I recently was contacted by a homeowner who was having problems with his new radiant floor heating installation. Despite the vague description of symptoms, I suspected that the installer may have fouled up the hydronic design, i.e. the piped delivery system of heated water to the pex tubing embedded in the floor.

That problem was fresh in my mind as I had just finished installing a radiant floor (as part of a complete kitchen remodel) and connecting it to an existing but flawed hydronic system the week prior. In that case, the HVAC contractor, who (a few months earlier) had installed a new boiler which serviced two other heating zones in the house, had left me pipe stubs on the supply and return manifolds to which to connect my zone. Unfortunately, they had improperly designed the manifolds so when I connected the radiant system, my circulating pump could not pull heated water through my mixing valve and on to the zone. In hydronic designs, there are certain things that you shouldn’t do that they had done. Pipe sizing and placement is especially critical in a multi-zoned system, otherwise one runs the risk of creating dysfunctional pressure profiles which, despite proper placement of circulating pumps, will not allow water to flow in its intended direction. I brought the contractor back in to remove an unnecessary mixing valve on one of the other zones which was drawing water directly from the return manifold (which can create negative pressure profiles in one or more of the other zones) instead of that zone’s return line upstream of the manifold. Once that issue was resolved, my radiant zone worked fine.

I was intrigued by the homeowner’s problem and could hardly wait to get on site to diagnose. What I discovered was worse than I had imagined and, frankly, heartbreaking… and maybe just a little comical –“Three Stooges” comical. For starters, the home was a beautifully refinished historic house and the radiant flooring project had included installing new tile – an ideal floor covering for radiant. Despite this, the radiant installer had opted to place the pex tubing on the underside of the subflooring, between the joists – known as “staple-up” in the industry. In my opinion, “staple up” should be a last resort, i.e. when replacement of the finished floor covering is not an option. Not only does placing the tubing on top of the subfloor reduce resistance to heat transfer to the intended space thus improving responsiveness to thermostat and heat load changes, it also enables greater flexibility in circuit design which can significantly improve temperature uniformity across the surface of the floor. To make matters worse, the installer did not use grooved metal plates, which dramatically improve heat transfer – a must for staple-up – to attach the tubing to the underside of the subfloor.

The radiant floor design certainly did not live up to its potential, yet that wasn’t the (only) problem. When the homeowner took me to the basement where the boiler and hydronic manifolds were, I learned that the installer had connected the radiant floor zone directly to manifold without including a mixing valve to reduce the temperature of the 180F water produced by the boiler. It’s no wonder that the homeowner complained that the floor was uncomfortably hot to walk on without shoes; ironically, the poor heat transfer inherent in this radiant floor design mitigated the problem enough to make it tolerable.

Sometime after the radiant installation, their HVAC contractor – a well-known and reputable company (by the way) – was on site for routine boiler maintenance and noticed the absence of a mixing valve for the radiant floor, so – here comes the comical part – installed one, but failed to realize that a circulating pump was required downstream of the mixing valve for it to function properly. As such, the heated water flowing to the mixing valve simply short-circuited and flowed out the cold inlet and directly to the return manifold rather than to the floor tubing, as intended. In addition, the short circuiting also reduced the maximum supply water temperature to 150F, uncomfortably close to point at which the risk of corrosion (and eventual premature failure) becomes significant in this non-condensing boiler.

The hydronic issue was easily fixable. Unfortunately, short of ripping out the tile and replacing it, radiant floor will forever remain sub par.

Radiant flooring can be the most comfortable and efficient means to heat a room… if designed and installed properly. Take the time to vet your contractor when it comes to hydronic and radiant floor system designs. Don’t assume that every heating & cooling company has that capability. At Lake Country Geothermal, in addition to residential geothermal heating & cooling systems, we specialize in hydronic (including radiant floor) design.


Pump, pipe, and mixing valve size and placement are critical to good hydronic design.
Hydronic Radiant Flooring
If you have to do “staple-up”, use metal heat plates to improve thermal contact to the subfloor.
Hydronics and Radiant Flooring Disasters

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