I'm in the process of getting quotes for having my 32 year old Weil-Mclain boiler replaced. I am looking at getting a high efficiency gas fired mod/con boiler. At the same time I want to replace my 12 year old gas fired domestic hot water heater with an indirect hot water tank connected to the boiler.
My ranch style house in the Chicago area was built in 1959. It is brick, with an attic and basement. I had approx 18 inches of insulation blown into the attic last year. I'm not sure what if any insulation is behind the plaster walls. The living area is heated by cast baseboard, and there is radiant floor heat in the basement floor. The windows are original single pane, with some large dual pane picture windows. Storm windows are installed on the double hung single pane windows. During the winter I also install that 3m plastic on the inside of the windows for an extra barrier.
The living area is approx 2200 square feet and the basement is about another 1800 square feet. I don't normally run the basement heat.
There are four heating zones, two in the living area and and two in the basement.
So I have had three contractors come through and given me estimates. One specd a 105,000 BTU Burnham Alpine, another specified a 210,000 BTU Lochinvar Knight and the third specified a 200,000 BTU Utica model.
The original Weil-Mclain boiler was rated for 175,000 BTU in and 140,000 BTU out. It has kept the house warm, which leads me to ask why would two of the contractors spec boilers that are specd for around 200,000 BTU's out? Neither of them measured the baseboard, just looked at the old boiler to determine the size of the replacement. Later, I questioned the guy who suggested the Lochinvar and he said that yeah a 150,000 BTU unit would work fine too... Doesn't inspire a lot of confidence.
I'm not a HVAC expert by any means but I want to make sure that I have the right size unit installed in my home. From what I read a heat loss calculation would help with the sizing but the sales guys don't seem inclined to do one when you mention it. Again, I'm not an expert but it seems critical to get the size right to make sure that the boiler condenses correctly to get as close to the 95% efficiency mark. The guys that quoted the 200,000 BTU models say that you can't oversize (and therefore short-cycle) a modulating boiler because it will adjust how much fuel it burns according to load.
The guy who specd the Burnham 105,000 determined how much btu's my baseboard can put out per foot per hour with 180 degree water (said it was around 75,000 BTU) and came up with the 105,000 BTU quote. He said that that would leave enough if I wanted ever to run the basement radiant floor. I looked at the specs on the Burnham Alpine 105,000 btu and it is rated for an output of 96,000 DOE and 83,000 IBR. I am not sure which number to use and it does seem like it is cutting it a little close... It seems like that boiler might be running close to full bore for a lot of the winter. Maybe that's right, I don't know.
Also, correct me if I'm wrong but these boilers should prioritize domestic hot water demands over the hydronic heat demands right? So is it correct not to include extra BTU's in sizing when including an indirect domestic hot water setup?
So here I am, scratching my head and hoping for some advice that would help me make the correct decision for my family's comfort and our financial well-being... :-) I have one more company coming out to give me a quote, I'll see what he says...
I'll be happy to gather any more info that anyone thinks would be necessary.
Thanks for reading!
Best to have a load calc done, to know what size boiler you need.
No real need to increase size of boiler for domestic hot water.