So, I just finished building a house … two story … 2400 sq ft each floor … 4800 total … in VT where it gets cold.

 

I have natural gas.

 

I choose a forced air furnace … sized it  … choose a 2 stage 120,000 btu unit (about 80,000 on low).

 

Works great … seldom ever goes to high.

 

So, I am now building a workshop.

 

3 bays isolated from each other .. 1000 sq ft each (20 x 50 each with 16 ft ceilings) .. plus an office  …. plus a 1000 sq ft loft … so about 4500 sq ft total.

 

Because there are now 5 distinct areas with one (the office / bathroom, so really 6 … two are much smaller than the others) … I was thinking about hot water heating.

 

Originally I thought just a single large furnace with zones (like I did to the house … I have three zones).

 

Then I thought about 3 or 4 simple single stage furnaces …one for each …but that get a bit pricey for a workshop.

 

Then I thought about a hot water system.  Simple enough … tankless boiler (like my hot water heater in the house) …. a bunch of PEX and a couple of ceiling hung radiators with fans in each area.

 

Now, here is my problem ….

 

I look at Home Depot … 199,000 BTUh boiler …. 9.3 gpm … 100 – 180 deg heating water.

 

I look at a Modine heat exchanger chart ….  HSB/HC 108 … at 9.7 GMP and 180 deg water, I am only getting out about 70,000 BTUh …why ?????

 

If the boiler has a 199,000 BTUh rating but using it’s full capacity I an only getting 70,000 BTUh out … what happened?????

 

I am a true DIY guy (thus it took 7 years to build a house) .. have put in a few furnaces (gas and electric) and A/C units … all worked great.

 

I have just never touched a hot water boiler before.

 

If I can get the full boiler heat out of the exchangers, I would have no problem … but less than half will never heat the building.

 

Can anyone explain what is going on or if I am making some kind of mistake reading the tables.

 

Thanks  so much …. Mike