Hydronic Radiant Slab Heating – Not As Efficient As Hoped
I’ve compared my winter heating bills between my old home (2-story 1450sqft) and new home (1-story 1550sqft) and the differences are a bit surprising. The old home had tall ceilings and an undersized furnace, it seemed like it was running all the time to keep the house at 70º. The “new” home has hydronic hot water radiant in-slab heat via a commercial hot water heater, but the gas usage (which are only used to heat hot water) is nearly DOUBLE. I thought hydronic radiant heat was supposed to be efficient?
My home inspectors said the hydronic radiant heat was in good working order…but these bills have me worried he missed something.
The new (to me) home was built in 1998, is a slab foundation, and the inspector was not able to determine insulation under slab or depth of radiant tubing in slab, although his thermal imaging looked normal. Good clear radiant lines throughout the house. The WIRSBO control system utilizes old school circa 1998 independent loop thermostats with floor sensors and a single Grundsfo circulating loop pump. Is it possible that single pump is too small for 1550sqft and four independent loops?
Now, granted, the difference in gas bills isn’t too bad ($75 current furnace vs $135 hydronic) but I’m a bit dismayed that in our case the commonly marketed “efficient” hydronic radiant heat is not so much.
One thing that confounds me is that I have no idea how the slab was insulated, and it very well may not be. Is there some way of inspecting the slab insulation via borehole? Would those costs even be worth it?
Other things to check and troubleshoot?
Replies
You could drill a half-inch hole under a door threshold or in some other out of the way place and see if the drill brings up any insulation. Shouldn't take more than 30 minutes.
How much usage? Not the dollars, the amounts of gas.
Best compare dates and weather data as well.
And, it takes a while to heat a slab, so running it regularly during the real heating season might sometimes show less usage per day.
And, the two story will accumulate heat from below. With a smaller attic footprint you could also have been benefitting from less heat loss.
We’ve been heating with in slab radiant with boiler for over thirty years. We use a masonry heater in the spring and fall as it’s way more efficient when conditions are such that you plan solar gain, milder daytime temps, etc.
Boiler start up is usually in early or mid December. Shut down, mid March when no leaves on the trees to block the low angle sun.
Gas usage statements always include how we’re doing compared to similar houses in the area. Every month we see our report as around 30% less than 25 most efficient, which of course might be aided by lowering the thermostat or freezing your rear off...... But, my wife really likes it toasty,
So, please describe your situation a little more.
Thanks.
What do you gain by finding out whether the slab is insulated or not? You can't add insulation to an existing slab...
How is your attic insulation in the new house? The lack thereof can make a big impact.
An yes that slab has a huge thermal mass so once you get it up to temperature you want to keep it there. Do not turn the heating on and off to try to save energy you will lose more than you gain.
I purchased the ECOsmart ECO 18 tankless water heater during one of their open box warehouse sales for a very cheap price. I’ve owned this unit for 2 weeks and installed this and upgraded my outdoor panel in one Saturday. I ran 3 runs of 8/2 with bare ground Romex on 3 40amp individual breakers. I planned to update to one of these units over a year ago, I work as a lineman and was able to run a 4/0 urd service to my home for a true 200 amp service I also upgraded my transformer for the “potential demand this unit would require 6months before installation. https://www.saveourh2o.org/best-water-heaters-for-radiant-floor-heat/