Boiler options for a addition?

[Kapara]

I am in the process of converting most of my garage into a studio. 280sqft. I am in an Alliance which is similar to a Eichler. Built in the 50's. Radiant in the slab. From what I have learned these slabs require the boiler to be set at around 140-150 Fahrenheit.

My unit is a Navien Combi Boiler NCB-240 which is multi zone capable.

My issue is that if I run a second zone for the garage what will I do about the boiler temperature? I dont think I can set different temperatures for different zones unless there is a way to reduce the temperature on its way back and mixing the return and less of the new heated water?

My attached garage is a shed style roof so at the lowest point from the garage floor to the top inside uninsulated roof is 8'4" 3/4. The minimum ceiling height on a conversion is 7' 6" which means I have only 10 3/4 inch for both raising the floor and insulating the ceiling.

My idea was to use either upanor fast-trak or Schluter Bekotec but someone said I have to now be careful about water temp as since the floor would be insulated compared to my 1950's floor I would probably have to lower the temp of the boiler to prevent damage to the concrete pour over the pex. For flooring I am on the fence between Tile and vinyl planks but vinyl recommends 80 degrees and no higher.

I am in Energy Zone 2 (California) and am going to get some info tomorrow on code regulation for ceiling insulation but I think because of the code exceptions on small additions so I might not have to put R30 in the ceiling. Also does using Radiant change the R value needed in the ceilings? Will ask building dept tomorrow.

I did find info on a WATTS MixTemp 180. The Hydronic person I spoke to did not mention it so I am wondering if he is not aware of what is possible or if I am just not reading what the MixTemp does.

So what is the minimum recommended depth of the radiant tubing?
What size tubing for 280 sq ft?
liquid screed or concrete pour over tubing?
Better of using heatply or sticking with Fast-Trak/Bekotec?

I hope I provided enough info to answer any questions.

Thanks,

Mark

 

[Dana]

Why does it need 140-150F water? You're WAY out over your skis here, trying to design the solution before you've even calculated the load!

I'd be surprised if ANY radiant slab in the Bay Area needed water that hot to heat with slab radiant (unless you keep the garage door open! ) If you're running it that hot you also get zero to negligible condensing efficiency out of it. If that's what it needs at design condition, the Navien should be run under outdoor reset control, not a fixed output temp.

In order to not short-cycle the NCB240 into low efficiency and a short lifespan there has to be enough radiation in the zone to emit the full ~17,000 BTU/hr. Assuming you have at least 250 square feet of slab available for radiant that would be 68 BTU/hr per square foot to balance perfectly, which would fry your feet if operated continuously. With above-the-slab radiant there also isn't sufficient thermal mass in radiation to keep it from short cycling during calls for heat. You COULD add a buffer tank to lengthen the minimum burn times, but it starts to get messy and expensive. How important is that warm floor to you?

A half ton Mitsubishi -FH06NA mini split can easily heat & cool that much space in your climate. Just sayin'... Your design heating load is probably about 2,500 BTU/hr (run the math) at San Rafael's 99% outside design temp of 31F , and the cooling load is probably less than 4000 BTU/hr. The FH06 can deliver 6000 BTU/hr cooling, 8700 BTU/hr heating at AHRI conditions, but can throttle back to 1600 BTU/hr under light load, so it would be able to modulate at extremely high efficiency much of the time.

Above the slab radiant can deliver 10 BTU/hr per square foot at temperatures well under 120F, not that it matters- you'd be short-cycling the Navien to death on zone calls with low mass radiation that emits less than 10,000 BTU/hr. If you really want to go there we can, but with caveats. You could try to run the whole house as a single zone, using a mixing valve to lower the water temp for the studio zone, but it may be difficult to achieve true room temperature balance over the entire range of heat load.

To get a handle on your actual whole house design load (prior to he extension) and BTU per square foot ratio at design condition for the existing house, run some fuel use load calculations on the wintertime gas bills. With your not-very-cool design temp and high solar gain there will be some error to factor in, but try it out using base 60F and base 65F weather data, which should provide upper and lower bounds on the heat load.