours providing the ultimate in efficiency without compromising comfort in any way.

In your own radiant-slab 1950's tract home, I would roll up all the rugs and carpet to the outside wall for a month and end all speculation. Next spring the perimeter would be hand-dug (preferably by someone else) and the proper rigid insulation installed with equally dramatic reduction in fuel usage to follow.

---------- FOLLOW-UP ----------

QUESTION: Thanks for the detailed information, Morgan. I shared it with the neighbor. She is not likely to remove the wall-to-wall and underpaddings so she will try very judicious setting of the temp of the thermostat in response to the environment (monthly trends) and we'll revise her thermostat programming to virtually eliminate trying to 'time' the temp settings except maybe for a minor set back at night. It is interesting that while the city runs our utilities and has an agressive conservation/education effort, they have not suggested the importance of  avoiding use of insulating floor coverings in radiant houses nor the ineffectuality of using a programmable thermostat either.

You can lead a horse to water...

One of the frustrating things about "Green" initiatives is the general lack of focus and priority among its most zealous advocates. Cost/benefit analysis is a sound business tool that should dictate most of our energy policy. Clearly this is lacking.

To be fair, radiant panel heating is an advanced skill common in Europe, Korea and other countries, but represents less than 1% of the US market. In fact I just had a client from here in Minnesota call to ask if a radiant floor heating system "could" actually heat a whole house. The irony is that the Twin Cities was really the bedrock of the modern radiant floor market starting in the mid-80s with polybutylent tubing.

If floor coverings that are not conducive to good heat transfer at low design water temperatures can not be avoided radiant walls or ceiling should be considered. In this particular case I would insulate the perimeter and watch my fuel bill go down in a hurry.  

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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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