The building details:
I recently built a 24 x 36 building behind my Prior Lake Minnesota home. The slab is on 4 inches of Owens Corning FOAMULAR (R20 total) with 1 inch of FOAMULAR (R5) around the outside of the slab.
The boiler is a 80,000 BTU Lars 95% efficient 10:1 Modulating boiler feeding three loops of tubing in the slab in parallel as one zone.
The walls are 10 feet high with R5 FOAMULAR under the siding and R23 Rockwool insulation on the inside. There is one insulate people door, one insulated 9 x 9 garage door and no windows.
The roof is a gambrel design which gives me a 12 x 36 "second floor" area but there are no walls ... it is simply a floor for storage. The roof itself has R23 Rockwool between the rafters and then 2 inches of FOAMULAR on the bottom of the rafters for R33 total.
The challenge:
The general contractor subbed the tubing and boiler installation. The sub has a great deal of plumbing experience but I am not sure he had a lot of radiant heat experience. His sizing calculations appear to have been done on-site after he stapled down the tubing. It was done on a cell phone in about 30 seconds, which did not inspire confidence. When I asked about cooling needs he said he did not have any information about that as "he didn't get into HVAC" stuff. When he installed the boiler he got the intake\exhaust backwards and also suggested there was no need for the outside reset sensor. He did leave it with me so I could install it if I wanted to. He set the outlet temperature at 120 and said the air thermostat would turn the boiler on and off as needed.
I've used multiple calculators to determine the heating and cooling specs for this building and they are all over the map. Cooling goes from 3000 BTU to 8000+ BTU and heating goes from 24000 BTU to over 65000 BTU. I believe the calculators are designed for houses with rooms, windows, basements, etc. and don't work well with a 24 x 36 windowless room that has around 14,000 cubic feed of air in it.
This fall was the first experience running this system. A air thermostat was set to 60 and it seemed like the boiler was on more than I thought it should be and that, on occasion, the indicated air temperature would fall as much as 5 degrees below 60.
I am now going to install the air conditioning and I thought I could resolve several issues with the correct choice.
I could go with one or two simple window air conditioners. There are two spots, one on each of the long walls, what were built into the wall to accommodate window air conditioning units. One unit of the correct capacity would probably do it but two units would mean smaller, more efficient units and two places distributing cool air rather than one ice-box area.
I could go with one or two mini-spits, with the same one\two analysis as the window units.
I could go with one or two mini-spit heat pumps. At the moment this seems to be the most appealing as they would provide cooling in the summer and cheaper heat than the natural gas used by the boiler. I could then do what ever it would take to control the slab temperature at 50 or so and the air temperature, most of the time, wold be controlled with the mini-split heat pump. As an example, two Mitsubishi 12,000 BTU mini-splits would provide adequate cooling ... as low as 2,500 BTU each ... and an additional 6,500 BTU of heating down to a 5 degree outside temperature and up to 12,500 BTU each at 35 degree outside temps.
Any thoughts on a reasonable path that gets me cooling in the summer and possibly heat in the winter?
Thanks,
Arvid
I recently built a 24 x 36 building behind my Prior Lake Minnesota home. The slab is on 4 inches of Owens Corning FOAMULAR (R20 total) with 1 inch of FOAMULAR (R5) around the outside of the slab.
The boiler is a 80,000 BTU Lars 95% efficient 10:1 Modulating boiler feeding three loops of tubing in the slab in parallel as one zone.
The walls are 10 feet high with R5 FOAMULAR under the siding and R23 Rockwool insulation on the inside. There is one insulate people door, one insulated 9 x 9 garage door and no windows.
The roof is a gambrel design which gives me a 12 x 36 "second floor" area but there are no walls ... it is simply a floor for storage. The roof itself has R23 Rockwool between the rafters and then 2 inches of FOAMULAR on the bottom of the rafters for R33 total.
The challenge:
The general contractor subbed the tubing and boiler installation. The sub has a great deal of plumbing experience but I am not sure he had a lot of radiant heat experience. His sizing calculations appear to have been done on-site after he stapled down the tubing. It was done on a cell phone in about 30 seconds, which did not inspire confidence. When I asked about cooling needs he said he did not have any information about that as "he didn't get into HVAC" stuff. When he installed the boiler he got the intake\exhaust backwards and also suggested there was no need for the outside reset sensor. He did leave it with me so I could install it if I wanted to. He set the outlet temperature at 120 and said the air thermostat would turn the boiler on and off as needed.
I've used multiple calculators to determine the heating and cooling specs for this building and they are all over the map. Cooling goes from 3000 BTU to 8000+ BTU and heating goes from 24000 BTU to over 65000 BTU. I believe the calculators are designed for houses with rooms, windows, basements, etc. and don't work well with a 24 x 36 windowless room that has around 14,000 cubic feed of air in it.
This fall was the first experience running this system. A air thermostat was set to 60 and it seemed like the boiler was on more than I thought it should be and that, on occasion, the indicated air temperature would fall as much as 5 degrees below 60.
I am now going to install the air conditioning and I thought I could resolve several issues with the correct choice.
I could go with one or two simple window air conditioners. There are two spots, one on each of the long walls, what were built into the wall to accommodate window air conditioning units. One unit of the correct capacity would probably do it but two units would mean smaller, more efficient units and two places distributing cool air rather than one ice-box area.
I could go with one or two mini-spits, with the same one\two analysis as the window units.
I could go with one or two mini-spit heat pumps. At the moment this seems to be the most appealing as they would provide cooling in the summer and cheaper heat than the natural gas used by the boiler. I could then do what ever it would take to control the slab temperature at 50 or so and the air temperature, most of the time, wold be controlled with the mini-split heat pump. As an example, two Mitsubishi 12,000 BTU mini-splits would provide adequate cooling ... as low as 2,500 BTU each ... and an additional 6,500 BTU of heating down to a 5 degree outside temperature and up to 12,500 BTU each at 35 degree outside temps.
Any thoughts on a reasonable path that gets me cooling in the summer and possibly heat in the winter?
Thanks,
Arvid