View Full Version : Used boiler vs new for radiant heat
02-05-2010, 10:52 PM
I built a garage (detached) last yr & put in infloor heat. Garage is 26' widex38' longx10' ceiling ;2"x6" walls & ceiling insulated, under concrete 2" insulation & insulation along outside of foundation has 2" of insulation down 2 ft.. Also 2- 8x 10 insulated overhead garage doors & 3 -3x3 windows and 1 steel insulated service door. I ran 4-250' runs of 1/2"pex tubing I just bought($250) a 2003 Lochinvar 45000 btu boiler 82% eff. which is 36900 btu output. The previous owner said he only put about 15 days use on the boiler because it was too small for what he was using it for. A couple plumbers said that this boiler would work fine for my use. I plan on setting the temp @ 55 -60degrees and leaving it. Ithougt about buying a new boiler but for the price of the used boiler vs. the better eff. of a new one it does not seem cost eff.( Approx. 10% savings a year) figuring to spend approx.$50 per month x 6 months=$300yr for a total of $30 a year savings. I will not be able to hook up my heat until spring because i need to bury a gas line from my house (about 100 ft) to my detached garage Any positive or negative feedback greatly appreciated!
02-06-2010, 05:40 AM
I think you made a good decision.
02-06-2010, 03:33 PM
I think you are fine with the choice of a used boiler. It still may be a little large for your well insulated garage and low space temperatures, but the high mass floor is is self buffering. There is a two stage Lochinvar and that would help if it is your model. Pay attention to combustion air requirements. One advantage of a new boiler other than AFUE is that you can get a sealed combustion unit which will lower your load with reduced infiltration, but It still may not be cost efficient for your space and temperatures.
Floor supply water temperatures will be pretty low for your application and be sure to include the mixing and bypass to keep boiler return temperatures above 140. Definitely download the manuals from Lochinvar if you don't have them and plumb it as recommended. If you used a newer modulating/condensing boiler it would simplify your bypass and plumbing requirements as you could serve the loops directly. Because of the low water temperatures, concrete floors are very efficiently heated with modulating/condensing boilers and I think you could expect efficiencies greater than 95% but even with the AFUE difference more like 15% than your 10% estimate your payback is limited.
I strongly recommend that you control this application with a slab sensor rather than air temperature or using dual slab/air inputs. Find a sensor location and floor temperature that keeps your space at the temperature you want to be and control to that point. In my experience it is a much more stable way to control a high mass radiant floor. You'll give up some air temperature control, but you won't be driving your boiler crazy trying to move temperatures around quickly. You may have to crank up the set point in colder weather.
Have fun with your project