Radiant Floor Heating Retrofit radiant heat on concrete spoiled in Korea, Korean radiant floor

Radiant Floor Heating Retrofit radiant heat on concrete spoiled in Korea, Korean radiant floor

Radiant Floor Heating /Retrofit radiant heat on concrete spoiled in Korea



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QUESTION: I live on the 4th floor of a 6 story condo with concrete floors and steel stud framing. My current heat system is forced air with a hot water heat exchanger. Last year I upgraded my traditional gas fired water heater to a tankless Rinnai water heater to power the heat.

I am now interested in a floor radiant heat system after visiting a foreign country that used them, and falling in love with them.

What kind of retrofitting can I use for existing suspended concrete floors that won’t make too big of a step up from the outer hallways into my unit?

I have a 1,000 sq foot unit with one bedroom. In the country that we visited we only turned the floor on in the living room. We preferred to sleep cool and didn’t use it in the bedroom or kitchen at all and we were plenty warm in winter. Is it advisable to only heat the living room? The kitchen and living room are only separated by a breakfast bar, not fully separate. That only leaves a bathroom and bedroom uncounted for.

And finally, in addition to the recommended system for retrofitting, will it work with the Rinnai water heater that I just installed?

I’ve looked at most of the youtube videos out there and still don’t know what system would be good for my situation.

Thank you for any help you can give.

Badger Radiant Designs

ANSWER: Here in Minneapolis the bulk of our field work is radiant floor heating in the renovation of old houses. However, we have of late been designing retrofit radiant floor heating systems for contractors and homeowners using several interesting systems made for renovation radiant systems such as you propose for your condominium.

As you suggest when installing a radiant floor heating system over an existing concrete slab, the first concern is the addition of height to the overall floor profile. Doors, thresholds and the always challenging dishwasher come to mind. But once you have a proper heat load you have the most important radiant floor design tool that you can have.

With a relatively low heat load presented by the typical condominium the output of the radiant panel is usually not a concern unless you will be covering it with shag carpeting or reclaimed 2x4s.

So let us assume your load is a reasonable 15,000 BTU’s and you will be using tile in the bath and a nice laminated, renewable bamboo flooring in the remainder of the condo. We often use Gypcete or other cementious materials to cover tubing attached to an existing slab. This method allows for precise PEX tubing placement and easy zone layout. However, Gypcrete can may not be poured thinner that 1 1/4″ in most circumstances and is not a durable surface, which necessitates some type of finished/durable floor covering. Total height; 1.5″ minimum.

Another system we use is one of a variety of radiant floor panel heating systems commonly referred to as «sandwich» radiant panels. This type of system offers relatively easy installation in most applications but does limit the pattern and potential output the designer may use when specifying the radiant panels.

The first is Thermal Board and produces a finished profile 5/8″ high to accommodate 1/2″ PEX tubing. This board comes in just a few patterns and must be carefully designed for the space it will serve. The board is quite sturdy and features a laminated sheet of aluminum (on top of the board) used to enhance uniform surface temperatures and greater output.

The second sandwich-type radiant floor heating system we install is made of interlocking panels with steel covers grooved again for the ever-popular 1/2″ PEX pipe. Mr. PEX calls their system RetroPanel����d we have found it to be quite effective. This panel also feature a small amount of insulation integrated to the bottom or the panel to help with slab-on-grade or basement radiant retrofits.

The last retrofit radiant panel I would mention is also the one with the thinnest finished profile, that being Wirsbo/Uponor’s Quik Trak����his panel is has a finished profile of just 1/2″ (requiring the uncommon 5/15″ Uponor hePEX tubing. This panel also incorporates a thin laminated aluminum sheet but attached to the bottom of the panel instead of the top as with the Thermal Board first mentioned.

As concrete is a very good heat transfer medium, insulating the slab in your suspended slab may be in order. Assuming the ceiling of the condominium below your is insulated the need for insulation below the radiant panel is reduced, though response time could be a factor if you have «big glass».

Finally, we have to address the control and heat source for your new radiant floor heating system. Here in Minneapolis we install Bosch, Eternal, Noritz, Rheem, Rinnai and Takagi tankless water heater but rarely for combination spaced and domestic hot water heating systems. Naturally we would never mix the two using potable hot water for space heating with the notable exception of tying a close-coupled fan-coil (furnace)to a domestic hot water heater. This practice is both common and accepted for condominiums here in the US and in Canada. But as you point out, you will not have the most comfortable heating system available if forced air is your only heat source.

Why can’t I use my new Rinnai tankless water heater for combo heating of radiant floors? Well ,you can do it, but it is not optimal. Since the average gas-fired tankless water heater is fired at 150,000btu’s (about 10 times what you will need to heat your space), you will find the tank-less short-cycling. Like highway vs. city miles the overall efficiency and reliability of a tankless water heater, pressed to what I would call micro-zone service, is not ideal.

The perfect heat source for a smaller space such as a garage, workshop, basement, or in your case a modest condominium, is really a tank-type or storage water heater. We use tank-type water heaters for almost all of our small radiant panels designs. The reasons for this are many but the primary advantage is that of the mass presented by the stored water. With small zones such as bathrooms and your typical bedroom calling regularly but presenting small loads any over-sized boiler or tank-less water heater (even with a modulating flame) will struggle to fire low enough to avoid the dreaded «short cycle».

If on the other hand you the burner must raise the temperature of an already warm 40 gallons of water «before» effecting the radiant floor panel, short cycles are far less likely and very small zones may be satisfied without even cycling the water heater. This is ideal.

But the tank-less water heater does away with standby losses and provides «endless» hot water! Yes, but a thick layer of foam insulation also eliminates significant standby losses and the burner (still 100,000 plus) will provide endless hot water with the extra advantages of being able to fill the big tub and provide the perfect heat source for small radiant floor panels.

Now I am not talking about your standard tank-type water heater mind you. We represent Bradford White here in the Twin Cities and often install their unique CombiCor water heater where applicable. This is a true combi with a built-in heat exchanger and conventional direct vent or gravity (silver pipe to the chimney) low-efficiency model.

More often we will upgrade older Combi-Cor water heaters or specify new radiant floors using a sealed-combustion, direct-vent, condensing water heater such as the old reliable stainless steel Polaris����de by American Water Heater since 1987 and the new glass-lined Vertex water heater made by A.O.Smith.

Both of these units are relatively quite, very efficient and when properly isolated from the the radiant heating system provide the ideal combination space and domestic hot water heat source for small renovation projects like you condo.

QUESTION: Thank you very much for the detailed response. If we did a full system in our condo with a mix of Wood in the kitchen, tile in the bath, and carpet with a very short pile in the bedroom and living/dinning, would that make you lean to one system over another?

I looked at your site and anticipate using you for a design of a system at the point of actually doing this. But for now while on an extended stay out of the country, I’m just researching. After living with this system in the country I’m in, it’s hard to believe we’ve lived with such a crazy forced air system all my life when the alternative is so much better.


I will not lean anyway until I see a CAD file.

The Koreans have had radiant floors for since ancient times. Frank Lloyd Wright rediscovered radiant floors while building the Imperial in Tokyo back in the 1920’s.

Unfortunately we in the US are more interested in drapes and floor covering than real comfort. But to be fare, if you don’t know what your missing.

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