addition with radiant
Last Post 10 Jul 2018 03:56 PM by Dana1. 3 Replies.
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kaparaUser is Offline
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10 Jul 2018 04:08 AM
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
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10 Jul 2018 01:22 PM
To properly design a HR floor heating system, you first need to have a room-by-room heat loss analysis (e.g., ACCA Manual J). This allows you to design the PEX spacing, circuit flow rates and circuit temps that can provide the required heat gain in each room for the locality temp design condition. Perhaps get a copy of John Siegenthaler’s “Modern Hydronic Heating” to learn what is involved in properly designing a HR floor heating system.

You don’t normally run a constant high supply temp (140-150F is quite high). You normally run a variable minimum supply temp that gets the job done efficiently (e.g., 85-95F is pretty typical for my slab HR system). The supply temp variability comes from the boiler adjusting the temp based on the actual outdoor temp (i.e., a method called outdoor reset). This variable supply temp allows the system to handle lower or higher building heat loss conditions (resulting from higher or lower outdoor temp) while still using the lowest supply temp and hence operate more efficiently.

If the system is designed properly, a single supply temp can usually be used for all rooms and circuits. In some cases, mixing valves may be needed. Again, it all come down to properly designing the HR floor heating system based on the room-by-room heat loss analysis.
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10 Jul 2018 02:04 PM
Yes, you can also control water temperature independent of the boiler through mixing valves like the Watts MixTemp. It blends return and supply to deliver the temp you want. And, yes, you can design a system with two water temperatures by using closely spaced tees. Google the term. Have you looked at European-style radiators? https://usa.hudsonreed.com/radiators It would make your refit a whole lot simpler.
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10 Jul 2018 03:56 PM
The min-fire output of the NCB-240 is about 17,000 BTU/hr, and I doubt there would be enough thermal mass in a 280' zone to keep it from short-cycling if operated as a separate zone.

Operating it as a lower-temp branch of the main zone can sometimes work, but you really need to do some of the math first to get that balance right. It may also be possible to design the radiant to match the water temperature requirements of the main house.

The notion that you need 140F or hotter water even for an uninsulated 1950s vintage radiant slab is misplaced, even if your subsoil was permafrost with an outside design temp in negative double-digits F, let alone a location as warm as CA climate zone 2.

A half ton or 3/4 ton minisplit, or a half-ton PTHP would probably be a cheaper/better solution, unless you're absolutely insisting on warm toes on those rare days when temps dip well into the +20s F.
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