Mean Radiant Temperature
Dear Comfort Advisor,
I have a hydronic radiant floor system in my home and also use a wood stove to help lower my fuel bills and I like the feel of if. My gas usage still seems amazingly high. What could be the problem?
Dear Toasty Toes Tom,
This is standard conversation that we have regularly with our customers. There can be a number if things that will run up fuel usage but the number one thing when using a radiant floor system and a wood stove is a matter of mass. Most radiant floor systems use embedded tubing in concrete (typical basement floor) and lightweight concrete (on upper levels) to transfer the heat to the structure (and you). It takes a lot of energy to warm up these slabs of mass and it happens relatively slowly. Once they are warm there is many hours of heat emitting that is simply wonderful. Radiant floors work best at a standard temperature day and night. Maintaining a consistent slab temperature takes much less energy (fuel) than varying the temperatures.
Enter the wood stove (not literally). My second favorite heat would have to be the good old wood stove. There is nothing like the glow and the massive amount of BTU’s that feel good all over. Most wood stoves have way more BTU output than your heating system and therefore do a great job of satisfying the heating load in the home. In this case, the boiler and the radiant slabs are offline and losing heat. You may ask yourself how is this possible when it 80 degrees in the home and everybody is running around in their underwear. The answer to that question is “Mean Radiant Temperature”.
The second law of thermodynamics states that heat energy flows from warmer to cooler, period. You need to consider that the heat energy stored in the lower level slab is in contact with the much cooler ground. Even if (hopefully) the slab is insulated this slows down the heat transfer but certainly does not eliminate it. As soon as the radiant floor thermostat turns off it is in the process of cooling down. If there was no heat transfer you wouldn’t need a boiler or radiant floor for that matter.
The longer the wood stove is on, the more the slab cools down. The typical situation is to run the wood stove until you go to bed and bank it to run as long as possible through the night. This works till 2:00 AM then the air temperature drops and the radiant floor is back heating the home. Unfortunately the slab has lost thousands of BTU’s and now needs to be recharged. The boiler fires, the meter spins, and by dawn the floor is warm again and you once again load up the wood stove (here we go again).
Most boiler systems are not equipped to compensate for this type of operation but fortunately can be modified to work better with a wood stove. Slab sensors that actually maintain the temperature of the mass, not the air work much better. Utilizing outdoor reset controls will certainly help as well. At AirWorks we can assess your particular situation and make recommendations to make your more comfortable and save operating costs. As always, estimates and good advice is free at AirWorks.