I wonder how often condensing boilers get installed with their factory settings or wrong settings like mine. If you have a condensing boiler that is sending 180 degree water to your radiators when it’s 60 degrees outside (which is what the old cast iron boilers were designed to do), you aren’t getting any of the efficiencies out of your condensing boiler akin to flooring your Prius at a stoplight while depressing your brake.
Let’s say you send out 150 degree water to your boiler- let’s expect that 10 degrees will dissipate from the radiators to heat your home. That means your return water is 140 degrees and you are right at the edge of getting a benefit from your condensing boiler. Any temperature below that is even better- efficiency for return temperature of 90 degrees is 95%! Compare that to efficiency of 85% for return temp of 170 degrees.
I am one who is quite curious about how new things work. After the installers left, I cranked on the system, waited a few minutes, and then felt the pipes which were way too hot on a warm 60 degree day. Reading the display, it showed it was cranking out 180 degree water. My HVAC contractor performed a manual j heat loss, but I wish they had programmed the boiler with that information as I had requested- I suspect they left the high settings they programmed in to test the boiler.
Determined to re-program the boiler, I geeked out on the installation manual for my Triangle Tube boiler and found out with a mini-computer and display built-in, it is quite easy to check and change the settings.
I used trial and error to program the outdoor reset curve so that I can maximize the time that I’m sending out lower temps to radiators (under 150 degrees) which will get the most benefit out of a condensing boiler. I first looked up the coldest day design temperature for Baltimore which is 17 degrees outdoor and programmed that in with 140 degree water going to the radiators- it automatically adjusts the curve for warmer days based on that info. As the fall season progressed, I found it was having trouble keeping up with heating the house at that setting so I upped the temperature to 150 degrees. 150 degrees is the sweet spot- the boiler kept the house at our desired indoor temp all season.
When the old cast iron radiators were installed when the house was built in 1933, they were sized for receiving 180 degree water from the boiler. Due in part to all the air sealing work, caulking and increased attic and knee wall insulation to R-49, we’ve been able to cut 30 degrees even on the coldest days meaning we don’t have to use near as much energy to heat the water being sent to the radiators. Last season we used 30% less fuel to heat the home after adjusting for outdoor temperature (heating degree days). See future article on how I calculated this.
As you can see, there is a lot more programming involved with the new condensing boilers. They can be installed correctly from a plumbing point of view and pass inspection, but if not programmed to match individual home characteristics and geographic location, will not maximize energy savings.
Side note: one HVAC contractor I met while I was interviewing potential contractors said I would not benefit from a condensing boiler. He said my existing radiators- the old floor standing cast iron ones- are designed to optimally operate at 180 degrees. He went on to state that if I put a condensing boiler in the house, I wouldn’t achieve any energy savings over a non condensing boiler since you can’t condense with return water that is above 140 degrees. He said some large homes with long returns in Roland Park (a community in Baltimore) could dissipate enough heat to send at 180 degrees and return below 140 but in our cape cod, no. I had never heard about this caveat before- so I dug up some information and found that you can operate old cast iron boilers at lower temps. They still put out heat at 70,80,90,100 degree water you just have to adjust the temperature settings on your boiler.