Cursed By Knowing The Numbers

Radiant heating is high on the list of wants in almost every one of the homes I have produced.  The comfort, the invisibility, and the economies of radiant heating make it an attractive alternative. 

There are ways of designing your system that optimize the installation. Use this post when discussing heating systems with your designer.

There are four five things to understand with any radiant system:

1. There is a one to four hour thermal lag because the system is heating up such a large mass--a stone floor, for example.
2. Because of the thermal lag, we don't interconnect this source of heat with the split system as radiant systems are not meant to cycle on and off.  We do include a second outdoor temp sensor that adjusts the water temperature automatically with changes in outdoor temperature.
3. In California or sunny western states where there is a large diurnal temperature range during parts of the year, the system is best suited for areas that are not influenced by these daily temperature swings--northern exposure and basements.
4. The system is most effective in spaces where there is not a lot of solar heat gain during the day, ie in northern/northeastern/basement spaces.  I am hesitant to install these units in spaces with a large amount of southern exposure, and believe they are actually counterproductive in spaces with a large amount of western exposure due to the thermal lag.
5. Spaces with a large amount of glass and high ceilings can develop cold drafty convection currents at the windows on cold days that can overwhelm the system.  Look carefully at tubing sizing and spacing to understand how to compensate for large expanses of glass. 

Radiant heating is best understood as a steady-state, set-it and forget it system.  Most of the jobs I do, we design the system to be set at 72 degrees.  We augment the radiant system with a split system that provides any necessary cooling in summer, and responds to calls for additional heat in the winter, and in spaces with potential for temperature swings.  We don't overthink the handoff (there is none) between the two systems--radiant provides the baseline, and the split system responds to your immediate requests.

You need the split system to provide the responsiveness, and the radiant system for quiet, clean comfort.

The heat in a radiant system can come from one of two sources--electrical heating mats or tubing with hot water from a boiler.  I use mats in small isolated areas--have one in my master bathroom floor, for example.  They are not efficient providers of heat on a $/BTU basis, but work in areas where you can't have leaks or if you just want comfort heating in small areas.

I prefer to put radiant heating under stone, tile or concrete rather than hardwood.  They are more tolerant of temperature shocks than wood.  Hardwood seems to like it best in the same comfort zone as us people--that is 65 to 80 degrees.  Warmer than that, and the hardwood dries out and shrinks.  Water temp in the tubing is typically 100 degrees F +/- 10 degrees.  If wood is the look you want, look at engineered flooring or floating floor systems that are more stable with these higher temperatures.  Carpet diminishes the efficiency of the system and traps heat in the floor, both no-nos.

Hydronic radiant systems designed around a PEX tubing system like Wirsbo should give you trouble free enjoyment for a number of years.  I caution that these systems will leak, it is a matter of when, not if.  In the interim, you will enjoy the comfort, even warmth and "invisibility" of this great heating alternative.

The numbers?  The systems I have put in have ranged from $12 to $22 PSF for hydronic systems.