During the Alberta construction boom of 2006, trades were impossible to lock down throughout the province. Our mechanical contractor for our house renovation was stretched thin. As a result, we had to wait a year for our heating system to be up and running. Lethbridge is not the warmest place in winter, but we survived with a wood burning stove and electrical unit heaters. Pete eventually showed up to finish the installation with a school looking fin radiator for the main space. “…after a week, you won’t even notice it’s here…” were Pete’s last words. There wasn’t a day that passed where I wouldn’t walk down the stairs and think to myself “…what a god-awful ugly thing….” when seeing the rad.

Two Years passed and one summers day I drained our hot water heating system, pulled out my saws-all and cussed Pete as I removed the radiator.

The replacement rad was our modification of an accepted practice. Heated floors take a long time to heat up, but radiate the better part of the day because of the radiation of the large heated physical mass. Our solution was to create a vertical mass mimicking the orientation and basic parameters of the fin radiator. Air had to flow behind and around it. The original radiator was a 350 series slant fin, measuring approximately 3-1/2”x 10”. I estimate it was offering 440btu per 12” with the boiler operating at 140 degrees farenheit. We have residential style slant fin radiation elsewhere that functions and is more aesthetically sensitive.

Our design was approximately 4” x 10” with a 3/4” space at the bottom to allow air flow. The only real trick was to allow an air bleed at the top of the loop. I used 3/4” Rahau H-pex (oxygen barrier) and zip tied it to 6” Re-mesh. I added 10m rebar as required. The only real trick is to add a shreader air bleed connection at the top of each run to eliminate air pockets.

Knowing I could not get the same efficiency of the fin radiator, I intuitively knew more sq ft area would be required, so we incorporated it into the design of our island. Structurally we had beefed up the structure significantly so there were no worries of structural failure.

Heating Load

Our design both functions physically and meets aesthetic desires. As we have a historic house, the concrete radiator has a commercial loft feel. We estimate the radiator pushes 200-220btu’s for the same sectional area equivalent of the fin radiator. In Lethbridge we use a rule of thumb of 90 btu’s per sq ft design temperature for a well sealed/well insulated house with reasonable but not excessive windows (this is prior to the adoption of the Alberta Energy Code). The radiator will usually radiate until 6-8hours after the heating cycle. There is a lag of 2-3 hours for the mass to reach full temperature. This means the boiler runs for an extended period in the morning, and temperature depending-might not need to turn on until the next day.

While we still are refining the design for use with some of our projects, we suggest leaving a 1.25” gap at the bottom and behind. Coordination with electrical services will be required and I would not use less than a 3/4” H-pex supply and return. As this solution creates long H-pex runs, be mindful of product maximum distances and head-pressure.

Post Construction Evaluation

We intuitively knew this project would have success but at the time we constructed it had no idea to what degree. The thermal imaging is the quickest way to understand where the thermal mass radiator sits compared to other heat delivery appliances. We keep our radiator set at 165 degrees Fahrenheit (73.3 Celsius), so the below values show the effectiveness of each. Cast Iron was the most successful at 149 degrees Fahrenheit (65 Celsius) with the site built concrete radiators slightly edging out the fin radiator at 109 degrees Fahrenheit (43 Celcius). Radiant Ceiling panels performed the least favorably at 102 degrees Fehrenheit (39 Celsius).

Any designer will tell you though, straight output is not always the prime factor. Cast Iron and Fin radiation are difficult to design around, and affect furniture layouts whereas ceiling radiant panels are the least obtrusive. We believe this might be an advantage for the Concrete Radiator solution as it might conform the easiest as we have hopefully proven.

Thermal lag has not been discussed and is an important subtext. While the fin radiator appears to be the least favourable in our study, it did respond the fastest in heat delivery. The cast iron likewise was efficient in this regard. The greater surface area of each facilitated this. The concrete was slow to heat up yet the slowest to cool down. In temperate heating climates this might be prone to cause overheating of spaces. In our climate it is beneficial. The boiler has longer run cycles but also longer periods between active heating. All the materials for this project are availible in Lethbridge, so our assumption is they are available throughout Alberta and elsewhere.

This is our design, and while we offer the idea freely, any use of this concept is at your own risk.